peres

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Author name code: peres
ADS astronomy entries on 2022-09-14
author:"Peres, Giovanni"

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Title: A Spatially Resolved Study of Hard X-Ray Emission in Kepler's
Supernova Remnant: Indications of Different Regimes of Particle
Acceleration
Authors: Sapienza, Vincenzo; Miceli, Marco; Bamba, Aya; Katsuda,
Satoru; Nagayoshi, Tsutomu; Terada, Yukikatsu; Bocchino, Fabrizio;
Orlando, Salvatore; Peres, Giovanni
2022ApJ...935..152S Altcode: 2022arXiv220706804S
Synchrotron X-ray emission in young supernova remnants (SNRs) is
a powerful diagnostic tool to study the population of high-energy
electrons accelerated at the shock front and the acceleration
process. We performed a spatially resolved spectral analysis of NuSTAR
and XMM-Newton observations of the young Kepler's SNR, aiming to
study in detail its nonthermal emission in hard X-rays. We selected
a set of regions all around the rim of the shell and extracted the
corresponding spectra. The spectra were analyzed by adopting a model
of synchrotron radiation in the loss-limited regime, to constrain
the dependence of the cutoff energy of the synchrotron radiation on
the shock velocity. We identify two different regimes of particle
acceleration, characterized by different Bohm factors. In the north,
where the shock interacts with a dense circumstellar medium (CSM),
we found a more efficient acceleration than in the south, where the
shock velocity is higher and there are no signs of shock interaction
with the dense CSM. Our results suggest an enhanced efficiency of
the acceleration process in regions where the shock-CSM interaction
generates an amplified and turbulent magnetic field. By combining hard
X-ray spectra with radio and γ-ray observations of Kepler's SNR, we
modeled the spectral energy distribution. In the light of our results
we propose that the observed γ-ray emission is mainly hadronic and
originates in the northern part of the shell.

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Title: The supernova remnant SN 1006 as a Galactic particle
accelerator
Authors: Giuffrida, Roberta; Miceli, Marco; Caprioli, Damiano;
Decourchelle, Anne; Vink, Jacco; Orlando, Salvatore; Bocchino,
Fabrizio; Greco, Emanuele; Peres, Giovanni
2022NatCo..13.5098G Altcode: 2022arXiv220814491G
The origin of cosmic rays is a pivotal open issue of high-energy
astrophysics. Supernova remnants are strong candidates to be the
Galactic factory of cosmic rays, their blast waves being powerful
particle accelerators. However, supernova remnants can power the
observed flux of cosmic rays only if they transfer a significant
fraction of their kinetic energy to the accelerated particles, but
conclusive evidence for such efficient acceleration is still lacking. In
this scenario, the shock energy channeled to cosmic rays should
induce a higher post-shock density than that predicted by standard
shock conditions. Here we show this effect, and probe its dependence
on the orientation of the ambient magnetic field, by analyzing deep
X-ray observations of the Galactic remnant of SN 1006. By comparing
our results with state-of-the-art models, we conclude that SN 1006 is
an efficient source of cosmic rays and obtain an observational support
for the quasi-parallel acceleration mechanism.

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Title: Additional Evidence for a Pulsar Wind Nebula in the Heart of
SN 1987A from Multiepoch X-Ray Data and MHD Modeling
Authors: Greco, Emanuele; Miceli, Marco; Orlando, Salvatore; Olmi,
Barbara; Bocchino, Fabrizio; Nagataki, Shigehiro; Sun, Lei; Vink,
Jacco; Sapienza, Vincenzo; Ono, Masaomi; Dohi, Akira; Peres, Giovanni
2022ApJ...931..132G Altcode: 2022arXiv220406804G
Since the day of its explosion, supernova (SN) 1987A has been closely
monitored to study its evolution and to detect its central compact
relic. In fact, the formation of a neutron star is strongly supported
by the detection of neutrinos from the SN. However, besides the
detection in the Atacama Large Millimeter/submillimeter Array (ALMA)
data of a feature that is compatible with the emission arising from a
protopulsar wind nebula (PWN), the only hint of the existence of such
an elusive compact object is provided by the detection of hard emission
in NuSTAR data up to ~20 keV. We report on the simultaneous analysis of
multiepoch observations of SN 1987A performed with Chandra, XMM-Newton,
and NuSTAR. We also compare the observations with a state-of-the-art
three-dimensional magnetohydrodynamic simulation of SN 1987A. A heavily
absorbed power law, consistent with the emission from a PWN embedded in
the heart of SN 1987A, is needed to properly describe the high-energy
part of the observed spectra. The spectral parameters of the best-fit
power law are in agreement with the previous estimate, and exclude
diffusive shock acceleration as a possible mechanism responsible for
the observed nonthermal emission. The information extracted from our
analysis is used to infer the physical characteristics of the pulsar
and the broadband emission from its nebula, in agreement with the ALMA
data. Analysis of the synthetic spectra also shows that, in the near
future, the main contribution to the Fe K emission line will originate
in the outermost shocked ejecta of SN 1987A.

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Title: Negative and positive feedback from a supernova remnant with
SHREC: a detailed study of the shocked gas in IC443
Authors: Cosentino, G.; Jiménez-Serra, I.; Tan, J. C.; Henshaw,
J. D.; Barnes, A. T.; Law, C. -Y.; Zeng, S.; Fontani, F.; Caselli,
P.; Viti, S.; Zahorecz, S.; Rico-Villas, F.; Megías, A.; Miceli,
M.; Orlando, S.; Ustamujic, S.; Greco, E.; Peres, G.; Bocchino, F.;
Fedriani, R.; Gorai, P.; Testi, L.; Martín-Pintado, J.
2022MNRAS.511..953C Altcode: 2022MNRAS.tmp...90C; 2022arXiv220103008C
Supernova remnants (SNRs) contribute to regulate the star formation
efficiency and evolution of galaxies. As they expand into the
interstellar medium (ISM), they transfer vast amounts of energy
and momentum that displace, compress, and heat the surrounding
material. Despite the extensive work in galaxy evolution models, it
remains to be observationally validated to what extent the molecular ISM
is affected by the interaction with SNRs. We use the first results of
the ESO-ARO Public Spectroscopic Survey SHREC to investigate the shock
interaction between the SNR IC443 and the nearby molecular clump G. We
use high-sensitivity SiO(2-1) and H13CO+(1-0)
maps obtained by SHREC together with SiO(1-0) observations obtained with
the 40-m telescope at the Yebes Observatory. We find that the bulk of
the SiO emission is arising from the ongoing shock interaction between
IC443 and clump G. The shocked gas shows a well-ordered kinematic
structure, with velocities blue-shifted with respect to the central
velocity of the SNR, similar to what observed towards other SNR-cloud
interaction sites. The shock compression enhances the molecular gas
density, n(H2), up to &gt;105 cm-3,
a factor of &gt;10 higher than the ambient gas density and similar to
values required to ignite star formation. Finally, we estimate that
up to 50 per cent of the momentum injected by IC443 is transferred
to the interacting molecular material. Therefore, the molecular ISM
may represent an important momentum carrier in sites of SNR-cloud
interactions.

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Title: Evidence for past interaction with an asymmetric circumstellar
shell in the young SNR Cassiopeia A
Authors: Orlando, S.; Wongwathanarat, A.; Janka, H. -T.; Miceli, M.;
Nagataki, S.; Ono, M.; Bocchino, F.; Vink, J.; Milisavljevic, D.;
Patnaude, D. J.; Peres, G.
2022arXiv220201643O Altcode:
Observations of the SNR Cassiopeia A (Cas A) show asymmetries in
the reverse shock that cannot be explained by models describing
a remnant expanding through a spherically symmetric wind of the
progenitor star. We investigate whether a past interaction of Cas A
with an asymmetric circumstellar shell can account for the observed
asymmetries. We performed 3D MHD simulations that describe the
remnant evolution from the SN to its interaction with a circumstellar
shell. The initial conditions are provided by a 3D neutrino-driven
SN model whose morphology resembles Cas A. We explored the parameter
space of the shell, searching for a set of parameters able to produce
reverse shock asymmetries at the age of 350 years analogous to those
observed in Cas A. The interaction of the remnant with the shell can
produce asymmetries resembling those observed in the reverse shock if
the shell was asymmetric with the densest portion in the nearside to
the northwest (NW). The reverse shock shows the following asymmetries
at the age of Cas A: i) it moves inward in the observer frame in the
NW region, while it moves outward in other regions; ii) the geometric
center of the reverse shock is offset to the NW from the geometric
center of the forward shock; iii) the reverse shock in the NW region
has enhanced nonthermal emission because, there, the ejecta enter
the reverse shock with a higher velocity (between 4000 and 7000 km/s)
than in other regions (below 2000 km/s). The asymmetries observed in
the reverse shock of Cas A can be interpreted as signatures of the
interaction of the remnant with an asymmetric circumstellar shell that
occurred between 180 and 240 years after the SN event. We suggest that
the shell was, most likely, the result of a massive eruption from the
progenitor star that occurred between $10^4$ and $10^5$ years prior
to core-collapse. We estimate a total mass of the shell of the order
2 Msun.

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Title: Correcting the effect of stellar spots on ARIEL transmission
spectra - II. The limb-darkening effect
Authors: Cracchiolo, G.; Micela, G.; Morello, G.; Peres, G.
2021MNRAS.507.6118C Altcode: 2021arXiv210812526C; 2021MNRAS.tmp.2273C
This paper is part of an effort to correct the transmission spectra of
a transiting planet orbiting an active star. In Paper I (Cracchiolo,
Micela &amp; Peres 2020), we have demonstrated a methodology to minimize
the potential bias induced by unocculted star spots on the transmission
spectrum, assuming a spot model parametrized by filling factor and
temperature. In this work, we introduce the limb-darkening effect,
therefore the position of the spot in the stellar disc and the impact
parameter of the transiting planet now play a key role. The method is
tested on simulations of planetary transits of three representative
kinds of planetary systems, at ARIEL resolution. We find that a
realistic treatment of the limb darkening is required to reliably
estimate both the spots parameters and the transmission spectrum of
the transiting planet. Furthermore, we show that the influence of
the spots on the retrieval of the planetary transmission spectrum
is significant for spots close to the centre of the star, covering a
fraction greater than 0.05 and with a temperature contrast greater than
$500\, {\rm K}$, and that for these cases our method can confidently
extract the transmission spectrum and the impact parameter of the
transiting planet for both cases of occulted and not occulted spots,
provided that we have an accurate characterization of the stellar
parameters and a reliable simulator of the instrument performances.

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Title: Modeling the remnants of core-collapse supernovae from luminous
blue variable stars
Authors: Ustamujic, S.; Orlando, S.; Miceli, M.; Bocchino, F.; Limongi,
M.; Chieffi, A.; Trigilio, C.; Umana, G.; Bufano, F.; Ingallinera,
A.; Peres, G.
2021A&A...654A.167U Altcode: 2021arXiv210801951U
Context. Luminous blue variable stars (LBVs) are massive evolved stars
that suffer sporadic and violent mass-loss events. They have been
proposed as the progenitors of some core-collapse supernovae (SNe),
but this idea is still debated because of a lack of strong evidence. As
supernova remnants (SNRs) can carry in their morphology the fingerprints
of the progenitor stars as well as of the inhomogeneous circumstellar
medium (CSM) sculpted by the progenitors, the study of SNRs from LBVs
could help to place core-collapse SNe in context with the evolution
of massive stars. Aims: We investigate the physical, chemical,
and morphological properties of the remnants of SNe originating from
LBVs in order to search for signatures in the ejecta distribution
and morphology of the remnants that could reveal the nature of the
progenitors. Methods: As a template of LBVs, we considered
the LBV candidate Gal 026.47+0.02. We selected a grid of models that
describe the evolution of a massive star with properties consistent
with those of Gal 026.47+0.02 and its final fate as a core-collapse
SN. We developed a three-dimensional hydrodynamic model that follows
the post-explosion evolution of the ejecta from the breakout of the
shock wave at the stellar surface to the interaction of the SNR with a
CSM characterized by two dense nested toroidal shells, parametrized in
agreement with multi-wavelength observations of Gal 026.47+0.02. 
Results: Our models show a strong interaction of the blast wave with
the CSM which determines an important slowdown of the expansion of the
ejecta in the equatorial plane where the two shells lay, determining
a high degree of asymmetry in the remnant. After ≈10 000 yr of
evolution, the ejecta show an elongated shape forming a broad jet-like
structure caused by the interaction with the shells and oriented along
the axis of the toroidal shells. Models with high explosion energy
show Fe-rich internal ejecta distributions surrounded by an elongated
Si-rich structure with a more diffuse O-rich ejecta all around. Models
with low explosion energy instead show a more homogeneous distribution
of chemical elements with a very low presence of Fe-group elements. Conclusions: The geometry and density distribution of the CSM where
a LBV star goes SN are fundamental in determining the properties of the
resulting SNR. For all the LBV-like progenitors explored here, we found
that the remnants show a common morphology, namely elongated ejecta with
an internal jet-like structure, which reflects the inhomogeneous and
dense pre-SN CSM surrounding the star. Movies are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202141569/olm">https://www.aanda.org</A>

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Title: Modeling particle acceleration and non-thermal emission in
supernova remnants
Authors: Orlando, S.; Miceli, M.; Ustamujic, S.; Tutone, A.; Greco,
E.; Petruk, O.; Bocchino, F.; Peres, G.
2021NewA...8601566O Altcode: 2020arXiv201213394O
According to the most popular model for the origin of cosmic
rays (CRs), supernova remnants (SNRs) are the site where CRs are
accelerated. Observations across the electromagnetic spectrum support
this picture through the detection of non-thermal emission that is
compatible with being synchrotron or inverse Compton radiation from high
energy electrons, or pion decay due to proton-proton interactions. These
observations of growing quantity and quality promise to unveil
many aspects of CRs acceleration and require more and more accurate
tools for their interpretation. Here, we show how multi-dimensional
MHD models of SNRs, including the effects on shock dynamics due to
back-reaction of accelerated CRs and the synthesis of non-thermal
emission, turned out to be very useful to investigate the signatures of
CRs acceleration and to put constraints on the acceleration mechanism
of high energy particles. These models have been used to interpret
accurately observations of SNRs in various bands (radio, X-ray and
γ-ray) and to extract from them key information about CRs acceleration.

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Title: X-ray emitting structures in the Vela SNR: ejecta anisotropies
and progenitor stellar wind residuals
Authors: Sapienza, V.; Miceli, M.; Peres, G.; Bocchino, F.; Orlando,
S.; Greco, E.; Combi, J. A.; García, F.; Sasaki, M.
2021A&A...649A..56S Altcode: 2021arXiv210309637S
Context. The Vela supernova remnant (SNR) shows several ejecta fragments
(or shrapnel) protruding beyond the forward shock, which are most
likely relics of anisotropies that developed during the supernova
(SN) explosion. Recent studies have revealed high Si abundance in two
shrapnel (shrapnel A and G), located in opposite directions with respect
to the SNR center. This suggests the possible existence of a Si-rich
jet-counterjet structure, similar to that observed in the SNR Cassiopea
A. Aims: We analyzed an XMM-Newton observation of a bright clump,
behind shrapnel G, which lies along the direction connecting shrapnel
A and G. The aim is to study the physical and chemical properties of
this clump to ascertain whether it is part of this putative jet-like
structure. Methods: We produced background-corrected and
adaptively-smoothed count-rate images and median photon energy maps,
and performed a spatially resolved spectral analysis. Results: We
identified two structures with different physical properties. The first
one is remarkably elongated along the direction connecting shrapnel
A and G. Its X-ray spectrum is much softer than that of the other
two shrapnel, to the point of hindering the determination of the Si
abundance; however, its physical and chemical properties are consistent
with those of shrapnel A and shrapnel G. The second structure, running
along the southeast-northwest direction, has a higher temperature
and appears similar to a thin filament. By analyzing the ROSAT data,
we have found that this filament is part of a very large and coherent
structure that we identified in the western rim of the shell. 
Conclusions: We obtained a thorough description of the collimated,
jet-like tail of shrapnel G in Vela SNR. In addition we discovered
a coherent and very extended feature roughly perpendicular to the
jet-like structure that we interpret as a signature of an earlier
interaction of the remnant with the stellar wind of its progenitor
star. The peculiar Ne/O ratio we found in the wind residual may be
suggestive of a Wolf-Rayet progenitor for Vela SNR, though further
analysis is required to address this point.

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Title: Modeling the mixed-morphology supernova remnant IC 443. Origins
of its complex morphology and X-ray emission
Authors: Ustamujic, S.; Orlando, S.; Greco, E.; Miceli, M.; Bocchino,
F.; Tutone, A.; Peres, G.
2021A&A...649A..14U Altcode: 2020arXiv201208017U
Context. The morphology and the distribution of material observed in
supernova remnants (SNRs) reflect the interaction of the supernova
(SN) blast wave with the ambient environment, the physical processes
associated with the SN explosion, and the internal structure of the
progenitor star. IC 443 is a mixed-morphology (MM) SNR located in a
quite complex environment: it interacts with a molecular cloud in the
northwestern and southeastern areas and with an atomic cloud in the
northeast. Aims: In this work, we aim to investigate the origin
of the complex morphology and multi-thermal X-ray emission observed
in SNR IC 443 through the study of the effect of the inhomogeneous
ambient medium in shaping its observed structure and an exploration
of the main parameters characterizing the remnant. Methods: We
developed a 3D hydrodynamic (HD) model for IC 443, which describes the
interaction of the SNR with the environment, parametrized in agreement
with the results of the multi-wavelength data analysis. We performed
an ample exploration of the parameter space describing the initial
blast wave and the environment, including the mass of the ejecta,
the energy and position of the explosion, as well as the density,
structure, and geometry of the surrounding clouds. From the simulations,
we synthesized the X-ray emission maps and spectra and compared them
with actual X-ray data collected by XMM-Newton. Results: Our
model explains the origin of the complex X-ray morphology of SNR IC 443
in a natural way, with the ability to reproduce, for the first time,
most of the observed features, including the centrally-peaked X-ray
morphology (characteristic of MM SNRs) when considering the origin
of the explosion at the position where the pulsar wind nebula CXOU
J061705.3+222127 was at the time of the explosion. In the model that
best reproduces the observations, the mass of the ejecta and the energy
of the explosion are ~7 M⊙ and ~1 × 1051
erg, respectively. From the exploration of the parameter space, we
find that the density of the clouds is n &gt; 300 cm−3
and that the age of SNR IC 443 is ~8000 yr. Conclusions: The
observed inhomogeneous ambient medium is the main property responsible
for the complex structure and the X-ray morphology of SNR IC 443,
resulting in a very asymmetric distribution of the ejecta due to the
off-centered location of the explosion inside the cavity formed by the
clouds. It can be argued that the centrally peaked morphology (typical
of MM SNRs) is a natural consequence of the interaction with the complex
environment. A combination of high resolution X-ray observations and
accurate 3D HD modeling is needed to confirm whether this scenario
is applicable to other MM SNRs. Movies are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202039940/olm">https://www.aanda.org</A>

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Title: Ariel: Enabling planetary science across light-years
Authors: Tinetti, Giovanna; Eccleston, Paul; Haswell, Carole; Lagage,
Pierre-Olivier; Leconte, Jérémy; Lüftinger, Theresa; Micela, Giusi;
Min, Michel; Pilbratt, Göran; Puig, Ludovic; Swain, Mark; Testi,
Leonardo; Turrini, Diego; Vandenbussche, Bart; Rosa Zapatero Osorio,
Maria; Aret, Anna; Beaulieu, Jean-Philippe; Buchhave, Lars; Ferus,
Martin; Griffin, Matt; Guedel, Manuel; Hartogh, Paul; Machado, Pedro;
Malaguti, Giuseppe; Pallé, Enric; Rataj, Mirek; Ray, Tom; Ribas,
Ignasi; Szabó, Robert; Tan, Jonathan; Werner, Stephanie; Ratti,
Francesco; Scharmberg, Carsten; Salvignol, Jean-Christophe; Boudin,
Nathalie; Halain, Jean-Philippe; Haag, Martin; Crouzet, Pierre-Elie;
Kohley, Ralf; Symonds, Kate; Renk, Florian; Caldwell, Andrew; Abreu,
Manuel; Alonso, Gustavo; Amiaux, Jerome; Berthé, Michel; Bishop,
Georgia; Bowles, Neil; Carmona, Manuel; Coffey, Deirdre; Colomé,
Josep; Crook, Martin; Désjonqueres, Lucile; Díaz, José J.; Drummond,
Rachel; Focardi, Mauro; Gómez, Jose M.; Holmes, Warren; Krijger,
Matthijs; Kovacs, Zsolt; Hunt, Tom; Machado, Richardo; Morgante,
Gianluca; Ollivier, Marc; Ottensamer, Roland; Pace, Emanuele; Pagano,
Teresa; Pascale, Enzo; Pearson, Chris; Møller Pedersen, Søren; Pniel,
Moshe; Roose, Stéphane; Savini, Giorgio; Stamper, Richard; Szirovicza,
Peter; Szoke, Janos; Tosh, Ian; Vilardell, Francesc; Barstow, Joanna;
Borsato, Luca; Casewell, Sarah; Changeat, Quentin; Charnay, Benjamin;
Civiš, Svatopluk; Coudé du Foresto, Vincent; Coustenis, Athena;
Cowan, Nicolas; Danielski, Camilla; Demangeon, Olivier; Drossart,
Pierre; Edwards, Billy N.; Gilli, Gabriella; Encrenaz, Therese; Kiss,
Csaba; Kokori, Anastasia; Ikoma, Masahiro; Morales, Juan Carlos;
Mendonça, João; Moneti, Andrea; Mugnai, Lorenzo; García Muñoz,
Antonio; Helled, Ravit; Kama, Mihkel; Miguel, Yamila; Nikolaou,
Nikos; Pagano, Isabella; Panic, Olja; Rengel, Miriam; Rickman,
Hans; Rocchetto, Marco; Sarkar, Subhajit; Selsis, Franck; Tennyson,
Jonathan; Tsiaras, Angelos; Venot, Olivia; Vida, Krisztián; Waldmann,
Ingo P.; Yurchenko, Sergey; Szabó, Gyula; Zellem, Rob; Al-Refaie,
Ahmed; Perez Alvarez, Javier; Anisman, Lara; Arhancet, Axel; Ateca,
Jaume; Baeyens, Robin; Barnes, John R.; Bell, Taylor; Benatti, Serena;
Biazzo, Katia; Błęcka, Maria; Bonomo, Aldo Stefano; Bosch, José;
Bossini, Diego; Bourgalais, Jeremy; Brienza, Daniele; Brucalassi,
Anna; Bruno, Giovanni; Caines, Hamish; Calcutt, Simon; Campante, Tiago;
Canestrari, Rodolfo; Cann, Nick; Casali, Giada; Casas, Albert; Cassone,
Giuseppe; Cara, Christophe; Carmona, Manuel; Carone, Ludmila; Carrasco,
Nathalie; Changeat, Quentin; Chioetto, Paolo; Cortecchia, Fausto;
Czupalla, Markus; Chubb, Katy L.; Ciaravella, Angela; Claret, Antonio;
Claudi, Riccardo; Codella, Claudio; Garcia Comas, Maya; Cracchiolo,
Gianluca; Cubillos, Patricio; Da Peppo, Vania; Decin, Leen; Dejabrun,
Clemence; Delgado-Mena, Elisa; Di Giorgio, Anna; Diolaiti, Emiliano;
Dorn, Caroline; Doublier, Vanessa; Doumayrou, Eric; Dransfield,
Georgina; Dumaye, Luc; Dunford, Emma; Jimenez Escobar, Antonio; Van
Eylen, Vincent; Farina, Maria; Fedele, Davide; Fernández, Alejandro;
Fleury, Benjamin; Fonte, Sergio; Fontignie, Jean; Fossati, Luca; Funke,
Bernd; Galy, Camille; Garai, Zoltán; García, Andrés; García-Rigo,
Alberto; Garufi, Antonio; Germano Sacco, Giuseppe; Giacobbe, Paolo;
Gómez, Alejandro; Gonzalez, Arturo; Gonzalez-Galindo, Francisco;
Grassi, Davide; Griffith, Caitlin; Guarcello, Mario Giuseppe;
Goujon, Audrey; Gressier, Amélie; Grzegorczyk, Aleksandra; Guillot,
Tristan; Guilluy, Gloria; Hargrave, Peter; Hellin, Marie-Laure;
Herrero, Enrique; Hills, Matt; Horeau, Benoit; Ito, Yuichi; Jessen,
Niels Christian; Kabath, Petr; Kálmán, Szilárd; Kawashima, Yui;
Kimura, Tadahiro; Knížek, Antonín; Kreidberg, Laura; Kruid, Ronald;
Kruijssen, J. M. Diederik; Kubelík, Petr; Lara, Luisa; Lebonnois,
Sebastien; Lee, David; Lefevre, Maxence; Lichtenberg, Tim; Locci,
Daniele; Lombini, Matteo; Sanchez Lopez, Alejandro; Lorenzani, Andrea;
MacDonald, Ryan; Magrini, Laura; Maldonado, Jesus; Marcq, Emmanuel;
Migliorini, Alessandra; Modirrousta-Galian, Darius; Molaverdikhani,
Karan; Molinari, Sergio; Mollière, Paul; Moreau, Vincent; Morello,
Giuseppe; Morinaud, Gilles; Morvan, Mario; Moses, Julianne I.; Mouzali,
Salima; Nakhjiri, Nariman; Naponiello, Luca; Narita, Norio; Nascimbeni,
Valerio; Nikolaou, Athanasia; Noce, Vladimiro; Oliva, Fabrizio;
Palladino, Pietro; Papageorgiou, Andreas; Parmentier, Vivien; Peres,
Giovanni; Pérez, Javier; Perez-Hoyos, Santiago; Perger, Manuel; Cecchi
Pestellini, Cesare; Petralia, Antonino; Philippon, Anne; Piccialli,
Arianna; Pignatari, Marco; Piotto, Giampaolo; Podio, Linda; Polenta,
Gianluca; Preti, Giampaolo; Pribulla, Theodor; Lopez Puertas, Manuel;
Rainer, Monica; Reess, Jean-Michel; Rimmer, Paul; Robert, Séverine;
Rosich, Albert; Rossi, Loic; Rust, Duncan; Saleh, Ayman; Sanna,
Nicoletta; Schisano, Eugenio; Schreiber, Laura; Schwartz, Victor;
Scippa, Antonio; Seli, Bálint; Shibata, Sho; Simpson, Caroline;
Shorttle, Oliver; Skaf, N.; Skup, Konrad; Sobiecki, Mateusz; Sousa,
Sergio; Sozzetti, Alessandro; Šponer, Judit; Steiger, Lukas; Tanga,
Paolo; Tackley, Paul; Taylor, Jake; Tecza, Matthias; Terenzi, Luca;
Tremblin, Pascal; Tozzi, Andrea; Triaud, Amaury; Trompet, Loïc; Tsai,
Shang-Min; Tsantaki, Maria; Valencia, Diana; Carine Vandaele, Ann;
Van der Swaelmen, Mathieu; Adibekyan, Vardan; Vasisht, Gautam; Vazan,
Allona; Del Vecchio, Ciro; Waltham, Dave; Wawer, Piotr; Widemann,
Thomas; Wolkenberg, Paulina; Hou Yip, Gordon; Yung, Yuk; Zilinskas,
Mantas; Zingales, Tiziano; Zuppella, Paola
2021arXiv210404824T Altcode:
Ariel, the Atmospheric Remote-sensing Infrared Exoplanet Large-survey,
was adopted as the fourth medium-class mission in ESA's Cosmic Vision
programme to be launched in 2029. During its 4-year mission, Ariel
will study what exoplanets are made of, how they formed and how they
evolve, by surveying a diverse sample of about 1000 extrasolar planets,
simultaneously in visible and infrared wavelengths. It is the first
mission dedicated to measuring the chemical composition and thermal
structures of hundreds of transiting exoplanets, enabling planetary
science far beyond the boundaries of the Solar System. The payload
consists of an off-axis Cassegrain telescope (primary mirror 1100
mm x 730 mm ellipse) and two separate instruments (FGS and AIRS)
covering simultaneously 0.5-7.8 micron spectral range. The satellite
is best placed into an L2 orbit to maximise the thermal stability
and the field of regard. The payload module is passively cooled via a
series of V-Groove radiators; the detectors for the AIRS are the only
items that require active cooling via an active Ne JT cooler. The Ariel
payload is developed by a consortium of more than 50 institutes from 16
ESA countries, which include the UK, France, Italy, Belgium, Poland,
Spain, Austria, Denmark, Ireland, Portugal, Czech Republic, Hungary,
the Netherlands, Sweden, Norway, Estonia, and a NASA contribution.

---------------------------------------------------------
Title: Correcting the effect of stellar spots on ARIEL transmission
spectra
Authors: Cracchiolo, G.; Micela, G.; Peres, G.
2021MNRAS.501.1733C Altcode: 2020MNRAS.tmp.3412C; 2020arXiv201110085C
The goal of this study is to assess the impact of the stellar spots
on the extraction of the planetary transmission spectra observed by
ARIEL. We develop a method to model the stellar spectrum of a star in
the presence of spots by using the out-of-transit observations. It is
based on a chi squared minimization procedure of the out-of-transit
spectrum on a grid of stellar spectra with different sizes and
temperatures of the spots. The approach allows us also to study
the temporal evolution of the spots when comparing stellar spectra
observed at different epochs. We also present a method to correct
the transit depth variations due to non-occulted stellar spots and
estimate the error we introduce if we apply the same correction
to crossings over the stellar spots. The method is tested on three
types of stellar targets that ARIEL will observe in its 4-yr mission
lifetime. In all the explored cases, the approach allows us to reliably
recover the spot parameters (size and temperature) from out-of-transit
observations and, for non-occulted spots, to confidently recover the
planetary atmosphere transmission spectrum within the noise level
(with average uncertainty of at most $3.3{{\ \rm per\ cent}}$ of
the planetary signal). Conversely, we find systematic biases in the
inferred planetary spectra due to the occulted spots, with measurable
effects for the brightest targets especially for more contrasted spots.

---------------------------------------------------------
Title: Indication of a Pulsar Wind Nebula in the Hard X-Ray Emission
from SN 1987A
Authors: Greco, Emanuele; Miceli, Marco; Orlando, Salvatore; Olmi,
Barbara; Bocchino, Fabrizio; Nagataki, Shigehiro; Ono, Masaomi; Dohi,
Akira; Peres, Giovanni
2021ApJ...908L..45G Altcode: 2021arXiv210109029G
Since the day of its explosion, SN 1987A (SN87A) was closely
monitored with the aim to study its evolution and to detect its
central compact relic. The detection of neutrinos from the supernova
strongly supports the formation of a neutron star (NS). However, the
constant and fruitless search for this object has led to different
hypotheses on its nature. To date, the detection in the Atacama Large
Millimeter/submillimeter Array data of a feature that is somehow
compatible with the emission arising from a proto-pulsar wind nebula
(PWN) is the only hint of the existence of such elusive compact
object. Here we tackle this 33 yr old issue by analyzing archived
observations of SN87A performed by Chandra and NuSTAR in different
years. We firmly detect nonthermal emission in the 10-20 kev energy
band, due to synchrotron radiation. The possible physical mechanism
powering such emission is twofold: diffusive shock acceleration
(DSA) or emission arising from an absorbed PWN. By relating a
state-of-the-art magnetohydrodynamic simulation of SN87A to the actual
data, we reconstruct the absorption pattern of the PWN embedded in the
remnant and surrounded by cold ejecta. We found that, even though the
DSA scenario cannot be firmly excluded, the most likely scenario that
well explains the data is that of PWN emission.

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Title: The fully developed remnant of a neutrino-driven
supernova. Evolution of ejecta structure and asymmetries in SNR
Cassiopeia A
Authors: Orlando, S.; Wongwathanarat, A.; Janka, H. -T.; Miceli, M.;
Ono, M.; Nagataki, S.; Bocchino, F.; Peres, G.
2021A&A...645A..66O Altcode: 2020A&A...645A..66O; 2020arXiv200901789O
Context. The remnants of core-collapse supernovae (SNe) are probes of
the physical processes associated with their parent SNe. Aims:
Here we aim to explore to which extent the remnant keeps memory of the
asymmetries that develop stochastically in the neutrino-heating layer
due to hydrodynamic instabilities (e.g., convective overturn and the
standing accretion shock instability; SASI) during the first second
after core bounce. Methods: We coupled a three-dimensional (3D)
hydrodynamic model of a neutrino-driven SN explosion, which has the
potential to reproduce the observed morphology of the Cassiopeia A (Cas
A) remnant, with 3D (magneto)-hydrodynamic simulations of the remnant
formation. The simulations cover ≈2000 yr of expansion and include
all physical processes relevant to describe the complexities in the SN
evolution and the subsequent interaction of the stellar debris with
the wind of the progenitor star. Results: The interaction of
large-scale asymmetries left from the earliest phases of the explosion
with the reverse shock produces, at the age of ≈350 yr, an ejecta
structure and a remnant morphology which are remarkably similar to those
observed in Cas A. Small-scale structures in the large-scale Fe-rich
plumes that were created during the initial stages of the SN, combined
with hydrodynamic instabilities that develop after the passage of the
reverse shock, naturally produce a pattern of ring- and crown-like
structures of shocked ejecta. The consequence is a spatial inversion
of the ejecta layers with Si-rich ejecta being physically interior to
Fe-rich ejecta. The full-fledged remnant shows voids and cavities in
the innermost unshocked ejecta, which are physically connected with
ring-like features of shocked ejecta in the main shell in most cases,
resulting from the expansion of Fe-rich plumes and their inflation due
to the decay of radioactive species. The asymmetric distributions of
44Ti and 56Fe, which are mostly concentrated in
the northern hemisphere, and pointing opposite to the kick velocity of
the neutron star, as well as their abundance ratio are both compatible
with those inferred from high-energy observations of Chandra and
NuSTAR. Finally, the simulations show that the fingerprints of the
SN can still be visible ≈2000 yr after the explosion. 
Conclusions: The main asymmetries and features observed in the
ejecta distribution of Cas A can be explained by the interaction
of the reverse shock with the initial large-scale asymmetries that
developed from stochastic processes (e.g., convective overturn and
SASI activity) that originate during the first seconds of the SN
blast. Movies associated to Figs. 7, 8, 12, 15 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202039335/olm">https://www.aanda.org</A>

---------------------------------------------------------
Title: Three-dimensional modeling from the onset of the SN to the
full-fledged SNR. Role of an initial ejecta anisotropy on matter
mixing
Authors: Tutone, A.; Orlando, S.; Miceli, M.; Ustamujic, S.; Ono,
M.; Nagataki, S.; Ferrand, G.; Greco, E.; Peres, G.; Warren, D. C.;
Bocchino, F.
2020A&A...642A..67T Altcode: 2020arXiv200901157T
Context. The manifold phases in the evolution of a core-collapse
(CC) supernova (SN) play an important role in determining the
physical properties and morphology of the resulting supernova remnant
(SNR). Thus, the complex morphology of SNRs is expected to reflect
possible asymmetries and structures developed during and soon after the
SN explosion. Aims: The aim of this work is to bridge the gap
between CC SNe and their remnants by investigating how post-explosion
anisotropies in the ejecta influence the structure and chemical
properties of the remnant at later times. Methods: We performed
three-dimensional magneto-hydrodynamical simulations starting soon
after the SN event and following the evolution of the system in the
circumstellar medium, which includes the wind of the stellar progenitor,
for 5000 yr, obtaining the physical scenario of a SNR. Here we focused
the analysis on the case of a progenitor red supergiant of 19.8
M⊙. We also investigated how a post-explosion large-scale
anisotropy in the SN affects the ejecta distribution and the matter
mixing of heavy elements in the remnant during the first 5000 yr of
evolution. Results: In the case of a spherically symmetric SN
explosion without large-scale anisotropies, the remnant roughly keeps
memory of the original onion-like layering of ejecta soon after the SN
event. Nevertheless, as the reverse shock hits the ejecta, the element
distribution departs from a homologous expansion because of the slowing
down of the outermost ejecta layers due to interaction with the reverse
shock. In the case of a large-scale anisotropy developed after the SN,
we found that the chemical stratification in the ejecta can be strongly
modified and the original onion-like layering is not preserved. The
anisotropy may cause spatial inversion of ejecta layers, for instance
leading to Fe/Si-rich ejecta outside the O shell, and may determine the
formation of Fe/Si-rich jet-like features that may protrude the remnant
outline. The level of matter mixing and the properties of the jet-like
feature are sensitive to the initial physical (density and velocity)
and geometrical (size and position) initial characteristics of the
anisotropy. Movies associated to Figs. 3 and 6 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202038336/olm">https://www.aanda.org</A>

---------------------------------------------------------
Title: Unveiling pure-metal ejecta X-ray emission in supernova
remnants through their radiative recombination continuum
Authors: Greco, Emanuele; Vink, Jacco; Miceli, Marco; Orlando,
Salvatore; Domček, Vladimir; Zhou, Ping; Bocchino, Fabrizio; Peres,
Giovanni
2020A&A...638A.101G Altcode: 2020arXiv200412924G
Context. Spectral analysis of X-ray emission from ejecta in supernova
remnants (SNRs) is hampered by the low spectral resolution of CCD
detectors, which typically creates a degeneracy between the best-fit
values of chemical abundances and the plasma emission measure. The
combined contribution of shocked ambient medium and ejecta to the
emerging X-ray emission further complicates the determination of
the ejecta mass and chemical composition. This degeneracy leads to
big uncertainties in mass estimates and can introduce a bias in the
comparison between the ejecta chemical composition derived from the
observations and the yields predicted by explosive nucleosynthesis
models. Aims: We explore the capabilities of present and future
spectral instruments with the aim of identifying a spectral feature that
may allow us to discriminate between metal-rich and pure-metal plasmas
in X-ray spectra of SNRs. Methods: We studied the behavior of
the most common X-ray emission processes of an optically thin plasma
in the high-abundance regime. We investigated spectral features of
bremsstrahlung, radiative recombination continua (RRC), and line
emission, by exploring a wide range of chemical abundances, plasma
temperatures, and ionization parameters. We then synthesized X-ray
spectra from a state-of-the-art 3D hydrodynamic simulation of Cas A,
by using the response matrix from the Chandra ACIS-S charged-coupled
device detector and that of the XRISM/Resolve X-ray calorimeter
spectrometer. Results: We found that a bright RRC shows up when
the plasma is made of pure-metal ejecta, and a high spectral resolution
is needed to actually identify this ejecta signature. We tested and
verified the applicability of our novel diagnostic tool and we propose
a promising target for the future detection of such spectral feature:
the southeastern Fe-rich clump of Cas A. Conclusions: While
there is no way to unambiguously reveal pure-metal ejecta emission
with CCD detectors, X-ray calorimeters will be able to pinpoint the
presence of pure-metal RRC and to recover correctly absolute mass and
the chemical composition of the ejecta, opening a new window on the
link between progenitor star, supernova and SNRs.

---------------------------------------------------------
Title: Hydrodynamic simulations unravel the
progenitor-supernova-remnant connection in SN 1987A
Authors: Orlando, S.; Ono, M.; Nagataki, S.; Miceli, M.; Umeda, H.;
Ferrand, G.; Bocchino, F.; Petruk, O.; Peres, G.; Takahashi, K.;
Yoshida, T.
2020A&A...636A..22O Altcode: 2019arXiv191203070O
Context. Massive stars end their lives in catastrophic supernova
(SN) explosions. Key information on the explosion processes and
on the progenitor stars can be extracted from observations of
supernova remnants (SNRs), which are the outcome of SNe. Deciphering
these observations, however, is challenging because of the complex
morphology of SNRs. Aims: We aim to link the dynamical and
radiative properties of the remnant of SN 1987A to the geometrical
and physical characteristics of the parent aspherical SN explosion
and to the internal structure of its progenitor star. Methods:
We performed comprehensive three-dimensional hydrodynamic simulations
which describe the long-term evolution of SN 1987A from the onset of
the SN to the full-fledged remnant at the age of 50 years, accounting
for the pre-SN structure of the progenitor star. The simulations include
all physical processes relevant for the complex phases of SN evolution
and for the interaction of the SNR with the highly inhomogeneous
ambient environment around SN 1987A. Furthermore, the simulations
follow the life cycle of elements from the synthesis in the progenitor
star through the nuclear reaction network of the SN to the enrichment
of the circumstellar medium as a result of the mixing of chemically
homogeneous layers of ejecta. From the simulations, we synthesize
observables that are to be compared with observations. Results:
By comparing the model results with observations, we constrained the
initial SN anisotropy causing Doppler shifts, observed in the emission
lines of heavy elements from ejecta, and leading to the remnant
evolution observed in the X-ray band in the last thirty years. In
particular, we found that the high mixing of ejecta unveiled by high
redshifts and broadenings of [Fe II] and 44Ti lines require
a highly asymmetric SN explosion channeling a significant fraction of
energy along an axis that is almost lying in the plane of the central
equatorial ring around SN 1987A, roughly along the line-of-sight,
but with an offset of 40°, with the lobe propagating away from
the observer slightly more energetic than the other. Furthermore,
we found unambiguously that the observed distribution of ejecta
and the dynamical and radiative properties of the SNR can be best
reproduced if the structure of the progenitor star was that of a blue
supergiant which had resulted from the merging of two massive stars. Movies associated to Figs. 4, 7, 9, and 10 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/201936718/olm">https://www.aanda.org</A>

---------------------------------------------------------
Title: Effects of intense flaring activity on accretion disk of
Classical T Tauri Stars
Authors: Colombo, S.; Orlando, S.; Peres, G.; Reale, F.; Argiroffi,
C.; Bonito, R.; Ibgui, L.; Stehle, C.
2020MmSAI..91..232C Altcode:
Classical T Tauri stars (CTTSs) are young low-mass stellar objects
that accrete mass from their circumstellar disks characterized by high
levels of coronal activity. Here, we investigate, using a 3D MHD model,
if an intense coronal activity due to flares that occur close to the
accretion disk may perturb the stability of the inner disk triggering
accretion episodes. We observe the formation of several loops that link
the star to the disk; all these loops build up a hot extended corona
with an X-ray luminosity comparable with typical values observed in
CTTSs. We found that the intense flaring activity close to the disk
can strongly perturb the disk stability, in fact accretion funnels
may be triggered by the flaring activity and thus contribute to the
mass accretion rate of the star.

---------------------------------------------------------
Title: Linking core-collapse supernova explosions to supernova
remnants through 3D MHD modeling
Authors: Orlando, S.; Wongwathanarat, A.; Janka, H. -T.; Miceli, M.;
Ono, M.; Nagataki, S.; Bocchino, F.; Peres, G.
2020MmSAI..91..325O Altcode: 2020arXiv201213630O
The structure and morphology of supernova remnants (SNRs) reflect the
properties of the parent supernovae (SNe) and the characteristics of the
inhomogeneous environments through which the remnants expand. Linking
the morphology of SNRs to anisotropies developed in their parent SNe can
be essential to obtain key information on many aspects of the explosion
processes associated with SNe. Nowadays, our capability to study the
SN-SNR connection has been largely improved thanks to multi-dimensional
models describing the long-term evolution from the SN to the SNR as
well as to observational data of growing quality and quantity across
the electromagnetic spectrum which allow to constrain the models. Here
we used the numerical resources obtained in the framework of the
“Accordo Quadro INAF-CINECA (2017)” together with a CINECA ISCRA
Award N.HP10BARP6Y to describe the full evolution of a SNR from the
core-collapse to the full-fledged SNR at the age of 2000 years. Our
simulations were compared with observations of SNR Cassiopeia A (Cas A)
at the age of ∼ 350 years. Thanks to these simulations we were able
to link the physical, chemical and morphological properties of a SNR to
the physical processes governing the complex phases of the SN explosion.

---------------------------------------------------------
Title: Non-LTE radiation hydrodynamics in PLUTO
Authors: Colombo, S.; Ibgui, L.; Orlando, S.; Rodríguez, R.; Espinosa,
G.; González, M.; Stehlé, C.; Peres, G.
2019A&A...631A..41C Altcode: 2019arXiv190704591C
Context. Modeling the dynamics of most astrophysical structures
requires an adequate description of the interaction of radiation and
matter. Several numerical (magneto-) hydrodynamics codes were upgraded
with a radiation module to fulfill this request. However, those that
used either the flux-limited diffusion (FLD) or the M1 radiation
moment approaches are restricted to local thermodynamic equilibrium
(LTE). This assumption may not be valid in some astrophysical
cases. Aims: We present an upgraded version of the LTE
radiation-hydrodynamics (RHD) module implemented in the PLUTO code,
which we have extended to handle non-LTE regimes. Methods:
Starting from the general frequency-integrated comoving-frame equations
of RHD, we have justified all the assumptions that were made to obtain
the non-LTE equations that are implemented in the module under the
FLD approximation. An operator-split method with two substeps was
employed: the hydrodynamics part was solved with an explicit method
by the solvers that are currently available in PLUTO, and the non-LTE
radiation diffusion and energy exchange part was solved with an implicit
method. The module was implemented in the PLUTO environment. It uses
databases of radiative quantities that can be provided independently
by the user: the radiative power loss, and the Planck and Rosseland
mean opacities. In our case, these quantities were determined from
a collisional-radiative steady-state model, and they are tabulated as
functions of temperature and density. Results: Our implementation
has been validated through different tests, in particular, radiative
shock tests. The agreement with the semi-analytical solutions (when
available) is good, with a maximum error of 7%. Moreover, we have
proved that a non-LTE approach is of paramount importance to properly
model accretion shock structures. Conclusion. Our radiation FLD
module represents a step toward a general non-LTE RHD modeling. The module is available at the CDS via anonymous ftp to <A
href="http://cdsarc.u-strasbg.fr/">http://cdsarc.u-strasbg.fr</A>
(ftp://130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/631/A41">http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/631/A41</A>
and upon request to the first author.

---------------------------------------------------------
Title: The Gaia-ESO survey: star formation history in Lambda Ori
star forming region
Authors: Prisinzano, Loredana; Trentacoste, Antonino; Peres, Giovanni;
Damiani, Francesco; Stelzer, Beate
2019lges.confE..36P Altcode:
Stellar age distributions of young stellar clusters are crucial to
assess the duration of the star formation process and how it depends on
the environment. The Lambda Ori star-forming region includes three
different clouds hosting the three young stellar clusters Collinder 69,
Barnard 30 and Barnard 35, that have been observed within the Gaia-ESO
survey (GES). Taking advantage of GES, Gaia DR2 and literature data
available for these subclusters, we derived stellar membership and
stellar ages from the HR diagram. We will present the comparison
of the star formation history of the entire Lambda Ori star forming
region in order to understand the effects of environment on the star
formation process.

---------------------------------------------------------
Title: VizieR Online Data Catalog: Radiation module for the code PLUTO
(Colombo+, 2019)
Authors: Colombo, S.; Ibgui, L.; Orlando, S.; Rodriguez, R.; Espinosa,
G.; Gonzalez, M.; Stehle, C.; Peres, G.
2019yCat..36310041C Altcode:
PLUTO is a freely-distributed software for the numerical solution
of mixed hyperbolic/parabolic systems of partial differential
equations (conservation laws) targeting high Mach number flows in
astrophysical fluid dynamics. The code is designed with a modular
and flexible structure whereby different numerical algorithms
can be separately combined to solve systems of conservation laws
using the finite volume or finite difference approach based on
Godunov-type schemes. See http://plutocode.ph.unito.it/ and
http://cerere.astropa.unipa.it/progetti_ricerca/HPC/resources.htm for
more details. (1 data file).

---------------------------------------------------------
Title: The Cherenkov Telescope Array
Authors: Williams, David; Balmaverde, Barbara; Benbow,
Wystan; Bucciantini, Niccolò; Buckley, James; Burton, Michael;
Capuzzo-Dolcetta, Roberto; Chaty, Sylvain; Coppi, Paolo; Costa,
Alessandro; D'Ammando, Filippo; De Cesare, Giovanni; Di Girolamo,
Tristano; Doro, Michele; Giglietto, Nicola; Hnatyk, Bohdan; Holder,
Jamie; Humensky, Brian; Kaaret, Phil; Kieda, David; Kohri, Kazunori;
Martinez, Manel; Minaya Flores, Ignacio A.; Mirzoyan, Razmik; Morselli,
Aldo; Mukherjee, Reshmi; Nieto Castaño, Daniel; Nikolajuk, Marek;
Ong, Rene; Otte, A. Nepomuk; Pareschi, Giovanni; Peres, Giovanni;
Reisenegger, Andreas; Romeo, Giuseppe; Santander, Marcos; Sciacca,
Eva; Schüssler, Fabian; Sergijenko, Olga; Vandenbroucke, Justin;
Vassiliev, Vladimir; Vorobiov, Serguei
2019BAAS...51g.291W Altcode: 2019astro2020U.291W
The Cherenkov Telescope Array (CTA), a unified worldwide effort and the
sole proposed implementation of the "Advanced Cherenkov Telescope Array"
recommended by the Astro2010 survey, is a ground-based instrument for
studies of very-high-energy gamma rays from 20 GeV to 300 TeV. This
white paper summarizes the CTA project and its science goals.

---------------------------------------------------------
Title: Effects of radiation in accretion regions of classical T
Tauri stars. Pre-heating of accretion column in non-LTE regime
Authors: Colombo, S.; Ibgui, L.; Orlando, S.; Rodriguez, R.; Espinosa,
G.; González, M.; Stehlé, C.; de Sá, L.; Argiroffi, C.; Bonito,
R.; Peres, G.
2019A&A...629L...9C Altcode: 2019arXiv190806799C
Context. Models and observations indicate that the impact of matter
accreting onto the surface of young stars produces regions at the base
of accretion columns where optically thin and thick plasma components
coexist. Thus, an accurate description of these impacts is necessary to
account for the effects of absorption and emission of radiation. Aims: We study the effects of radiation emerging from shock-heated
plasma in impact regions on the structure of the pre-shock down-falling
material. We investigate whether a significant absorption of radiation
occurs and if it leads to a pre-shock heating of the accreting gas. Methods: We developed a radiation hydrodynamics model describing
an accretion column impacting onto the surface of a classical T Tauri
star. The model takes into account the stellar gravity, the thermal
conduction, and the effects of radiative losses and of absorption
of radiation by matter in the nonlocal thermodynamic equilibrium
regime. Results: After the impact, a hot slab of post-shock
plasma develops at the base of the accretion column. Part of the
radiation emerging from the slab is absorbed by the pre-shock accreting
material. As a result, the pre-shock accretion column gradually heats
up to temperatures of 105 K, forming a radiative precursor
of the shock. The precursor has a thermal structure with the hottest
part at T ≈ 105 K, with a size comparable to that of
the hot slab, above the post-shock region. At larger distances the
temperature gradually decreases to T ≈ 104 K. 
Conclusions: Our model predicts that ≈70% of the radiation emitted
by the post-shock plasma is absorbed by the pre-shock accretion column
immediately above the slab and is re-emitted in the UV band. This
may explain why accretion rates derived from UV observations are
systematically higher than rates inferred from X-ray observations.

---------------------------------------------------------
Title: Discovery of a jet-like structure with overionized plasma in
the SNR IC 443
Authors: Greco, E.; Miceli, M.; Orlando, S.; Peres, G.; Troja, E.;
Bocchino, F.
2019sros.confE.198G Altcode:
IC 443 is a supernova remnant (SNR) located in a complex environment
and interacting with nearby clouds. Indications for the presence
of overionized plasma have been found though the possible physical
causes of overionization are still debated. Moreover, because of
its peculiar position and proper motion, it is not clear if the
pulsar wind nebula (PWN) within the remnant is the relic of the IC
443 progenitor star or just a rambling one seen in projection on the
remnant. We addressed the study of the IC 443 X-ray emission in order
to investigate the relationship between the PWN and the remnant, the
presence of overionization and its origin. We identified an elongated
(jet-like) structure with Mg-rich plasma in overionization. The
head of the jet is interacting with a molecular cloud and the jet is
aligned with the position of the PWN at the instant of the supernova
explosion. Interestingly, the direction of the jet of ejecta is
somehow consistent with the direction of the PWN jet. Our discovery of
a jet of ejecta in IC 443 enlarge the sample of core-collapse SNRs with
collimated ejecta structures. IC 443's jet is the first one which shows
overionized plasma, possibly associated with the adiabatic expansion of
ejecta. The match between the jet's direction and the original position
of the PWN strongly supports the association between the neutron star
and IC 443.

---------------------------------------------------------
Title: Studying the radiative recombination continua in the X-ray
spectra of pure ejecta and of overionized plasmas
Authors: Greco, E.; Vink, J.; Miceli, M.; Domcek, V.; Zhou, P.;
Orlando, S.; Peres, G.; Bocchino, F.
2019sros.confE.144G Altcode:
Several Supernova Remnants (SNRs) show signatures of over ionization. A
plasma is over ionized when its ionization state is higher than
that expected from its electron temperature, namely when it is
non-equilibrium of ionization and in recombining phase. The physical
origin of over ionized plasma in SNRs is largely debated in the
literature. The characteristic feature of a recombining plasma in the
X-rays is the presence of prominent Radiative Recombination Continua
(RRC), associated with electron recombining with ions. However, the
intensity of the RRCs in the X-rays also increases with the plasma
metallicity. We here present a thorough spectral study to investigate
whether the bright RRC originating from a pure ejecta plasma in
collisional ionization equilibrium can mimic that of an over ionized
plasma in non-equilibrium of ionization. We performed series of spectral
simulations in the X-rays, by exploring different values of temperature,
emission measure, and chemical abundances. We also considered spectra
emerging from a combination of interstellar medium and pure ejecta
and compared them against actual observations of recombining SNRs. We
present here some preliminary results and discuss in which conditions
pure ejecta plasma in collisional ionization equilibrium might mimic
features usually attributed to over ionized plasma in non-equilibrium
of ionization.

---------------------------------------------------------
Title: A Virtual Reality Environment for Scientific Exploitation of
3D MHD Astrophysical Simulations
Authors: Bocchino, Fabrizio; Orlando, S.; Pillitteri, I.; Miceli,
M.; Peres, G.
2019sros.confE..76B Altcode:
Virtual Reality (VR) hardware and software are now routinely used
in several fields for public outreach and education with excellent
feedback. For this purpose, YouTube and online multimedia digital
stores have several VR titles of great impact in the Astro physics
and Space categories of their catalogues. However, scientific
use of VR environments are still at their beginning. Fully 3D
Magneto-hydrodynamical simulations of astrophysical phenomena represent
a challenge in standard data visualization for scientific purposes, for
the amount of processed data and the wealth of scientific information
they contain. In this poster, we present a VR platform based on
Oculus Rift for data visualization of 3D MHD models of Supernova
Remnants and Young Stellar Objects developed at INAF-Osservatorio
Astronomico di Palermo. We show that fully immersive space-walks inside
properly configured data cubes may be of great importance to study the
distribution of hot plasmas, cold materials and the magnetic fields
of these environments.

---------------------------------------------------------
Title: Measuring the post-shock temperatures of heavy ions in SN 1987A
Authors: Miceli, M.; Orlando, S.; Bocchino, F.; Park, S.; Pazhayath
Ravi, A.; Burrows, D.; Frank, K.; Argiroffi, C.; Peres, G.; Greco,
E.; Petruk, O.
2019sros.confE.108M Altcode:
The shock waves originated from supernova explosions provide crucial
information on the physics of shock heating. Astrophysical shocks are
typically collisionless and electrons, protons, and ions are expected
to be heated at different temperatures, but the actual dependence of
the post-shock temperature on the particle mass is still debated. We
tackle this long standing issue through the analysis of high-resolution
observations of SN 1987A, made with the Chandra X-ray telescope in
2007 and 2011, and we also include the latest 2018 observation in
our analysis. We study the observed spectra in close comparison with
a dedicated full 3-D hydrodynamic simulation. The simulation is able
to reproduce self-consistently the whole broadening of the spectral
lines of many ions altogether. We could therefore measure the post
shock temperature of protons and selected ions. We found that the ion
to proton temperature ratio is always significantly higher than one and
increases linearly with the ion mass for a wide range of masses. Thanks
to the new 2018 deep observation we are also able to further extend
our results for a wide range of shock parameters.

---------------------------------------------------------
Title: Three-dimensional MHD modeling of SNR IC 443: effects of the
inhomogeneous medium in shaping the remnant morphology
Authors: Ustamujic, S.; Orlando, S.; Greco, E.; Miceli, M.; Bocchino,
F.; Peres, G.
2019sros.confE.202U Altcode:
In this study we aim at investigating the effects of the inhomogeneous
medium in shaping the remnant morphology of IC 443 after the supernova
(SN) explosion. The distribution of the interstellar medium in the
vicinity of the supernova remnant (SNR) plays a fundamental role for
our understanding of the morphology and later evolution. In particular,
IC443 is a SNR located in a quite complex environment: it interacts with
a molecular cloud in the northwestern and southeastern areas and with
an atomic cloud in the northeast. We have developed a 3D MHD model for
SNR IC~443 describing the interaction of the SNR with the environment,
parametrized in agreement with the results of the multiwavelength data
analysis. In this poster we present our preliminary results.

---------------------------------------------------------
Title: 3D MHD Simulations from the Onset of the SN to the Full-fledged
SNR: Role of Ejecta Clumps on Matter Mixing
Authors: Tutone, Antonio; Orlando, Salvatore; Miceli, Marco; Peres,
Giovanni; Ono, Masaomi; Nagataki, Shigehiro; Ferrand, Gilles
2019sros.confE..91T Altcode:
The physical properties and the morphology of supernova remnants
(SNRs) partially reflect the structure of the stellar progenitor and
possible asymmetries developed soon after their parent supernova (SN)
explosions. The aim of this work is to bridge the gap between SNe and
their remnants by investigating how the various chemically homogeneous
layers at the time of the explosion map into the resulting abundance
pattern observed when the remnant is fully developed. To this end, we
have performed three-dimensional magneto-hydrodynamical simulations
starting soon after the SN and following the interaction of the SN
ejecta with the CSM (consisting in the wind of the stellar progenitor),
obtaining the physical scenario of a SNR. As stellar progenitor, we
have considered the case of a 19 M⊙ red supergiant. We investigated
how the evolution of a post-explosion large-scale anisotropy in the
ejecta and the role of its initial parameters (position, dimension,
density and velocity contrast) can affect the ejecta distribution
and the matter mixing of heavy elements in the remnant, covering 5000
years of evolution.

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Title: A stellar flare-coronal mass ejection event revealed by X-ray
plasma motions
Authors: Argiroffi, C.; Reale, F.; Drake, J. J.; Ciaravella, A.;
Testa, P.; Bonito, R.; Miceli, M.; Orlando, S.; Peres, G.
2019NatAs...3..742A Altcode: 2019NatAs.tmp..328A; 2019arXiv190511325A
Coronal mass ejections (CMEs), often associated with
flares1-3, are the most powerful magnetic phenomena occurring
on the Sun. Stars show magnetic activity levels up to ten thousand times
higher4, and CME effects on stellar physics and circumstellar
environments are predicted to be substantial5-9. However,
stellar CMEs remain observationally unexplored. Using time-resolved
high-resolution X-ray spectroscopy of a stellar flare on the
active star HR 9024 observed with the High Energy Transmission
Grating Spectrometer onboard the Chandra X-ray Observatory space
telescope, we distinctly detected Doppler shifts in S xvi, Si xiv
and Mg xii lines that indicate upward and downward motions of hot
plasmas (around 10-25 MK) within the flaring loop, with velocities
of 100-400 km s-1, in agreement with a model of a flaring
magnetic tube. Most notably, we also detected a later blueshift in
the O viii line that reveals an upward motion, with velocity 90 ± 30
km s-1, of cool plasma (about 4 MK), that we ascribe to a
CME coupled to the flare. From this evidence we were able to derive
a CME mass of 1 .2-0.8+2.6×1 021
g and a CME kinetic energy of 5 .2-3.6+27.7×1
034 erg. These values provide clues in the extrapolation of
the solar case to higher activity levels in other stars, suggesting that
CMEs could indeed be a major cause of mass and angular momentum loss.

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Title: New view of the corona of classical T Tauri stars: Effects
of flaring activity in circumstellar disks
Authors: Colombo, S.; Orlando, S.; Peres, G.; Reale, F.; Argiroffi,
C.; Bonito, R.; Ibgui, L.; Stehlé, C.
2019A&A...624A..50C Altcode: 2019arXiv190207048C
Context. Classical T Tauri stars (CTTSs) are young low-mass stellar
objects that accrete mass from their circumstellar disks. They are
characterized by high levels of coronal activity, as revealed by
X-ray observations. This activity may affect the disk stability and
the circumstellar environment. Aims: Here we investigate if
an intense coronal activity due to flares that occur close to the
accretion disk may perturb the stability of the inner disk, disrupt
the inner part of the disk, and might even trigger accretion phenomena
with rates comparable with those observed. Methods: We modeled
a magnetized protostar surrounded by an accretion disk through 3D
magnetohydrodinamic simulations. The model takes into account the
gravity from the central star, the effects of viscosity in the disk,
the thermal conduction (including the effects of heat flux saturation),
the radiative losses from optically thin plasma, and a parameterized
heating function to trigger the flares. We explored cases characterized
by a dipole plus an octupole stellar magnetic field configuration
and different density of the disk or by different levels of flaring
activity. Results: As a result of the simulated intense flaring
activity, we observe the formation of several loops that link the
star to the disk; all these loops build up a hot extended corona
with an X-ray luminosity comparable with typical values observed in
CTTSs. The intense flaring activity close to the disk can strongly
perturb the disk stability. The flares trigger overpressure waves
that travel through the disk and modify its configuration. Accretion
funnels may be triggered by the flaring activity and thus contribute
to the mass accretion rate of the star. Accretion rates synthesized
from the simulations are in a range between 10-10 and
10-9 M⊙ yr-1. The accretion columns
can be perturbed by the flares, and they can interact with each other;
they might merge into larger streams. As a result, the accretion pattern
can be rather complex: the streams are highly inhomogeneous, with a
complex density structure, and clumped. Movies are available at <A
href="https://www.aanda.org/10.1051/0004-6361/201834342/olm">https://www.aanda.org</A>

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Title: 3D MHD modeling of the expanding remnant of SN 1987A. Role
of magnetic field and non-thermal radio emission
Authors: Orlando, S.; Miceli, M.; Petruk, O.; Ono, M.; Nagataki,
S.; Aloy, M. A.; Mimica, P.; Lee, S. -H.; Bocchino, F.; Peres, G.;
Guarrasi, M.
2019A&A...622A..73O Altcode: 2018arXiv181200021O
 Aims: We investigate the role played by a pre-supernova (SN)
ambient magnetic field in the dynamics of the expanding remnant of
SN 1987A, and the origin and evolution of the radio emission from the
remnant, in particular during the interaction of the blast wave with
the nebula surrounding the SN. Methods: We modeled the evolution
of SN 1987A from the breakout of the shock wave at the stellar surface
to the expansion of its remnant through the surrounding nebula using
three-dimensional magnetohydrodynamic simulations. The model considers
the radiative cooling, the deviations from equilibrium of ionization,
the deviation from temperature-equilibration between electrons and ions,
and a plausible configuration of the pre-SN ambient magnetic field. We
explore the strengths of the pre-SN magnetic field ranging between 1
and 100 μG at the inner edge of the nebula and we assume an average
field strength at the stellar surface B0 ≈ 3 kG. From
the simulations, we synthesize the thermal X-ray and the non-thermal
radio emission and compare the model results with observations. 
Results: The presence of an ambient magnetic field with strength in the
range considered does not change significantly the overall evolution of
the remnant. Nevertheless, the magnetic field reduces the erosion and
fragmentation of the dense equatorial ring after the impact of the SN
blast wave. As a result, the ring survives the passage of the blast,
at least during the time covered by the simulations (40 yr). Our model
is able to reproduce the morphology and lightcurves of SN 1987A in both
X-ray and radio bands. The model reproduces the observed radio emission
if the flux originating from the reverse shock is heavily suppressed. In
this case, the radio emission originates mostly from the forward shock
traveling through the H II region and this may explain why the radio
emission seems to be insensitive to the interaction of the blast with
the ring. Possible mechanisms for the suppression of emission from the
reverse shock are investigated. We find that synchrotron self-absorption
and free-free absorption have negligible effects on the emission during
the interaction with the nebula. We suggest that the emission from the
reverse shock at radio frequencies might be limited by highly magnetized
ejecta. Movies associated to Figs. 2 and 4 are available at <A
href="http://www.aanda.org/10.1051/0004-6361/201834487/olm">http://www.aanda.org</A>

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Title: Mass Accretion Impacts in Classical T Tauri Stars: A
Multi-disciplinary Approach
Authors: Orlando, S.; Argiroffi, C.; Bonito, R.; Colombo, S.; Peres,
G.; Reale, F.; Miceli, M.; Ibgui, L.; Stehlé, C.; Matsakos, T.
2019ASSP...55...43O Altcode:
No abstract at ADS

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Title: Collisionless shock heating of heavy ions in SN 1987A
Authors: Miceli, Marco; Orlando, Salvatore; Burrows, David N.; Frank,
Kari A.; Argiroffi, Costanza; Reale, Fabio; Peres, Giovanni; Petruk,
Oleh; Bocchino, Fabrizio
2019NatAs...3..236M Altcode: 2019NatAs.tmp..194M; 2019arXiv190110336M
Astrophysical shocks at all scales, from those in the heliosphere up
to cosmological shock waves, are typically `collisionless', because the
thickness of their jump region is much shorter than the collisional mean
free path. Across these jumps, electrons, protons and ions are expected
to be heated at different temperatures. Supernova remnants (SNRs)
are ideal targets to study collisionless processes because of their
bright post-shock emission and fast shocks, but the actual dependence
of the post-shock temperature on the particle mass is still widely
debated1. We tackle this longstanding issue through the
analysis of deep multi-epoch and high-resolution observations, made with
the Chandra X-ray telescope, of the youngest nearby supernova remnant,
SN 1987A. We introduce a data analysis method by studying the observed
spectra in close comparison with a dedicated full three-dimensional
hydrodynamic simulation that self-consistently reproduces the broadening
of the spectral lines of many ions together. We measure the post-shock
temperature of protons and ions through comparison of the model with
observations. Our results show that the ratio of ion temperature
to proton temperature is always significantly higher than one and
increases linearly with the ion mass for a wide range of masses and
shock parameters.

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Title: Flaring activity on the disk of Classical T Tauri Stars:
effects on disk stability
Authors: Colombo, S.; Orlando, S.; Peres, G.; Reale, F.; Argiroffi,
C.; Bonito, R.; Ibgui, L.; Stehlé, C.
2018sf2a.conf..121C Altcode:
Classical T Tauri Stars (CTTSs) are young stellar objects surrounded
by a circumstellar disk with which they exchange mass and angular
momentum through accretion. Despite this process is a crucial aspect
of star formation, some issues are still not clear; in particular how
the material loses angular momentum and falls into the star. CTTSs
are also characterized by strong X-ray emission. Part of this X-ray
emission comes from the heated plasma in the external regions of
the stellar corona with temperature between 1 and 100 MK. The plasma
heating is presumably due to the strong magnetic field (Feigelson and
Montmerle, 1999) in the form of high energetic flares in proximity
of the stellar surface. This energetic phenomena may influence the
circumstellar environment. Recently, Reale et al. (2018) proved that
long flares may connect the disk to the stellar surface. Moreover a
study of Orlando et al. (2011) has shown that an intense flare close
to the disk may strongly perturb its stability, inducing accretion
episodes. Starting from these lines of evidence, here we investigate
the effects of multiple flares with low-to-medium intensity on the disk
stability, and check if they may be responsible for triggering accretion
episodes. To this end, we developed a 3D magnetohydrodynamics model
describing a CTTS surrounded by an accretion disk subject to intense
flaring activity. The flares occur randomly in proximity of a thick
disk. We found that the flaring activity determines the formation of
a hot extended corona that links the disk to the stellar surface. In
addition, the flares strongly perturb the disk and trigger accretion
phenomena with a mass accretion rate comparable with those inferred
by X-ray observations.

---------------------------------------------------------
Title: Discovery of a jet-like structure with overionized plasma in
the SNR IC 443
Authors: Greco, Emanuele; Miceli, Marco; Orlando, Salvatore; Peres,
Giovanni; Troja, Eleonora; Bocchino, Fabrizio
2018A&A...615A.157G Altcode: 2018arXiv180406714G
Context. IC 443 is a supernova remnant (SNR) located in a quite complex
environment since it interacts with nearby clouds. Indications for the
presence of overionized plasma have been found though the possible
physical causes of overionization are still debated. Moreover,
because of its peculiar position and proper motion, it is not clear
if the pulsar wind nebula (PWN) within the remnant is the relic of
the IC 443 progenitor star or just a rambling one seen in projection
on the remnant. Aims: Here we address the study of IC 443
plasma in order to clarify the relationship PWN-remnant, the presence
of overionization and the origin of the latter. Methods: We
analyzed two XMM-Newton observations producing background-subtracted,
vignetting-corrected and mosaicked images in two different energy
bands and we performed a spatially resolved spectral analysis of
the X-ray emission. Results: We identified an elongated
(jet-like) structure with Mg-rich plasma in overionization. The
head of the jet is interacting with a molecular cloud and the jet is
aligned with the position of the PWN at the instant of the supernova
explosion. Interestingly, the direction of the jet of ejecta is somehow
consistent with the direction of the PWN jet. Conclusions:
Our discovery of a jet of ejecta in IC 443 enlarge the sample of
core-collapse SNRs with collimated ejecta structures. IC 443's jet
is the first one which shows overionized plasma, possibly associated
with the adiabatic expansion of ejecta. The match between the jet's
direction and the original position of the PWN strongly supports the
association between the neutron star and IC 443.

---------------------------------------------------------
Title: Chemical Evolution of Interstellar Methanol Ice Analogs upon
Ultraviolet Irradiation: The Role of the Substrate
Authors: Ciaravella, A.; Jiménez-Escobar, A.; Cosentino, G.;
Cecchi-Pestellini, C.; Peres, G.; Candia, R.; Collura, A.; Barbera,
M.; Di Cicca, G.; Varisco, S.; Venezia, A. M.
2018ApJ...858...35C Altcode:
An important issue in the chemistry of interstellar ices is the
role of dust materials. In this work, we study the effect of an
amorphous water-rich magnesium silicate deposited onto ZnSe windows
on the chemical evolution of ultraviolet-irradiated methanol
ices. For comparison, we also irradiate similar ices deposited
onto bare ZnSe windows. Silicates are produced at relatively
low temperatures exploiting a sol-gel technique. The chemical
composition of the synthesized material reflects the forsterite
stoichiometry. Si-OH groups and magnesium carbonates are incorporated
during the process. The results show that the substrate material
does affect the chemical evolution of the ice. In particular, the
CO2/CO ratio within the ice is larger for methanol ices
deposited onto the silicate substrate as a result of concurrent effects:
the photolysis of carbonates present in the adopted substrate as a
source of CO2, CO, and carbon and oxygen atoms; reactions
of water molecules and hydroxyl radicals released from the substrate
with the CO formed in the ice by the photolysis of the methanol ice;
and changes in the structure and energy of the silicate surface by
ultraviolet irradiation, leading to more favorable conditions for
chemical reactions or catalysis at the grain surface. The results of
our experiments allow such chemical effects contributed by the various
substrate material components to be disentangled.

---------------------------------------------------------
Title: Analysis of the XMM-Newton observations of IC443
Authors: Greco, E.; Miceli, M.; Peres, G.; Orlando, S.; Troja, E.;
Bocchino, F.
2017xru..conf..276G Altcode:
We analyze for the first time the full set of archive XMM-Newton
EPIC observations of the Galactic Supernova Remnant IC 443. We aim at
identifying the contribution of the shocked ejecta and interstellar
medium and at the describing the physical and chemical properties of the
shocked plasma. We also aim at addressing the presence of overionized
plasma and its physical origin. We trace the morphology of Si- and
S-rich ejecta with unprecedented spatial resolution, by adopting a novel
method to produce maps of equivalent width. We describe in detail the
method adopted and the results obtained and present preliminary results
of a spatially resolved spectral analysis performed on selected regions,
chosen on the basis of our image analysis.

---------------------------------------------------------
Title: Redshifted X-rays from the material accreting onto TW Hydrae:
Evidence of a low-latitude accretion spot
Authors: Argiroffi, C.; Drake, J. J.; Bonito, R.; Orlando, S.; Peres,
G.; Miceli, M.
2017A&A...607A..14A Altcode: 2017arXiv170803158A
Context. High resolution spectroscopy, providing constraints on plasma
motions and temperatures, is a powerful means to investigate the
structure of accretion streams in classical T Tauri stars (CTTS). In
particular, the accretion shock region, where the accreting material
is heated to temperatures of a few million degrees as it continues
its inward bulk motion, can be probed by X-ray spectroscopy. 
Aims: In an attempt to detect for the first time the motion of this
X-ray-emitting post-shock material, we searched for a Doppler shift
in the deep Chandra High Energy Transmission Grating observation of
the CTTS TW Hya. This test should unveil the nature of this X-ray
emitting plasma component in CTTS and constrain the accretion stream
geometry. Methods: We searched for a Doppler shift in the
X-ray emission from TW Hya with two different methods: by measuring
the position of a selected sample of emission lines and by fitting
the whole TW Hya X-ray spectrum, allowing the line-of-sight velocity
to vary. Results: We found that the plasma at T 2 - 4 MK has a
line-of-sight velocity of 38.3 ± 5.1 km s-1 with respect
to the stellar photosphere. This result definitively confirms that
this X-ray-emitting material originates in the post-shock region, at
the base of the accretion stream, and not in coronal structures. The
comparison of the observed velocity along the line of sight, 38.3 ±
5.1 km s-1, with the inferred intrinsic velocity of the post
shock of TW Hya, vpost ≈ 110 - 120 km s-1,
indicates that the footpoints of the accretion streams on TW Hya are
located at low latitudes on the stellar surface. Conclusions:
Our results indicate that complex magnetic field geometries, such as
those of TW Hya, permit low-latitude accretion spots. Moreover, since
on TW Hya the redshift of the soft X-ray emission is very similar to
that of the narrow component of the C iv resonance doublet at 1550 Å,
then the plasma at 2 - 4 MK and that at 0.1 MK likely originate in
the same post-shock regions.

---------------------------------------------------------
Title: Accretion disk coronae of intermediate polar cataclysmic
variables. 3D magnetohydrodynamic modelling and thermal X-ray emission
Authors: Barbera, E.; Orlando, S.; Peres, G.
2017A&A...600A.105B Altcode: 2017arXiv170204702B
Context. Intermediate polar cataclysmic variables (IPCV) contain a
magnetic, rotating white dwarf surrounded by a magnetically truncated
accretion disk. To explain their strong flickering X-ray emission,
accretion has been successfully taken into account. Nevertheless,
observations suggest that accretion phenomena might not be the only
process behind it. An intense flaring activity occurring on the
surface of the disk may generate a corona, contribute to the thermal
X-ray emission, and influence the system stability. Aims: Our
purposes are: investigating the formation of an extended corona above
the accretion disk, due to an intense flaring activity occurring on the
disk surface; studying the effects of flares on the disk and stellar
magnetosphere; assessing its contribution to the observed thermal X-ray
flux. Methods: We have developed a 3D magnetohydrodynamic (MHD)
model of a IPCV system. The model takes into account gravity, disk
viscosity, thermal conduction, radiative losses, and coronal flare
heating through heat injection at randomly chosen locations on the
disk surface. To perform a parameter space exploration, several system
conditions have been considered, with different magnetic field intensity
and disk density values. From the results of the evolution of the model,
we have synthesized the thermal X-ray emission. Results: The
simulations show the formation of an extended corona, linking disk and
star. The flaring activity is capable of strongly influencing the disk
configuration and possibly its stability, effectively deforming the
magnetic field lines. Hot plasma evaporation phenomena occur in the
layer immediately above the disk. The flaring activity gives rise to
a thermal X-ray emission in both the [ 0.1-2.0 ] keV and the [ 2.0-10
] keV X-ray bands. Conclusions: An intense coronal activity
occurring on the disk surface of an IPCV can affect the structure
of the disk depending noticeably on the density of the disk and the
magnetic field of the central object. Moreover, the synthesis of the
thermal X-ray fluxes shows that this flaring activity may contribute
to the observed flickering thermal X-ray emission.

---------------------------------------------------------
Title: Hydrodynamic modelling of accretion impacts in classical T
Tauri stars: radiative heating of the pre-shock plasma
Authors: Costa, G.; Orlando, S.; Peres, G.; Argiroffi, C.; Bonito, R.
2017A&A...597A...1C Altcode: 2016arXiv160901059C; 2016A&A...597A...1C
Context. It is generally accepted that, in classical T Tauri stars,
the plasma from the circumstellar disc accretes onto the stellar
surface with free-fall velocity and the impact generates a shock. The
impact region is expected to contribute to emission in different
spectral bands; many studies have confirmed that the X-rays arise
from the post-shock plasma but, otherwise, there are no studies in the
literature investigating the origin of the observed UV emission which
is apparently correlated to accretion. Aims: We investigated the
effect of radiative heating of the infalling material by the post-shock
plasma at the base of the accretion stream, with the aim to identify in
which region a significant part of the UV emission originates. 
Methods: We developed a one-dimensional hydrodynamic model describing
the impact of an accretion stream onto the stellar surface; the model
takes into account the gravity, the radiative cooling of an optically
thin plasma, the thermal conduction, and the heating due to absorption
of X-ray radiation. The latter term represents the heating of the
infalling plasma due to the absorption of X-rays emitted from the
post-shock region. Results: We found that the radiative heating
of the pre-shock plasma plays a non-negligible role in the accretion
phenomenon. In particular, the dense and cold plasma of the pre-shock
accretion column is gradually heated up to a few 105K
due to irradiation of X-rays arising from the shocked plasma at the
impact region. This heating mechanism does not affect significantly
the dynamics of the post-shock plasma. On the other hand, a region
of radiatively heated gas (that we consider a precursor) forms in
the unshocked accretion column and contributes significantly to UV
emission. Our model naturally reproduces the luminosity of UV emission
lines correlated to accretion and shows that most of the UV emission
originates from the precursor.

---------------------------------------------------------
Title: 3D MHD modeling of twisted coronal loops
Authors: Reale, F.; Orlando, S.; Guarrasi, M.; Mignone, A.; Peres,
G.; Hood, A. W.; Priest, E. R.
2016ApJ...830...21R Altcode: 2016arXiv160705500R
We perform MHD modeling of a single bright coronal loop to include the
interaction with a non-uniform magnetic field. The field is stressed
by random footpoint rotation in the central region and its energy is
dissipated into heating by growing currents through anomalous magnetic
diffusivity that switches on in the corona above a current density
threshold. We model an entire single magnetic flux tube in the solar
atmosphere extending from the high-β chromosphere to the low-β corona
through the steep transition region. The magnetic field expands from
the chromosphere to the corona. The maximum resolution is ∼30 km. We
obtain an overall evolution typical of loop models and realistic loop
emission in the EUV and X-ray bands. The plasma confined in the flux
tube is heated to active region temperatures (∼3 MK) after ∼2/3
hr. Upflows from the chromosphere up to ∼100 km s-1
fill the core of the flux tube to densities above 109
cm-3. More heating is released in the low corona than the
high corona and is finely structured both in space and time.

---------------------------------------------------------
Title: X-Raying the Dark Side of Venus—Scatter from Venus’
Magnetotail?
Authors: Afshari, M.; Peres, G.; Jibben, P. R.; Petralia, A.; Reale,
F.; Weber, M.
2016AJ....152..107A Altcode: 2016arXiv160706697A
We analyze significant X-ray, EUV, and UV emission coming from
the dark side of Venus observed with Hinode/XRT and Solar Dynamics
Observatory/Atmospheric Imaging Assembly (SDO/AIA) during a transit
across the solar disk that occurred in 2012. As a check we have
analyzed an analogous Mercury transit that occurred in 2006. We have
used the latest version of the Hinode/XRT point spread function to
deconvolve Venus and Mercury X-ray images, to remove instrumental
scattering. After deconvolution, the flux from Venus’ shadow remains
significant while that of Mercury becomes negligible. Since stray
light contamination affects the XRT Ti-poly filter data we use, we
performed the same analysis with XRT Al-mesh filter data, not affected
by the light leak. Even the latter data show residual flux. We have
also found significant EUV (304 Å, 193 Å, 335 Å) and UV (1700 Å)
flux in Venus’ shadow, measured with SDO/AIA. The EUV emission from
Venus’ dark side is reduced, but still significant, when deconvolution
is applied. The light curves of the average flux of the shadow in the
X-ray, EUV, and UV bands appear different as Venus crosses the solar
disk, but in any of them the flux is, at any time, approximately
proportional to the average flux in a ring surrounding Venus, and
therefore proportional to that of the solar regions around Venus’
obscuring disk line of sight. The proportionality factor depends on
the band. This phenomenon has no clear origin; we suggest that it may
be due to scatter occurring in the very long magnetotail of Venus.

---------------------------------------------------------
Title: Impacts of fragmented accretion streams onto classical T
Tauri stars: UV and X-ray emission lines
Authors: Colombo, S.; Orlando, S.; Peres, G.; Argiroffi, C.; Reale, F.
2016A&A...594A..93C Altcode: 2016arXiv160703009C
Context. The accretion process in classical T Tauri stars (CTTSs) can be
studied through the analysis of some UV and X-ray emission lines which
trace hot gas flows and act as diagnostics of the post-shock downfalling
plasma. In the UV-band, where higher spectral resolution is available,
these lines are characterized by rather complex profiles whose origin is
still not clear. Aims: We investigate the origin of UV and X-ray
emission at impact regions of density structured (fragmented) accretion
streams. We study if and how the stream fragmentation and the resulting
structure of the post-shock region determine the observed profiles
of UV and X-ray emission lines. Methods: We modeled the impact
of an accretion stream consisting of a series of dense blobs onto the
chromosphere of a CTTS through two-dimensional (2D) magnetohydrodynamic
(MHD) simulations. We explored different levels of stream fragmentation
and accretion rates. From the model results, we synthesize C IV (1550
Å) and O VIII (18.97 Å) line profiles. Results: The impacts of
accreting blobs onto the stellar chromosphere produce reverse shocks
propagating through the blobs and shocked upflows. These upflows, in
turn, hit and shock the subsequent downfalling fragments. As a result,
several plasma components differing for the downfalling velocity,
density, and temperature are present altoghether. The profiles of C
IV doublet are characterized by two main components: one narrow and
redshifted to speed ≈ 50 km s-1 and the other broader and
consisting of subcomponents with redshift to speed in the range 200-400
km s-1. The profiles of O VIII lines appear more symmetric
than C IV and are redshifted to speed ≈ 150 km s-1. Conclusions: Our model predicts profiles of C IV line remarkably
similar to those observed and explains their origin in a natural
way as due to stream fragmentation. Movies are available at <A
href="http://www.aanda.org/10.1051/0004-6361/201628858/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: Modeling the shock-cloud interaction in SN 1006: Unveiling
the origin of nonthermal X-ray and γ-ray emission
Authors: Miceli, M.; Orlando, S.; Pereira, V.; Acero, F.; Katsuda,
S.; Decourchelle, A.; Winkler, F. P.; Bonito, R.; Reale, F.; Peres,
G.; Li, J.; Dubner, G.
2016A&A...593A..26M Altcode: 2016arXiv160608748M
Context. The supernova remnant SN 1006 is a source of high-energy
particles and its southwestern limb is interacting with a dense ambient
cloud, thus is a promising region for γ-ray hadronic emission. 
Aims: We aim at describing the physics and the nonthermal emission
associated with the shock-cloud interaction to derive the physical
parameters of the cloud (poorly constrained by the data analysis), to
ascertain the origin of the observed spatial variations in the spectral
properties of the X-ray synchrotron emission, and to predict spectral
and morphological features of the resulting γ-ray emission. 
Methods: We performed 3D magnetohydrodynamic simulations modeling
the evolution of SN 1006 and its interaction with the ambient cloud,
and explored different model setups. By applying the REMLIGHT code on
the model results, we synthesized the synchrotron X-ray emission and
compared it with actual observations to constrain the parameters of the
model. We also synthesized the leptonic and hadronic γ-ray emission
from the models, deriving constraints on the energy content of the
hadrons accelerated at the southwestern limb. Results: We found
that the impact of the SN 1006 shock front with a uniform cloud with
density 0.5 cm-3 can explain the observed morphology, the
azimuthal variations of the cutoff frequency of the X-ray synchrotron
emission, and the shock proper motion in the interaction region. Our
results show that the current upper limit for the total hadronic energy
in the southwestern limb is 2.5 × 1049 erg.

---------------------------------------------------------
Title: UV And X-Ray Emission from Impacts of Fragmented Accretion
Streams on Classical T Tauri Stars
Authors: Colombo, Salvatore; Orlando, Salvatore; Peres, Giovanni;
Argiroffi, Costanza; Reale, Fabio
2016csss.confE..64C Altcode:
According to the magnetoshperic accretion scenario, during their
evo- lution, Classical T Tauri stars accrete material from
their circumstellar disk. The accretion process is regulated
by the stellar magnetic eld and produces hot and dense
post-shocks on the stellar surface as a result of impacts of
the downfalling material. The impact regions are expected 
to strongly radiate in UV and X-rays. Several lines of evidence
support the magnetospheric accretion scenario, especially
in optical and infrared bands. However several points still
remain unclear as, for instance,where the complex-pro le UV
lines originate, or whether and how UV and X-ray emission is
produced in the same shock region. The analysis of a large solar
eruption has shown that EUV excesses might be e ectively produced by the impact of dense fragments onto the stellar surface. Since
a steady accretion stream does not reprouce observations, in
this work we investi- gate the e ects of a fragmented accretion
stream on the uxes and pro les of C IV and O VIII emission
lines. To this end we model the impact of a fragmented accretion
stream onto the chromosphere of a CTTS with 2D axysimmetric
magneto-hydrodynamic simulations. Our model takes into account
of the gravity, the stellar magnetic eld, the thermal conduction and the radiative cooling from an optically thin plasma. 
From the model results, we synthesize the UV and X-ray emission 
including the e ect of Doppler shift along the line of sight. We nd
that a fragmented accretion stream produces complex pro les of
UV emission lines which consists of multiple components with di
erent Doppler shifts. Our model predicts line pro les that are
consistent with those observed and explain their origin as due
to the stream fragmentation.

---------------------------------------------------------
Title: EUV Flickering of Solar Coronal Loops: A New Diagnostic of
Coronal Heating
Authors: Tajfirouze, E.; Reale, F.; Peres, G.; Testa, P.
2016ApJ...817L..11T Altcode: 2016arXiv160103935T
A previous work of ours found the best agreement between EUV light
curves observed in an active region core (with evidence of super-hot
plasma) and those predicted from a model with a random combination
of many pulse-heated strands with a power-law energy distribution. We
extend that work by including spatially resolved strand modeling and
by studying the evolution of emission along the loops in the EUV 94 Å
and 335 Å channels of the Atmospheric Imaging Assembly on board the
Solar Dynamics Observatory. Using the best parameters of the previous
work as the input of the present one, we find that the amplitude of the
random fluctuations driven by the random heat pulses increases from the
bottom to the top of the loop in the 94 Å channel and from the top
to the bottom in the 335 Å channel. This prediction is confirmed by
the observation of a set of aligned neighboring pixels along a bright
arc of an active region core. Maps of pixel fluctuations may therefore
provide easy diagnostics of nanoflaring regions.

---------------------------------------------------------
Title: ADAHELI: exploring the fast, dynamic Sun in the x-ray, optical,
and near-infrared
Authors: Berrilli, Francesco; Soffitta, Paolo; Velli, Marco; Sabatini,
Paolo; Bigazzi, Alberto; Bellazzini, Ronaldo; Bellot Rubio, Luis
Ramon; Brez, Alessandro; Carbone, Vincenzo; Cauzzi, Gianna; Cavallini,
Fabio; Consolini, Giuseppe; Curti, Fabio; Del Moro, Dario; Di Giorgio,
Anna Maria; Ermolli, Ilaria; Fabiani, Sergio; Faurobert, Marianne;
Feller, Alex; Galsgaard, Klaus; Gburek, Szymon; Giannattasio, Fabio;
Giovannelli, Luca; Hirzberger, Johann; Jefferies, Stuart M.; Madjarska,
Maria S.; Manni, Fabio; Mazzoni, Alessandro; Muleri, Fabio; Penza,
Valentina; Peres, Giovanni; Piazzesi, Roberto; Pieralli, Francesca;
Pietropaolo, Ermanno; Martinez Pillet, Valentin; Pinchera, Michele;
Reale, Fabio; Romano, Paolo; Romoli, Andrea; Romoli, Marco; Rubini,
Alda; Rudawy, Pawel; Sandri, Paolo; Scardigli, Stefano; Spandre,
Gloria; Solanki, Sami K.; Stangalini, Marco; Vecchio, Antonio;
Zuccarello, Francesca
2015JATIS...1d4006B Altcode:
Advanced Astronomy for Heliophysics Plus (ADAHELI) is a project concept
for a small solar and space weather mission with a budget compatible
with an European Space Agency (ESA) S-class mission, including launch,
and a fast development cycle. ADAHELI was submitted to the European
Space Agency by a European-wide consortium of solar physics research
institutes in response to the "Call for a small mission opportunity
for a launch in 2017," of March 9, 2012. The ADAHELI project builds
on the heritage of the former ADAHELI mission, which had successfully
completed its phase-A study under the Italian Space Agency 2007 Small
Mission Programme, thus proving the soundness and feasibility of
its innovative low-budget design. ADAHELI is a solar space mission
with two main instruments: ISODY: an imager, based on Fabry-Pérot
interferometers, whose design is optimized to the acquisition of
highest cadence, long-duration, multiline spectropolarimetric images
in the visible/near-infrared region of the solar spectrum. XSPO: an
x-ray polarimeter for solar flares in x-rays with energies in the 15
to 35 keV range. ADAHELI is capable of performing observations that
cannot be addressed by other currently planned solar space missions,
due to their limited telemetry, or by ground-based facilities, due to
the problematic effect of the terrestrial atmosphere.

---------------------------------------------------------
Title: Accretion in young stars: measure of the stream velocity of
TW Hya from the X-ray Doppler shift
Authors: Argiroffi, Costanza; Bonito, Rosaria; Orlando, Salvatore;
Miceli, Marco; Peres, Giovanni
2015eheu.conf...57A Altcode:
High-resolution X-ray spectra are a unique tool to investigate the
accretion process in young stars. In fact X-rays allow to investigate
the accretion-shock region, where the infalling material is heated by
strong shocks due to the impact with the denser stellar atmosphere. Here
we show for the first time that it is possible to constrain the
velocity of the accretion stream by measuring the Doppler shift of
the emitted X-rays. To this aim we analyzed the deep Chandra/HETGS
observation of the accreting young star TW Hya. We selected a sample of
emission lines free from significant blends, fitted them with gaussian
profiles, computed the radial velocity corresponding to each line,
and averaged these velocities to obtain an accurate estimate of the
global velocity of the X-ray emitting plasma. After correcting for
Earth's motion, we compared this observed velocity with the photospheric
radial velocity. In order to check this procedure we applied the same
technique to other Chandra/HETGS spectra of single stars, whose X-rays
are due only to coronal plasma. While spectra of pure coronal sources
provide Doppler shifts in agreement with the known stellar radial
velocity, we found that the X-ray spectrum of TW Hya is red-shifted
by ~30-40 km/s with respect to the stellar photosphere. This proves
that the X-ray emitting plasma on TW Hya is moving with respect to
the stellar surface, definitively confirming that it originates in the
accretion-shock region. The observed velocity suggests that the base of
the accretion region is located at low latitudes of the stellar surface.

---------------------------------------------------------
Title: VizieR Online Data Catalog: UV variability and accretion in
NGC 2264 (Venuti+, 2015)
Authors: Venuti, L.; Bouvier, J.; Irwin, J.; Stauffer, J.; Hillenbrand,
L. A.; Rebull, L.; Cody, A. M.; Alencar, S. H. P.; Micela, G.;
Flaccomio, E.; Peres, G.
2015yCat..35810066V Altcode:
Our study aims at characterizing the variability signatures at short
wavelengths for several hundred accreting and non-accreting members of
the star-forming region NGC 2264 (~3Myr). We performed simultaneous,
2-week-long UV (u-band) and optical (r-band) monitoring of the cluster
with CFHT/MegaCam, with several flux measurements per observing night to
retrace the variability features on timescales from hours to a couple
of weeks. Photometric amplitudes and color behaviors are explored
to investigate the physical origin of the observed variability in
the different cases. In table2, we present the average u and
r photometry measured for individual sources in our sample, their
amplitudes of variability, the amount of variability measured via the
light curve rms and Stetson's (1996PASP..108..851S) J-index indicator,
the r-to-(u-r) amplitude ratio and the uncertainty associated with
the latter. (1 data file).

---------------------------------------------------------
Title: UV variability and accretion dynamics in the young open
cluster NGC 2264
Authors: Venuti, L.; Bouvier, J.; Irwin, J.; Stauffer, J. R.;
Hillenbrand, L. A.; Rebull, L. M.; Cody, A. M.; Alencar, S. H. P.;
Micela, G.; Flaccomio, E.; Peres, G.
2015A&A...581A..66V Altcode: 2015arXiv150606858V
Context. Photometric variability is a distinctive feature of young
stellar objects; exploring variability signatures at different
wavelengths provides insight into the physical processes at work in
these sources. Aims: We explore the variability signatures at
ultraviolet (UV) and optical wavelengths for several hundred accreting
and non-accreting members of the star-forming region NGC 2264 (~3
Myr). Methods: We performed simultaneous monitoring of u- and
r-band variability for the cluster population with CFHT/MegaCam. The
survey extended over two full weeks, with several flux measurements
per observing night. A sample of about 750 young stars is probed
in our study, homogeneously calibrated and reduced, with internally
consistently derived stellar parameters. Objects span the mass range
0.1-2 M⊙; about 40% of them show evidence for active
accretion based on various diagnostics (Hα, UV, and
IR excesses). Results: Statistically distinct variability
properties are observed for accreting and non-accreting cluster
members. The accretors exhibit a significantly higher level of
variability than the non-accretors, in the optical and especially
in the UV. The amount of u-band variability is found to correlate
statistically with the median amount of UV excess in disk-bearing
objects, which suggests that mass accretion and star-disk interaction
are the main sources of variability in the u band. Spot models are
applied to account for the amplitudes of variability of accreting
and non-accreting members, which yields different results for each
group. Cool magnetic spots, several hundred degrees colder than the
stellar photosphere and covering from 5 to 30% of the stellar surface,
appear to be the leading factor of variability for the non-accreting
stars. In contrast, accretion spots with a temperature a few thousand
degrees higher than the photospheric temperature and that extend over
a few percent of the stellar surface best reproduce the variability of
accreting objects. The color behavior is also found to be different
between accreting and non-accreting stars. While objects commonly
become redder when fainter, typical amplitudes of variability for
accreting members rapidly increase from the r to the u band, which
indicates a much stronger contrast at short wavelengths; a lower
color dependence in the photometric amplitudes is instead measured for
diskless stars. Finally, we compare the u-band variability monitored
here on two-week timescales with that measured on both shorter (hours)
and longer (years) timescales. We find that variability on timescales
of hours is typically ~10% of the peak-to-peak variability on day
timescales, while longer term variability on a timescale of years is
consistent with amplitudes measured over weeks. Conclusions: We
conclude that for both accreting and non-accreting stars, the mid-term
rotational modulation by hot and cold spots is the leading timescale for
a variability of up to several years. In turn, this suggests that the
accretion process is essentially stable over years, although it exhibits
low-level shorter term variations in single accretion events. Based
on observations obtained with MegaPrime/MegaCam, a joint project of
CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT)
which is operated by the National Research Council (NRC) of Canada,
the Institut National des Sciences de l'Univers of the Centre National
de la Recherche Scientifique (CNRS) of France, and the University of
Hawaii.Full Table 2 is only available at the CDS via anonymous ftp to <A
href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A>
(ftp://130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/581/A66">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/581/A66</A>

---------------------------------------------------------
Title: Accretion Shocks in Young Stars: the Role of Local Absorption
on the X-ray Emission
Authors: Bonito, R.; Argiroffi, C.; Orlando, S.; Miceli, M.; Peres,
G.; Matsakos, T.; Stehle, C.; Ibgui, L.
2015csss...18..199B Altcode:
We investigate the X-ray emission from accretion shocks in classical
T Tauri stars, due to the infalling material impacting the stellar
surface. Several aspects in both observations and models of the
accretion process are still unclear: the observed X-ray luminosity
of the post-shock plasma is below the predicted value, the density vs
temperature structure of the shocked plasma, with increasing densities
at higher temperature, is opposite of what expected from simple
accretion shock models. To address these issues we performed numerical
magnetohydrodynamic simulations describing the impact of an accretion
stream onto the stellar surface and considered the local absorption
due to the surrounding medium. We explored the effects of absorption
for different viewing angles and for the He-like line triplets commonly
used for density diagnostic. From the model results we synthesize the
X-ray emission from the accretion shock, producing maps and spectra. We
perform density and temperature diagnostics on the synthetic spectra,
and we directly compare our results with the observations. Our model
shows that the X-ray fluxes detected are lower than expected because of
the local absorption. The emerging spectra suggest a complex density vs
temperature distribution proving that a detailed model accounting for
a realistic treatment of the local absorption is needed to interpret
the observations of X-ray emitting accretion shocks.

---------------------------------------------------------
Title: Mass Accretion Processes in Young Stellar Objects: Role of
Intense Flaring Activity
Authors: Orlando, Salvatore; Reale, Fabio; Peres, Giovanni; Mignone,
Andrea
2014mbhe.conf..108O Altcode:
According to the magnetospheric accretion scenario, young low-mass
stars are surrounded by circumstellar disks which they interact
with through accretion of mass. The accretion builds up the star
to its final mass and is also believed to power the mass outflows,
which may in turn have a signicant role in removing the excess
angular momentum from the star-disk system. Although the process
of mass accretion is a critical aspect of star formation, some of
its mechanisms are still to be fully understood. On the other hand,
strong flaring activity is a common feature of young stellar objects
(YSOs). In the Sun, such events give rise to perturbations of the
interplanetary medium. Similar but more energetic phenomena occur
in YSOs and may influence the circumstellar environment. In fact, a
recent study has shown that an intense flaring activity close to the
disk may strongly perturb the stability of circumstellar disks, thus
inducing mass accretion episodes (Orlando et al. 2011). Here we review
the main results obtained in the field and the future perspectives.

---------------------------------------------------------
Title: Magnetohydrodynamic Modeling of the Accretion Shocks in
Classical T Tauri Stars: The Role of Local Absorption in the X-Ray
Emission
Authors: Bonito, R.; Orlando, S.; Argiroffi, C.; Miceli, M.; Peres,
G.; Matsakos, T.; Stehle, C.; Ibgui, L.
2014ApJ...795L..34B Altcode: 2014arXiv1410.2775B
We investigate the properties of X-ray emission from accretion shocks
in classical T Tauri stars (CTTSs), generated where the infalling
material impacts the stellar surface. Both observations and models
of the accretion process reveal several aspects that are still
unclear: the observed X-ray luminosity in accretion shocks is below
the predicted value, and the density versus temperature structure of
the shocked plasma, with increasing densities at higher temperature,
deduced from the observations, is at odds with that proposed in the
current picture of accretion shocks. To address these open issues,
we investigate whether a correct treatment of the local absorption
by the surrounding medium is crucial to explain the observations. To
this end, we describe the impact of an accretion stream on a CTTS by
considering a magnetohydrodynamic model. From the model results, we
synthesize the X-ray emission from the accretion shock by producing
maps and spectra. We perform density and temperature diagnostics
on the synthetic spectra, and we directly compare the results with
observations. Our model shows that the X-ray fluxes inferred from the
emerging spectra are lower than expected because of the complex local
absorption by the optically thick material of the chromosphere and of
the unperturbed stream. Moreover, our model, including the effects of
local absorption, explains in a natural way the apparently puzzling
pattern of density versus temperature observed in the X-ray emission
from accretion shocks.

---------------------------------------------------------
Title: Mapping accretion and its variability in the young open
cluster NGC 2264: a study based on u-band photometry
Authors: Venuti, L.; Bouvier, J.; Flaccomio, E.; Alencar, S. H. P.;
Irwin, J.; Stauffer, J. R.; Cody, A. M.; Teixeira, P. S.; Sousa,
A. P.; Micela, G.; Cuillandre, J. -C.; Peres, G.
2014A&A...570A..82V Altcode: 2014arXiv1408.0432V
Context. The accretion process has a central role in the formation of
stars and planets. Aims: We aim at characterizing the accretion
properties of several hundred members of the star-forming cluster
NGC 2264 (3 Myr). Methods: We performed a deep ugri mapping as
well as a simultaneous u-band+r-band monitoring of the star-forming
region with CFHT/MegaCam in order to directly probe the accretion
process onto the star from UV excess measurements. Photometric
properties and stellar parameters are determined homogeneously for
about 750 monitored young objects, spanning the mass range ~0.1-2
M⊙. About 40% of the sample are classical (accreting)
T Tauri stars, based on various diagnostics (Hα, UV and IR
excesses). The remaining non-accreting members define the (photospheric
+ chromospheric) reference UV emission level over which flux excess
is detected and measured. Results: We revise the membership
status of cluster members based on UV accretion signatures, and report
a new population of 50 classical T Tauri star (CTTS) candidates. A
large range of UV excess is measured for the CTTS population,
varying from a few times 0.1 to ~3 mag. We convert these values to
accretion luminosities and accretion rates, via a phenomenological
description of the accretion shock emission. We thus obtain mass
accretion rates ranging from a few 10-10 to ~10-7
M⊙/yr. Taking into account a mass-dependent detection
threshold for weakly accreting objects, we find a &gt;6σ correlation
between mass accretion rate and stellar mass. A power-law fit, properly
accounting for censored data (upper limits), yields Ṁacc
∝ M*1.4±0.3. At any given stellar mass,
we find a large spread of accretion rates, extending over about 2
orders of magnitude. The monitoring of the UV excess on a timescale
of a couple of weeks indicates that its variability typically
amounts to 0.5 dex, i.e., much smaller than the observed spread in
accretion rates. We suggest that a non-negligible age spread across the
star-forming region may effectively contribute to the observed spread
in accretion rates at a given mass. In addition, different accretion
mechanisms (like, e.g., short-lived accretion bursts vs. more stable
funnel-flow accretion) may be associated to different Ṁacc
regimes. Conclusions: A huge variety of accretion properties
is observed for young stellar objects in the NGC 2264 cluster. While
a definite correlation seems to hold between mass accretion rate
and stellar mass over the mass range probed here, the origin of the
large intrinsic spread observed in mass accretion rates at any given
mass remains to be explored. Based on observations obtained
with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at
the Canada-France-Hawaii Telescope (CFHT) which is operated by the
National Research Council (NRC) of Canada, the Institut National
des Sciences de l'Univers of the Centre National de la Recherche
Scientifique (CNRS) of France, and the University of Hawaii.Full
Tables 2-4 are only available at the CDS via anonymous ftp to <A
href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A>
(ftp://130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/570/A82">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/570/A82</A>

---------------------------------------------------------
Title: 3D YSO accretion shock simulations: a study of the magnetic,
chromospheric and stochastic flow effects
Authors: Matsakos, T.; Chièze, J. -P.; Stehlé, C.; González, M.;
Ibgui, L.; de Sá, L.; Lanz, T.; Orlando, S.; Bonito, R.; Argiroffi,
C.; Reale, F.; Peres, G.
2014IAUS..302...66M Altcode:
The structure and dynamics of young stellar object (YSO) accretion
shocks depend strongly on the local magnetic field strength and
configuration, as well as on the radiative transfer effects responsible
for the energy losses. We present the first 3D YSO shock simulations
of the interior of the stream, assuming a uniform background magnetic
field, a clumpy infalling gas, and an acoustic energy flux flowing at
the base of the chromosphere. We study the dynamical evolution and
the post-shock structure as a function of the plasma-beta (thermal
pressure over magnetic pressure). We find that a strong magnetic field
(~hundreds of Gauss) leads to the formation of fibrils in the shocked
gas due to the plasma confinement within flux tubes. The corresponding
emission is smooth and fully distinguishable from the case of a weak
magnetic field (~tenths of Gauss) where the hot slab demonstrates
chaotic motion and oscillates periodically.

---------------------------------------------------------
Title: VizieR Online Data Catalog: Mapping accretion variability in
NGC 2264 (Venuti+, 2014)
Authors: Venuti, L.; Bouvier, J.; Flaccomio, E.; Alencar, S. H. P.;
Irwin, J.; Stauffer, J. R.; Cody, A. M.; Teixeira, P. S.; Sousa,
A. P.; Micela, G.; Cuillandre, J. -C.; Peres, G.
2014yCat..35700082V Altcode: 2014yCat..35709082V
We present homogeneous multi-band (optical+UV) photometry (table2) and
stellar parameters (table3: accreting/non-accreting status, spectral
type, Av, bolometric luminosity, stellar mass, stellar radius and age
estimate) for 757 young stars in the cluster (706 confirmed members +
51 newly identified candidates from our survey). In table4, we present
the UV excess measurements, u-band excess luminosity, median accretion
rates and their measured variability ranges for 237 accreting objects
in our sample. (3 data files).

---------------------------------------------------------
Title: X-rays from accretion shocks in classical T Tauri stars:
2D MHD modeling and the role of local absorption
Authors: Argiroffi, C.; Bonito, R.; Orlando, S.; Miceli, M.; Reale,
F.; Peres, G.; Matsakos, T.; Sthelé, C.; Ibgui, L.
2014IAUS..302...48A Altcode:
In classical T Tauri stars (CTTS) strong shocks are formed where the
accretion funnel impacts with the denser stellar chromosphere. Although
current models of accretion provide a plausible global picture of
this process, some fundamental aspects are still unclear: the observed
X-ray luminosity in accretion shocks is order of magnitudes lower than
predicted; the observed density and temperature structures of the hot
post-shock region are puzzling and still unexplained by models. To address these issues we performed 2D MHD simulations describing an
accretion stream impacting onto the chromosphere of a CTTS, exploring
different configurations and strengths of the magnetic field. From
the model results we then synthesized the X-ray emission emerging
from the hot post-shock, taking into account the local absorption
due to the pre-shock stream and surrounding atmosphere. We find
that the different configurations and strengths of the magnetic field
profoundly affect the hot post-shock properties. Moreover the emerging
X-ray emission strongly depends also on the viewing angle under which
accretion is observed. Some of the explored configuration are able to
reproduce the observed features of X-ray spectra of CTTS.

---------------------------------------------------------
Title: Accretion impacts studied on the Sun
Authors: Reale, F.; Orlando, S.; Testa, P.; Peres, G.; Landi, E.;
Schrijver, C.
2014xru..confE.169R Altcode:
Accretion in star-forming regions is a hot topic. The Sun has recently
offered an interesting opportunity to study accretion impacts observed
in great detail at high energies (Reale et al. 2013, Science, 341,
6143, 251). After the eruption of a dense filament triggered by an
energetic flare on June 7, 2011 part of the ejected material falls
back onto the solar surface. The impact of the downfalling plasma is
similar to that of accretion flows on young stellar objects, and was
imaged in the EUV by the Atmospheric Imaging Assembly (AIA) on-board
the Solar Dynamics Observatory (SDO). Hydrodynamic simulations confirm
that the high energy emission is produced by the impact of high-density
plasma at the highest free-fall speeds and show the importance of
the absorption in reducing the X-ray emission and of fragmentation
in explaining the line broadenings. Impacts such as these present
a laboratory for stellar astronomers to study the impact of dense
(accreting) circumstellar material in unique detail.

---------------------------------------------------------
Title: Filling the gap between supernova explosions and their
remnants: the Cassiopeia A laboratory
Authors: Orlando, S.; Miceli, M.; Pumo, M.; Bocchino, F.; Reale, F.;
Peres, G.
2014xru..confE.298O Altcode:
Supernova remnats (SNRs) show a complex morphology characterized by
an inhomogeneous spatial distribution of ejecta, believed to reflect
pristine structures and features of the progenitor supernova (SN)
explosion. Filling the gap between SN explosions and their remnants
is very important for a comprehension of the origin of present-day
structure of ejecta in SNRs and to probe and constraint current models
of SN explosions. The SNR Cassiopeia A (Cas A) is an attractive
laboratory for studying the SNe-SNRs connection, being one of the
best studied SNRs for which its 3D structure is known. We present a
three-dimensional hydrodynamic model describing the evolution of Cas
A from the immediate aftermath of the SN explosion to its expansion
through the interstellar medium, taking into account the distribution
of element abundances of the ejecta, the backreaction of accelerated
cosmic rays at the shock front, and the deviations from equilibrium
of ionizazion for the most important elements. We use the model to
derive the physical parameters characterizing the SN explosion and
reproducing the today morphology of Cas A.

---------------------------------------------------------
Title: A New Look at Spitzer Primary Transit Observations of the
Exoplanet HD 189733b
Authors: Morello, G.; Waldmann, I. P.; Tinetti, G.; Peres, G.; Micela,
G.; Howarth, I. D.
2014ApJ...786...22M Altcode: 2014arXiv1403.2874M
Blind source separation techniques are used to reanalyze two
exoplanetary transit light curves of the exoplanet HD 189733b recorded
with the IR camera IRAC on board the Spitzer Space Telescope at
3.6 μm during the "cold" era. These observations, together with
observations at other IR wavelengths, are crucial to characterize
the atmosphere of the planet HD 189733b. Previous analyses of the
same data sets reported discrepant results, hence the necessity of
the reanalyses. The method we used here is based on the Independent
Component Analysis (ICA) statistical technique, which ensures a high
degree of objectivity. The use of ICA to detrend single photometric
observations in a self-consistent way is novel in the literature. The
advantage of our reanalyses over previous work is that we do not have
to make any assumptions on the structure of the unknown instrumental
systematics. Such "admission of ignorance" may result in larger
error bars than reported in the literature, up to a factor 1.6. This
is a worthwhile tradeoff for much higher objectivity, necessary for
trustworthy claims. Our main results are (1) improved and robust values
of orbital and stellar parameters, (2) new measurements of the transit
depths at 3.6 μm, (3) consistency between the parameters estimated
from the two observations, (4) repeatability of the measurement within
the photometric level of ~2 × 10-4 in the IR, and (5) no
evidence of stellar variability at the same photometric level within
one year.

---------------------------------------------------------
Title: A new look at Spitzer primary transit observations of the
exoplanet HD189733b
Authors: Morello, G.; Waldmann, I. P.; Tinetti, G.; Peres, G.; Micela,
G.; Howarth, I. D.
2014EPSC....9..462M Altcode:
Blind source separation techniques are used to reanalyse two
exoplanetary transit lightcurves of the exoplanet HD189733b recorded
with the IR camera IRAC on board the Spitzer Space Telescope at
3.6μm during the "cold" era. These observations, together with
observations at other IR wavelengths, are crucial to characterise
the atmosphere of the planet HD189733b. Previous analyses of the
same datasets reported discrepant results, hence the necessity of
the reanalyses. The method we used here is based on the Independent
Component Analysis (ICA) statistical technique, which ensures a high
degree of objectivity. The use of ICA to detrend single photometric
observations in a self-consistent way is novel in the literature. The
advantage of our reanalyses over previous work is that we do not have
to make any assumptions on the structure of the unknown instrumental
systematics. Such "admission of ignorance" may result in larger
error bars than reported in the literature, up to a factor 1.6. This
is a worthwhile trade-off for much higher objectivity, necessary for
trustworthy claims. Our main results are (1) improved and robust values
of orbital and stellar parameters, (2) new measurements of the transit
depths at 3.6μm, (3) consistency between the parameters estimated from
the two observations, (4) repeatability of the measurement within the
photometric level of ~ 2 × 10-4 in the IR, (5) no evidence of stellar
variability at the same photometric level within 1 year.

---------------------------------------------------------
Title: Role of local absorption on the X-ray emission from MHD
accretion shocks in classical T Tauri stars
Authors: Bonito, R.; Orlando, S.; Argiroffi, C.; Miceli, M.; Reale,
F.; Peres, G.; Matsakos, T.; Stehle, H. C.; Ibgui, L.
2014EPJWC..6405004B Altcode:
Accretion processes onto classical T Tauri stars (CTTSs) are believed to
generate shocks at the stellar surface due to the impact of supersonic
downflowing plasma. Although current models of accretion streams provide
a plausible global picture of this process, several aspects are still
unclear. For example, the observed X-ray luminosity in accretion shocks
is, in general, well below the predicted value. A possible explanation
discussed in the literature is in terms of significant absorption of the
emission due to the thick surrounding medium. Here we consider a 2D MHD
model describing an accretion stream propagating through the atmosphere
of a CTTS and impacting onto its chromosphere. The model includes all
the relevant physics, namely the gravity, the thermal conduction, and
the radiative cooling, and a realistic description of the unperturbed
stellar atmosphere (from the chromosphere to the corona). From the model
results, we synthesize the X-ray emission emerging from the hot slab
produced by the accretion shock, exploring different configurations
and strengths of the stellar magnetic field. The synthesis includes
the local absorption by the thick surrounding medium and the Doppler
shift of lines due to the component of plasma velocity along the
line-of-sight. We explore the effects of absorption on the emerging
X-ray spectrum, considering different inclinations of the accretion
stream with respect to the observer. Finally we compare our results
with the observations.

---------------------------------------------------------
Title: Mass Accretion Processes in Young Stellar Objects: Role of
Intense Flaring Activity
Authors: Orlando, S.; Reale, F.; Peres, G.; Mignone, A.
2014arXiv1401.0408O Altcode:
According to the magnetospheric accretion scenario, young low-mass
stars are surrounded by circumstellar disks which they interact
with through accretion of mass. The accretion builds up the star
to its final mass and is also believed to power the mass outflows,
which may in turn have a significant role in removing the excess
angular momentum from the star-disk system. Although the process
of mass accretion is a critical aspect of star formation, some of
its mechanisms are still to be fully understood. On the other hand,
strong flaring activity is a common feature of young stellar objects
(YSOs). In the Sun, such events give rise to perturbations of the
interplanetary medium. Similar but more energetic phenomena occur
in YSOs and may influence the circumstellar environment. In fact, a
recent study has shown that an intense flaring activity close to the
disk may strongly perturb the stability of circumstellar disks, thus
inducing mass accretion episodes (Orlando et al. 2011). Here we review
the main results obtained in the field and the future perspectives.

---------------------------------------------------------
Title: 3D numerical modeling of YSO accretion shocks
Authors: Matsakos, T.; Chièze, J. -P.; Stehlé, C.; González, M.;
Ibgui, L.; de Sá, L.; Lanz, T.; Orlando, S.; Bonito, R.; Argiroffi,
C.; Reale, F.; Peres, G.
2014EPJWC..6404003M Altcode:
The dynamics of YSO accretion shocks is determined by radiative
processes as well as the strength and structure of the magnetic
field. A quasi-periodic emission signature is theoretically expected
to be observed, but observations do not confirm any such pattern. In
this work, we assume a uniform background field, in the regime of
optically thin energy losses, and we study the multi-dimensional shock
evolution in the presence of perturbations, i.e. clumps in the stream
and an acoustic energy flux flowing at the base of the chromosphere. We
perform 3D MHD simulations using the PLUTO code, modelling locally the
impact of the infalling gas onto the chromosphere. We find that the
structure and dynamics of the post-shock region is strongly dependent
on the plasma-beta (thermal over magnetic pressure), different values
of which may give distinguishable emission signatures, relevant for
observations. In particular, a strong magnetic field effectively
confines the plasma inside its flux tubes and leads to the formation
of quasi-independent fibrils. The fibrils may oscillate out of phase
and hence the sum of their contributions in the emission results in
a smooth overall profile. On the contrary, a weak magnetic field is
not found to have any significant effect on the shocked plasma and the
turbulent hot slab that forms is found to retain its periodic signature.

---------------------------------------------------------
Title: XIPE: the X-ray imaging polarimetry explorer
Authors: Soffitta, Paolo; Barcons, Xavier; Bellazzini, Ronaldo; Braga,
João; Costa, Enrico; Fraser, George W.; Gburek, Szymon; Huovelin,
Juhani; Matt, Giorgio; Pearce, Mark; Poutanen, Juri; Reglero, Victor;
Santangelo, Andrea; Sunyaev, Rashid A.; Tagliaferri, Gianpiero;
Weisskopf, Martin; Aloisio, Roberto; Amato, Elena; Attiná, Primo;
Axelsson, Magnus; Baldini, Luca; Basso, Stefano; Bianchi, Stefano;
Blasi, Pasquale; Bregeon, Johan; Brez, Alessandro; Bucciantini,
Niccoló; Burderi, Luciano; Burwitz, Vadim; Casella, Piergiorgio;
Churazov, Eugene; Civitani, Marta; Covino, Stefano; Curado da Silva,
Rui Miguel; Cusumano, Giancarlo; Dadina, Mauro; D'Amico, Flavio; De
Rosa, Alessandra; Di Cosimo, Sergio; Di Persio, Giuseppe; Di Salvo,
Tiziana; Dovciak, Michal; Elsner, Ronald; Eyles, Chris J.; Fabian,
Andrew C.; Fabiani, Sergio; Feng, Hua; Giarrusso, Salvatore; Goosmann,
René W.; Grandi, Paola; Grosso, Nicolas; Israel, Gianluca; Jackson,
Miranda; Kaaret, Philip; Karas, Vladimir; Kuss, Michael; Lai, Dong;
La Rosa, Giovanni; Larsson, Josefin; Larsson, Stefan; Latronico, Luca;
Maggio, Antonio; Maia, Jorge; Marin, Frédéric; Massai, Marco Maria;
Mineo, Teresa; Minuti, Massimo; Moretti, Elena; Muleri, Fabio; O'Dell,
Stephen L.; Pareschi, Giovanni; Peres, Giovanni; Pesce, Melissa;
Petrucci, Pierre-Olivier; Pinchera, Michele; Porquet, Delphine; Ramsey,
Brian; Rea, Nanda; Reale, Fabio; Rodrigo, Juana Maria; Różańska,
Agata; Rubini, Alda; Rudawy, Pawel; Ryde, Felix; Salvati, Marco;
de Santiago, Valdivino Alexandre; Sazonov, Sergey; Sgró, Carmelo;
Silver, Eric; Spandre, Gloria; Spiga, Daniele; Stella, Luigi; Tamagawa,
Toru; Tamborra, Francesco; Tavecchio, Fabrizio; Teixeira Dias, Teresa;
van Adelsberg, Matthew; Wu, Kinwah; Zane, Silvia
2013ExA....36..523S Altcode: 2013arXiv1309.6995S; 2013ExA...tmp...32S
Abstract X-ray polarimetry, sometimes alone, and sometimes coupled to
spectral and temporal variability measurements and to imaging, allows
a wealth of physical phenomena in astrophysics to be studied. X-ray
polarimetry investigates the acceleration process, for example,
including those typical of magnetic reconnection in solar flares,
but also emission in the strong magnetic fields of neutron stars and
white dwarfs. It detects scattering in asymmetric structures such
as accretion disks and columns, and in the so-called molecular torus
and ionization cones. In addition, it allows fundamental physics in
regimes of gravity and of magnetic field intensity not accessible to
experiments on the Earth to be probed. Finally, models that describe
fundamental interactions (e.g. quantum gravity and the extension of
the Standard Model) can be tested. We describe in this paper the X-ray
Imaging Polarimetry Explorer (XIPE), proposed in June 2012 to the first
ESA call for a small mission with a launch in 2017. The proposal was,
unfortunately, not selected. To be compliant with this schedule, we
designed the payload mostly with existing items. The XIPE proposal
takes advantage of the completed phase A of POLARIX for an ASI small
mission program that was cancelled, but is different in many aspects:
the detectors, the presence of a solar flare polarimeter and photometer
and the use of a light platform derived by a mass production for
a cluster of satellites. XIPE is composed of two out of the three
existing JET-X telescopes with two Gas Pixel Detectors (GPD) filled
with a He-DME mixture at their focus. Two additional GPDs filled with
a 3-bar Ar-DME mixture always face the Sun to detect polarization from
solar flares. The Minimum Detectable Polarization of a 1 mCrab source
reaches 14 % in the 2-10 keV band in 105 s for pointed
observations, and 0.6 % for an X10 class solar flare in the 15-35 keV
energy band. The imaging capability is 24 arcsec Half Energy Width
(HEW) in a Field of View of 14.7 arcmin × 14.7 arcmin. The spectral
resolution is 20 % at 6 keV and the time resolution is 8 μs. The
imaging capabilities of the JET-X optics and of the GPD have been
demonstrated by a recent calibration campaign at PANTER X-ray test
facility of the Max-Planck-Institut für extraterrestrische Physik (MPE,
Germany). XIPE takes advantage of a low-earth equatorial orbit with
Malindi as down-link station and of a Mission Operation Center (MOC)
at INPE (Brazil). The data policy is organized with a Core Program
that comprises three months of Science Verification Phase and 25 %
of net observing time in the following 2 years. A competitive Guest
Observer program covers the remaining 75 % of the net observing time.

---------------------------------------------------------
Title: Radiative accretion shocks along nonuniform stellar magnetic
fields in classical T Tauri stars
Authors: Orlando, S.; Bonito, R.; Argiroffi, C.; Reale, F.; Peres,
G.; Miceli, M.; Matsakos, T.; Stehlé, C.; Ibgui, L.; de Sa, L.;
Chièze, J. P.; Lanz, T.
2013A&A...559A.127O Altcode: 2013arXiv1309.5038O
Context. According to the magnetospheric accretion model, hot
spots form on the surface of classical T Tauri stars (CTTSs) in
regions where accreting disk material impacts the stellar surface at
supersonic velocity, generating a shock. Aims: We investigate
the dynamics and stability of postshock plasma that streams along
nonuniform stellar magnetic fields at the impact region of accretion
columns. We study how the magnetic field configuration and strength
determine the structure, geometry, and location of the shock-heated
plasma. Methods: We model the impact of an accretion stream
onto the chromosphere of a CTTS by 2D axisymmetric magnetohydrodynamic
simulations. Our model considers the gravity, the radiative cooling,
and the magnetic-field-oriented thermal conduction (including the
effects of heat flux saturation). We explore different configurations
and strengths of the magnetic field. Results: The structure,
stability, and location of the shocked plasma strongly depend on
the configuration and strength of the magnetic field. In the case
of weak magnetic fields (plasma β ≳ 1 in the postshock region),
a large component of B may develop perpendicular to the stream at
the base of the accretion column, which limits the sinking of the
shocked plasma into the chromosphere and perturbs the overstable shock
oscillations induced by radiative cooling. An envelope of dense and cold
chromospheric material may also develop around the shocked column. For
strong magnetic fields (β &lt; 1 in the postshock region close to
the chromosphere), the field configuration determines the position of
the shock and its stand-off height. If the field is strongly tapered
close to the chromosphere, an oblique shock may form well above the
stellar surface at the height where the plasma β ≈ 1. In general,
we find that a nonuniform magnetic field makes the distribution of
emission measure vs. temperature of the postshock plasma at T &gt;
106 K lower than when there is uniform magnetic field. Conclusions: The initial magnetic field strength and configuration
in the region of impact of the stream are expected to influence
the chromospheric absorption and, therefore, the observability of
the shock-heated plasma in the X-ray band. In addition, the field
strength and configuration also influence the energy balance of the
shocked plasma with its emission measure at T &gt; 106 K,
which is lower than expected for a uniform field. The above effects
contribute to underestimating the mass accretion rates derived in
the X-ray band. Movies are available in electronic form at <A
href="http://www.aanda.org">http://www.aanda.org</A>

---------------------------------------------------------
Title: YSO accretion shocks: magnetic, chromospheric or stochastic
flow effects can suppress fluctuations of X-ray emission
Authors: Matsakos, T.; Chièze, J. -P.; Stehlé, C.; González, M.;
Ibgui, L.; de Sá, L.; Lanz, T.; Orlando, S.; Bonito, R.; Argiroffi,
C.; Reale, F.; Peres, G.
2013A&A...557A..69M Altcode: 2013arXiv1307.5389M
Context. Theoretical arguments and numerical simulations of radiative
shocks produced by the impact of the accreting gas onto young stars
predict quasi-periodic oscillations in the emitted radiation. However,
observational data do not show evidence of such periodicity. 
Aims: We investigate whether physically plausible perturbations in
the accretion column or in the chromosphere could disrupt the shock
structure influencing the observability of the oscillatory behavior. Methods: We performed local 2D magneto-hydrodynamical simulations
of an accretion shock impacting a chromosphere, taking optically thin
radiation losses and thermal conduction into account. We investigated
the effects of several perturbation types, such as clumps in the
accretion stream or chromospheric fluctuations, and also explored
a wide range of plasma-β values. Results: In the case of
a weak magnetic field, the post-shock region shows chaotic motion
and mixing, smoothing out the perturbations and retaining a global
periodic signature. On the other hand, a strong magnetic field confines
the plasma in flux tubes, which leads to the formation of fibrils
that oscillate independently. Realistic values for the amplitude,
length, and time scales of the perturbation are capable of bringing
the fibril oscillations out of phase, suppressing the periodicity of
the emission. Conclusions: The strength of a locally uniform
magnetic field in YSO accretion shocks determines the structure of the
post-shock region, namely, whether it will be somewhat homogeneous or
if it will split up to form a collection of fibrils. In the second
case, the size and shape of the fibrils is found to depend strongly
on the plasma-β value but not on the perturbation type. Therefore,
the actual value of the protostellar magnetic field is expected to
play a critical role in the time dependence of the observable emission.

---------------------------------------------------------
Title: A new look at the Spitzer primary transit observations of
the exoplanet HD189733b
Authors: Morello, G.; Peres, G.; Micela, G.; Waldmann, I. P.; Tinetti,
G.; Howarth, I. D.
2013EPSC....8..997M Altcode:
New blind source separation techniques are used to analyse uniformly
eight primary transit lightcurves of the exoplanet HD189733b recorded
with the infrared camera IRAC on board the Spitzer Space Telescope
at 3.6μm. The observations were performed between 2006 and 2011:
two of them were obtained in the"Cold Spitzer" regime, the others
were taken in the "Warm Spitzer" period. The techniques we used to
process the data are based on an Independent Component Analysis (ICA)
approach, i.e. a computational method to disentangle the 'original
source signals' from a set of observations/ recordings in which they
are mixed. ICA assumes only the mutual statistical independence and the
non-gaussianity of the source signals. Our objective was to extract
the transit components by removing instrumental systematic effects
and possibly other sources of astrophysical noise, such as background
and stellar activity. The novelty of the algorithms used is their
ability to extract the exoplanet signal in a single observation. In
this presentation we will present the results obtained, detail the
methods adopted and critically discuss the conclusions of our work by
comparing said results to the ones obtained in the literature.

---------------------------------------------------------
Title: Bright Hot Impacts by Erupted Fragments Falling Back on the
Sun: A Template for Stellar Accretion
Authors: Reale, Fabio; Orlando, Salvatore; Testa, Paola; Peres,
Giovanni; Landi, Enrico; Schrijver, Carolus J.
2013Sci...341..251R Altcode:
Impacts of falling fragments observed after the eruption of a filament
in a solar flare on 7 June 2011 are similar to those inferred for
accretion flows on young stellar objects. As imaged in the ultraviolet
(UV)-extreme UV range by the Atmospheric Imaging Assembly onboard
the Solar Dynamics Observatory, many impacts of dark, dense matter
display uncommonly intense, compact brightenings. High-resolution
hydrodynamic simulations show that such bright spots, with plasma
temperatures increasing from ~104 to ~106
kelvin, occur when high-density plasma (&gt;&gt;1010
particles per cubic centimeter) hits the solar surface at several
hundred kilometers per second, producing high-energy emission as in
stellar accretion. The high-energy emission comes from the original
fragment material and is heavily absorbed by optically thick plasma,
possibly explaining the lower mass accretion rates inferred from x-rays
relative to UV-optical-near infrared observations of young stars.

---------------------------------------------------------
Title: Hydrodynamic modelling of ejecta shrapnel in the Vela
supernova remnant
Authors: Miceli, M.; Orlando, S.; Reale, F.; Bocchino, F.; Peres, G.
2013MNRAS.430.2864M Altcode: 2013arXiv1301.3085M; 2013MNRAS.tmp..743M
Many supernova remnants (SNRs) are characterized by a knotty ejecta
structure. The Vela SNR is an excellent example of remnant in which
detached clumps of ejecta are visible as X-ray emitting bullets that
have been observed and studied in great detail. We aim at modelling
the evolution of ejecta shrapnel in the Vela SNR, investigating the
role of their initial parameters (position and density) and addressing
the effects of thermal conduction and radiative losses. We performed
a set of 2D hydrodynamic simulations describing the evolution of a
density inhomogeneity in the ejecta profile. We explored different
initial setups. We found that the final position of the shrapnel is
very sensitive to its initial position within the ejecta, while the
dependence on the initial density contrast is weaker. Our model also
shows that moderately overdense knots can reproduce the detached
features observed in the Vela SNR. Efficient thermal conduction
produces detectable effects by determining an efficient mixing of the
ejecta knot with the surrounding medium and shaping a characteristic
elongated morphology in the clump.

---------------------------------------------------------
Title: Hot Plasma Detected in Active Regions by HINODE/XRT and SDO/AIA
Authors: Reale, F.; Testa, P.; Guarrasi, M.; DeLuca, E.; Peres, G.;
Golub, L.
2012ASPC..456..129R Altcode:
Multiple ratios of Hinode/XRT filters showed evidence of a minor
very hot emission measure component in active regions. Recently also
SDO/AIA detected hot plasma in the core of an active region. Here
we provide estimates showing that the amount of emission measure of
the hot component detected with SDO is consistent with that detected
with Hinode/XRT.

---------------------------------------------------------
Title: Generation of Knots in a Randomly Pulsed Protostellar Jet:
Synthesis of the X-ray Emission
Authors: Bonito, R.; Orlando, S.; Miceli, M.; Eislöffel, J.; Peres,
G.; Favata, F.
2011ASPC..448..559B Altcode: 2011csss...16..559B
X-rays from protostellar jets have been discovered first in HH 2
and HH 154 and are now considered as a general feature of jets. HH
154 is among the best studied X-ray emitting jet: up to now it is
the only jet whose X-ray source proper motion has been measured. By
observing this jet in X-rays, a complex morphology of the detected
source has been revealed. Here we discuss the results of modeling a
randomly ejected pulsed jet traveling into an initially homogeneous
medium. Our model allows us to directly compare the model predictions
and the observations.

---------------------------------------------------------
Title: X-ray emission from protostellar jet HH 154: first evidence
of a diamond shock?
Authors: Bonito, Rosaria; Orlando, Salvatore; Miceli, Marco; Peres,
Giovanni; Micela, Giusi; Favata, Fabio
2011xru..conf..190B Altcode:
No abstract at ADS

---------------------------------------------------------
Title: X-Ray Emission from Protostellar Jet HH 154: The First Evidence
of a Diamond Shock?
Authors: Bonito, R.; Orlando, S.; Miceli, M.; Peres, G.; Micela, G.;
Favata, F.
2011ApJ...737...54B Altcode: 2011arXiv1105.4081B
X-ray emission from about 10 protostellar jets has been discovered and
it appears as a feature common to the most energetic jets. Although
X-ray emission seems to originate from shocks internal to jets,
the mechanism forming these shocks remains controversial. One of the
best-studied X-ray jets is HH 154, which has been observed by Chandra
over a time base of about 10 years. We analyze the Chandra observations
of HH 154 by investigating the evolution of its X-ray source. We show
that the X-ray emission consists of a bright stationary component and a
faint elongated component. We interpret the observations by developing
a hydrodynamic model describing a protostellar jet originating from
a nozzle and compare the X-ray emission synthesized from the model
with the X-ray observations. The model takes into account the thermal
conduction and radiative losses and shows that the jet/nozzle leads
to the formation of a diamond shock at the nozzle exit. The shock is
stationary over the period covered by our simulations and generates an
X-ray source with luminosity and spectral characteristics in excellent
agreement with the observations. We conclude that the X-ray emission
from HH 154 is consistent with a diamond shock originating from a
nozzle through which the jet is launched into the ambient medium. We
suggest that the physical origin of the nozzle could be related to
the dense gas in which the HH 154 driving source is embedded and/or
to the magnetic field at the jet launching/collimation region.

---------------------------------------------------------
Title: Mass accretion to young stars triggered by flaring activity
in circumstellar discs
Authors: Orlando, Salvatore; Reale, Fabio; Peres, Giovanni; Mignone,
Andrea
2011MNRAS.415.3380O Altcode: 2011arXiv1104.5107O; 2011MNRAS.tmp..846O
Young low-mass stars are characterized by ejection of collimated
outflows and by circumstellar discs which they interact with through
accretion of mass. The accretion builds up the star to its final mass
and is also believed to power the mass outflows, which may in turn
remove the excess angular momentum from the star-disc system. However,
although the process of mass accretion is a critical aspect of star
formation, some of its mechanisms are still to be fully understood. A
point not considered to date and relevant for the accretion process
is the evidence of very energetic and frequent flaring events in
these stars. Flares may easily perturb the stability of the discs,
thus influencing the transport of mass and angular momentum. Here we
report on 3D magnetohydrodynamic modelling of the evolution of a flare
with an idealized non-equilibrium initial condition occurring near the
disc around a rotating magnetized star. The model takes into account
the stellar magnetic field, the gravitational force, the viscosity of
the disc, the magnetic-field-oriented thermal conduction (including the
effects of heat flux saturation), the radiative losses from optically
thin plasma and the coronal heating. We show that, during its first
stage of evolution, the flare gives rise to a hot magnetic loop linking
the disc to the star. The disc is strongly perturbed by the flare: disc
material evaporates under the effect of the thermal conduction and an
overpressure wave propagates through the disc. When the overpressure
reaches the opposite side of the disc, a funnel flow starts to develop
there, accreting substantial disc material on to the young star from
the side of the disc opposite to the flare.

---------------------------------------------------------
Title: Solar Dynamics Observatory Discovers Thin High Temperature
Strands in Coronal Active Regions
Authors: Reale, Fabio; Guarrasi, Massimiliano; Testa, Paola; DeLuca,
Edward E.; Peres, Giovanni; Golub, Leon
2011ApJ...736L..16R Altcode: 2011arXiv1106.1591R
One scenario proposed to explain the million degree solar corona
is a finely stranded corona where each strand is heated by a rapid
pulse. However, such fine structure has neither been resolved through
direct imaging observations nor conclusively shown through indirect
observations of extended superhot plasma. Recently, it has been shown
that the observed difference in the appearance of cool and warm coronal
loops (~1 MK and ~2-3 MK, respectively)—warm loops appearing "fuzzier"
than cool loops—can be explained by models of loops composed of
subarcsecond strands, which are impulsively heated up to ~10 MK. That
work predicts that images of hot coronal loops (gsim 6 MK) should
again show fine structure. Here we show that the predicted effect is
indeed widely observed in an active region with the Solar Dynamics
Observatory, thus supporting a scenario where impulsive heating of
fine loop strands plays an important role in powering the active corona.

---------------------------------------------------------
Title: Multiwavelength diagnostics of accretion in an X-ray selected
sample of CTTSs
Authors: Curran, R. L.; Argiroffi, C.; Sacco, G. G.; Orlando, S.;
Peres, G.; Reale, F.; Maggio, A.
2011A&A...526A.104C Altcode: 2010arXiv1011.5915C
Context. High resolution X-ray spectroscopy has revealed soft X-rays
from high density plasma in classical T Tauri stars (CTTSs), probably
arising from the accretion shock region. However, the mass accretion
rates derived from the X-ray observations are consistently lower than
those derived from UV/optical/NIR studies. Aims: We aim to test
the hypothesis that the high density soft X-ray emission originates
from accretion by analysing, in a homogeneous manner, optical accretion
indicators for an X-ray selected sample of CTTSs. Methods:
We analyse optical spectra of the X-ray selected sample of CTTSs and
calculate the accretion rates based on measuring the Hα, Hβ, Hγ, He
ii 4686 Å, He i 5016 Å, He i 5876 Å, O i 6300 Å, and He i 6678 Å
equivalent widths. In addition, we also calculate the accretion rates
based on the full width at 10% maximum of the Hα line. The different
optical tracers of accretion are compared and discussed. The derived
accretion rates are then compared to the accretion rates derived from
the X-ray spectroscopy. Results: We find that, for each CTTS
in our sample, the different optical tracers predict mass-accretion
rates that agree within the errors, albeit with a spread of ≈ 1
order of magnitude. Typically, mass-accretion rates derived from Hα
and He i 5876 Å are larger than those derived from Hβ, Hγ, and O
i. In addition, the Hα full width at 10%, whilst a good indicator of
accretion, may not accurately measure the mass-accretion rate. When
the optical mass-accretion rates are compared to the X-ray derived
mass-accretion rates, we find that: a) the latter are always lower
(but by varying amounts); b) the latter range within a factor of ≈ 2
around 2 × 10-10 M⊙ yr-1, despite
the former spanning a range of ≈ 3 orders of magnitude. We suggest
that the systematic underestimate of the X-ray derived mass-accretion
rates could depend on the density distribution inside the accretion
streams, where the densest part of the stream is not visible in the
X-ray band because of the absorption by the stellar atmosphere. We
also suggest that a non-negligible optical depth of X-ray emission
lines produced by post-shock accreting plasma may explain the almost
constant mass-accretion rates derived in X-rays if the effect is larger
in stars with higher optical mass-accretion rates.

---------------------------------------------------------
Title: Science Objectives for an X-Ray Microcalorimeter Observing
the Sun
Authors: Laming, J. Martin; Adams, J.; Alexander, D.; Aschwanden, M;
Bailey, C.; Bandler, S.; Bookbinder, J.; Bradshaw, S.; Brickhouse,
N.; Chervenak, J.; Christe, S.; Cirtain, J.; Cranmer, S.; Deiker, S.;
DeLuca, E.; Del Zanna, G.; Dennis, B.; Doschek, G.; Eckart, M.; Fludra,
A.; Finkbeiner, F.; Grigis, P.; Harrison, R.; Ji, L.; Kankelborg,
C.; Kashyap, V.; Kelly, D.; Kelley, R.; Kilbourne, C.; Klimchuk, J.;
Ko, Y. -K.; Landi, E.; Linton, M.; Longcope, D.; Lukin, V.; Mariska,
J.; Martinez-Galarce, D.; Mason, H.; McKenzie, D.; Osten, R.; Peres,
G.; Pevtsov, A.; Porter, K. Phillips F. S.; Rabin, D.; Rakowski, C.;
Raymond, J.; Reale, F.; Reeves, K.; Sadleir, J.; Savin, D.; Schmelz,
J.; Smith, R. K.; Smith, S.; Stern, R.; Sylwester, J.; Tripathi, D.;
Ugarte-Urra, I.; Young, P.; Warren, H.; Wood, B.
2010arXiv1011.4052L Altcode:
We present the science case for a broadband X-ray imager with
high-resolution spectroscopy, including simulations of X-ray spectral
diagnostics of both active regions and solar flares. This is part of
a trilogy of white papers discussing science, instrument (Bandler et
al. 2010), and missions (Bookbinder et al. 2010) to exploit major
advances recently made in transition-edge sensor (TES) detector
technology that enable resolution better than 2 eV in an array that
can handle high count rates. Combined with a modest X-ray mirror, this
instrument would combine arcsecondscale imaging with high-resolution
spectra over a field of view sufficiently large for the study of
active regions and flares, enabling a wide range of studies such as
the detection of microheating in active regions, ion-resolved velocity
flows, and the presence of non-thermal electrons in hot plasmas. It
would also enable more direct comparisons between solar and stellar
soft X-ray spectra, a waveband in which (unusually) we currently have
much better stellar data than we do of the Sun.

---------------------------------------------------------
Title: On the observability of T Tauri accretion shocks in the
X-ray band
Authors: Sacco, G. G.; Orlando, S.; Argiroffi, C.; Maggio, A.; Peres,
G.; Reale, F.; Curran, R. L.
2010A&A...522A..55S Altcode: 2010arXiv1007.2423S
Context. High resolution X-ray observations of classical T Tauri
stars (CTTSs) show a soft X-ray excess due to high density plasma
(ne = 1011-113 cm-3). This
emission has been attributed to shock-heated accreting material
impacting onto the stellar surface. Aims: We investigate the
observability of the shock-heated accreting material in the X-ray band
as a function of the accretion stream properties (velocity, density,
and metal abundance) in the case of plasma-β ≪ 1 (thermal pressure
≪ magnetic pressure) in the post-shock zone. Methods: We use
a 1-D hydrodynamic model describing the impact of an accretion stream
onto the chromosphere of a CTTS, including the effects of radiative
cooling, gravity stratification and thermal conduction. We explore the
space of relevant parameters and synthesize from the model results the
X-ray emission in the [0.5-8.0] keV band and in the resonance lines
of O vii (21.60 Å) and Ne ix (13.45 Å), taking into account the
absorption from the chromosphere. Results: The accretion stream
properties largely influence the temperature and the stand-off height
of the shocked slab and its sinking in the chromosphere, determining
the observability of the shocked plasma affected by chromospheric
absorption. Our model predicts that X-ray observations preferentially
detect emission from low density and high velocity shocked accretion
streams due to the large absorption of dense post-shock plasma. In
all the cases examined, the post-shock zone exhibits quasi-periodic
oscillations due to thermal instabilities with periods ranging from
3×10-2 to 4×103 s. In the case of inhomogeneous
streams and β ≪ 1, the shock oscillations are hardly detectable. Conclusions: We suggest that, if accretion streams are inhomogeneous,
the selection effect introduced by the absorption on observable plasma
components may easily explain the discrepancy between the accretion
rate measured by optical and X-ray data as well as the different
densities measured using different He-like triplets in the X-ray band.

---------------------------------------------------------
Title: Chronology of star formation and disk evolution in the
Eagle Nebula
Authors: Guarcello, M. G.; Micela, G.; Peres, G.; Prisinzano, L.;
Sciortino, S.
2010A&A...521A..61G Altcode: 2010arXiv1008.0422G
Context. Massive star-forming regions are characterized by intense
ionizing fluxes, strong stellar winds and, occasionally, supernovae
explosions, all of which have important effects on the surrounding
media, on the star-formation process and on the evolution of young
stars and their circumstellar disks. We present a multiband study
of the massive young cluster NGC 6611 and its parental cloud (the
Eagle Nebula) with the aim of studying how OB stars affect the
early stellar evolution and the formation of other stars. 
Aims: We search for evidence of triggering of star formation by the
massive stars inside NGC 6611 on a large spatial scale (~10 parsec)
and ongoing disk photoevaporation in NGC 6611 and how its efficiency
depends on the mass of the central stars. Methods: We assemble
a multiband catalog of the Eagle Nebula with photometric data, ranging
from B band to 8.0 μm, and X-ray data obtained with two new and one
archival Chandra/ACIS-I observation. We select the stars with disks
from infrared photometry and disk-less ones from X-ray emission, which
are associated both with NGC 6611 and the outer region of the Eagle
Nebula. We study induced photoevaporation searching for the spatial
variation of disk frequency for distinct stellar mass ranges. The
triggering of star formation by OB stars has been investigated by
deriving the history of star formation across the nebula. 
Results: We find evidence of sequential star formation in the Eagle
Nebula going from the southeast (2.6 Myears) to the northwest (0.3
Myears), with the median age of NGC 6611 members ~1 Myear. In NGC 6611,
we observe a drop of the disk frequency close to massive stars (up to
an average distance of 1 parsec), without observable effects at larger
distances. Furthermore, disks are more frequent around low-mass stars
(≤ 1 M⊙) than around high-mass stars, regardless of
the distance from OB stars. Conclusions: The star-formation
chronology we find in the Eagle Nebula does not support the hypothesis
of a large-scale process triggered by OB stars in NGC 6611. Instead,
we speculate that it was triggered by the encounter (about 3 Myears
ago) with a giant molecular shell created by supernovae explosions
about 6 Myears ago. We find evidence of disk photoevaporation close
to OB stars, where disks are heated by incident extreme ultraviolet
(EUV) radiation. No effects are observed at large distances from OB
stars, where photoevaporation is induced by the far ultraviolet (FUV)
radiation, and long timescales are usually required to completely
dissipate the disks.

---------------------------------------------------------
Title: Pre-main sequence stars with disks in the Eagle Nebula observed
in scattered light
Authors: Guarcello, M. G.; Damiani, F.; Micela, G.; Peres, G.;
Prisinzano, L.; Sciortino, S.
2010A&A...521A..18G Altcode: 2010arXiv1006.3813G
Context. NGC 6611 and its parental cloud, the Eagle Nebula (M 16),
are well-studied star-forming regions, thanks to their large content
of both OB stars and stars with disks and the observed ongoing star
formation. In our previous studies of the Eagle Nebula, we identified
834 disk-bearing stars associated with the cloud, after detecting their
excesses in NIR bands from J band to 8.0 μ m. Aims: In this
paper, we study in detail the nature of a subsample of disk-bearing
stars that show peculiar characteristics. They appear older than
the other members in the V vs. V-I diagram, and/or they have one
or more IRAC colors at pure photospheric values, despite showing
NIR excesses, when optical and infrared colors are compared. Methods: We confirm the membership of these stars to M 16 by a
spectroscopic analysis. The physical properties of these stars with
disks are studied by comparing their spectral energy distributions
(SEDs) with the SEDs predicted by models of T Tauri stars with disks
and envelopes. Results: We show that the age of these stars
estimated from the V vs. V-I diagram is unreliable since their V-I
colors are altered by the light scattered by the disk into the line
of sight. Only in a few cases their SEDs are compatible with models
with excesses in V band caused by optical veiling. Candidate members
with disks and photospheric IRAC colors are selected by the used
NIR disk diagnostic, which is sensitive to moderate excesses, such
as those produced by disks with low masses. In 1/3 of these cases,
scattering of stellar flux by the disks can also be invoked. 
Conclusions: The photospheric light scattered by the disk grains into
the line of sight can affect the derivation of physical parameters
of Class II stars from photometric optical and NIR data. Besides,
the disks diagnostic we defined are useful for selecting stars with
disks, even those with moderate excesses or whose optical colors are
altered by veiling or photospheric scattered light. Table with
the data of the stars is only available in electronic form at the
CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via <A
href="http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/521/A18">http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/521/A18</A>

---------------------------------------------------------
Title: Coronal Fuzziness Modeled with Pulse-heated Multi-stranded
Loop Systems
Authors: Guarrasi, Massimiliano; Reale, Fabio; Peres, Giovanni
2010ApJ...719..576G Altcode: 2010arXiv1006.3495G
Coronal active regions are observed to get increasingly fuzzy (i.e.,
increasingly confused and uniform) in increasingly hard energy bands or
lines. We explain this as evidence of fine multi-temperature structure
of coronal loops. To this end, we model bundles of loops made of thin
strands, each heated by short and intense heat pulses. For simplicity,
we assume that the heat pulses are all equal and triggered only once
in each strand at a random time. The pulse intensity and cadence
are selected so as to have steady active region loops (~3 MK) on
average. We compute the evolution of the confined heated plasma with a
hydrodynamic loop model. We then compute the emission along each strand
in several spectral lines, from cool (&lt;=1 MK), to warm (2-3 MK)
lines, detectable with Hinode/Extreme-ultraviolet Imaging Spectrometer,
to hot X-ray lines. The strands are then put side by side to construct
an active region loop bundle. We find that in the warm lines (2-3 MK)
the loop emission fills all the available image surface. Therefore,
the emission appears quite uniform and it is difficult to resolve the
single loops, while in the cool lines the loops are considerably more
contrasted and the region is less fuzzy. The main reasons for this
effect are that, during their evolution, i.e., pulse heating and slow
cooling, each strand spends a relatively long time at temperatures
around 2-3 MK and it has a high emission measure during that phase,
so the whole region appears more uniform or smudged. We predict that
fuzziness should be reduced in the hot UV and X-ray lines.

---------------------------------------------------------
Title: VizieR Online Data Catalog: Stars associated to Eagle Nebula
(M16=NGC6611) (Guarcello+ 2010)
Authors: Guarcello, M. G.; Micela, G.; Peres, G.; Prisinzano, L.;
Sciortino, S.
2010yCat..35210061G Altcode: 2010yCat..35219061G
This catalog contains coordinates and both optical and infrared
photometry, plus usefull tags, of the candidate stars associated to
the Eagle Nebula (M16), bost disk-less and disk-bearing, selected
in Guarcello et al. 2010: "Chronology of star formation and disks
evolution in the Eagle Nebula". The optical photometry in BVI
bands comes from observations with WFI@ESO (Guarcello et al. 2007,
Cat. J/A+A/462/245); JHK photometry have been obtained from 2MASS/PSC
(Bonatto et al. 2006A&amp;A...445..567B, Guarcello et al. 2007,
Cat. J/A+A/462/245) and UKIDSS/GPS catalogs (Guarcello et al., 2010,
in prep.) ; IRAC data are from GLIMPSE public survey (Indebetouw
2007ApJ...666..321I, Guarcello et al., 2009, Cat. J/A+A/496/453);
X-ray data from three observations with Chandra/ACIS-I (Linsky et
al., 2007, Cat. J/ApJ/654/347, Guarcello et al., 2007, J/A+A/462/245,
Guarcello et al. 2010, in prep.). (1 data file).

---------------------------------------------------------
Title: Generation of radiative knots in a randomly pulsed protostellar
jet. II. X-ray emission
Authors: Bonito, R.; Orlando, S.; Miceli, M.; Eislöffel, J.; Peres,
G.; Favata, F.
2010A&A...517A..68B Altcode: 2010arXiv1005.2125B
Context. Protostellar jets are known to emit in a wide range of bands,
from radio to IR to optical bands, and to date about ten jets that
also emit X-rays have been detected, with a rate of discovery of about
one per year. Aims: We aim at investigating the mechanism
leading to the X-ray emission detected in protostellar jets and in
particular at constraining the physical parameters that describe
the jet/ambient interaction by comparing our model predictions with
observations available from the literature. Methods: We perform
2D axisymmetric hydrodynamic simulations of the interaction between
a supersonic jet and the ambient medium. The jet is described as a
train of plasma blobs randomly ejected by the stellar source along
the jet axis. We explore the parameter space by varying the ejection
rate, the initial Mach number of the jet, and the initial density
contrast between the ambient medium and the jet. We synthesize the
X-ray emission from the model as it would be observed with the current
X-ray telescopes. Results: The mutual interactions among the
ejected blobs themselves and of the blobs with the ambient medium
lead to complex X-ray-emitting structures within the jet. The X-ray
sources consist of several components: irregular chains of knots,
isolated knots with measurable proper motion, apparently stationary
knots, and reverse shocks. The predicted X-ray luminosity strongly
depends on the ejection rate and on the initial density contrast
between the ambient medium and the jet, with a lesser dependence on
the jet Mach number. Conclusions: Our model represents the
first attempt to describe the X-ray properties of all X-ray-emitting
protostellar jets discovered so far. The comparison between our model
predictions and the observations can provide a useful diagnostic tool,
which is necessary for a proper interpretation of the observations. We
specifically suggest that the observable quantities derived from the
spectral analysis of X-ray observations can be used to constrain the
ejection rate, a parameter explored in our model that is not measurable
by current observations in all wavelength bands.

---------------------------------------------------------
Title: Large Scale Properties of Coronal Heating along the Solar Cycle
Authors: Peres, G.; Argiroffi, C.; Orlando, S.; Reale, F.
2010ASPC..428..139P Altcode:
We discuss various studies of the global properties of coronal
heating. Some of them find power laws tying the X-ray luminosity with
the magnetic flux of individual structures, of the whole Sun, and of
active solar-type stars. Others are based on methods to model the Sun
as an X-ray star. We also briefly discuss solar-like active stars and
how the Sun fits in the whole scenario. We use a new model, including
all flares, of the Sun as an X-ray star to describe the evolution of
the corona along the solar cycle and the implications on the heating of
closed coronal structures. We point out that, as activity increases,
more heating is released into the confined coronal plasma and such
a heating has to be, on average, more intense in order to explain
the widespread evidence of a temperature increase with activity. By
the same token, nanoflare heating (if existent) has to increase and
decrease along the cycle differently from flares.

---------------------------------------------------------
Title: VizieR Online Data Catalog: Blue stars with disk photometry
in NGC 6611 (Guarcello+, 2010)
Authors: Guarcello, M. G.; Damiani, F.; Micela, G.; Peres, G.;
Prisinzano, L.; Sciortino, S.
2010yCat..35210018G Altcode: 2010yCat..35219018G
This catalog contains coordinates and both optical and infrared
photometry, plus usefull tags, of the Blue Stars With Disk (BWE stars)
discussed in detail in Guarcello et al. (2010, in prep): "Pre-main
sequence stars with disks in the Eagle nebula observed in scattered
light". The optical photometry in BVI bands comes from observations with
WFI@ESO (Guarcello et al. 2007, Cat. J/A+A/462/245); JHK photometry have
been obtained from 2MASS/PSC and UKIDSS/GPS catalogs (Bonatto et al.,
2006A&amp;A...445..567B, Guarcello et al., 2007, Cat. J/A+A/462/245 and
2010, in prep); IRAC data are from GLIMPSE public survey (Indebetouw
2007ApJ...666..321I, Guarcello et al., 2009, Cat. J/A+A/496/453);
X-ray data from observations with Chandra/ACIS-I (Linsky et al. 2007,
Cat. J/ApJ/654/347, Guarcello et al., 2007, Cat. J/A+A/462/245,
Guarcello et al., 2010, in prep). BWE stars have been studied by
analizing their Spectral Energy Distribution (SED) using the grid of
YSO models developed by Robitaille et al. 2006ApJS..167..256R. (1
data file).

---------------------------------------------------------
Title: Observability and diagnostics in the X-ray band of shock-cloud
interactions in supernova remnants
Authors: Orlando, S.; Bocchino, F.; Miceli, M.; Zhou, X.; Reale, F.;
Peres, G.
2010A&A...514A..29O Altcode: 2010arXiv1001.2240O
Context. X-ray emitting features originating from the interaction of
supernova shock waves with small interstellar gas clouds are revealed
in many X-ray observations of evolved supernova remnants (e.g., Cygnus
Loop and Vela), but their interpretation is not straightforward. Aims: We develop a self-consistent method for the analysis and
interpretation of shock-cloud interactions in middle-aged supernova
remnants, which can provide the key parameters of the system and the
role of relevant physical effects such as thermal conduction, without
the need to perform ad-hoc numerical simulations and bother about
morphology details. Methods: We explore all the possible values
of the shock speed and cloud density contrast relevant to middle-aged
SNRs with a set of hydrodynamic simulations of shock-cloud interaction
including the effects of thermal conduction and radiative cooling. From
the simulations, we synthesize spatially and spectrally resolved
focal-plane data as they would be collected with XMM-Newton/EPIC,
an X-ray instrument commonly used in these studies. Results:
We develop and calibrate two diagnostic tools, the first based on the
mean photon energy versus count-rate scatter plot and the second on
the spectral analysis of the interaction region, that can be used to
highlight the effects of thermal conduction and to derive the shock
speed in case of efficient conduction at work. These tools can be used
to ascertain information from X-ray observations, without the need to
develop detailed and ad-hoc numerical models for the interpretation
of the data.

---------------------------------------------------------
Title: Generation of radiative knots in a randomly pulsed protostellar
jet. I. Dynamics and energetics
Authors: Bonito, R.; Orlando, S.; Peres, G.; Eislöffel, J.; Miceli,
M.; Favata, F.
2010A&A...511A..42B Altcode: 2009arXiv0912.2757B
Context. Herbig-Haro objects are characterized by a complex knotty
morphology detected mainly along the axis of protostellar jets in a
wide range of bands: from radio to IR to optical bands, with X-rays
knots also detected in the past few years. Evidence of interactions
between knots formed in different epochs have been found, suggesting
that jets may result from the ejection of plasma blobs from the stellar
source. Aims: We aim at investigating the physical mechanism
leading to the irregular knotty structure observed in protostellar jets
in different wavelength bands and the complex interactions occurring
among blobs of plasma ejected from the stellar source. Methods:
We performed 2D axisymmetric hydrodynamic numerical simulations of
a randomly ejected pulsed jet. The jet consists of a train of blobs
that ram with supersonic speed into the ambient medium. The initial
random velocity of each blob follows an exponential distribution. We
explored the ejection rate parameter to derive constraints on the
physical properties of protostellar jets by comparing model results
with observations. Our model takes the effects of radiative losses
and thermal conduction into account. Results: We find that
the mutual interactions of blobs ejected at different epochs and with
different speeds lead to a variety of plasma components not described by
current models of jets. The main features characterizing the randomly
pulsed jet scenario are: single high-speed knots, showing a measurable
proper motion in nice agreement with optical and X-rays observations;
irregular chains of knots aligned along the jet axis and possibly
interacting with each other; reverse shocks interacting with outgoing
knots; oblique shock patterns produced by the reflection of shocks
at the cocoon surrounding the jet. All these structures work together
to help determining the morphology of the jet in different wavelength
bands. We also find that the thermal conduction plays a crucial role
in damping out hydrodynamic instabilities that would develop within
the cocoon and that contribute to the jet breaking.

---------------------------------------------------------
Title: X-ray emitting MHD accretion shocks in classical T Tauri
stars. Case for moderate to high plasma-β values
Authors: Orlando, S.; Sacco, G. G.; Argiroffi, C.; Reale, F.; Peres,
G.; Maggio, A.
2010A&A...510A..71O Altcode: 2009arXiv0912.1799O
Context. Plasma accreting onto classical T Tauri stars (CTTS)
is believed to impact the stellar surface at free-fall velocities,
generating a shock. Current time-dependent models describing accretion
shocks in CTTSs are one-dimensional, assuming that the plasma moves and
transports energy only along magnetic field lines (β ≪ 1). 
Aims: We investigate the stability and dynamics of accretion shocks in
CTTSs, considering the case of β ⪆ 1 in the post-shock region. In
these cases the 1D approximation is not valid and a multi-dimensional
MHD approach is necessary. Methods: We model an accretion
stream propagating through the atmosphere of a CTTS and impacting onto
its chromosphere by performing 2D axisymmetric MHD simulations. The
model takes into account the stellar magnetic field, the gravity, the
radiative cooling, and the thermal conduction (including the effects
of heat flux saturation). Results: The dynamics and stability
of the accretion shock strongly depend on the plasma β. In the case
of shocks with β &gt; 10, violent outflows of shock-heated material
(and possibly MHD waves) are generated at the base of the accretion
column and intensely perturb the surrounding stellar atmosphere and
the accretion column itself (therefore modifying the dynamics of the
shock). In shocks with β ≈ 1, the post-shock region is efficiently
confined by the magnetic field. The shock oscillations induced by
cooling instability are strongly influenced by β: for β &gt; 10, the
oscillations may be rapidly dumped by the magnetic field, approaching a
quasi-stationary state, or may be chaotic with no obvious periodicity
due to perturbation of the stream induced by the post-shock plasma
itself; for β≈ 1 the oscillations are quasi-periodic, although their
amplitude is smaller and the frequency higher than those predicted by
1D models. Three movies are only available in electronic form at
<A href="http://www.aanda.org">http://www.aanda.org</A>

---------------------------------------------------------
Title: The EM(T) of stellar coronae
Authors: Argiroffi, Costanza; Peres, Giovanni; Orlando, Salvatore;
Reale, Fabio
2010cosp...38.2900A Altcode: 2010cosp.meet.2900A
Studying the solar corona, due to its vicinity, is the starting point
to understand stellar activity. The emission measure distribution vs
temperature, EM(T), is a useful tool to study coronal plasmas, in fact
it allows: to investigate the energy balance of coronal plasmas, to
easily compare different stars, and also to compare the solar corona to
that of other active stars irrespective of the very different observing
techniques. The EM(T) of the solar corona differs significantly, in
terms of average plasma temperatures, peak temperatures, and total
emission measure, with respect to that of active stars. In this work
it is discussed how the evaluation of the EM(T) of the solar corona,
and of its components (quiescent plasma, active regions, flares, etc.),
parallel to the reconstruction of the EM(T) of stars at different
activity levels, can be used to investigate coronal physics.

---------------------------------------------------------
Title: Diagnostics of the coronal fine structuring
Authors: Guarrasi, Massimiliano; Reale, Fabio; Peres, Giovanni
2010cosp...38.2832G Altcode: 2010cosp.meet.2832G
Coronal structures are believed to consist of bundles of fibrils below
the resolution of the present-day X-ray and UV telescopes. We model
coronal structures as collections of individual thin and independent
loop atmospheres, each ignited by a heat pulse, and investigate possible
indirect diagnostics of the fine structuring. In particular, we propose
that the substructuring is the explanation of the different appearance
of the structures in different spectral lines and bands.

---------------------------------------------------------
Title: X-ray optical depth diagnostics of T Tauri accretion shocks
Authors: Argiroffi, C.; Maggio, A.; Peres, G.; Drake, J. J.;
López-Santiago, J.; Sciortino, S.; Stelzer, B.
2009A&A...507..939A Altcode: 2009arXiv0909.0218A
Context: In classical T Tauri stars, X-rays are produced by two plasma
components: a hot low-density plasma, with frequent flaring activity,
and a high-density lower temperature plasma. The former is coronal
plasma related to the stellar magnetic activity. The latter component,
never observed in non-accreting stars, could be plasma heated by the
shock formed by the accretion process. However its nature is still being
debated. Aims: Our aim is to probe the soft X-ray emission from
the high-density plasma component in classical T Tauri stars to check
whether this plasma is heated in the accretion shock or whether it is
coronal plasma. Methods: High-resolution X-ray spectroscopy allows
us to measure individual line fluxes. We analyze X-ray spectra of the
classical T Tauri stars <ASTROBJ>MP Muscae</ASTROBJ> and <ASTROBJ>TW
Hydrae</ASTROBJ>. Our aim is to evaluate line ratios to search for
optical depth effects, which are expected in the accretion-driven
scenario. We also derive the plasma emission measure distributions EMD,
to investigate whether and how the EMD of accreting and non accreting
young stars differ. The results are compared to those obtained for
the non-accreting weak-line T Tauri star <ASTROBJ>TWA 5</ASTROBJ>. Results: We find evidence of resonance scattering in the strongest
lines of MP Mus, supporting the idea that soft X-rays are produced
by plasma heated in the accretion shock. We also find that the EMD
of MP Mus has two peaks: a cool peak at temperatures expected for
plasma heated in the accretion shock, and a hot peak typical of coronal
plasma. The shape of the EMD of MP Mus appears to be the superposition
of the EMD of a pure coronal source, like TWA 5, and an EMD alike that
of TW Hya, which is instead dominated by shock-heated plasma.

---------------------------------------------------------
Title: Modeling accretion shocks on CTTSs and their X-ray emission
Authors: Sacco, Giuseppe; Orlando, Salvatore; Argiroffi, Costanza;
Maggio, Antonio; Peres, Giovanni; Reale, Fabio
2009cfdd.confE.189S Altcode:
Recent high spectral resolution X-ray observations of some CTTSs show
the presence of high density plasma (ne=10^{11}-10^{13} cm^{-3}) at
temperature T=2-3 MK. This plasma is likely heated up by an accretion
shock on the star surface. We investigate this issue by an accurate
modelling of the impact of an accretion stream onto the stellar
chromosphere. Specifically, we present a large set of 1D hydrodynamical
simulations aimed at investigating the physical properties of the
system as a function of the density, and the velocity of the accretion
stream and of the abundances of the heavy elements. We also synthesize
the plasma X-ray emission from the simulations results, in order to
link the observed properties of the CTTSs with the accretion shock
physics. Furthermore, we present results of 2D magneto-hydrodynamical
simulations aimed at investigating the role of the magnetic field in
confining the shocked plasma at the base of the accretion column for
different magnetic field strength.

---------------------------------------------------------
Title: Multi-wavelength diagnostics of accretion in an X-ray selected
sample of CTTSs
Authors: Curran, Rachel; Argiroffi, Costanza; Sacco, Giuseppe Germano;
Orlando, Salvatore; Reale, Fabio; Peres, Giovanni; Maggio, Antonio
2009cfdd.confE..81C Altcode:
The majority of CTTSs observed to date with high spectral resolution
X-ray spectroscopy reveal soft X-ray emission (E&lt;0.7 KeV) which
originates from cool (1-5 MK), high density (n ∼ 10^{11}-10^{13}
cm^{-3}) plasma. This is currently interpreted to be due to mass
accretion. Supporting this interpretation is the fact that this
plasma component is too dense to have a coronal origin, and it has
never been observed in non-accreting stars. Synthesized X-ray spectra
from detailed hydrodynamical modelling of the interaction between
the accretion flow and the stellar chromosphere also confirm this
interpretation. However, the mass accretion rates derived from X-ray
data are consistently underestimated when compared to mass accretion
rates derived from UV/optical data. We test the hypothesis that this
soft X-ray emission originates from accretion by analysing optical,
NIR and X-ray data for an X-ray selected sample of CTTSs. We derive
mass accretion rates for the sample based on Hα, He I, O I and Ca II
emission lines, along with the X-ray data. We draw comparisons between
these mass accretion rates to understand the underestimation of the
X-ray derived mass accretion rates. We discuss the possibilities of
a) the X-ray emission being partially absorbed, b) the optical/NIR
emission arising from different parts of the accretion stream and c)
the uncertainties involved in the estimation of the mass accretion
rates from different spectroscopic diagnostics.

---------------------------------------------------------
Title: Modeling the X-ray emission from jets observed with Chandra
Authors: Bonito, Rosaria; Orlando, S.; Peres, G.; Favata, F.; Miceli,
M.; Eisloffel, J.
2009cfdd.confE...2F Altcode:
In the last decade, X-rays from jets have been discovered thanks to
the unprecedented resolving power of Chandra. The first convincing
evidence of X-ray jets came from HH2 (2000), HH154 following shortly
thereafter. While more sources were detected later, HH154 remains a
unique source being the nearest and the most luminous among the nearest
jets: the details of the relevant X-ray morphology can be studied to a
level impossible with more distant objects.Our group has investigated
the mechanisms of X-ray emission from jets both analyzing multi-epochs
Chandra data of HH154 and developing numerical models of jets where
X-rays are generated by jets shocking onto the circumstellar medium. The
analysis of Chandra data of HH 154 in different epochs allowed us to
study for the first time the morphology and the evolution of the X-ray
source on a time scale of 4years. We found that the source consists of
an unresolved, point-like component with no detectable proper motion
and an elongated component with a proper motion consistent with a shock
moving away from the parent star.We present here the comparison between
the Chandra observations of HH jets and our hydrodynamic model of a
randomly ejected pulsed jet which reproduces the knotty morphology
observed.

---------------------------------------------------------
Title: Modeling SNR shock waves expanding through the magnetized
inhomogeneous interstellar medium
Authors: Orlando, S.; Bocchino, F.; Miceli, M.; Reale, F.; Peres, G.
2009NCimC..32b..45O Altcode:
No abstract at ADS

---------------------------------------------------------
Title: VizieR Online Data Catalog: Photometry of candidate members
of NGC6611 (Guarcello+, 2009)
Authors: Guarcello, M. G.; Micela, G.; Damiani, F.; Peres, G.;
Prisinzano, L.; Sciortino, S.
2009yCat..34960453G Altcode:
Optical BVI photometry have been obtained from 2.2m WFI@ESO
observations; JHK data are from 2MASS Point Sources Catalog; IRAC
data are from GLIMPSE catalog. X-ray data have been obtained from
a CHANDRA/ACIS-I observation of a field o f 17'x17' centered on the
cluster. (1 data file).

---------------------------------------------------------
Title: Correlation between the spatial distribution of
circumstellar disks and massive stars in the young open cluster NGC
6611. II. Cluster members selected with Spitzer/IRAC
Authors: Guarcello, M. G.; Micela, G.; Damiani, F.; Peres, G.;
Prisinzano, L.; Sciortino, S.
2009A&A...496..453G Altcode: 2008arXiv0812.0945G
Context: The observations of the proplyds in the Orion Nebula Cluster,
exhibiting clear evidence of ongoing photoevaporation, have provided
clear proof of the role of externally induced photoevaporation in
the evolution of circumstellar disks. NGC 6611 is an open cluster
suitable for study of disk photoevaporation, due to its significant
population of massive members and stars with disk. In a previous paper,
we obtained evidence of the influence of the strong UV field generated
by the massive cluster members on the evolution of disks around low-mass
Pre-Main Sequence members. Our study was based on a multiband BVIJHK
and X-ray catalog compiled for the purpose of selecting cluster members
with and without disk. Aims: We attempt to complete the list of
candidate cluster members, using data at longer wavelengths obtained
with Spitzer/IRAC, and we reinvestigate the effects of UV radiation on
the evolution of disks in NGC 6611. Methods: In a field of view
of 33'×34' centered on the cluster, we select the candidate members
with disks of NGC 6611 using IRAC color-color diagrams and suitable
reddening-free color indices. Using the X-ray data to select Class
III cluster members, we also estimate disks frequency relative to the
intensity of the incident radiation emitted by massive members. Results: We identify 458 candidate members with circumstellar disks,
among which 146 had not been discovered previously. By comparing all
color indices used to select cluster members with disk, we claim that
these indices measure the excess of radiation due to the emission
of the same physical region of the disk (i.e. the inner rim at the
dust sublimation radius). Our new results confirm that UV radiation
from massive stars affects the evolution of nearby circumstellar
disks. Optical-infrared catalog of the candidate members of the
open cluster NGC 6611 is only available in electronic form at the
CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via
http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/496/453

---------------------------------------------------------
Title: The Sun as a benchmark of flaring activity in stellar coronae
Authors: Argiroffi, C.; Peres, G.; Orlando, S.; Reale, F.
2009AIPC.1094..200A Altcode: 2009csss...15..200A
The solar corona is a template to study and understand stellar
activity. However the solar corona differs from that of active stars:
the Sun has lower X-ray luminosity, and on average cooler plasma
temperatures. Active stellar coronae have a hot peak in their emission
measure distribution, EM (T), at 8-20 MK, while the non-flaring solar
corona has a peak at 1-2 MK. In the solar corona significant amounts of
plasma at temperature ~10 MK are observed only during flares. To
investigate what is the time-averaged effect of solar flares we measure
the disk-integrated time-averaged emission measure, EMF
(T), of an unbiased sample of solar flares. To this aim we analyze
uninterrupted GOES/XRS light curves over time intervals of one month. We
also obtain the EMQ (T) of the quiescent corona for the same
time intervals from Yohkoh/SXT data. To investigate variations due
to the solar cycle we evaluate EMF (T) and EMQ
(T) at different phases of the cycle between December 1991 and April
1998. Irrespective of the solar cycle phase, EMF
(T) appears as a peak in the distribution, and it is significantly
larger than the values of EMQ (T) for T~5-10 MK. Adding
EMF (T) and EMQ (T) we obtain for the first time
a time average EM (T) of the entire solar corona: it is double-peaked,
with the hot peak, due to time-averaged flares, being located at
temperatures similar to those of active stars, but less enhanced. In the assumption that the heating of the corona is entirely due
to flares, from nano- to macro-flares, a two peak EM (T) distribution
suggests that then either the flare distribution or the confined plasma
response to flares, or both, are bimodal. Moreover the EMF
(T) shape supports the hypothesis that the hot EM (T) peak of active
coronae is due to unresolved solar-like flares. If this is the case,
quiescent and flare components should follow different scaling laws
for increasing stellar activity.

---------------------------------------------------------
Title: Accretion shock on CTTSs and its X-ray emission
Authors: Sacco, G. G.; Argiroffi, C.; Orlando, S.; Maggio, A.; Peres,
G.; Reale, F.
2009AIPC.1094..329S Altcode: 2009csss...15..329S
High spectral resolution X-ray observations of classical T Tauri stars
(CTTSs) demonstrate the presence of plasma at T~2-3×106
K and ne~1011-1013 cm-3. Stationary
models suggest that this emission is due to shock-heated accreting
material. We address this issue by a 1-D hydrodynamic model of the
impact of the accretion flow onto a chromosphere of a CTTS with the
aim of investigating the stability of accretion shock and the role
of the chromosphere. Our simulations include the effects of gravity,
radiative losses from optically thin plasma, the thermal conduction
and a detailed modeling of the stellar chromosphere. Here we present
the results of a simulation based on the parameters of the CTTS MP Mus.

---------------------------------------------------------
Title: The nearest X-ray emitting protostellar jet observed with HST
Authors: Bonito, R.; Fridlund, C. V. M.; Favata, F.; Micela, G.;
Peres, G.; Djupvik, A. A.; Liseau, R.
2009AIPC.1094..349B Altcode: 2009csss...15..349B
The HH 154 jet coming from the YSO binary L1551 IRS5 is one of the
closest (about 150 pc) astrophysical jet known. It is therefore a
unique laboratory for studies of outflow mechanisms and of the shocks
forming at the interaction front between the expanding material and
the ambient medium. The substructures (knots) observed within the HH
154 jet were imaged in several spectral bands using the Hubble Space
Telescope. This allows us to derive a simple characterization of the
physical conditions in different structures as well as to measure the
proper motion of the knots in the jet, their flux variability and shock
emission over a time base of about ten years. These knots in the jet
undergo signicant morphological variations: some of them disappear
in a few years and collision between different knots may occur. Our
results suggest the presence of a shock front at the base of the jet
identified with an internal working surface; more important, the knot at
the base of the jet is coincident with the X-ray source discovered in
HH 154. In fact the optical data are compared to X-rays data collected
with Chandra with the goal of understanding the energetics of this jet,
in particular, and of low-mass star jets, in general.

---------------------------------------------------------
Title: The Complex Morphology of the X-ray and Optical Emission from
HH 154: The Pulsed Jet Scenario
Authors: Bonito, Rosaria; Orlando, Salvatore; Peres, Giovanni; Favata,
Fabio; Eislöffel, Jochen
2009ASSP...13..353B Altcode: 2009pjc..book..353B
We study the optical and X-ray emission from protostellar jets,
focusing, in particular, on the case of HH 154. This project consists
of two different and complementary approaches: the development of
hydrodynamical models of the jet/ambient interaction, and the analysis
of multi-wavelength observations. Comparing the results derived from the
simulations with the observations we can infer the physical mechanisms
leading to the complex morphology of the X-rays source observed at
the base of HH 154.

---------------------------------------------------------
Title: MHD modeling of supernova remnants expanding through
inhomogeneous interstellar medium .
Authors: Orlando, S.; Bocchino, F.; Miceli, M.; Reale, F.; Peres, G.
2009MSAIS..13...97O Altcode:
We model the expansion of a blast wave of a supernova explosion
through a magnetized and inhomogeneous interstellar medium (ISM). The
aim is to explore the role of an ambient magnetic field and/or a
non-uniform ISM in determining the morphology of supernova remnants
(SNRs) as observed in different bands. In particular, we investigate:
1) whether the morphology of bilateral SNRs (BSNRs) observed in
the radio band is mainly determined by a non-uniform ISM or by
a non-uniform ambient magnetic field and 2) the role of thermal
conduction and of non-uniform ISM in determining the morphology and
physical characteristics of mixed-morphology (MM) SNRs. We present 3-D
hydrodynamic and MHD simulations of a spherical SNR shock propagating
through an inhomogeneous ISM. We discuss the origin of the asymmetries
observed in BSNRs and of the unexpected morphology observed in MM-SNRs.

---------------------------------------------------------
Title: Effects of Flaring Activity on Dynamics of Accretion Disks
in YSOs
Authors: Yelenina, Tatiana G.; Orlando, Salvatore; Reale, Fabio;
Peres, Giovanni; Mignone, Andrea; Matsakos, Titos
2009ASSP...13..631Y Altcode: 2009pjc..book..631Y
We investigate the effects of strong flares on the accretion
phenomena in YSOs. Among all classical assumptions, the model accounts
magnetic-field oriented thermal conduction. We study the global dynamics
of the system for two positions of the heating release triggering
the flare.

---------------------------------------------------------
Title: Hydrodynamic Modeling of Accretion Shock on CTTSs
Authors: Sacco, Germano G.; Argiroffi, Constanza; Orlando, Salvatore;
Maggio, Antonio; Peres, Giovanni; Reale, Fabio
2009ASSP...13..607S Altcode: 2009pjc..book..607S
High resolution (R ~ 600) X-ray observations of some classical T
Tauri stars (CTTSs) (TW Hya, BP Tau, V4046 Sgr, MP Mus and RU Lupi)
have shown the presence of X-ray plasma at T ~ 2-3 × 106
K and denser than n e ~ 1011 cm-3 [1,
2, 3, 4, 5], which suggests an origin different from the coronal one (n
 e ~ 1010 cm_3). Stationary models demonstrated that X-ray
emission from CTTSs could also be produced by the accreting material
[6]. We address this issue with the aid of a time-dependent hydrodynamic
numerical model describing the impact of an accretion stream onto the
chromosphere of a CTTS (see [7] for more details). Our simulations
include the effects of gravity, radiative losses from optically thin
plasma, the thermal conduction and a detailed modeling of the stellar
chromosphere. The gravity and the parameters describing the accretion
stream (density n e D 1011 cm-3
and velocity v D 450 km s-1) were chosen in order to match
the X-ray properties of the CTTSs MP Mus [4].

---------------------------------------------------------
Title: Round-table discussion
Authors: Pasian, F.; Peres, G.; Longo, G.
2009MSAIS..13..125P Altcode:
These notes summarise the main topics touched upon during the
Round-Table discussion held during the HPC-Grid 08 Workshop, INAF
headquarters, March 12th 2008.

---------------------------------------------------------
Title: X-ray emission from dense plasma in classical T Tauri stars:
hydrodynamic modeling of the accretion shock
Authors: Sacco, G. G.; Argiroffi, C.; Orlando, S.; Maggio, A.; Peres,
G.; Reale, F.
2008A&A...491L..17S Altcode: 2008arXiv0810.0192S
Context: High spectral resolution X-ray observations of classical
T Tauri stars (CTTSs) demonstrate the presence of plasma
at temperature T∼ 2-3× 106 K and density n_e∼
1011-1013 cm-3, which are unobserved
in non-accreting stars. Stationary models suggest that this emission
is due to shock-heated accreting material, but do not allow us to
analyze the stability of the material and its position in the stellar
atmosphere. Aims: We investigate the dynamics and stability of
shock-heated accreting material in classical T Tauri stars and the role
of the stellar chromosphere in determining the position and thickness
of the shocked region. Methods: We perform one-dimensional
hydrodynamic simulations of the impact of an accretion flow on the
chromosphere of a CTTS, including the effects of gravity, radiative
losses from optically thin plasma, thermal conduction and a well tested
detailed model of the stellar chromosphere. We present the results of
a simulation based on the parameters of the CTTS MP Mus. Results:
We find that the accretion shock generates an hot slab of material above
the chromosphere with a maximum thickness of 1.8 × 109
cm, density n_e∼ 1011-1012 cm-3,
temperature T∼ 3× 106 K, and uniform pressure equal to
the ram pressure of the accretion flow (~450 dyn cm-2). The
base of the shocked region penetrates the chromosphere and remains at a
position at which the ram pressure is equal to the thermal pressure. The
system evolves with quasi-periodic instabilities of the material in the
slab leading to cyclic disappearance and re-formation of the slab. For
an accretion rate of ~10-10~M⊙ yr-1,
the shocked region emits a time-averaged X-ray luminosity of L_X≈
7× 1029 erg s-1, which is comparable with the
X-ray luminosity observed in CTTSs of identical mass. Furthermore,
the X-ray spectrum synthesized from the simulation reproduces in detail
all the main features of the O VIII and O VII lines of the star MP Mus.

---------------------------------------------------------
Title: The flaring and quiescent components of the solar corona
Authors: Argiroffi, C.; Peres, G.; Orlando, S.; Reale, F.
2008A&A...488.1069A Altcode: 2008arXiv0805.2685A
Context: The solar corona is a template to understand stellar
activity. The Sun is a moderately active star, and its corona differs
from that of active stars: for instance, active stellar coronae have
a double-peaked emission measure distribution EM(T) with a hot peak
at 8-20 MK, while the non-flaring solar corona has one peak at 1-2 MK
and, typically, much cooler plasma. Aims: We study the average
contribution of flares to the solar emission measure distribution to
investigate indirectly the hypothesis that the hot peak in the EM(T) of
active stellar coronae is due to a large number of unresolved solar-like
flares, and to infer properties about the flare distribution from
nano- to macro-flares. Methods: We measure the disk-integrated
time-averaged emission measure, EM_F(T), of an unbiased sample of
solar flares, analyzing uninterrupted GOES/XRS light curves over time
intervals of one month. We obtain the EM_Q(T) of quiescent corona for
the same time intervals from Yohkoh/SXT data. To investigate how EM_F(T)
and EM_Q(T) vary during the solar cycle, we evaluate them at different
phases of the cycle between December 1991 and April 1998. Results:
Irrespective of the solar cycle phase, EM_F(T) appears as a peak in the
distribution, and it is significantly larger than the values of EM_Q(T)
for T∼5-10 MK. As a result, the time-averaged EM(T) of the entire
solar corona is double-peaked, with the hot peak, due to time-averaged
flares, being located at temperatures similar to those of active stars,
but less enhanced. The EM_F(T) shape supports the hypothesis that
the hot EM(T) peak of active coronae is due to unresolved solar-like
flares. If this is the case, quiescent and flare components should
follow different scaling laws for increasing stellar activity. In the
assumption that the heating of the corona is entirely due to flares,
from nano- to macro-flares, then either the flare distribution or the
confined plasma response to flares, or both, are bimodal.

---------------------------------------------------------
Title: Hinode/XRT Diagnostics of Loop Thermal Structure
Authors: Reale, F.; Parenti, S.; Reeves, K. K.; Weber, M.; Bobra,
M. G.; Barbera, M.; Kano, R.; Narukage, N.; Shimojo, M.; Sakao, T.;
Peres, G.; Golub, L.
2008ASPC..397...50R Altcode:
We investigate possible diagnostics of the thermal structure of coronal
loops from Hinode/XRT observations made with several filters. We
consider the observation of an active region with five filters. We
study various possible combinations of filter data to optimize for
sensitivity to thermal structure and for signal enhancement.

---------------------------------------------------------
Title: The nearest X-ray emitting protostellar jet (HH 154) observed
with Hubble
Authors: Bonito, R.; Fridlund, C. V. M.; Favata, F.; Micela, G.;
Peres, G.; Djupvik, A. A.; Liseau, R.
2008A&A...484..389B Altcode:
Context: The jet coming from the YSO binary <ASTROBJ>L1551</ASTROBJ>
<ASTROBJ>IRS5</ASTROBJ> is the closest astrophysical jet known. It is
therefore a unique laboratory for studies of outflow mechanisms and of
the shocks occurring when expanding material hits the ambient medium as
well as of how the related processes influence the star- (and planet-)
forming process. Aims: The optical data are related to other
data covering the spectrum from the optical band to X-rays with goal
of understanding the energetics of low-mass star jets, in general, and
of this jet in particular. We study the time evolution of the jet, by
measuring the proper motions of knots as they progress outwards from
the originating source. Methods: The nebulosities associated
with the jet(s) from the protostellar binary <ASTROBJ>L1551</ASTROBJ>
<ASTROBJ>IRS5</ASTROBJ> were imaged in a number of spectral bands using
the Hubble Space Telescope. This allows the proper motion to be measured
and permits a simple characterization of the physical conditions in
different structures. To this end we developed a reproducible method
of data analysis, which allows us to define the position and shape
of each substructure observed within the protostellar jet. Using
this approach, we derive the proper motion of the knots in the jet,
as well as their flux variability and shock emission. Results:
The time base over which HST observations were carried out is now
about ten years. The sub-structures within the jet undergo significant
morphological variations: some knots seem to disappear in a few years
and collision between different knots, ejected at different epochs and
maybe with different speed, may occur. The velocities along the jet
vary between ~100 km s-1 and over 400 km s-1,
with the highest speed corresponding to the knots at the base of the
jet. Conclusions: There are indications that the <ASTROBJ>HH
154</ASTROBJ> jet has been active relatively recently. Our results
suggest the presence of a new shock front at the base of the jet
identified with an internal working surface. From the analysis of the
terminal and internal working surfaces within the jet, we find that
the more likely scenario for the <ASTROBJ>HH 154</ASTROBJ> jet is
that of a jet traveling through a denser ambient medium (a “light
jet”). These results are consistent with the Bonito et al. (2007)
model predictions. Furthermore, there is strong evidence that the knots
at the base of the northern jet correspond to the location where the
highest velocity and the highest excitation component are measured
along the jet. More important, this is the location where the X-ray
source has been discovered. Based on observations made with the
NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science
Institute, which is operated by the Association of Universities for
Research in Astronomy, Inc., under NASA contract NAS 5-26555. These
observations are associated with programs #6127, #6411 &amp; #10351.

---------------------------------------------------------
Title: The Importance of Magnetic-Field-Oriented Thermal Conduction
in the Interaction of SNR Shocks with Interstellar Clouds
Authors: Orlando, S.; Bocchino, F.; Reale, F.; Peres, G.; Pagano, P.
2008ApJ...678..274O Altcode: 2008arXiv0801.1403O
We explore the importance of magnetic-field-oriented thermal conduction
in the interaction of supernova remnant (SNR) shocks with radiative
gas clouds and in determining the mass and energy exchange between the
clouds and the hot surrounding medium. We perform 2.5-dimensional MHD
simulations of a shock impacting on an isolated gas cloud, including
anisotropic thermal conduction and radiative cooling; we consider the
representative case of a Mach 50 shock impacting on a cloud 10 times
denser than the ambient medium. We consider different configurations
of the ambient magnetic field and compare MHD models with or without
thermal conduction. The efficiency of thermal conduction in the
presence of a magnetic field is, in general, reduced with respect to
the unmagnetized case. The reduction factor strongly depends on the
initial magnetic field orientation, and it is at a minimum when the
magnetic field is initially aligned with the direction of the shock
propagation. Thermal conduction contributes to the suppression of
hydrodynamic instabilities, reducing the mass mixing of the cloud
and preserving the cloud from complete fragmentation. Depending on
the magnetic field orientation, the heat conduction may determine a
significant energy exchange between the cloud and the hot surrounding
medium which, while remaining always at levels less than those in the
unmagnetized case, leads to a progressive heating and evaporation of
the cloud. This additional heating may offset the radiative cooling of
some parts of the cloud, preventing the onset of thermal instabilities.

---------------------------------------------------------
Title: A coronal explosion on the flare star CN Leonis
Authors: Schmitt, J. H. M. M.; Reale, F.; Liefke, C.; Wolter, U.;
Fuhrmeister, B.; Reiners, A.; Peres, G.
2008A&A...481..799S Altcode: 2008arXiv0801.3752S
We present simultaneous high-temporal and high-spectral resolution
observations of the nearby flare star CN Leo at optical and soft X-ray
wavelengths. During our observing campaign a major flare occurred,
raising the star's instantaneous energy output by almost three orders
of magnitude. The flare shows the often observed impulsive behavior,
with a rapid rise and slow decay in the optical and a broad soft X-ray
maximum about 200 seconds after the optical flare peak. In addition
to this usually encountered flare phenomenology we find, however, an
extremely short (τ _dec ≈ 2 s) soft X-ray peak, which is very likely
of thermal, rather than nonthermal nature and coincides temporally
with the optical flare peak. While at hard X-ray energies nonthermal
bursts are routinely observed on the Sun at flare onset, thermal
soft X-ray bursts on time scales of seconds have never been observed
in a solar, nor stellar context. Time-dependent, one-dimensional
hydrodynamic modeling of this event requires an extremely short energy
deposition time scale τ _dep of a few seconds to reconcile theory with
observations, thus suggesting that we are witnessing the results of
a coronal explosion on CN Leo. Thus the flare on CN Leo provides the
opportunity to observationally study the physics of the long-sought
“micro-flares” thought to be responsible for coronal heating.

---------------------------------------------------------
Title: Numerical Simulations and Diagnostics in Astrophysics: A few
Magnetohydrodynamics Examples
Authors: Peres, G.; Bonito, R.; Orlando, S.; Reale, F.
2008mss..conf...66P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Fine Thermal Structure of a Coronal Active Region
Authors: Reale, Fabio; Parenti, Susanna; Reeves, Kathy K.; Weber,
Mark; Bobra, Monica G.; Barbera, Marco; Kano, Ryouhei; Narukage,
Noriyuki; Shimojo, Masumi; Sakao, Taro; Peres, Giovanni; Golub, Leon
2007Sci...318.1582R Altcode:
The determination of the fine thermal structure of the solar corona is
fundamental to constraining the coronal heating mechanisms. The Hinode
X-ray Telescope collected images of the solar corona in different
passbands, thus providing temperature diagnostics through energy
ratios. By combining different filters to optimize the signal-to-noise
ratio, we observed a coronal active region in five filters, revealing
a highly thermally structured corona: very fine structures in the
core of the region and on a larger scale further away. We observed
continuous thermal distribution along the coronal loops, as well as
entangled structures, and variations of thermal structuring along the
line of sight.

---------------------------------------------------------
Title: VizieR Online Data Catalog: BVI photometry in NGC 6611
(Guarcello+, 2007)
Authors: Guarcello, M. G.; Prisinzano, L.; Micela, G.; Damiani, F.;
Peres, G.; Sciortino, S.
2007yCat..34620245G Altcode:
The optical data have been taken in the BVI bands with the Wield Field
Camera (WFI), mounted on the 2.2m telescope of the European Southern
Observatory (ESO) in La Silla (Chile). The optical images used in this
work were taken on 29 July 2000. (1 data file).

---------------------------------------------------------
Title: On X-Ray Optical Depth in the Coronae of Active Stars
Authors: Testa, Paola; Drake, Jeremy J.; Peres, Giovanni; Huenemoerder,
David P.
2007ApJ...665.1349T Altcode: 2007arXiv0706.4080T
We have investigated the optical thickness of the coronal plasma
through the analysis of high-resolution X-ray spectra of a large sample
of active stars observed with the High Energy Transmission Grating
Spectrometer on Chandra. In particular, we probed for the presence of
significant resonant scattering in the strong Lyman series lines arising
from hydrogen-like oxygen and neon ions. The active RS CVn-type binaries
II Peg and IM Peg and the single M dwarf EV Lac show significant
optical depth. For these active coronae, the Lyα/Lyβ ratios are
significantly depleted as compared with theoretical predictions and
with the same ratios observed in similar active stars. Interpreting
these decrements in terms of resonance scattering of line photons out
of the line of sight, we are able to derive an estimate for the typical
size of coronal structures, and from these we also derive estimates of
coronal filling factors. For all three sources we find that both the
photon path length as a fraction of the stellar radius and the implied
surface filling factors are very small and amount to a few percent at
most. The measured Lyα/Lyβ ratios are in good agreement with APED
theoretical predictions, thus indicating negligible optical depth,
for the other sources in our sample. We discuss the implications for
coronal structuring and heating flux requirements. For the stellar
sample as a whole, the data suggest increasing quenching of Lyα
relative to Lyβ as a function of both LX/Lbol
and the density-sensitive Mg XI forbidden-to-intercombination line
ratio, as might generally be expected.

---------------------------------------------------------
Title: On the origin of asymmetries in bilateral supernova remnants
Authors: Orlando, S.; Bocchino, F.; Reale, F.; Peres, G.; Petruk, O.
2007A&A...470..927O Altcode: 2007arXiv0704.0890O
Aims:We investigate whether the morphology of bilateral supernova
remnants (BSNRs) observed in the radio band is determined mainly either
by a non-uniform interstellar medium (ISM) or by a non-uniform ambient
magnetic field. Methods: We perform 3D MHD simulations of a
spherical SNR shock propagating through a magnetized ISM. Two cases
of shock propagation are considered: 1) through a gradient of ambient
density with a uniform ambient magnetic field; 2) through a homogeneous
medium with a gradient of ambient magnetic field strength. From the
simulations, we synthesize the synchrotron radio emission, making
different assumptions about the details of acceleration and injection
of relativistic electrons. Results: We find that asymmetric BSNRs
are produced if the line-of-sight is not aligned with the gradient
of ambient plasma density or with the gradient of ambient magnetic
field strength. We derive useful parameters to quantify the degree
of asymmetry of the remnants that may provide a powerful diagnostic
of the microphysics of strong shock waves through the comparison
between models and observations. Conclusions: BSNRs with two
radio limbs of different brightness can be explained if a gradient of
ambient density or, most likely, of ambient magnetic field strength
is perpendicular to the radio limbs. BSNRs with converging similar
radio arcs can be explained if the gradient runs between the two arcs.

---------------------------------------------------------
Title: Recent X-ray studies of stellar cycles and long-term
variability
Authors: Peres, Giovanni; Micela, Giuseppina; Favata, Fabio
2007HiA....14..287P Altcode:
We discuss recent X-ray studies of stellar cycles and long-term
variability.

---------------------------------------------------------
Title: The X-Ray Telescope (XRT) for the Hinode Mission
Authors: Golub, L.; DeLuca, E.; Austin, G.; Bookbinder, J.; Caldwell,
D.; Cheimets, P.; Cirtain, J.; Cosmo, M.; Reid, P.; Sette, A.; Weber,
M.; Sakao, T.; Kano, R.; Shibasaki, K.; Hara, H.; Tsuneta, S.; Kumagai,
K.; Tamura, T.; Shimojo, M.; McCracken, J.; Carpenter, J.; Haight,
H.; Siler, R.; Wright, E.; Tucker, J.; Rutledge, H.; Barbera, M.;
Peres, G.; Varisco, S.
2007SoPh..243...63G Altcode:
The X-ray Telescope (XRT) of the Hinode mission provides an
unprecedented combination of spatial and temporal resolution in solar
coronal studies. The high sensitivity and broad dynamic range of XRT,
coupled with the spacecraft's onboard memory capacity and the planned
downlink capability will permit a broad range of coronal studies over
an extended period of time, for targets ranging from quiet Sun to
X-flares. This paper discusses in detail the design, calibration, and
measured performance of the XRT instrument up to the focal plane. The
CCD camera and data handling are discussed separately in a companion
paper.

---------------------------------------------------------
Title: X-ray emission from MP Muscae: an old classical T Tauri star
Authors: Argiroffi, C.; Maggio, A.; Peres, G.
2007A&A...465L...5A Altcode: 2007astro.ph..1765A
Aims:We study the properties of X-ray emitting plasma of <ASTROBJ>MP
Mus</ASTROBJ>, an old classical T Tauri star. We check whether
an accretion process could produce the observed X-ray emission
and we derive the accretion parameters and the characteristics of
the shock-heated plasma. We compare the properties of <ASTROBJ>MP
Mus</ASTROBJ> with those of younger classical T Tauri stars to
test whether age is related to the properties of the X-ray emitting
plasma. Methods: XMM-Newton X-ray spectra allow us to measure
plasma temperatures, abundances, and electron density. The density
of cool plasma probes whether X-ray emission is produced by plasma
heated in the accretion process. Results: X-ray emission from
<ASTROBJ>MP Mus</ASTROBJ> originates from high density cool plasma
but a hot flaring component is also present, suggesting that both
coronal magnetic activity and accretion contribute to the observed
X-ray emission. We find a Ne/O ratio similar to that observed in the
much younger classical T Tauri star <ASTROBJ>BP Tau</ASTROBJ>. From the
soft part of the X-ray emission, mostly produced by plasma heated in the
accretion shock, we derive a mass accretion rate of 5×10-11
M_⊙ yr-1.

---------------------------------------------------------
Title: MHD evolution of a fragment of a CME core in the outer
solar corona
Authors: Pagano, P.; Reale, F.; Orlando, S.; Peres, G.
2007A&A...464..753P Altcode:
Context: Detailed hydrodynamic modeling explained several features of a
fragment of the core of a Coronal Mass Ejection observed with SoHO/UVCS
at 1.7 R⊙ on 12 December 1997, but some questions remained
unsolved. Aims: We investigate the role of the magnetic fields in
the thermal insulation and the expansion of an ejected fragment (cloud)
traveling upwards in the outer corona. Methods: We perform MHD
simulations including the effects of thermal conduction and radiative
losses of a dense spherical or cylindrical cloud launched upwards in the
outer corona, with various assumptions on the strength and topology of
the ambient magnetic field; we also consider the case of a cylindrical
cloud with an internal magnetic field component along its axis. Results: We find that a weak ambient magnetic field (β ∼ 20) with
open topology provides both significant thermal insulation and large
expansion. The cylindrical cloud expands more than the spherical one.

---------------------------------------------------------
Title: X-rays from protostellar jets: emission from continuous flows
Authors: Bonito, R.; Orlando, S.; Peres, G.; Favata, F.; Rosner, R.
2007A&A...462..645B Altcode: 2006astro.ph.10507B
Context: Recently X-ray emission from protostellar jets has been
detected with both XMM-Newton and Chandra satellites, but the physical
mechanism which can give rise to this emission is still unclear. Aims: We performed an extensive exploration of the parameter space for
the main parameters influencing the jet/ambient medium interaction. Aims
include: 1) to constrain the jet/ambient medium interaction regimes
leading to the X-ray emission observed in Herbig-Haro objects in
terms of the emission by a shock forming at the interaction front
between a continuous supersonic jet and the surrounding medium;
2) to derive detailed predictions to be compared with optical and
X-ray observations of protostellar jets; 3) to get insight into the
protostellar jet's physical conditions. Methods: We performed
a set of two-dimensional hydrodynamic numerical simulations, in
cylindrical coordinates, modeling supersonic jets ramming into a uniform
ambient medium. The model takes into account the most relevant physical
effects, namely thermal conduction and radiative losses. Results:
Our model explains the observed X-ray emission from protostellar
jets in a natural way. In particular, we find that a protostellar
jet that is less dense than the ambient medium well reproduces the
observations of the nearest Herbig-Haro object, HH 154, and allows us
to make detailed predictions of a possible X-ray source proper motion
(v_sh ≈500 km s-1) detectable with Chandra. Furthermore,
our results suggest that the simulated protostellar jets which best
reproduce the X-rays observations cannot drive molecular outflows.

---------------------------------------------------------
Title: MHD evolution of a fragment of a CME core in the outer
solar corona
Authors: Pagano, P.; Reale, F.; Orlando, S.; Peres, G.
2007astro.ph..1145P Altcode:
Detailed hydrodynamic modeling explained several features of a fragment
of the core of a Coronal Mass Ejection observed with SoHO/UVCS at
1.7 Ro on 12 December 1997, but some questions remained unsolved. We
investigate the role of the magnetic fields in the thermal insulation
and the expansion of an ejected fragment (cloud) traveling upwards in
the outer corona. We perform MHD simulations including the effects
of thermal conduction and radiative losses of a dense spherical or
cylindrical cloud launched upwards in the outer corona, with various
assumptions on the strength and topology of the ambient magnetic field;
we also consider the case of a cylindrical cloud with an internal
magnetic field component along its axis. We find that a weak ambient
magnetic field (beta~20) with open topology provides both significant
thermal insulation and large expansion. The cylindrical cloud expands
more than the spherical one.

---------------------------------------------------------
Title: Magnetic activity and the solar corona: first results from
the Hinode satellite .
Authors: Reale, Fabio; Parenti, Susanna; Reeves, Kathy K.; Weber,
Mark; Bobra, Monica G.; Barbera, Marco; Kano, Ryohei; Narukage,
Noriyuki; Shimojo, Masumi; Sakao, Taro; Peres, Giovanni; Golub, Leon
2007MmSAI..78..591R Altcode:
The structure, dynamics and evolution of the solar corona are governed
by the magnetic field. In spite of significant progresses in our insight
of the physics of the solar corona, several problems are still under
debate, e.g. the role of impulsive events and waves in coronal heating,
and the origin of eruptions, flares and CMEs. The Hinode mission has
started on 22 september 2006 and aims at giving new answers to these
questions. The satellite contains three main instruments, two high
resolution telescopes, one in the optical and one in the X-ray band,
and an EUV imaging spectrometer. On the Italian side, INAF/Osservatorio
Astronomico di Palermo has contributed with the ground-calibration
of the filters of the X-ray telescope. We present some preliminary
mission results, with particular attention to the X-ray telescope data.

---------------------------------------------------------
Title: VizieR Online Data Catalog: X-ray variability of NGC 2516
stars in the XMM-Newton observations.
Authors: Marino, A.; Micela, G.; Pillitteri, I.; Peres, G.
2007yCat..34560977M Altcode:
We present the characteristics of the X-ray variability of stars in the
cluster NGC 2516 as derived from XMM-Newton/EPIC/pn data. The X-ray
variations on short (hours), medium (months), and long (years) time
scales have been explored. We detected 303 distinct X-ray sources by
analysing six EPIC/pn observations; 194 of them are members of the
cluster. Stars of all spectral types, from the early-types to the
late-M dwarfs, were detected. (2 data files).

---------------------------------------------------------
Title: Correlation between the spatial distribution of circumstellar
disks and massive stars in the open cluster NGC 6611. Compiled
catalog and cluster parameters
Authors: Guarcello, M. G.; Prisinzano, L.; Micela, G.; Damiani, F.;
Peres, G.; Sciortino, S.
2007A&A...462..245G Altcode: 2006astro.ph.10401G
Context: The observation of young stars with circumstellar disks
suggests that the disks are dissipated, starting from the inner region,
by the radiation of the central star and eventually by the formation
of rocky planetesimals, over a time scale of several million years. It
was also shown that strong UV radiation emitted by nearby massive
stars can heat a circumstellar disk up to some thousand degrees,
inducing the photoevaporation of the gas. This process strongly
reduces the dissipation time scale. Aims: We study whether there
exists a correlation between the spatial distribution of stars with
circumstellar disks and the position of massive stars with spectral
class earlier than B5, in the open cluster NGC 6611. Methods:
We created a multiband catalog of the cluster, down to V∼ 23^m,
using optical data from a WFI observation at 2.2 m of ESO in the
BVI bands, the 2MASS public point source catalog and an archival
X-ray observation made with CHANDRA/ACIS. We selected the stars with
infrared excess (due to the emission of a circumstellar disk) using
suitable color indices independent of extinction, and studied their
spatial distribution. Results: The spatial distribution of the
stars with K band excess (due to the presence of a circumstellar disk)
is anti correlated with that of the massive stars: the disks are more
frequent at large distances from these stars. We argue that this is in
agreement with the hypothesis that the circumstellar disks are heated
by the UV radiation from the massive stars and photoevaporated. Based on observations made with the European Observatory telescopes
obtained from the ESO/ST-ECF Science Archive Facility. Table 3 and
Appendix are only available in electronic form at http://www.aanda.org

---------------------------------------------------------
Title: XMM-Newton survey of two upper Scorpius regions
Authors: Argiroffi, C.; Favata, F.; Flaccomio, E.; Maggio, A.; Micela,
G.; Peres, G.; Sciortino, S.
2006A&A...459..199A Altcode: 2006astro.ph..7298A
Aims.We studied X-ray emission from young stars by analyzing the deep
XMM-Newton observations of two regions of the Upper Scorpius association
with an age of 5 Myr. Methods: .Based on near infrared and optical
photometry we identified 22 Upper Scorpius photometric members among
the 224 detected X-ray sources. We derived coronal properties of Upper
Scorpius stars by performing X-ray spectral and timing analyses. The
study of four strong and isolated stellar flares allowed us to derive
the length of the flaring loops. Results: .Among the 22 Upper
Scorpius stars, 13 were identified as Upper Scorpius photometric members
for the first time. The sample includes 7 weak-line T Tauri stars and
1 classical T Tauri star, while the nature of the remaining sources
is unknown. Except for the intermediate mass star HD 142578, all the
detected USco sources are low-mass stars of spectral types ranging from
G to late M. The X-ray emission spectrum of the most intense Upper
Scorpius sources indicates metal depleted plasma with temperature of
~10 MK, resembling the typical coronal emission of active main sequence
stars. At least 59% of the detected members of the association have
variable X-ray emission, and the flaring coronal structures appear
shorter than or comparable to the stellar radii already at the Upper
Scorpius age. We also found indications of increasing plasma metallicity
(up to a factor 20) during strong flares. We identified a new galaxy
cluster among the 224 X-ray source detected: the X-ray spectrum of
its intra cluster medium indicates a redshift of ~0.41 ± 0.02.

---------------------------------------------------------
Title: Crushing of interstellar gas clouds in supernova
remnants. II. X-ray emission
Authors: Orlando, S.; Bocchino, F.; Peres, G.; Reale, F.; Plewa, T.;
Rosner, R.
2006A&A...457..545O Altcode: 2006astro.ph..7252O
Context: .X-ray observations of evolved supernova remnants (e.g. the
Cygnus loop and the Vela SNRs) reveal emission originating from the
interaction of shock waves with small interstellar gas clouds. Aims: .We study and discuss the time-dependent X-ray emission
predicted by hydrodynamic modeling of the interaction of a SNR shock
wave with an interstellar gas cloud. The scope includes: 1) to study
the correspondence between modeled and X-ray emitting structures, 2)
to explore two different physical regimes in which either thermal
conduction or radiative cooling plays a dominant role, and 3) to
investigate the effects of the physical processes at work on the
emission of the shocked cloud in the two different regimes. 
Methods: .We use a detailed hydrodynamic model, including thermal
conduction and radiation, and explore two cases characterized
by different Mach numbers of the primary shock: M= 30 (post-shock
temperature T_psh ≈ 1.7 MK) in which the cloud dynamics is dominated
by radiative cooling and M= 50 (T_psh ≈ 4.7 MK) dominated by thermal
conduction. From the simulations, we synthesize the expected X-ray
emission, using available spectral codes. Results: .The morphology
of the X-ray emitting structures is significantly different from that of
the flow structures originating from the shock-cloud interaction. The
hydrodynamic instabilities are never clearly visible in the X-ray
band. Shocked clouds are preferentially visible during the early phases
of their evolution. Thermal conduction and radiative cooling lead to
two different phases of the shocked cloud: a cold cooling dominated
core emitting at low energies and a hot thermally conducting corona
emitting in the X-ray band. The thermal conduction makes the X-ray
image of the cloud smaller, more diffuse, and shorter-lived than that
observed when thermal conduction is neglected.

---------------------------------------------------------
Title: VizieR Online Data Catalog: XMM-Newton survey of two Upper
Scorpius regions (Argiroffi+ 2006)
Authors: Argiroffi, C.; Favata, F.; Flaccomio, E.; Maggio, A.; Micela,
G.; Peres, G.; Sciortino, S.
2006yCat..34590199A Altcode:
Portions of the Upper Scorpius association were observed with the
Curtis Schmidt (CTIO) telescope, with the Danish 1.54m (ESO) telescope,
and with the XMM-Newton satellite. The tables contain all the sources
detected with the different instruments, and their characteristics. (4 data files).

---------------------------------------------------------
Title: X-ray variability of NGC 2516 stars in the XMM-Newton
observations
Authors: Marino, A.; Micela, G.; Pillitteri, I.; Peres, G.
2006A&A...456..977M Altcode: 2008arXiv0801.2300M
We present the characteristics of the X-ray variability of stars in
the cluster NGC 2516 as derived from XMM-Newton/EPIC/pn data. The
X-ray variations on short (hours), medium (months), and long (years)
time scales have been explored. We detected 303 distinct X-ray sources
by analysing six EPIC/pn observations; 194 of them are members of the
cluster. Stars of all spectral types, from the early-types to the late-M
dwarfs, were detected. The Kolmogorov-Smirnov test applied to the X-ray
photon time series shows that, on short time scales, only a relatively
small fraction (ranging from 6% to 31% for dG and dF, respectively) of
the members of NGC 2516 are variable with a confidence level ≥99%;
however, it is possible that the fraction is small only because of
the poor statistics. The time X-ray amplitude distribution functions
(XAD) of a set of dF7-dK2 stars, derived on short (hours) and medium
(months) time scales, seem to suggest that medium-term variations,
if present, have a much smaller amplitude than those on short time
scales; a similar result is also obtained for dK3-dM stars. The
amplitude variations of late-type stars in NGC 2516 are consistent
with those of the coeval Pleiades stars. Comparing these data with
those of ROSAT/PSPC, collected 7-8 years earlier, and of ROSAT/HRI,
just 4-5 years earlier, we find no evidence of significant variability
on the related time scales, suggesting that long-term variations due to
activity cycles similar to the solar cycle are not common among young
stars. Indications of spectral variability was found in one star whose
spectra at three epochs were available.

---------------------------------------------------------
Title: Recent X-ray Observations of Stellar Cycles and Long Term
Variability
Authors: Peres, G.; Micela, G.; et al.
2006IAUJD...8E..63P Altcode:
We will discuss some aspects relevant to the detection of coronal
activity cycles in late-type stars. The effects of the activity solar
cycle manifest themselves in many bands, and most notably in the X-ray
band. Since strong X-ray emission and other forms of activity are
very evident in active late-type stars, one would expect detection of
analogous X-ray cycles on these stars. However this is not the case. So
to which extent can we apply the Solar-stellar connection in the coronal
context? Certainly the Sun fits in the general late-type-stars "trend"
of activity vs. rotation, age, temperature, flux etc.; on the other
side extreme stellar activity shows "saturation" in rapidly rotating
stars and the above "trend" may change significantly or break for very
active stars, suggesting the action of a mechanism different from the
solar-type dynamo. In this context, proving the presence of coronal
solar-like cycles and determining their characteristics, hopefully for
a large stellar sample, would provide fundamental tests. Detecting
stellar coronal cycles is difficult, given the limited availability
of present day X-ray telescopes for this purpose. Also, since X-ray
observations aimed at cycle determinations cover time intervals much
shorter than cycles themselves, and separated by years, it is hard to
disentangle the mix of short term variability and cycles. Nonetheless
there is some evidence of long term variability in some samples and
in relatively old solar-mass stars (but not in young solar-mass stars
neither in M stars). Some projects dedicated to find X-ray cycles using
present day telescopes (e.g. those driven by Schmitt and by Favata)
have started yielding results. A possibly more fruitful approach to
detecting X-ray cycles is devising a relatively small X-ray satellite
entirely dedicated to a long observing program to monitor active stars,
like SADE (recently proposed by P. Martens and collaborators). One of
the important goals of this research would be tracing the evolutionary
path of the cyclic activity and of dynamo physics by measuring the
activity of solar analogs in different stages of their life-cycles
and for different stellar masses.

---------------------------------------------------------
Title: The Corona of the Sun as a Star
Authors: Peres, Giovanni; Orlando, Salvatore; Reale, Fabio
2006AIPC..848...41P Altcode:
We study the physics of the solar corona as a whole, i.e. of the Sun
as a Star, in order to understand its global features and to provide a
template for stellar coronae. In this process we strive to understand
the features of various structures which compose the solar corona. This
process in not straightforward given the problems of observing the Sun
as a whole: e.g., no recent X-ray wide-band, medium-resolution, spectrum
of the Sun is avaible, unlike stars and no X-ray spectral monitoring of
the Sun at various activity phases is available. The presentation will
discuss our work in this field; we present the method we have devised,
based on Yohkoh/SXT data, to derive the Differential Emission Measure
vs. Temperature, and the X-ray spectrum, of the solar corona, and the
related main results. Finally we present some new results on flares
and microflares and the implications on solar-stellar coronal heating.

---------------------------------------------------------
Title: VizieR Online Data Catalog: XMM spectroscopy of TWA 5
(Argiroffi+, 2005)
Authors: Argiroffi, C.; Maggio, A.; Peres, G.; Stelzer, B.;
Neuhaeuser, R.
2006yCat..34391149A Altcode:
TWA 5 was observed for ~30ks with XMM-Newton on January 9, 2003. (1 data file).

---------------------------------------------------------
Title: The discovery of an expanding X-ray source in the HH 154
protostellar jet
Authors: Favata, F.; Bonito, R.; Micela, G.; Fridlund, M.; Orlando,
S.; Sciortino, S.; Peres, G.
2006A&A...450L..17F Altcode: 2006astro.ph..3186F
Context: .Protostellar jets are a new class of X-ray sources which
has been discovered with both XMM-Newton and Chandra. The mechanism
responsible for the X-ray emission is still not clear. Self-shocking
in jets, shocks where the jet hits the surrounding medium, reflected
or scattered stellar X-ray emission have all been invoked as possible
explanations. Aims: .One key diagnostic discriminating among
physical emission mechanisms is the motion of the X-ray source:
hydrodynamical numerical models of continuous protostellar jets
plowing through a uniform medium show an X-ray emitting shock front
moving at several hundreds km s-1. In the nearest X-ray
emitting protostellar jet, HH 154, this is detectable, with the
spatial resolution of the Chandra X-ray observatory, over a few
years baseline, allowing a robust discrimination among different
mechanisms. Methods: .We have performed, in October 2005, a
deep Chandra X-ray observation of HH 154. Comparison with the previous
(2001) Chandra observation allows to detect proper motion down to the
level predicted by models of X-ray emitting shocks in the jet. 
Results: .The 2005 Chandra observation of HH 154 shows unexpected
morphological changes of the X-ray emission in comparison with the 2001
data. Two components are present: a stronger, point-like component
with no detectable motion and a weaker component which has expanded
in size by approximately 300 AU over the 4 years time base of the
two observations. This expansion corresponds to approximately 500
km s-1, very close to the velocity of the X-ray emitting
shock in the simple theoretical models. Conclusions: .The 2005
data show a more complex system than initially thought (and modeled),
with multiple components with different properties. The observed
morphology is possibly indicating a pulsed jet propagating through
a non-homogeneous medium, likely with medium density decreasing with
distance from the driving source. Detailed theoretical modeling and
deeper X-ray observations will be needed to understand the physics of
this fascinating class of sources.

---------------------------------------------------------
Title: VizieR Online Data Catalog: XMM observations of IC 2391 stars
(Marino+, 2005)
Authors: Marino, A.; Micela, G.; Peres, G.; Pillitteri, I.;
Sciortino, S.
2006yCat..34300287M Altcode:
We present the analysis of the Guaranteed Time XMM-Newton/EPIC
observation pointed on the young open cluster IC 2391. The observation
(Obs. No. 0112420101), centered on (RA=8:42:00, DE=-53:00:36), was
performed on Nov. 20, 2001 during orbit 357 of XMM-Newton. (2
data files).

---------------------------------------------------------
Title: Interaction of SNR shocks with thermally conducting,
radiative clouds
Authors: Orlando, S.; Bocchino, F.; Peres, G.; Reale, F.
2006cosp...36.3172O Altcode: 2006cosp.meet.3172O
The shock wave of shell supernova remnants SNRs offers the unique
opportunity to detect directly the structures of the interstellar
medium both at large and small scales On the other hand the several
physical effects at work during the interaction between the shock and
the medium along with the superposition effects along a given light
of sight require detailed modeling and high resolution observations in
order to derive useful diagnostics Here we study the interaction of an
evolved SNR shock front impacting on a small interstellar gas cloud
through a 3D hydrodynamic model which takes into account the effects
of radiative losses and thermal conduction We explore the interplay
between the physical processes at work during the cloud evolution and
their effect on the mass and energy exchange with the surrounding medium
We also investigate the time-dependent X-ray emission originating from
the shock-cloud interactions for two prototypical cases in which either
thermal conduction or radiative cooling plays a dominant role in the
cloud dynamics and energetic We study the correspondence between ideal
and observable structures identifing a set of diagnostic tools which
may be useful in the comparison with XMM-Newton and Chandra data of
SNR shells

---------------------------------------------------------
Title: X-ray properties of NGC 2516 open cluster .
Authors: Marino, A.; Pillitteri, I.; Micela, G.; Peres, G.
2006MSAIS...9..256M Altcode:
We present the results of a study of X-ray properties of the NGC 2516,
obtained with a series of XMM-Newton observations. Deep X-ray photometry
has been obtained by summing up six EPIC observations, for a total
of 105 ks, thus increasing the sensitivity in the X-ray band in this
region by a factor 5 with respect to all previous surveys. Coronal
spectra of solar mass stars are generally well described by one or two
plasma thermal model, with typical temperatures ranging in 0.5-2.0 keV
interval. We have studied the X-ray luminosity functions for different
spectral types, confirming that G-M type stars are statistically
less luminous than stars in the coeval Pleiades. Analysis of X-ray
light-curves shows that only a small fraction of cluster members are
variable on short time scales. X-ray variations are also studied on
longer time scales.

---------------------------------------------------------
Title: X-ray Emission Mechanisms in Herbig - Haro objects .
Authors: Bonito, R.; Orlando, S.; Peres, G.; Favata, F.; Rosner, R.
2006MSAIS...9..226B Altcode:
X-ray emission in Herbig - Haro objects is a quite recent and uncommon
finding still waiting full explanation. With the scope of explaining
this X-ray emission, our project is devoted to model the interaction
between a supersonic jet originating from a young stellar object
and the ambient medium. We have performed a wide exploration of the
parameter space to infer the configuration(s) which can give rise to
X-ray emission very similar to what recently observed.

---------------------------------------------------------
Title: Stationary and Flaring Heating Effect on the Coronal Emission
Measure .
Authors: Peres, G.; Orlando, S.; Di Matteo, V.; Reale, F.
2006MSAIS...9...97P Altcode:
Our scope is finding the relative role of "steady" vs. flare coronal
heating through the analysis of steady and flaring emission measures,
finding whether or not the latter appear as the high-temperature
tail of the former and if there is a bi-modality suggesting different
heating mechanisms. To this end, we have derived the emission measure
vs. temperature for the steady corona, and for flares, from Yohkoh/SXT
dataset plus GOES flare data cross-calibrated with the Yohkoh/SXT
data. The comparison between the two datasets and the ensuing emission
measure distributions vs. temperature is discussed.

---------------------------------------------------------
Title: X-raying the interstellar medium: the study of SNR shells at
the OAPa .
Authors: Bocchino, F.; Miceli, M.; Reale, F.; Maggio, A.; Orlando,
S.; Peres, G.
2006MSAIS...9..223B Altcode:
The shock wave of shell supernova remnant (SNR) offers the unique
opportunity to detect directly the structures of the interstellar
medium, both at large and small scale. Unfortunately, the several
physical effects at work in the interaction between the shock and
the medium, along with the superposition effects along a given light
of sight, require detailed modeling and high resolution observations
of the systems in order to understand its physical behavior At the
Osservatorio Astronomico di Palermo, we have started a long-term
project which will lead, for the first time, to a self-consistent
methodological approach for comparison of new accurate numerical models
and high resolution multi-wavelength observations. In this poster, we
introduce the observation we have already analyzed,while in the talk
of S. Orlando the new numerical models we have developed are presented

---------------------------------------------------------
Title: Hydrodynamic interaction of SNR shocks with thermally
conducting, radiative clouds .
Authors: Orlando, S.; Peres, G.; Reale, F.; Bocchino, F.; Plewa, T.;
Rosner, R.
2006MSAIS...9..208O Altcode:
Supernova remnants (SNRs) are privileged laboratories to investigate
the physical and chemical evolution of the galactic interstellar
medium (ISM) and the mass distribution of the plasma in the Galaxy. Here, we study the interaction of an evolved SNR shock front with
on a small interstellar gas cloud. Our model takes into account the
hydrodynamics and the effects of the radiative losses and of the
thermal conduction. We study the interplay between the radiative
cooling and the thermal conduction during the cloud evolution and
their effect on the mass and energy exchange between the cloud and the
surrounding medium. We find that in cases dominated by the radiative
losses the cloud fragments into cold, dense, and compact filaments
surrounded by a hot corona which is ablated by the thermal conduction;
instead, in cases dominated by the thermal conduction, the shocked
cloud evaporates into the ISM in a few dynamical time-scales. In all
the cases analyzed we find that the thermal conduction suppresses the
hydrodynamic instabilities at the cloud boundaries.

---------------------------------------------------------
Title: Mhd-Modeling of the Propagation of a Coronal Mass Ejection
Authors: Pagano, P.; Reale, F.; Orlando, S.; Peres, G.
2005ESASP.600E.161P Altcode: 2005dysu.confE.161P; 2005ESPM...11..161P
No abstract at ADS

---------------------------------------------------------
Title: Crushing of interstellar gas clouds in supernova
remnants. I. The role of thermal conduction and radiative losses
Authors: Orlando, S.; Peres, G.; Reale, F.; Bocchino, F.; Rosner,
R.; Plewa, T.; Siegel, A.
2005A&A...444..505O Altcode: 2005astro.ph..8638O
We model the hydrodynamic interaction of a shock wave of an evolved
supernova remnant with a small interstellar gas cloud like the ones
observed in the Cygnus loop and in the Vela SNR. We investigate the
interplay between radiative cooling and thermal conduction during cloud
evolution and their effect on the mass and energy exchange between
the cloud and the surrounding medium. Through the study of two cases
characterized by different Mach numbers of the primary shock (M= 30 and
50, corresponding to a post-shock temperature T≈ 1.7× 106
K and ≈ 4.7× 106 K, respectively), we explore two very
different physical regimes: for M= 30, the radiative losses dominate
the evolution of the shocked cloud which fragments into cold, dense,
and compact filaments surrounded by a hot corona which is ablated by
the thermal conduction; instead, for M= 50, the thermal conduction
dominates the evolution of the shocked cloud, which evaporates in a few
dynamical time-scales. In both cases we find that the thermal conduction
is very effective in suppressing the hydrodynamic instabilities that
would develop at the cloud boundaries.

---------------------------------------------------------
Title: Modeling a Coronal Loop Heated by Magnetohydrodynamic
Turbulence Nanoflares
Authors: Reale, F.; Nigro, G.; Malara, F.; Peres, G.; Veltri, P.
2005ApJ...633..489R Altcode: 2005astro.ph..6694R
We model the hydrodynamic evolution of the plasma confined in a coronal
loop, 30,000 km long, subject to the heating of nanoflares due to
intermittent magnetic dissipative events in the MHD turbulence produced
by loop footpoint motions. We use the time-dependent distribution of
energy dissipation along the loop obtained from a hybrid shell model,
occurring for a magnetic field of about 10 G in the corona; the relevant
heating per unit volume along the loop is used in the Palermo-Harvard
loop plasma hydrodynamic model. We describe the results, focusing on
the effects produced by the most intense heat pulses, which lead to
loop temperatures between 1 and 1.5 MK.

---------------------------------------------------------
Title: XMM-Newton spectroscopy of the metal depleted T Tauri star
<ASTROBJ>TWA 5</ASTROBJ>
Authors: Argiroffi, C.; Maggio, A.; Peres, G.; Stelzer, B.;
Neuhäuser, R.
2005A&A...439.1149A Altcode: 2005astro.ph..5075A
We present results of X-ray spectroscopy for <ASTROBJ>TWA
5</ASTROBJ>, a member of the young TW Hydrae association, observed
with XMM-Newton. <ASTROBJ>TWA 5</ASTROBJ> is a multiple system which
shows Hα emission, a signature typical of classical T Tauri stars,
but no infrared excess. From this analysis of the RGS and EPIC spectra,
we have derived the emission measure distribution vs. temperature of the
X-ray emitting plasma, its abundances, and the electron density. The
characteristic temperature and density of the plasma suggest a corona
similar to that of weak-line T Tauri stars and active late-type main
sequence stars. <ASTROBJ>TWA 5</ASTROBJ> also shows low iron abundance
(~0.1 times the solar photospheric one) and a pattern of increasing
abundances for elements with increasing first ionization potential
reminiscent of the inverse FIP effect observed in highly active
stars. The especially high ratio Ne/Fe∼10 is similar to that of the
classical T Tauri star <ASTROBJ>TW Hya</ASTROBJ>, where the accreting
material has been held responsible for the X-ray emission. We discuss
the possible role of an accretion process in this scenario. Since all
T Tauri stars in the TW Hydrae association studied so far have very
high Ne/Fe ratios, we also propose that environmental conditions may
cause this effect.

---------------------------------------------------------
Title: XMM-Newton observations of the Upper Scorpius association
Authors: Argiroffi, Costanza; Favata, F.; Maggio, A.; Micela, G.;
Peres, G.; Sciortino, S.
2005sfet.confE..47A Altcode:
The 5 Myr old Upper Scorpius Association, due to its proximity (145 pc)
and low interstellar absorption, allows a detailed study of the X-ray
emission from PMS stars. We present the results of the analysis of
XMM observations of two Upper Scorpius regions. We detected 224 X-ray
sources among which we identified 21 Upper Scorpius probable members
on the basis of the 2MASS and DENIS photometry. The selected Upper
Scorpius sample includes 7 WTTS, while the nature of the remaining
14 sources it is not known. Except the high mass star HD 142578
(spectral type A2), all the other detected Upper Scorpius sources are
low mass stars of spectral type ranging from K to late M. The spectral
analysis of the most intense sources indicates plasma temperature
of ∼10 MK, resembling the typical coronal emission of active main
sequence stars. We study the abundance vs. temperature pattern of
Upper Scorpius sources and compare it with that observed for active
stars. The Kolmogorov-Smirnov test proves that 67% of the detected
X-ray Upper Scorpius sources were variable during the observation,
and some of them displayed large flare-like events.

---------------------------------------------------------
Title: X-ray emission mechanisms in protostellar jets
Authors: Bonito, R.; Orlando, S.; Peres, G.; Favata, F.; Rosner, R.
2005ESASP.560..185B Altcode: 2005csss...13..185B
No abstract at ADS

---------------------------------------------------------
Title: The structure of coronal plasma in active stellar coronae
from density measurements
Authors: Testa, P.; Drake, J. J.; Peres, G.
2005ESASP.560..997T Altcode: 2005csss...13..997T
No abstract at ADS

---------------------------------------------------------
Title: High resolution X-ray spectroscopy of pre-main-sequence stars:
TWA 5 and PZ Tel
Authors: Argiroffi, C.; Drake, J. J.; Harnden, F. R.; Maggio, A.;
Peres, G.; Sciortino, S.; Stelzer, B.; Neuhäuser, R.
2005ESASP.560..399A Altcode: 2004astro.ph.10264A; 2005csss...13..399A
We report on the analysis of high resolution X-ray spectra of two
pre-main-sequence stars: TWA 5 (observed with XMM-Newton) and PZ
Telescopii (observed with Chandra/HETGS). TWA 5 is a classical T Tauri
star in the TW Hydrae association while PZ Tel is a rapidly rotating
weak-lined T Tauri star in the beta-Pictoris moving group. For both
stars we have reconstructed the emission measure distribution and
derived the coronal abundances to check for possible patterns of the
abundances related to the first ionization potential of the various
elements. We have also derived estimates of the plasma density from the
analysis of the He-like triplets. We compare the characteristics of our
targets with those of other pre-main sequence stars previously analyzed
by other authors: TW Hya, HD 98800 and HD 283572. Our findings suggest
that X-ray emission from classical T Tauri and weak-lined T Tauri stars
is produced in all cases by magnetically-heated coronae, except for
TW Hya which has unique plasma temperatures and densities. Moreover we
derive that TWA 5 has the same peculiar Ne/Fe abundance ratio as TW Hya.

---------------------------------------------------------
Title: Emission Measure Distribution in Loops Impulsively Heated at
the Footpoints
Authors: Testa, Paola; Peres, Giovanni; Reale, Fabio
2005ApJ...622..695T Altcode: 2004astro.ph.12482T
This work is prompted by evidence of sharply peaked emission measure
distributions in active stars and by the claims of isothermal loops
in solar coronal observations, at variance with the predictions
of hydrostatic loop models with constant cross section and
uniform heating. We address the problem with loops heated at the
footpoints. Since steady heating does not allow static loop model
solutions, we explore whether pulse-heated loops can exist and appear
as steady loops on a time average. We simulate pulse-heated loops,
using the Palermo-Harvard 1-D hydrodynamic code, for different initial
conditions corresponding to typical coronal temperatures of stars
ranging from intermediate to active [T~(3-10)×106 K]. We
find long-lived quasi-steady solutions even for heating concentrated at
the footpoints over a spatial region of the order of ~1/5 of the loop
half-length and broader. These solutions yield an emission measure
distribution with a peak at high temperature, and the cool side
of the peak is as steep as ~T5, in contrast to the usual
~T3/2 of hydrostatic models with constant cross section and
uniform heating. Such peaks are similar to those found in the emission
measure distribution of active stars around 107 K.

---------------------------------------------------------
Title: X-ray spectral and timing properties of the IC 2391 stars
Authors: Marino, A.; Micela, G.; Peres, G.; Pillitteri, I.;
Sciortino, S.
2005ESASP.560..787M Altcode: 2005csss...13..787M
No abstract at ADS

---------------------------------------------------------
Title: Coronal properties of active G-type stars in different
evolutionary phases
Authors: Scelsi, L.; Maggio, A.; Peres, G.; Pallavicini, R.
2005ESASP.560..939S Altcode: 2005csss...13..939S
No abstract at ADS

---------------------------------------------------------
Title: Coronal properties of G-type stars in different evolutionary
phases
Authors: Scelsi, L.; Maggio, A.; Peres, G.; Pallavicini, R.
2005A&A...432..671S Altcode: 2005astro.ph..1631S
We report on the analysis of XMM-Newton observations of three G-type
stars in very different evolutionary phases: the weak-lined T Tauri
star <ASTROBJ>HD 283572</ASTROBJ>, the Zero Age Main Sequence star
<ASTROBJ>EK Dra</ASTROBJ> and the Hertzsprung-gap giant star <ASTROBJ>31
Com</ASTROBJ>. They all have high X-ray luminosity (~1031
erg s-1 for HD 283572 and 31 Com and ~1030
erg s-1 for EK Dra). We compare the Emission Measure
Distributions (EMDs) of these active coronal sources, derived from
high-resolution XMM-Newton grating spectra, as well as the pattern
of elemental abundances vs. First Ionzation Potential (FIP). We also
perform time-resolved spectroscopy of a flare detected by XMM from
EK Dra. We interpret the observed EMDs as the result of the emission
of ensembles of magnetically confined loop-like structures with
different apex temperatures. Our analysis indicates that the coronae
of HD 283572 and 31 Com are very similar in terms of dominant coronal
magnetic structures, in spite of differences in the evolutionary phase,
surface gravity and metallicity. In the case of EK Dra the distribution
appears to be slightly flatter than in the previous two cases, although
the peak temperature is similar.

---------------------------------------------------------
Title: Size of coronal structures in active stellar coronae from
the detection of X-ray resonant scattering
Authors: Testa, P.; Drake, J. J.; Peres, G.; Deluca, E. E.
2005ESASP.560...43T Altcode: 2005csss...13...43T
No abstract at ADS

---------------------------------------------------------
Title: X-ray spectral and timing characteristics of the stars in
the young open cluster IC 2391
Authors: Marino, A.; Micela, G.; Peres, G.; Pillitteri, I.;
Sciortino, S.
2005A&A...430..287M Altcode: 2004astro.ph.10099M
We present X-ray spectral and timing analysis of members of the young
open cluster IC 2391 observed with the XMM-Newton observatory. We
detected 99 X-ray sources by analysing the summed data obtained from
MOS1, MOS2 and pn detectors of the EPIC camera; 24 of them are members,
or probable members, of the cluster. Stars of all spectral types have
been detected, from the early-types to the late-M dwarfs. Despite the
capability of the instrument to recognize up to 3 thermal components,
the X-ray spectra of the G, K and M members of the cluster are
well described with two thermal components (at kT1 ∼
0.3-0.5 keV and kT2 ∼ 1.0-1.2 keV respectively) while
the X-ray spectra of F members require only a softer 1-T model. The
Kolmogorov-Smirnov test applied to the X-ray photon time series shows
that approximately 46% of the members of IC 2391 are variable with a
confidence level &gt;99%. The comparison of our data with those obtained
with ROSAT/PSPC, nine years earlier, and ROSAT/HRI, seven years earlier,
shows that there is no evidence of significant variability on these
time scales, suggesting that long-term variations due to activity
cycles similar to that on the Sun are not common, if present at all,
among these young stars.

---------------------------------------------------------
Title: The Density of Coronal Plasma in Active Stellar Coronae
Authors: Testa, Paola; Drake, Jeremy J.; Peres, Giovanni
2004ApJ...617..508T Altcode: 2004astro.ph..5019T
We have analyzed high-resolution X-ray spectra of a sample of 22 active
stars observed with the High Energy Transmission Grating Spectrometer on
Chandra in order to investigate their coronal plasma density. Densities
were investigated using the lines of the He-like ions O VII, Mg XI,
and Si XIII. Si XIII lines in all stars of the sample are compatible
with the low-density limit (i.e., ne&lt;~1013
cm-3), casting some doubt on results based on lower
resolution Extreme Ultraviolet Explorer (EUVE) spectra finding densities
ne&gt;1013 cm-3. Mg XI lines betray
the presence of high plasma densities up to a few times 1012
cm-3 for most of the sources with higher X-ray luminosity
(&gt;~1030 ergs s-1) stars with higher
LX and LX/Lbol tend to have higher
densities at high temperatures. Ratios of O VII lines yield much lower
densities of a few times 1010 cm-3, indicating
that the “hot” and “cool” plasma resides in physically different
structures. In the cases of EV Lac, HD 223460, Canopus, μ Vel, TY Pyx,
and IM Peg, our results represent the first spectroscopic estimates
of coronal density. No trends in density-sensitive line ratios with
stellar parameters effective temperature and surface gravity were found,
indicating that plasma densities are remarkably similar for stars with
pressure scale heights differing by up to 3 orders of magnitude. Our
findings imply remarkably compact coronal structures, especially for
the hotter (~7 MK) plasma emitting the Mg XI lines characterized by
the coronal surface filling factor, fMgXI, ranging from
10-4 to 10-1, while we find fOVII
values from a few times 10-3 up to ~1 for the cooler (~2
MK) plasma emitting the O VII lines. We find that fOVII
approaches unity at the same stellar surface X-ray flux level as
characterizes solar active regions, suggesting that these stars become
completely covered by active regions. At the same surface flux level,
fMgXI is seen to increase more sharply with increasing
surface flux. These results appear to support earlier suggestions that
hot 107 K plasma in active coronae arises from flaring
activity and that this flaring activity increases markedly once the
stellar surface becomes covered with active regions. Comparison of our
measured line fluxes with theoretical models suggests that significant
residual model inaccuracies might be present and, in particular,
that cascade contributions to forbidden and intercombination lines
resulting from dielectronic recombination might be to blame.

---------------------------------------------------------
Title: Calibration of the XRT-SOLARB flight filters at the XACT
facility of INAF-OAPA
Authors: Barbera, Marco; Artale, Maria Antonella; Candia, Roberto;
Collura, Alfonso; Lullo, Giuseppe; Peres, Giovanni; Perinati, Emanuele;
Varisco, Salvatore; Bookbinder, J. A.; Cheimets, Peter N.; Cosmo,
Mario L.; DeLuca, Edward E.; Golub, Leon; Weber, Mark A.
2004SPIE.5488..423B Altcode:
The X-Ray Telescope (XRT) experiment on-board the Japanese satellite
SOLAR-B (launch in 2006) aimed at providing full Sun field of view at ~
1.5" angular resolution, will be equipped with two wheels of focal-plane
filters to select spectral features of X-ray emission from the Solar
corona, and a front-end filter to significantly reduce the visible
light contamination. We present the results of the X-ray calibrations
of the XRT flight filters performed at the X-ray Astronomy Calibration
and Testing (XACT) facility of INAF-OAPA. We describe the instrumental
set-up, the adopted measurement technique, and present the transmission
vs. energy and position measurements.

---------------------------------------------------------
Title: Calibration of the XRT-SOLARB flat mirror samples at the XACT
Facility of INAF-OAPA
Authors: Artale, Maria Antonella; Barbera, Marco; Collura, Alfonso;
Di Cicca, Gaspare; Peres, Giovanni; Varisco, Salvatore; Bookbinder,
J. A.; Cheimets, Peter N.; Cosmo, Mario L.; DeLuca, Edward E.; Golub,
Leon; Weber, Mark A.
2004SPIE.5488..440A Altcode:
The X-Ray Telescope (XRT) experiment on-board the Japanese satellite
SOLAR-B (launch in 2006) is equipped with a modified Wolter I grazing
incidence X-ray telescope (focal length 2700 mm) to image the full Sun
at ~ 1.5" angular resolution onto a 2048 x 2048 back illuminated CCD
focal plane detector. The X-ray telescope consisting of one single
reflecting shell is coated with ion beam sputtered Iridium over a
binding layer of Chromium to provide nearly 5 square centimetres
effective area at 60 Å. We present preliminary results of X-ray
calibrations of the XRT flat mirror samples performed at the X-ray
Astronomy Calibration and Testing (XACT) facility of INAF-OAPA. We
describe the instrumental set-up, the adopted measurement technique,
and present the measured reflectivity vs. angle of incidence at few
energies.

---------------------------------------------------------
Title: The Sun as an X-ray star: Active region evolution, rotational
modulation, and implications for stellar X-ray variability
Authors: Orlando, S.; Peres, G.; Reale, F.
2004A&A...424..677O Altcode:
We study the contribution of an active region and its core to the
luminosity and the spectrum of the Sun in the X-ray band and to the
relevant solar emission measure vs. temperature distribution, EM(T). We
also study the relevant changes in the course of four solar rotations,
and the solar rotational modulation due to this active region, the
only one present at that time. To this end, we have used a large
sample of full-disk Yohkoh/SXT observations taken between July and
October 1996, covering most of the active region evolution. From the
Yohkoh/SXT data we have synthesized the X-ray spectra of the whole
solar corona, and the focal plane data as they would be collected with
Rosat/PSPC, XMM-Newton/EPIC and Chandra/ACIS. This work is part of a
project to study the Sun as an X-ray star, using the solar data as a
guide and a template for stellar observations. We found that the
active region contributes significantly to the X-ray spectrum of the
Sun mainly during the first month of its evolution. The rotational
modulation due to the active region causes a significant variability
of the average X-ray flux, with only moderate spectral variation,
in the pass-bands of Rosat/PSPC, XMM-Newton/EPIC and Chandra/ACIS. We investigated the characteristics of the X-ray variability due to
the rotational modulation, to the solar cycle, and to flares together
with the possible implications on stellar X-ray variability. We derived
the diagram of X-ray surface flux vs. spectral hardness ratio in the
Rosat/PSPC band; we studied the variability due to the solar cycle and
to the rotational modulation and we found that both lead to the same
correlation between F_pspc and HR_pspc with a very steep slope. The
variability due to the evolution of flares again produces a correlation
between F_pspc and HR_pspc but with a much flatter slope than in the
other two cases. Analogous results have been found when analyzing data
in the format of XMM-Newton/EPIC and Chandra/ACIS.

---------------------------------------------------------
Title: X-Ray Spectroscopy of the Unsteady Quiescent Corona of AD
Leonis with Chandra
Authors: Maggio, A.; Drake, J. J.; Kashyap, V.; Harnden, F. R., Jr.;
Micela, G.; Peres, G.; Sciortino, S.
2004ApJ...613..548M Altcode: 2004astro.ph..5580M
We present the analysis and interpretation of an observation of the
flare star AD Leo (dM3e) with the Low Energy Transmission Grating
of Chandra. The high-resolution X-ray spectrum-dominated by emission
lines from O VII-VIII, Ne IX-X, and Fe XVII-Fe XIX-allowed us to infer
the plasma emission measure distribution (EMD) versus temperature,
as well as the abundances of individual elements in the corona of
this magnetically active star, during a typical state characterized
by significant variability but no evident flaring event. We have also
measured plasma densities at various temperatures using spectroscopic
diagnostics provided by He-like triplets and Fe XXI lines. We show
that the present EMD is similar, in terms of overall shape and
temperature of the peak, to those previously obtained from Extreme
Ultraviolet Explorer (EUVE) spectra during quiescent and flaring
states confirming the long-term stability of the corona of AD Leo. At
variance with the case of other active stars, the EMD of AD Leo is
characterized by a significantly shallower slope, compatible with
that predicted by static models of isobaric loops with constant cross
section and uniform heating. We discuss such coronal modeling to infer
the average properties of the corona in terms of loop populations,
including estimates of the surface filling factor derived by comparison
between the model and the observed EMD. We also show that the EMD is
compatible with the model of a corona continuously heated by flares,
which predicts an EMD slope slightly steeper than observed, but that can
be accommodated by observational uncertainties. The coronal composition
is such that the element abundances, relative to solar values, tend to
increase with the first ionization potential, with few exceptions. The
line-to-continuum ratios suggest a nearly solar metallicity, a result
difficult to reconcile with previous determinations based on global
fitting of X-ray spectra.

---------------------------------------------------------
Title: The X-ray emission mechanism in the protostellar jet HH 154
Authors: Bonito, R.; Orlando, S.; Peres, G.; Favata, F.; Rosner, R.
2004A&A...424L...1B Altcode:
We study the mechanism causing the X-ray emission recently detected in
protostellar jets, by performing a detailed modeling of the interaction
between a supersonic jet originating from a young stellar object and the
ambient medium, for various values of density contrast, ν, between the
ambient density and the jet, and of Mach number, M; radiative losses
and thermal conduction have been taken into account. Here we report a
representative case which reproduces, without any ad hoc assumption,
the characteristics of the X-ray emission recently observed in the
protostellar jet HH 154. We find that the X-ray emission originates
from a localized blob, consistent with observations, which moves with
velocity v ∼ 500 km s-1; we therefore predict the X-ray
source to have a detectable proper motion.

---------------------------------------------------------
Title: Are Coronae of Late-Type Stars Made of Solar-like
Structures? The X-Ray Surface Flux versus Hardness Ratio Diagram
and the Pressure-Temperature Correlation
Authors: Peres, G.; Orlando, S.; Reale, F.
2004ApJ...612..472P Altcode: 2004astro.ph..5281P
This work is dedicated to the solar-stellar connection, i.e., the close
similarity of the Sun and late-type stars; in particular, this work
shows that stellar coronae can be composed of X-ray-emitting structures
similar to those present in the solar corona. To this end we use a large
set of ROSAT PSPC observations of late-type stars of all spectral types
and activity levels and a large set of solar X-ray data collected with
Yohkoh SXT. Solar data have been analyzed and formatted to study the
Sun as an X-ray star; they include observations of the solar corona
at various phases of the solar cycle and data on various kinds of
X-ray coronal structures, from flares to the background corona, i.e.,
the most quiet regions. We use the X-ray surface flux (FX)
versus spectral hardness ratio (HR) diagram as a fundamental tool for
our study. We find that FX is strongly correlated to HR in
stellar coronae, in the solar corona at all phases of the solar cycle,
and in the individual solar coronal structures; all the above follow
the same law. Schmitt found the same correlation in stellar coronae. We
therefore claim that coronae of late-type stars are formed with X-ray
structures very similar to those in the Sun, since their behavior is
identical to that of the solar coronal structures and of the whole
solar corona. The spatial location of the X-ray structures on the
star, however, could be very different from those on the Sun. In
this scenario, the fraction of the stellar surface covered with
active regions and their bright cores increases with activity; the
most active stars are brighter and hotter than if they were entirely
covered with active regions, so they can be explained only with the
additional presence of several flares (or flarelike structures) at
any time. On the basis of the FX versus HR correlation,
corresponding to FX~T6, we then derive a set of
new laws relating the temperature, pressure, volumetric heating, and
characteristic loop length of the coronal plasma on all the late-type
stars. In addition, individual solar coronal structures and the whole
solar corona follow the same laws. These laws also agree with recent
findings of higher plasma density at higher temperatures in stellar
coronae. We claim that the strong correlation between surface flux and
temperature and the laws mentioned above are just the effect of more
fundamental physical mechanisms driving the coronal structures of all
the late-type stars from the emergence of new magnetic structures to
their dispersal and dissipation.

---------------------------------------------------------
Title: High-Resolution X-Ray Spectroscopy of the Post-T Tauri Star
PZ Telescopii
Authors: Argiroffi, C.; Drake, J. J.; Maggio, A.; Peres, G.; Sciortino,
S.; Harnden, F. R.
2004ApJ...609..925A Altcode: 2004astro.ph..3017A
We present an analysis of the Chandra High Energy Transmission
Grating Spectrometer observation of the rapidly rotating
(Prot=0.94days) post-T Tauri (~20 Myr old) star PZ
Telescopii, in the Tucana association. Using two different methods,
we have derived the coronal emission measure distribution EM(T)
and chemical abundances. The EM(T) peaks at logT=6.9 and exhibits a
significant emission measure at temperatures logT&gt;7. The coronal
abundances are generally ~0.5 times the solar photospheric values,
which are presumed fairly representative of the composition of the
underlying star. A minimum in abundance is seen at a first ionization
potential (FIP) of 7-8 eV, with evidence for higher abundances at
both lower and higher FIP, similar to patterns seen in other active
stars. From an analysis of the He-like triplet of Mg XI, we have
estimated electron densities of ~1012-1013
cm-3. All the coronal properties found for PZ Tel are much
more similar to those of AB Dor, which is slightly older than PZ Tel,
than to those of the younger T Tauri star TW Hya. These results support
earlier conclusions that the soft X-ray emission of TW Hya is likely
dominated by accretion activity rather than by a magnetically heated
corona. Our results also suggest that the coronae of pre-main-sequence
stars rapidly become similar to those of older active main-sequence
stars soon after the accretion stage has ended.

---------------------------------------------------------
Title: Detection of X-Ray Resonance Scattering in Active Stellar
Coronae
Authors: Testa, Paola; Drake, Jeremy J.; Peres, Giovanni; DeLuca,
Edward E.
2004ApJ...609L..79T Altcode: 2004astro.ph..5520T
An analysis of Lyman series lines arising from hydrogen-like oxygen
and neon ions in the coronae of the active RS CVn-type binaries II Peg
and IM Peg, observed using the Chandra High Resolution Transmission
Grating Spectrograph, shows significant decrements in the Lyα/Lyβ
ratios as compared with theoretical predictions and with the same ratios
observed in similar active binaries. We interpret these decrements in
terms of resonance scattering of line photons out of the line of sight;
these observations present the first strong evidence of this effect in
active stellar coronae. The net line photon loss implies a nonuniform
and asymmetric surface distribution of emitting structures on these
stars. Escape probability arguments, together with the observed line
ratios and estimates of the emitting plasma density, imply typical
line-of-sight sizes of the coronal structures that dominate the X-ray
emission of 1010 cm at temperatures of 3×106 K
and of 108 cm at 107 K. These sizes are an order
of magnitude larger than predicted by simple quasi-static coronal loops
models but are still very small compared to the several 1011
cm radii of the underlying stars.

---------------------------------------------------------
Title: Crushing of Interstellar Gas Clouds in Supernova Remnants:
the Role of Thermal Conduction and Radiative Losses
Authors: Peres, G.; Orlando, S.; Reale, F.; Rosner, R.; Plewa, T.;
Siegel, A.
2004AIPC..703..326P Altcode:
We model hydrodynamic interactions of an old supernova remnant shock
wave with a small interstellar gas cloud, taking into account the
effects of thermal conduction and radiative losses. In particular,
we consider a representative case of a Mach 30 shock impacting on an
isolated cloud with density contrast χ = 10 with respect to the ambient
medium. Thermal conduction appears to be effective in suppressing the
Kelvin-Helmholtz and Rayleigh-Taylor instabilities which would develop
at the cloud boundaries. We demonstrate that the radiative losses
play a crucial role in the dynamics of the shock-cloud interaction,
dominating evolution of the shocked cloud medium.

---------------------------------------------------------
Title: Modeling an X-ray flare on Proxima Centauri: Evidence of
two flaring loop components and of two heating mechanisms at work
Authors: Reale, F.; Güdel, M.; Peres, G.; Audard, M.
2004A&A...416..733R Altcode: 2003astro.ph.12267R
We model in detail a flare observed on Proxima Centauri with the
EPIC-PN on board XMM-Newton at high statistics and high time resolution
and coverage. Time-dependent hydrodynamic loop modeling is used to
describe the rise and peak of the light curve, and a large fraction of
the decay, including its change of slope and a secondary maximum, over
more than 2 h. The light curve, the emission measure and the temperature
derived from the data allow us to constrain the loop morphology and the
heating function and to show that this flare can be described with two
components: a major one triggered by an intense heat pulse injected in
a single flaring loop with half-length ≈1.0 × 1010 cm,
the other one by less intense heat pulses released about 1/2 h after
the first one in related loop systems, probably arcades, with the same
half-length. The heat functions of the two loop systems appear very
similar: an intense pulse located at the loop footpoints followed by
a low gradual decay distributed in the coronal part of the loop. The
latter result and the similarity to at least one solar event (the
Bastille Day flare in 2000) indicate that this pattern may be common
to solar and stellar flares. Based on observations obtained with
XMM-Newton, an ESA science mission with instruments and contributions
directly funded by ESA Member states and the USA (NASA).

---------------------------------------------------------
Title: Long-Term X-Ray Spectral Variability of the Nucleus of M81
Authors: La Parola, V.; Fabbiano, G.; Elvis, M.; Nicastro, F.; Kim,
D. W.; Peres, G.
2004ApJ...601..831L Altcode: 2003astro.ph.10461L
We have analyzed the soft X-ray emission from the nuclear source of
the nearby spiral galaxy M81, using the available data collected with
ROSAT, ASCA, BeppoSAX, and Chandra. The source flux is highly variable
(sometimes dramatic: a factor of 4 in 20 days), showing variability at
different timescales, from 2 days to 4 yr, and in particular a steady
increase of the flux by a factor of &gt;~2 over 4 yr, broken by rapid
flares. After accounting for the extended component resolved by Chandra,
the nuclear soft X-ray spectrum (from ROSAT/PSPC, BeppoSAX/LECS, and
Chandra data) cannot be fitted well with a single absorbed power-law
model. Acceptable fits are obtained by adding an extra component,
either a multicolor blackbody (MCBB) or an absorption feature. In
the MCBB case, the inner accretion disk would be far smaller than the
Schwartzchild radius for the 3-60×106 Msolar
nucleus, requiring a strictly edge-on inclination of the disk, even if
the nucleus is a rotating Kerr black hole. The temperature is 0.27 keV,
larger than expected from the accretion disk of a Schwartzchild black
hole but consistent with that expected from a Kerr black hole. In the
power law+absorption feature model, we have either high-velocity (0.3c)
infalling C V clouds or neutral C I absorption at rest. In both cases
the C:O overabundance is a factor of 10.

---------------------------------------------------------
Title: Using AMR to Simulate the 3-D Hydrodynamic Interaction of
Supernova Shocks with Interstellar Gas Clouds
Authors: Orlando, S.; Peres, G.; Reale, F.; Plewa, T.; Rosner, R.;
Siegel, A.
2004MSAIS...4...82O Altcode:
We study the 3-D hydrodynamic interaction of supernova shock fronts
with interstellar clouds to investigate the evolution, the morphology
and the deviations from equilibrium of ionization. To this end, we
use the FLASH code including PARAMESH, an advanced and versatile
parallel adaptive mesh refinement package. We present here the
preliminary results obtained modeling a representative case of a Mach
50 shock impacting on an isolated cloud with density contrast chi =
10 with respect to the ambient medium. The preliminary analysis of the
non-equilibrium ionization (NEI) effects on the ionization stages of
oxygen, and iron is discussed.

---------------------------------------------------------
Title: X-ray spectroscopy of the Hertzsprung-gap giant star 31 Com,
observed with XMM-Newton
Authors: Scelsi, L.; Maggio, A.; Peres, G.; Gondoin, Ph.
2004A&A...413..643S Altcode:
We have analysed the XMM-Newton X-ray spectra of the yellow giant 31 Com
with the aim of deriving information on the coronal structures of this
archetypical Hertzsprung-gap star. To determine the emission measure
distribution vs. temperature, EM(T), and the elemental abundances
of the coronal plasma, with an accurate line-based approach, we have
developed a new method for simple and accurate line measurements, based
on rebinning and co-adding the two RGS spectra. We have reconstructed
the EM(T) independently with both APED and CHIANTI atomic databases in
order to investigate possible differences in the final outcome of the
analysis, and we have obtained consistent results. The derived emission
measure distribution has a well defined peak at T ∼ 107
K and a significant amount of plasma at higher temperatures; there is
also evidence for plasma at temperatures below ∼106.5 K,
with a mean electron density of ∼3 × 1010 cm-3,
as inferred from the line ratio of the O Vii triplet. We have made a
global fitting of the EPIC spectra, using multi-component isothermal
(3-T) model, and then compared the results with the EM(T), looking for
a consistent multi-temperature description of both the RGS and EPIC
spectra, over the whole spectral range. While the EM(T) and the 3-T
models individually provide a good description of the data set on which
they are based, none of them describes adequately the data of all the
other instruments; the disagreements may be related, at least in part,
to cross-calibration problems. Finally, we have used the EM(T) to derive
information about the properties of the coronal structures. Our results
indicate that the corona of 31 Com is dominated by a class of magnetic
loops with peak temperature ∼107 K and apparently more
isothermal than the solar ones. Table 3 and Appendix A are only
available in electronic form at http://www.edpsciences.org

---------------------------------------------------------
Title: Study of the young open cluster IC2391: discovery of X-ray
rotational modulation in a supersaturated star
Authors: Micela, G.; Marino, A.; Peres, G.; Pillitteri, I.;
Sciortino, S.
2004MmSAI..75..442M Altcode:
We present the analysis of the Guarantee Time XMM-Newton/EPIC
observation pointed on the young open cluster IC 2391. We find evidence,
for the first time, of X-ray rotational modulation on a supersaturated
star, member of the cluster, implying the presence of structural
inhomogeneities. We also present preliminary results of the spectral
analysis of the X-ray brightest cluster stars.

---------------------------------------------------------
Title: SADE: The starspot and dynamo explorer
Authors: Martens, P. C. H.; Acton, L. W. A.; Klumpar, D.; Kankelborg,
C.; Stern, R. A.; Peres, G.; Culhane, J. L.
2003AdSpR..32.1123M Altcode:
We propose a mission called SADE, the Starspot And Dynamo Explorer, to
study dynamo activity in nearby late-type stars. The onboard instruments
will be a Ca-K telescope for magnetically dominated chromospheric
emission, and an X-ray grazing incidence telescope to study coronal
emission. We design the mission for a life-time of 15 years or longer
to capture a full activity cycle for most solar-type stars. We aim to
firmly establish the spectrum of the relation between chromospheric
and corona' emission in late-type stars, and capture one or more stars
going into or coming out of a Maunder type minimum. Operation costs will
be kept to a minimum by automating mission operations to a maximum,
and have the science operations be carried out by students at Montana
State University.

---------------------------------------------------------
Title: Viewing the sun as an X-ray star
Authors: Orlando, S.; Peres, G.; Reale, F.
2003AdSpR..32..955O Altcode:
The Sun is the late-type star we can study with the highest level of
detail. In the interpretation of stellar data, therefore, it is often
assumed that the physical processes of the coronae of late-type stars
are similar to those of the solar corona, i.e. the "solar-stellar
analogy". In order to investigate the validity of this assumption,
we have started a program to study systematically the Sun as an X-ray
star. Our program aims to explore how far the solar model can be
applied to other stars. In this paper we review the results obtained
from these studies and, in particular, we discuss the variability
of a star identical to the Sun during its cycle, the contribution
of different coronal regions to the whole corona X-ray spectrum, and
the role of flares. We find that the non-flaring Sun during its cycle
fits well in the stellar scenario as a low-activity star. Our results
suggest that the coronae of stars from low to intermediate activity may
be explained as the effect of structures similar to those present on the
Sun, changing the relative weight of the different kinds of non-flaring
coronal regions; the coronae of very active stars may be explained
if a continuous flaring activity characterizes these stars. Possible
implications in stellar data interpretation are discussed.

---------------------------------------------------------
Title: X-ray variability of Pleiades late-type stars as observed
with the ROSAT-PSPC
Authors: Marino, A.; Micela, G.; Peres, G.; Sciortino, S.
2003A&A...406..629M Altcode:
We present a comprehensive analysis of X-ray variability of
the late-type (dF7-dM) Pleiades stars, detected in all ROSAT-PSPC
observations; X-ray variations on short (hours) and medium (months) time
scales have been explored. We have grouped the stars in two samples:
89 observations of 42 distinct dF7-dK2 stars and 108 observations
of 61 dK3-dM stars. The Kolmogorov-Smirnov test applied on all X-ray
photon time series show that the percentage of cases of significant
variability is quite similar on both samples, suggesting that the
presence of variability does not depend on mass for the time scales and
mass range explored. The comparison between the Time X-ray Amplitude
Distribution functions (XAD) of the set of dF7-dK2 and of the dK3-dM
show that, on short time scales, dK3-dM stars show larger variations
than dF7-dK2. A subsample of eleven dF7-dK2 and eleven dK3-dM Pleiades
stars allows the study of variability on longer time scales: we found
that variability on medium - long time scales is relatively more common
among dF7-dK2 stars than among dK3-dM ones. For both dF7-dK2 Pleiades
stars and dF7-dK2 field stars, the variability on short time scales
depends on Lx while this dependence has not been observed
among dK3-dM stars. It may be that the variability among dK3-dM stars
is dominated by flares that have a similar luminosity distribution
for stars of different Lx, while flaring distribution
in dF7-dK2 stars may depend on X-ray luminosity. The lowest mass
stars show significant rapid variability (flares?) and no evidence
of rotation modulation or cycles. On the contrary, dF7-dK2 Pleiades
stars show both rapid variability and variations on longer time scales,
likely associated with rotational modulation or cycles.

---------------------------------------------------------
Title: X-ray rotational modulation of a supersaturated star in IC 2391
Authors: Marino, A.; Micela, G.; Peres, G.; Sciortino, S.
2003A&A...407L..63M Altcode: 2003astro.ph..7170M
We present evidence of X-ray rotational modulation on VXR45, a young
fast rotator star, member of IC 2391. It is a dG9 spectral type star
whose rotational period and X-ray luminosity make it a supersaturated
star. Our X-ray observation, made with EPIC/PN on XMM-Newton,
covers about two photometric rotational periods. The detection
of X-ray rotational modulation implies the presence of structural
inhomogeneities. Possible interpretations are presented and discussed.

---------------------------------------------------------
Title: CDS/SoHO multi-line observation of a solar active region:
Detection of a hot stable loop and of a cool dynamic loop
Authors: Di Giorgio, S.; Reale, F.; Peres, G.
2003A&A...406..323D Altcode:
We analyze a space-, time- and spectral-resolved SoHO/CDS observation of
the evolution of an active region over a time lapse of approximately
three hours in various spectral lines emitted in the interval of
temperature 1.3 x 104 &lt; T &lt; 2.5 x 106
K. We identify and characterize two structures of interest: a longer
coronal loop (~ 5.5*E9 cm), relatively steady and well
visible in lines forming at coronal temperatures (e.g. Fe XIV 334.17
Å, Fe XVI 360.76 Å) and a smaller one (~ 1.8 x 109 cm),
transient and visible only in cooler lines (O IV 554.51 Å, O V 629.73
Å). In the hot lines, the longer loop has a bright apex and an emission
distribution of constant shape, but of moderately variable absolute
intensity; the region around the loop apex shows a distinct brightening
practically in all lines. In the hot lines, the brightening appears
as a minor perturbation over a steadily high emission level. In the
same region the emission measure vs temperature of the hottest lines
indicates a temperature of ~ 2 MK, lower than the temperature obtained
from Yohkoh data taken just before the CDS observation. Comparison
with steady-state loop scaling laws and with plasma time scales,
and connection to cooling or heating episodes are discussed. As for
the cool loop, its whole evolution, from ignition to disappearance,
is directly observed, confirming the highly transient nature of such
structures. The O V line is blue-shifted at one footpoint, indicating
an upflow associated with the loop ignition.

---------------------------------------------------------
Title: X-Ray Rotational Modulation in VXR45
Authors: Marino, A.; Micela, G.; Peres, G.; Sciortino, S.
2003IBVS.5427....1M Altcode:
We present evidence of X-ray rotational modulation on VXR45, a young
very fast rotator star member of IC2391 open cluster, observed with
XMM-Newton observatory.

---------------------------------------------------------
Title: On coronal structures and their variability in active stars:
The case of Capella observed with Chandra/LETGS
Authors: Argiroffi, C.; Maggio, A.; Peres, G.
2003A&A...404.1033A Altcode:
In this paper we present a detailed analysis of two X-ray spectra of
Capella, taken eleven months apart with the Low Energy Transmission
Grating Spectrometer (LETGS) of the Chandra Observatory. We have studied
variability of the coronal emission over different time scales, both
in the whole X-ray band and in narrow temperature ranges identified
by lines. The comparison of the two observations shows that the
whole coronal emission of Capella in March 2000 was 3% higher than in
February 2001; there also appears to be a tendency, albeit a marginal
one, for the hottest lines to show the largest changes between the
two observations. A detailed search for short-term variability (on
time scales ranging from 102 to 104 s) in the
emission of individual lines shows that in all cases the emission is
compatible with a constant source; the firm upper limits of 5%-10% to
the source variability on short time scales suggests that the intense
X-ray emission is due to stable coronal structures and not to flaring
activity. We have also determined the coronal thermal structure,
as described with the emission measure distribution vs. temperature
and with the help of plasma density, derived from the analysis
of the O Vii, Ne Ix, Mg Xi and Si Xiii He-like ion triplets. The
emission measure distribution, em(T), and the element abundances,
have been reconstructed with the Markov-Chain Monte Carlo method by
\citeauthor{KashyapDrake1998}; the em(T) presents a previously known
sharp peak around log T=6.8-6.9, but we have also found evidence of a
small amount of plasma at T&gt;107 K. With the help of the
em(T) and the density values we have estimated the pressure and volume
of the emitting plasma at different temperatures, and we have derived
information about the structure of individual loops and about the
population of loops having different maximum temperatures. Our results
indicate that loops with higher maximum temperature have higher pressure
and smaller volume than lower temperature loops. Appendices A and
B are only available in electronic form at http://www.edpsciences.org

---------------------------------------------------------
Title: EBIT diagnostics using X-ray spectra of highly ionized Ne
Authors: Matranga, Marco; Barbera, Marco; Maggio, Antonio; Peres,
Giovanni; Serio, Salvatore; Takács, Endre; Silver, Eric; Gillaspy,
John; Schnopper, Herbert; Laming, Martin; Beeman, Jeff; Haller, Eugen;
Madden, Norman
2003NIMPB.205..244M Altcode:
We have carried out a detailed analysis of highly ionized neon
spectra collected at the NIST EBIT using an NTD germanium X-ray
microcalorimeter developed at the Harvard-Smithsonian Center for
Astrophysics [Nucl. Instr. and Meth. A 444 (2000) 156]. Our attention
was focused especially on the Ne IX He-like triplet to check electron
density diagnostics through the intercombination/forbidden line
ratio. We have investigated possible effects of the ion dynamics on
the plasma emission line intensities, looking at the dependence of
the count-rate and the charge state distribution on the electron beam
energy and current. The temperature and spatial distribution of the
neon ions, and hence the overlap between the electron beam and the ion
cloud, depend on the electron beam operating parameters. The overlap
affects the average electron density seen by the ions, and in turn
the measured line ratio. These results underscore the value of future
improved studies of the trapped ion dynamics, both for understanding the
EBIT performance and for allowing experimenters to take full advantage
of its potential for astrophysical plasma diagnostics.

---------------------------------------------------------
Title: Hydrodynamic Model of Loops Heated by Microflares at the
Footpoints
Authors: Testa, P.; Peres, G.
2003SPD....34.1702T Altcode: 2003BAAS...35..837T
We simulate loops maintained in coronal conditions by random heat pulses
concentrated close to the footpoints, by using the Palermo-Harvard
1-D hydrodynamic code. We have explored the existence of dynamic
but quasi-static solutions when the heating is very concentrated at
footpoints whereas the heating concentrated at footpoints does not allow
static loop models solutions. We studied the characteristics of the
solutions (in terms of stability, density and temperature structure,
emission measure distribution) as a function of the parameters that
define the heating function, for three different loop lengths. We
found stable solutions even for heating concentrated over spatial
regions of the order of L/5 and higher values. For these stable
solutions, the average temperature profiles as a function of the loop
coordinate show a flatter or even inverted profile (for the case with
more concentrated heating) with respect to the standard static models;
the emission measure distribution as a function of temperature is
much steeper (up to ∼ T5) than the usual behavior as
T3/2 of the hydrostatic standard models.

---------------------------------------------------------
Title: ROSAT HRI Investigation of the NGC 507 X-Ray Halo
Authors: Paolillo, M.; Fabbiano, G.; Peres, G.; Kim, D. -W.
2003ApJ...586..850P Altcode: 2002astro.ph.11438P
We present an X-ray investigation of the elliptical galaxy NGC
507 in the Pisces cluster. We make use of archival ROSAT HRI and
Chandra data, and of previously published PSPC data, to connect
the large-scale structure of the halo to the core morphology. Our
analysis, based on a bidimensional double beta model of the halo
surface brightness, shows that the halo core (r&lt;2-3re)
and the external halo (r&gt;3re) are characterized by
different dynamical properties and suggests a different origin of the
two components. The halo core has a complex morphology with a main
X-ray emission peak, coincident with the center of the optical galaxy,
and several secondary peaks. The spatial and spectral analysis of the
central peak shows that this feature is produced by denser hot gas
in the galaxy core. While both homogeneous and inhomogeneous cooling
flow models predict a deposited mass exceeding the observed amount,
our data support the scenario in which the gas is kinetically heated
by stellar mass losses. Comparison with previously published studies
suggests that the core of an X-ray extended galaxy is associated with
the stellar distribution and has properties similar to the X-ray halos
of compact galaxies. The secondary peaks are due instead to interactions
between the radio-emitting plasma and the surrounding ISM, producing
density fluctuations in the hot gas. We find that the energy input by
the central radio source in the ISM is large enough to prevent gas
cooling and may explain the failure of the cooling flow models. The
total mass profile derived from the bidimensional model shows that
a significant amount of dark matter is present at large radii. The
dark halo extends on cluster scales and is likely associated with the
whole cluster rather than with NGC 507. This structure is typical of
many X-ray-bright early-type galaxies and may explain the spatial and
spectral differences with X-ray compact galaxies largely debated in the
literature. The large-scale surface brightness distribution is irregular
and more extended in the northeast direction. The displacement of the
cluster halo from the optical galaxy and the filamentary structures
observed in the halo core further suggest that the galaxy may be
slowly moving within the group potential. Finally, we found that ~20%
of the sources detected by Kim &amp; Fabbiano in the NGC 507 halo are
due to point sources, while the nature of the remaining population is
not clear. If associated with NGC 507, they could be either accreting
binaries hosting a massive black hole or density clumps of the X-ray
halo.

---------------------------------------------------------
Title: Monitoring the stability of thin and medium back-up filters
of the Newton-XMM EPIC camera
Authors: Barbera, Marco; Collura, Alfonso; Artale, Marinella A.;
Varisco, Salvatore; Peres, Giovanni; Sciortino, Salvatore; Serio,
Salvatore; Villa, Gabriele E.
2003SPIE.4851..264B Altcode:
We are conducting a measurement program on back-up filters of the
XMM-Newton EPIC camera aimed at monitoring possible aging effects
during the mission lifetime. One thin and one medium EPIC back-up
filters have been stored since 1997 in an environment similar to that
one of the flight filters (dry nitrogen box before launch, high vacuum
after launch). The transmission of the two filters has been measured
periodically in the 1900-10000 angstrom wavelength range where effects
of aging would be clearly evident. The preliminary results, after
5 years of monitoring, show that a slight aging effect has occurred
on both filters which, however, has no significant impact onto the
EPIC calibration for the correct analysis of the X-ray astrophysical
observations.

---------------------------------------------------------
Title: Spectral Variability of the Nucleus of M33 in a Chandra/ACIS
Observation
Authors: La Parola, V.; Damiani, F.; Fabbiano, G.; Peres, G.
2003ApJ...583..758L Altcode: 2002astro.ph.10174L
We have analyzed a 90 ks long observation of the bright nucleus of
M33 made with Chandra/ACIS. We detected low-amplitude (~10%) highly
significant variability on timescales of ~5000 s. We also found
associated spectral variability. The two main spectral components
[a power law with Γ~2 and a multicolor disk (MCD) with kT~0.9-1.2
keV] vary in relative flux. The MCD temperature also increases with
increasing MCD flux. The pattern of variability is reminiscent of
(but not identical to) Galactic black hole binaries. An &gt;~5
Msolar accreting black hole may explain this source.

---------------------------------------------------------
Title: Physics of the outer atmosphere of the Sun and of stars along
their evolutionary track
Authors: Peres, G.; Randich, S.
2003MmSAI..74..403P Altcode:
We discuss the activity of a Co-financed project dedicated to study
stellar activity and, more in general, stellar properties, through
observations in X-ray, UV, optical etc. bands and through modeling. A
key aspect of the project is the study of activity of stars in various
phases of their evolution so as to ascertain the role of evolution in
determining activity and the role of activity in influencing evolution,
e.g. through the significant loss of angular momentum. An even more
detailed study, thanks to its proximity, of the Sun is a fundamental
part of this project; the relevant information is indeed both important
for solar physics `per se' and to help defining with a high level
of detail, and therefore of insight, the physical conditions in the
solar atmosphere which thus is an important reference for the studies
of stellar activity.

---------------------------------------------------------
Title: Development and Application of Numerical Modules for FLASH
in Palermo: Two Astrophysical Examples
Authors: Orlando, S.; Peres, G.; Reale, F.; Rosner, R.; Plewa, T.;
Siegel, A.
2003MSAIS...1...45O Altcode:
We collaborate with the Flash Center at the University of Chicago to
help upgrading and to apply extensively the FLASH code to astrophysical
problems. In particular, we have developed new modules for FLASH
which extend the field of applicability of the code to some areas
in astrophysics, like solar and stellar coronae, and supernova
remnants. The new modules so far developed and tested describe:
the non-equilibrium ionization effects of the most abundant elements
in astrophysical plasmas, the thermal conduction and the viscosity
according to the formulation of Spitzer (1962), and the radiative losses
from an optically thin plasma according to the Raymond spectral code,
and to Peres et al. (1982) for the chromosphere. We show some selected
results for a coronal flare and for a supernova remnant, obtained with
the version of FLASH 2.0 code including the new modules.

---------------------------------------------------------
Title: Non-Equilibrium Ionization Effects Induced During Coronal
Flares
Authors: Orlando, S.; Peres, G.; Reale, F.; Rosner, R.; Siegel, A.
2003MmSAI..74..643O Altcode:
We present preliminary results of hydrodynamic modeling of flares
occurring in plasma confined in coronal loops. Our analysis focuses on
the deviations from ionization equilibrium on the population fractions
of the most abundant elements in astrophysical plasmas, and on the
possible implications for plasma diagnostics.

---------------------------------------------------------
Title: HPC in Astronomy: overview and perspectives
Authors: Peres, G.
2003MSAIS...1..223P Altcode:
I present a personal overview of italian high performance computing
in astronomy, as given with the presentation of this meeting, then
suggest ways for a more fruitful interaction with the rest of Astronomy;
finally and more important, I suggest a qualitative improvement of high
performance computing in astronomy through the support of challenging
and competitive key projects.

---------------------------------------------------------
Title: Experimental activity in Palermo related to Solar-B and
CALOS satellites
Authors: Peres, G.; Barbera, M.; Orlando, S.; Ciaravella, A.; Reale,
F.; Serio, S.
2003MmSAI..74..831P Altcode:
We report on the experimental activities related to the calibration
of the XRT telescope on board the Solar-B satellite and on the design
and studies related to the X-ray-calorimeters-based satellite CALOS
(Calorimetri per Osservazioni Solari).

---------------------------------------------------------
Title: X-ray variability in ROSAT-PSPC observations of dF7-dK2 stars
Authors: Marino, A.; Peres, G.; Micela, G.; Sciortino, S.
2002ASPC..277..539M Altcode: 2002sccx.conf..539M
No abstract at ADS

---------------------------------------------------------
Title: AD Leo observed with Chandra LETG: emission measure
distribution density, and element abundances of the coronal plasma
Authors: Maggio, A.; Drake, J. J.; Kashyap, V.; Micela, G.; Sciortino,
S.; Peres, G.; Harnden, F. R., Jr.; Murray, S. S.
2002ASPC..277...57M Altcode: 2002sccx.conf...57M
No abstract at ADS

---------------------------------------------------------
Title: XMM-Newton Observations of X-ray Luminous G-type Stellar
Coronae
Authors: Scelsi, L.; Maggio, A.; Peres, G.; Sanz-Forcada, J.; Gondoin,
Ph.; Pallavicini, R.
2002hrxs.confE..22S Altcode:
We present results obtained from the analysis of XMM-Newton spectra
of the Hertzsprung-gap giant 31 Com and of the pre-main-sequence star
HD 283572, two G-type stars in very different evolutionary phases,
but having similar high X-ray luminosities. We discuss methodological
issues related to the data analysis, as well as the results on the
inferred properties of the stellar coronae.

---------------------------------------------------------
Title: Temperature and Density Structure of Hot and Cool Loops
Derived from the Analysis of TRACE Data
Authors: Testa, P.; Peres, G.; Reale, F.; Orlando, S.
2002ApJ...580.1159T Altcode:
We study the transversal structure (in particular the filamentation)
and the longitudinal plasma stratification in two sets of solar
coronal loops observed with TRACE in the 171 and 195 Å passbands. The
density stratification and the thermal structuring of the plasma along
the fibrils that form the loops are derived using two techniques
simultaneously: (1) a filter ratio diagnostic (195/171 Å) and
(2) modeling intensity profiles along the fibrils with hydrostatic
models; in both techniques we remove the background flux. We find
consistent values of temperature and density with both methods in the
coronal structures selected. We find evidence of a very warm fibril
(T~5×106 K) and of rather cold ones (T~2×105
K). The hot fibril appears to have a thermal structure with a maximum
at the apex compatible with temperature and density stratification,
dictated by energy balance, typical of a nonisothermal hydrostatic
loop. The cold fibrils appear to be isothermal and are probably in
dynamical conditions.

---------------------------------------------------------
Title: Observing the Sun as an X-ray Star: Active Region Evolution
and Rotational Modulation
Authors: Orlando, S.; Peres, G.; Reale, F.
2002ASPC..277..341O Altcode: 2002sccx.conf..341O
No abstract at ADS

---------------------------------------------------------
Title: Temperature and density structure of hot and cool loops
derived from the analysis of TRACE data
Authors: Testa, P.; Peres, G.; Reale, F.; Orlando, S.
2002ESASP.505..203T Altcode: 2002solm.conf..203T; 2002IAUCo.188..203T
We address the plasma structuring both across and along the magnetic
field in two sets of solar coronal loops, observed with TRACE in the 171
Å and 195 Å passbands. We derive, after proper background removal,
the density stratification and the thermal structure of the plasma in
the fibrils forming the loops with two techniques: a) filter ratio
diagnostic (195 Å/171 Å) and b) modeling intensity profiles along
the fibrils with hydrostatic models. We find evidence of a hot fibril
(T ~ 5×106K), with temperature and density stratification
well-described with a typical non-isothermal hydrostatic loop model,
and evidence of rather cold fibrils (T ~ 2×105K), isothermal
and probably in dynamic conditions.

---------------------------------------------------------
Title: Non-equilibrium ionization effects during flares in coronal
loops
Authors: Orlando, Salvatore; Peres, Giovanni; Reale, Fabio; Rosner,
Robert
2002ESASP.505..517O Altcode: 2002solm.conf..517O; 2002IAUCo.188..517O
We present preliminary results of hydrodynamic modeling of flares
occurring in plasma magnetically confined in coronal loops, using the
adaptive-mesh astrophysical hydrodynamic code FLASH. We also investigate
the deviations from ionization equilibrium on the population fractions
of the most abundant elements in astrophysical plasmas and the relevant
consequences on observed spectra.

---------------------------------------------------------
Title: Modeling non-confined coronal flares: Dynamics and X-ray
diagnostics
Authors: Reale, F.; Bocchino, F.; Peres, G.
2002A&A...383..952R Altcode: 2001astro.ph.12333R
Long-lasting, intense, stellar X-ray flares may approach conditions
of breaking magnetic confinement and evolving in open space. In the
perspective of searching for possible tracers of non-confinement,
we explore this hypothesis with hydrodynamic simulations of flares
occurring in a non-confined corona: model flares are triggered by a
transient impulsive heating injected in a plane-parallel stratified
corona. The plasma evolution is described by means of a numerical 2-D
model in cylindrical geometry R,Z. We explore the space of fundamental
parameters. As a reference model, we consider a flare triggered by a
heating pulse of 10 erg cm-3 s-1 lasting 150 s
and released in a region ~ 109 cm wide and at a height ~ 2
x 109 cm from the base of the stellar surface. The pressure
at the base of the corona of the unperturbed atmosphere is 0.1 dyne
cm-2. The heating would cause a 20 MK flare if delivered
in a 40 000 km long closed loop. The modeled plasma evolution in the
heating phase involves the propagation of a 10 MK conduction front
and the evaporation of a shocked bow density front upwards from the
chromosphere. As the heating is switched off, the temperature drops
in few seconds while the density front still propagates, expanding,
and gradually weakening. This kind of evolution is shared by other
simulations with different coronal initial pressure, and location,
duration and intensity of the heating. The X-ray emission, spectra and
light curves at the ASCA/SIS focal plan, and in two intense X-ray lines
(Mg XI at 9.169 Åand Fe XXI at 128.752 Å), have been synthesized
from the models. The results are discussed and compared to features
of confined events, and scaling laws are derived. The light curves
invariably show a very rapid rise, a constant phase as long as the
constant heating is on, and then a very fast decay, on time scales of
few seconds, followed by a more gradual one (few minutes). We show
that this evolution of the emission, and especially the fast decay,
together with other potentially observable effects, are intrinsic to
the assumption of non-confinement. Their lack indicates that observed
long-lasting stellar X-ray flares should involve plasma strongly
confined by magnetic fields.

---------------------------------------------------------
Title: CALOS: an experiment to study the solar corona with an array
of NTD Ge microcalorimeters
Authors: Barbera, M.; Peres, G.; Orlando, S.; Reale, F.; Collura,
A.; Serio, S.; Silver, E.; Bandler, S.; Schnopper, H. W.; Costa, E.;
Bellazzini, R.
2002AIPC..605..547B Altcode:
In response to the Italian Space Agency announcement “New Ideas for
Space Missions,” we have proposed an observatory “CALorimetri per
Osservazioni Solari” (CALOS) that will perform spatially resolved
(Δθ~2') X-ray spectroscopy of the solar corona over the
0.1-10 keV band using an array of NTD germanium microcalorimeters. The
observatory will also include an X-ray polarimeter of radically
new design that will study the hard X-ray solar emission and its
polarization and will serve as a flare alarm. .

---------------------------------------------------------
Title: Deep ROSAT HRI Observations of the NGC 1399/NGC 1404 Region:
Morphology and Structure of the X-Ray Halo
Authors: Paolillo, M.; Fabbiano, G.; Peres, G.; Kim, D. -W.
2002ApJ...565..883P Altcode: 2001astro.ph..9342P
We present the analysis of a deep (167 ks) ROSAT HRI observation
of the cD galaxy NGC 1399 in the Fornax Cluster, comparing it with
previous work on this galaxy and with recent Chandra data. We find,
in agreement with previous observations, an extended and asymmetric
gaseous halo with a luminosity (in the 0.1-2.4 keV energy band)
of LX=(5.50+/-0.04)×1041 ergs s-1
within 46 kpc (assuming a distance of D=19 Mpc). Using both HRI and,
at larger radii, archival PSPC data, we find that the radial behavior
of the X-ray surface brightness profile is not consistent with a
simple β-model and suggests instead three distinct components. We use
a multicomponent bidimensional model to study in detail these three
components, which we identify respectively with the cooling flow region,
the galactic halo, and the cluster halo. From these data we derive a
binding mass distribution in agreement with that suggested by optical
dynamical indicators, with an inner core dominated by luminous matter
and an extended dark halo differently distributed on galactic and
cluster scales. The HRI data and a preliminary analysis of Chandra
public data allow us to detect significant density fluctuations in
the halo. We discuss possible nonequilibrium scenarios to explain the
hot halo structure, including tidal interactions with neighboring
galaxies, ram stripping from the intracluster medium, and merging
events. In the innermost region of NGC 1399, the comparison between
the X-ray and radio emission suggests that the radio-emitting plasma
is displacing and producing shocks in the hot X-ray-emitting gas. We
do not detect the nuclear source in X-rays, and we pose an upper limit
of ~4×1039 ergs s-1 (0.1-2.4 keV) to its X-ray
luminosity. We found that the NGC 1404 halo is well represented by a
single symmetric β-model and follows the stellar light profile within
the inner 8 kpc. The mass distribution is similar to the “central”
component of the NGC 1399 halo. At larger radii, ram pressure stripping
from the intracluster gas produces strong asymmetries in the galactic
halo. Finally, we discuss the properties of the point-source population,
finding evidence of correlation between the source excess and NGC 1399.

---------------------------------------------------------
Title: Deep ROSAT-HRI Observation of the cD Galaxy NGC 1399 in the
Fornax Cluster: Morphology and Dynamical Status of the X-Ray Halo
Authors: Paolillo, M.; Peres, G.; Fabbiano, G.; Kim, D. -W.
2002ASPC..268..425P Altcode: 2002tceg.conf..425P
No abstract at ADS

---------------------------------------------------------
Title: SADE, the Student Astrophysical Dynamo Explorer
Authors: Martens, P.; Acton, L.; Klumpar, D.; Stern, R.; Peres, G.;
Culhane, L.
2002cosp...34E1298M Altcode: 2002cosp.meetE1298M
In soft x-rays the solar coronal radiance varies by a factor of 10-30
over the solar activity cycle. A similar variation in most stars in
the existing x-ray database has not been found (Stern 2001); even
stars which exhibit chromospheric activity cycles show only marginal
evidence for X-ray cycles. This is rather puzzling as the time span
and multiple coverage of the x-ray sky should reveal at least a hint of
such a pronounced cyclical variation. We propose a mission called the
Student Astrophysical Dynamo Explorer to measure the x-ray brightness
of about 75 stars once every 5 days for up to 15 years. Selection of
prime stars takes into account location (avoid eclipse), rotation rate,
Ca-K observations, and magnetic field strength, to focus on the best
candidates for dynamo studies. We baseline a nested 4-5 mirror system
with 200 cm^2 geometric area, with a 1.5 to 2 meter focal length, 15
arcsec on-axis resolution, and Au or Ni coatings. The strawman detector
is a back-illuminated CCD of 512x512 pixels, with pixels that can be
large as a 15 arcseconds. Available exposure time per star per visit
is about an hour and a half. We are exploring the option of adding
a visible light detector for astroseismology. To minimize operations
cost for this long duration mission we envisage tracking and commanding
from a simple ground station at Montana State University, operated by
students under the auspices of MSU's Space Science and Engineering Lab
(SSEL).

---------------------------------------------------------
Title: Spectral and time analysis of the coronal X-ray emission from
Capella observed with Chandra/LETG
Authors: Argiroffi, C.; Maggio, A.; Peres, G.
2002cosp...34E3034A Altcode: 2002cosp.meetE3034A
We present a detailed analysis of two X-ray spectra of Capella,
taken eleven months apart, obtained with the LETGS of the handra
Observatory. We have identified the strongest emission lines, including
several Fe lines with ionization level from FeXVI to FeXXIII. We
have determined the electron density Ne and the relevant temperature
T using the line ratios of the OVII, NeIX, MgXI and SiXIII He-like
triplets, finding Ne values in the range 10^10-10^12 cm-3; from the
Ne and T values we have derived volumes and pressures of the emitting
plasma vs temperatures. Emission measure distribution em(T), with
element abundances, has been reconstructed for each observation with
the Markov-Chain Monte Carlo method proposed by Kashyap &amp;Drake
(1998). The em(T) distribution obtained has a sharp peak around log
T=6.8-6.9 and no significant amount of plasma has been found for log
T&gt;7.2. From the em(T) shape we have derived information about the
population of loops at different temperatures. We have compared the
results obtained from each observation finding that during the first
one the whole coronal emission was 3% stronger; there also appears
to be a tendency, albeit a marginal one, for hotter lines to show the
largest changes between the two observations. We have also performed
a detailed search for short-term variability with the Collura et
al. (1987) method testing photon arrival times of individual lines
(the investigated time scale range is 10^2-10^4 s), thus testing the
variability of coronal plasma in a narrow temperature range. In all
the cases analysed we have found that emission was compatible with
a constant source, therefore we can only pose upper limits of 5%-10%
to the source variability, suggesting that the intense X- ray emission
is due to stable structures like loops and not to flare activity.

---------------------------------------------------------
Title: Viewing the Sun as an X-ray star
Authors: Orlando, S.; Peres, G.; Reale, F.
2002cosp...34E.906O Altcode: 2002cosp.meetE.906O
Thanks to its distance from the Earth, the Sun is the late-type star
we can study with the highest level of detail. It is not surprising,
therefore, that many stellar data are interpreted in the light of
the so-called “solar-stellar analogy", i.e. the assumption that
the coronae of late-type stars are the result of physical processes
similar to those leading to the solar corona. In this paper we review
the results obtained from studying the Sun as an X-ray star in the
attempt to explore how far the solar model can be applied to other
stars, and how the underlying processes may differ. In particular, we
discuss how a star identical to the Sun appears to change during its
cycle, how different coronal regions resolved on the Sun contribute
to the whole corona X-ray spectrum, and how flares may affect the
stellar-like spectrum of the Sun. We discuss also the implications
on the X-ray variability we should observe on a star identical to
the Sun due to different phenomena, e.g. stellar cycle, flares and
rotational modulation.

---------------------------------------------------------
Title: On X-ray variability in ROSAT-PSPC observations of F7-K2 stars
Authors: Marino, A.; Micela, G.; Peres, G.; Sciortino, S.
2002A&A...383..210M Altcode:
We have analyzed the X-ray variability of dF7-dK2 stars in the solar
neighborhood detected with the pointed ROSAT-PSPC observations. Our data
base is the sample of all stars listed in the CNS3 catalog (Gliese &amp;
Jahreibeta 1991) having a B-V color between 0.5 and 0.9; it includes 70
pointed observations of 40 distinct stars or multiple systems. We have
applied the unbinned Kolmogorov-Smirnov test on all X-ray photon time
series of our sample: only 10 observations relative to 8 distinct stars
are variable at a confidence level greater than 99% and 4 of them belong
to multiple systems. For the subsample of 9 stars observed both at the
beginning and at the end of the mission, we can study the variability
on time scale of years and compare amplitude variations at short and
long time scales. Our analysis suggests that, for these stars, the X-ray
variability is more likely on longer time scale. All the stars variable
on long time scale, and not on short time scale, are relatively quiet
and similar to the Sun, suggesting that the variations may be due to
cycles. The comparison of our results with those previously obtained
for dM stars shows that the amplitude of variability of X-ray emission
from dF7-dK2 stars is smaller than that observed in dM stars.

---------------------------------------------------------
Title: The Distribution of the Emission Measure, and of the Heating
Budget, among the Loops in the Corona
Authors: Peres, G.; Orlando, S.; Reale, F.; Rosner, R.
2001ApJ...563.1045P Altcode: 2001astro.ph.11192P
The aim of this paper is to validate a methodology for connecting the
emission measure of individual solar coronal loops to the integrated
emission measure of the entire solar corona and using this connection
to deduce the energetic properties of the corona; we then show how
this methodology can be applied to observations of solar-like stellar
coronae. The solar validation is carried out by using spatially resolved
X-ray observations of the Sun obtained from the Yohkoh satellite. This
work is a further step in our effort to place the “solar-stellar
connection” on a quantitative footing. In particular, we show how this
analysis procedure can be used in the context of archival Einstein,
ROSAT, and EUVE data, as well as for Chandra and XMM-Newton data, as
a complementary analysis tool to existing multithermal component models.

---------------------------------------------------------
Title: Coronal loop hydrodynamics. The solar flare observed on
November 12, 1980 revisited: The UV line emission
Authors: Betta, R. M.; Peres, G.; Reale, F.; Serio, S.
2001A&A...380..341B Altcode: 2001astro.ph.10514B
We revisit a well-studied solar flare whose X-ray emission originating
from a simple loop structure was observed by most of the instruments
on board SMM on November 12, 1980. The X-ray emission of this flare,
as observed with the XRP, was successfully modeled previously. Here
we include a detailed modeling of the transition region and we compare
the hydrodynamic results with the UVSP observations in two EUV lines,
measured in areas smaller than the XRP rasters, covering only some
portions of the flaring loop (the top and the foot-points). The single
loop hydrodynamic model, which fits well the evolution of coronal lines
(those observed with the XRP and the Fe XXI 1354.1 Å line observed
with the UVSP) fails to model the flux level and evolution of the O
V 1371.3 Åline.

---------------------------------------------------------
Title: The Sun as an X-Ray Star. IV. The Contribution of Different
Regions of the Corona to Its X-Ray Spectrum
Authors: Orlando, S.; Peres, G.; Reale, F.
2001ApJ...560..499O Altcode:
We study X-ray-synthesized spectra of solar regions as templates
to interpret analogous stellar spectra. We define three classes of
coronal structures of different brightness, low (background quiet
corona), medium (active regions), and high (active region cores),
and determine their contribution to the solar X-ray emission measure
versus temperature, EM(T), luminosity, and spectrum. This study defines
the extent of the solar analogy quantitatively and accurately. To this
end, we have selected a large sample of full-disk Yohkoh soft X-ray
telescope observations taken between the maximum and the minimum
of solar cycle 22, obtaining the contribution of each class to the
whole Sun's EM(T). From the EM(T) distributions, we synthesize
the X-ray spectra of the Sun and of the single classes of solar
coronal regions as they would be collected with the ROSAT Position
Sensitive Proportional Counter (PSPC) and ASCA Solid-State Imaging
Spectrometer. We find that the Sun during the cycle fits well in the
stellar scenario as a low-activity star. The ROSAT PSPC hardness ratio
(HR) and surface X-ray flux, FPSPC, both increase going from
the background corona to the active regions and the cores of the active
regions, and range between the values of low and intermediate activity
stars. We suggest that the coronae of these stars may be explained
as the effect of structures similar to those present on the Sun and
that the various levels of X-ray luminosity, HR, and FPSPC
are achieved by changing the surface coverage of the different classes
of coronal regions.

---------------------------------------------------------
Title: Contribution of various coronal structures to the emission
measure vs. temperature distribution and X-ray spectrum of the corona
Authors: Orlando, Salvatore; Peres, Giovanni; Reale, Fabio
2001ESASP.493..301O Altcode: 2001sefs.work..301O
No abstract at ADS

---------------------------------------------------------
Title: The OAPA/DPSFA: solar physics, instrumental expertise, and
the XACT facility
Authors: Barbera, M.; Collura, A.; Peres, G.; Reale, F.; Orlando,
S.; Serio, S.
2001ESASP.493..167B Altcode: 2001sefs.work..167B
No abstract at ADS

---------------------------------------------------------
Title: The Sun as an X-Ray Star. III. Flares
Authors: Reale, F.; Peres, G.; Orlando, S.
2001ApJ...557..906R Altcode: 2001astro.ph..4021R
In previous works we have developed a method to convert solar X-ray
data, collected with the Yohkoh/SXT, into templates of stellar coronal
observations. Here we apply the method to several solar flares, for
comparison with stellar X-ray flares. Eight flares, from weak (GOES
class C5.8) to very intense ones (X9) are selected as representative of
the flaring Sun. The emission measure distribution versus temperature,
EM(T), of the flaring regions is derived from Yohkoh/SXT observations
in the rise, peak, and decay of the flares. The EM(T) is rather peaked
and centered around T~107 K for most of the time. Typically,
it grows during the rise phase of the flare, and then it decreases
and shifts toward lower temperatures during the decay, more slowly if
there is sustained heating. The most intense flare we studied shows
emission measure even at very high temperatures (T~108
K). Time-resolved X-ray spectra both unfiltered and filtered through
the instrumental responses of the nonsolar instruments ASCA/SIS and
ROSAT/PSPC are then derived. Synthesized ASCA/SIS and ROSAT/PSPC
spectra are generally well fitted with single thermal components at
temperatures close to that of the EM(T) maximum, albeit two thermal
components are needed to fit some flare decays. ROSAT/PSPC spectra
show that solar flares are in a 2 orders of magnitude flux range
(106-108 ergs cm-2 s-1)
and a narrow PSPC hardness ratio range, however, higher than that of
typical nonflaring solar-like stars.

---------------------------------------------------------
Title: 1997 December 12 Helical Coronal Mass Ejection. II. Density,
Energy Estimates, and Hydrodynamics
Authors: Ciaravella, A.; Raymond, J. C.; Reale, F.; Strachan, L.;
Peres, G.
2001ApJ...557..351C Altcode:
We use Ultraviolet Coronagraph and Spectrometer (UVCS) spectra to
investigate the density range of the plasma ejected during the
coronal mass ejection (CME) on 1997 December 12. Time-dependent
ionization states for several phenomenological models, with the
boundary conditions derived from the EUV Imaging Telescope (EIT)
and UVCS observations, were computed and constraints on the density
and temperature of the plasma at the early stage of the ejection are
obtained. The role of physical mechanisms such as thermal conduction,
radiation, and heating is also studied with a two-dimensional
hydrodynamics simulation. The kinetic, thermal, and gravitational
energies are estimated as well as the plasma heating. Whenever the
ejected plasma has a density &gt;=109cm-3, a
continuous supply of heat is required to meet the conditions observed
at 1.7 Rsolar. Moreover heating mechanisms that release
energy gradually during the outward motion of the plasma seem to be
more appropriate than those that dump most of the energy when the
plasma is lower in the corona. Our simulations also indicate that
a three-dimensional self-similar expansion does not fit the UVCS
observations. Comparisons with some CME models from the dynamical and
energetics points of view are discussed.

---------------------------------------------------------
Title: The Ultraluminous M81 X-9 Source: 20 Years' Variability and
Spectral States
Authors: La Parola, V.; Peres, G.; Fabbiano, G.; Kim, D. W.;
Bocchino, F.
2001ApJ...556...47L Altcode: 2001astro.ph..3250L
The source X-9 was discovered with the Einstein Observatory in the
field of M81 and is located in the dwarf galaxy Holmberg IX. X-9
has a 0.2-4.0 keV luminosity in excess of the Eddington limit
for a 1 Msolar compact accreting object, if it is at
the same distance as Holmberg IX (3.4 Mpc). Past hypotheses on the
nature of this super-Eddington source included a supernova remnant or
supershell, an accreting compact object, and a background QSO. To shed
light on the nature of this source, we have analyzed archival data,
including the Einstein data, 23 ROSAT observations, and BeppoSAX
and ASCA pointings. Our analysis reveals that most of the emission
of X-9 arises from a pointlike highly variable source (0.5-2.4 keV
LX~2-8×1039 ergs s-1) and that lower
luminosity extended emission may be associated with it. The spectrum
of this source changes between low- and high-intensity states, in a
way reminiscent of the spectra of galactic black hole candidates. Our
result strongly suggest that X-9 is not a background QSO, but a bona
fide “super-Eddington” source in Ho IX, a dwarf companion of M81.

---------------------------------------------------------
Title: Deep ROSAT-HRI observation of the elliptical galaxy NGC 1399
Authors: Paolillo, M.; Fabbiano, G.; Peres, G.; Kim, D. -W.
2001astro.ph..6309P Altcode:
We present the preliminary results of a deep (167 ks) ROSAT HRI
observation of the cD galaxy NGC1399 in the Fornax cluster. We find,
in agreement with previous observations, an extended (41 Kpc adopting
a distance of 19 Mpc) gaseous halo with a luminosity of L_X=(4.41\pm
0.04)x10^{41} erg/s. The 5 arcsec resolution of the data allows us to
detect a very complex and asymmetric structure of the halo with respect
to the optical galaxy. Moreover the analysis of the radial structure
reveals the presence of a multi-component profile not consistent
with a simple King model over the whole 40 Kpc. We do not detect the
presence of a central source and pose an upper limit to the luminosity
of a possible active nucleus. Due to the length of the observation,
comparable to that of a deep survey, we detect a large number of sources
within the HRI FOV, in slight excess with respect to the estimates based
on previous surveys. We study the flux distribution of the sources,
their temporal behaviour and their spatial distribution with respect
to the central galaxy.

---------------------------------------------------------
Title: Spectral and temporal properties of X-ray emission from the
ultra-luminous source X-9 in M81
Authors: La Parola, V.; Fabbiano, G.; Kim, D. W.; Peres, G.
2001astro.ph..2111L Altcode:
We have analysed the spectra and the variability of individual X-ray
sources in the M-81 field using data from the available ROSAT-PSPC and
ROSAT-HRI observations of this nearby spiral galaxy. Here we present the
results on the second brightest source in the field (X-9 - Fabbiano,
1988 ApJ 325 544), whose identification and interpretation is still
unclear. Our work includes the study of the shape of X-9 from HRI data,
the light curve and hardness ratio evolution, and the spectral analysis.

---------------------------------------------------------
Title: Spectra and Evolution of Two X-Ray Sources in M81
Authors: La Parola, V.; Peres, G.; Fabbiano, G.; Kim, D. W.
2001ASPC..230..397L Altcode: 2001gddg.conf..397L; 2001astro.ph..2088L
We analyzed the spectral and temporal features of X-ray sources in M81
using data from all the relevant ROSAT-PSPC/HRI observations. We discuss
the main features of the point-like nucleus and of the second brightest
source in the field (X-9), whose identification is still unclear.

---------------------------------------------------------
Title: The Solar Corona vs. Stellar Coronae
Authors: Peres, G.
2001ASPC..234...41P Altcode: 2001xras.conf...41P
No abstract at ADS

---------------------------------------------------------
Title: Recent results on the study of the Sun as an X-ray star
Authors: Peres, Giovanni; Orlando, Salvatore; Reale, Fabio
2001MmSAI..72..583P Altcode:
The Sun is an important test-bed for the physics of stellar
coronae. However the context, the methods and the scope of solar studies
are different from those of stellar ones and a direct comparison among
the two is hard. In order to bridge the gap between solar and stellar
coronal physics, we have developed and tested a method to compare
directly X-ray data relevant to the solar and the stellar coronae;
it is based on the wide band X ray data collected with Yohkoh/SXT. The
intermediate results of this analysis (emission measure distributions
vs. temperature, solar X-ray spectra, their evolution etc.) are
important also in the context of solar physics. We discuss the solar
X-ray characteristic in the stellar context, its variability during
the cycle, solar X-ray flares, and how these results compare with
those of stellar coronae.

---------------------------------------------------------
Title: On Stellar Coronae and Solar Active Regions
Authors: Drake, Jeremy J.; Peres, Giovanni; Orlando, Salvatore;
Laming, J. Martin; Maggio, Antonio
2000ApJ...545.1074D Altcode:
Based on Yohkoh Soft X-Ray Telescope (SXT) observations of the Sun
near peak activity level obtained on 1992 January 6, we search for
coronal structures that have emission measure distributions EM(T) that
match the observed stellar coronal emission measure distributions
derived for the intermediate-activity stars ɛ Eri (K2 V) and
ξ Boo A (G8 V) from Extreme Ultraviolet Explorer spectroscopic
observations. We find that the temperatures of the peaks of the
observed stellar distributions EM(T), as well as their slopes in the
temperature range 6.0&lt;~logT&lt;~6.5, are very similar to those
obtained for the brightest of the solar active regions in the 1992
January 6 SXT images. The observed slopes correspond approximately to
EM~Tβ with β~4, which is much steeper than predicted by
static, uniformly heated loop models. Plasma densities in the coronae
of ɛ Eri and ξ Boo A are also observed to be essentially the same
as the plasma densities typical of solar active regions. These data
provide the best observational support yet obtained for the hypothesis
that solar-like stars up to the activity levels of ɛ Eri (K2 V)
and ξ Boo A are dominated by active regions similar to, though
possibly considerably larger than, those observed on the Sun. The
surface filling factor of bright active regions needed to explain
the observed stellar emission measures is approximately unity. We
speculate on the scenario in which small-scale “nanoflares” dominate
the heating of active regions up to activity levels similar to those
of ɛ Eri (K2 V) and ξ Boo A. At higher activity levels still, the
interactions of the active regions themselves may lead to increasing
flaring on larger scales that is responsible for heating plasma to
the observed coronal temperatures of T&gt;~107 K on very
active stars. Observations of X-ray and EUV light curves using more
sensitive instruments than are currently available, together with
determinations of plasma densities over the full range of coronal
temperatures (106-107 K and higher), will be
important to confirm flare heating hypotheses and to elicit further
details concerning coronal structures at solar-like active region
temperatures (T&lt;~5×106 K) and the temperatures that
characterize the most active stars (T&gt;~107 K).

---------------------------------------------------------
Title: Multi-Thread Modeling of Coronal Loops with TRACE Data
Authors: Nightingale, R. W.; Aschwanden, M. J.; Alexander, D.; Reale,
F.; Peres, G.
2000SPD....31.0211N Altcode: 2000BAAS...32..812N
The temperature Te(s) and density structure ne(s)
of active region loops in EUV observed with TRACE is modeled with a
multi-thread model. The model loops are synthesized from the summed
emission of many loop threads that have a distribution of maximum
temperatures and that satisfy the steady-state Rosner-Tucker-Vaiana
(RTV) scaling law, modified by Serio et al. for gravitational
stratification (RTVSp). From model-fitting of the 171 and
195 Angstroms fluxes of 41 loops, which have loop half lengths in the
range of L=4-320 Mm, we find: (1) The EUV loops can be explained by
near-isothermal loop threads in the temperature range of Te
~ 0.8-1.6 MK with substantially smaller temperature gradients than
predicted by the RTVSp model, (2) the loop base pressure,
p0 ~ 0.3+/- 0.1 dyne cm-2, is independent of
the loop length L, it agrees with the RTVSp model for
the shortest loops, but exceeds the RTVSp model up to a
factor of 35 for the largest loops, and (3) the pressure scale height
is consistent with hydrostatic equilibrium for the shortest loops,
but exceeds the temperature scale height up to a factor of ~ 3 for
the largest loops. This work was supported by the TRACE project at
LMSAL (contract NAS5-38099). Ref.: Aschwanden,M.J., Nightingale,R.W.,
Alexander,D., Reale,F., and Peres,G. 2000, ApJ, subm., “Evidence for
Nonuniform Heating of Coronal Loops Inferred from Multi-Thread Modeling
of TRACE Data”, URL="ftp://sag.lmsal.com/pub/aschwand/2000_reale.ps.gz"

---------------------------------------------------------
Title: A Brightening Coronal Loop Observed by TRACE. I. Morphology
and Evolution
Authors: Reale, F.; Peres, G.; Serio, S.; DeLuca, E. E.; Golub, L.
2000ApJ...535..412R Altcode:
We analyze the transient brightening of a solar coronal loop observed,
at high time cadence (30 s) and spatial resolution (0.5" pixel size),
with the Transition Region and Coronal Explorer (TRACE) in the 171 Å
band on 1998 June 26. The loop, located in AR 8253, is ~1010
cm long and inclined with respect to the vertical to the solar
surface. Its geometry and shape do not change significantly during the
brightening, which lasts for ~2 hr and is preceded by highly dynamic
events in nearby and perhaps interacting loops. The loop footpoints
brighten first; after ~10 minutes, moving brightness fronts rise
initially from the northern footpoint, and after another ~7 minutes
from the southern one, at an apparent speed ~100 km s-1, the
whole loop becoming clearly visible afterward. During the rising phase
the loop evolves coherently as a single magnetic tube. The brightness
profile is asymmetric with respect to the loop apex at all times; the
brightness contrast between the footpoints and the apex decreases with
time from a ratio of ~10 to ~3. After the loop has become all visible,
the several parallel filaments which form it follow an independent
evolution. Assuming a plasma temperature of ~1 MK, we infer a plasma
density of ~6×108 cm -3 and a pressure of ~0.2
dyn cm-2 close to the loop apex at the luminosity maximum. A
companion paper is devoted to modeling the rising phase of this event.

---------------------------------------------------------
Title: A Brightening Coronal Loop Observed by TRACE. II. Loop Modeling
and Constraints on Heating
Authors: Reale, F.; Peres, G.; Serio, S.; Betta, R. M.; DeLuca, E. E.;
Golub, L.
2000ApJ...535..423R Altcode:
This is the second of two papers dedicated to the brightening of a
coronal loop observed by the Transition Region and Coronal Explorer
(TRACE) on 1998 June 26; it aims at hydrodynamic modeling of the
brightening. Since the loop geometry is practically unchanged during
the brightening, the evolution of the plasma confined in the loop is
described with a one-dimensional hydrodynamic time-dependent numerical
model, and from the results the emission along the loop in the TRACE
171 Å band is synthesized. The information from Paper I is used
to derive the geometry and the initial configuration of the loop as
well as for comparison with the results of the model. The modeling is
focused to determine the amount, spatial distribution, and evolution
of the heating deposited in the loop to make the modeled evolution
close to that observed with TRACE. We find that, in order to match the
observed evolution and distribution of the brightness along the loop,
the heating has to be nonsymmetrical in the loop, in particular,
deposited between the apex and one footpoint (3×109 cm
from the southern footpoint). A reasonable match with observations
is obtained by assuming that the heating is switched on abruptly and
then kept constant for the whole rising phase. An even better match
is obtained with the heating high and constant for 100 s and then
decaying exponentially with an e-folding time of 300 s. We discuss
the resulting physical scenario; a bright irregular structure close
to the loop in the TRACE images may be a tracer of the heating release.

---------------------------------------------------------
Title: What TRACE Observations tell us about Heating of Coronal Loops
Authors: Aschwanden, M. J.; Nightingale, R. W.; Alexander, D.; Reale,
F.; Peres, G.
2000SPD....31.0210A Altcode: 2000BAAS...32..812A
We analyzed the temperature Te(s) and density structure
ne(s) of active region loops in EUV observed with TRACE (see
SPD abstract by Nightingale et al.). The observational data indicate
that cool EUV loops with maximum temperatures of Tmax ~
0.8-1.6 MK cannot be explained with the static steady-state scaling
law of Rosner, Tucker, &amp; Vaiana (1978) or Serio et al. (1981),
in terms of uniform heating. However, they are fully consistent with
Serio's model (which includes gravitation and a heating scale height)
in the case of nonuniform heating, with heating scale heights in the
range of sH=17 +/- 6 Mm. This heating function provides
almost uniform heating for small loops (L &lt; 20 Mm), but restricts
heating to the footpoints of large loops (L ~ 50-300 Mm). Another
observational result of cool EUV loops is that the pressure scale
height exceeds the hydrostatic scale height by a factor of ql
~ 1-3. This suggests that the pressure balance of these EUV
loops may not be governed by hydrostatic equilibrium, but rather
indicates a non-steady state, e.g. caused by dynamic mass flows and/or
intermittent heating. Chromospheric upflows may explain the extended
scale heights as well as the quasi-isothermal temperature structure
of EUV loops. We review and discuss the differences between cool (T ~
1-2 MK) EUV loops and hot (T ~ 2-8 MK) soft X-ray loops concerning
loop scaling laws, radiative equilibrium, hydrostatic equilibrium,
and heating function. - This work is supported by the TRACE project at
LMSAL (contract NAS5-38099) Ref.: Aschwanden,M.J., Nightingale,R.W.,
Alexander,D., Reale,F., and Peres,G. 2000, ApJ, subm., “Evidence for
Nonuniform Heating of Coronal Loops Inferred from Multi-Thread Modeling
of TRACE Data”

---------------------------------------------------------
Title: Models of dynamic coronal loops
Authors: Peres, Giovanni
2000SoPh..193...33P Altcode:
This paper reviews the basic ideas underlying one-dimensional fluid
dynamic models of coronal loops and presents some of their most recent
applications. These models are an important theoretical support to
explore the new scenario provided by the data of Yohkoh, SOHO, and
TRACE, and are useful to interpret observations, when supplemented
by appropriate spectral synthesis codes. Possible developments are
also discussed.

---------------------------------------------------------
Title: The XACT facility at OAPA : ongoing projects and future
developments
Authors: Barbera, M.; Collura, A.; Artale, M.; Candia, R.; Cavadi,
A.; Mirabello, F.; Peres, G.; Perinati, E.; Sciortino, S.; Serio, S.;
Varisco, S.; Bandler, S.; Murray, S. S.; Schnopper, H. W.; Silver,
E.; Zombeck, M. V.
2000MmSAI..71.1127B Altcode:
No abstract at ADS

---------------------------------------------------------
Title: A systematic analysis of X-ray variability of dM stars
Authors: Marino, A.; Micela, G.; Peres, G.
2000A&A...353..177M Altcode:
We have systematically analyzed X-ray variability of dM stars. Our
data base is the sample of all dM stars listed in the CNS3 (Gliese
&amp; Jahreiss 1991) catalog which have been observed with the ROSAT
PSPC. Our data sample includes 86 pointed observations of 55 distinct
stars or multiple systems. A large fraction of stars shows significant
variations, regardless of their quiescent flux. Variability is detected
on all observable time scales. The amplitudes of these variations are
independent of both stellar X-ray and visual luminosity. Compared
to solar X-ray variability properties our results suggest that the
amplitude distribution of X-ray variability in dM stars is consistent
with the analogous distribution for solar flares. We discuss the
effect of variability on the spread observed in the X-ray luminosity
function of M stars. The comparison of our data with those obtained
with Einstein IPC shows that variations on time scales shorter than
a few month are more common than long term variations comparable to,
e.g., the 11 years solar cycle.

---------------------------------------------------------
Title: The Sun as an X-Ray Star. I. Deriving the Emission Measure
Distribution versus Temperature of the Whole Solar Corona from
theYohkoh/Soft X-Ray Telescope Data
Authors: Orlando, S.; Peres, G.; Reale, F.
2000ApJ...528..524O Altcode:
The scope of this work is to obtain the emission measure distributions
versus temperature, EM(T), of the whole solar corona from Yohkoh
Soft X-ray Telescope images. As discussed in Paper II, the EM(T)
is our starting point for studying the Sun as an X-ray star. To
this purpose, we need to extract as much information as possible
from the Yohkoh/SXT data covering the whole range of the Yohkoh/SXT
temperature sensitivity, i.e., 5.5&lt;logT(K)&lt;8. In particular at
low photon counts and temperatures below 106 K, errors on
the temperature and emission measure determination are expected to be
large. To this end, we have made an extensive set of simulations to
explore the nominal performance of the entire system (instrument and
data analysis system) in the determination of the plasma temperature
and emission measure at low, intermediate, and high photon counts per
pixel. We have shown that low-count data with a number of photons per
pixel nphot&lt;10 are affected by large errors and lead
to the derivation of an unrealistic EM(T) characterized by a steep
negative slope. As a result, we have devised an analysis method that
minimizes the instrumental and statistical effects on the determination
of EM(T) and allows us to determine the global coronal EM(T). As a
first application to real SXT data, we have derived the EM(T) of the
Sun close to the maximum of the solar cycle, a challenging case. The
low-temperature part is in agreement with analogous studies made in
the UV band, and it shows a well-defined maximum at T~2 MK.

---------------------------------------------------------
Title: The Sun as an X-Ray Star. II. Using theYohkoh/Soft X-Ray
Telescope-derived Solar Emission Measure versus Temperature to
Interpret Stellar X-Ray Observations
Authors: Peres, G.; Orlando, S.; Reale, F.; Rosner, R.; Hudson, H.
2000ApJ...528..537P Altcode:
This paper is the second of a project dedicated to using solar
Yohkoh/SXT data as a guide and a template to interpret data on
stellar coronae. In the light of the large differences in scope
and approach between solar and stellar studies, we have developed a
method to translate Yohkoh/SXT data of the whole solar corona into
stellar-like data, i.e., to put them in the same format and context
as the stellar ones. First from the Yohkoh/SXT images we derive
the whole-Sun X-ray emission measure versus temperature [EM(T)],
in the range 105.5-108 K, during the specific
observation. Then, we synthesize the solar X-ray spectrum; finally,
we fold the spectrum through the instrumental response of nonsolar
X-ray observatories, for instance, ROSAT/PSPC and ASCA/SIS. Finally,
we analyze such solar coronal data in the same band and with the
same methods used for stellar observations, allowing a direct
and homogeneous comparison with them. In this paper we present in
detail our method and, as an example of results, we show and discuss
EM(T) and stellar-like spectra for three phases of the solar cycle:
maximum, intermediate phase, and minimum. The total amount and the
distribution of the emission measure change dramatically during the
cycle, in particular at temperatures above 106 K. We also
show the EM(T) of the whole solar corona during a large flare. The
ROSAT/PSPC- and ASCA/SIS-like X-ray spectra of the Sun as a star
that we obtain are discussed in the context of stellar coronal
physics. The Sun's coronal total luminosity in the ROSAT/PSPC band
ranges from ~2.7×1026 ergs s-1 (at minimum)
to ~4.7×1027 ergs s-1 (at maximum). We discuss
future developments and possible applications of our method.

---------------------------------------------------------
Title: TRACE-derived Temperature and Emission Measure Profiles along
Long-lived Coronal Loops: The Role of Filamentation
Authors: Reale, F.; Peres, G.
2000ApJ...528L..45R Altcode: 1999astro.ph.11096R
In a recent Letter, Lenz et al. have shown evidence of uniform
temperature along steady long coronal loops observed by TRACE in two
different passbands (171 and 195 Å filters). We propose that such
a piece of evidence can be explained by the subarcsecond structuring
of the loops across the magnetic field lines. In this perspective, we
present a model of a bundle of six thin parallel hydrostatic filaments
with temperature stratification dictated by detailed energy balance
and with temperatures at their apex ranging between 0.8 and 5 MK. If
analyzed as a single loop, the bundle would appear isothermal along
most of its length.

---------------------------------------------------------
Title: X-ray spectra of the Sun as a star: how different coronal
regions contribute to the observed X-ray spectrum
Authors: Orlando, S.; Peres, G.; Reale, F.; Rosner, R.; Hudson, H.
2000ASPC..198..479O Altcode: 2000scac.conf..479O
No abstract at ADS

---------------------------------------------------------
Title: On the Dynamics of Siphon Flows in Coronal Loops
Authors: Betta, R.; Peres, G.; Serio, S.; Orlando, S.
1999ESASP.448..475B Altcode: 1999ESPM....9..475B; 1999mfsp.conf..475B
No abstract at ADS

---------------------------------------------------------
Title: Fast solar wind acceleration by Alfvén waves: observable
effects on the EUV lines detected by SOHO/UVCS
Authors: Ventura, R.; Orlando, S.; Peres, G.; Spadaro, D.
1999A&A...352..670V Altcode:
SOHO/UVCS observations of the most intense EUV spectral lines emitted by
the solar corona have been providing us a good opportunity to study in
detail the acceleration regions of the solar wind. In this work we aim
at deriving useful diagnostics and identifying possible signatures of
Alfvén waves momentum deposition. More specifically we investigate,
with the help of a detailed wind model (Orlando et al. 1996), the
insight and the constraints that these observations give on the presence
of Alfvén waves, as deduced from the influence of the waves on the
solar wind structure and dynamics. The model developed by Orlando et
al. (1996) accounts for the momentum deposition by a spectrum of non-WKB
Alfvén waves, generated in the Sun's lower atmosphere and undergoing
significant reflection across the transition region. We compute a
set of wind solutions characterized by different physical conditions,
synthesize, from them, the emission in the Lyalpha , Lybeta and O VI
doublet (1032 Ä, 1038 Ä) lines and derive possible diagnostics. We
finally compare our results with the most recent SOHO/UVCS data.

---------------------------------------------------------
Title: Long-lived Coronal Loop Profiles from TRACE
Authors: Lenz, Dawn D.; DeLuca, Edward E.; Golub, Leon; Rosner, Robert;
Bookbinder, Jay A.; Litwin, Christof; Reale, Fabio; Peres, Giovanni
1999SoPh..190..131L Altcode:
An initial study of long-lived loops observed with TRACE (Lenz et al.,
1999) shows that they have no significant temperature stratification
and that they are denser than the classic loop model predicts. Models
that agree better with the observations include a loop consisting of a
bundle of filaments at different temperatures and a loop with momentum
input by MHD waves. Some implications for coronal heating models and
mechanisms are discussed.

---------------------------------------------------------
Title: The Dynamics Of The Plasma Confined In Coronal Loops Subject
To A Random Heating
Authors: Betta, R.; Reale, F.; Peres, G.
1999ESASP.446..179B Altcode: 1999soho....8..179B
We model the dynamics of the plasma in the corona and transition region
between the chromosphere and the corona using the Palermo-Harvard 1-D
hydrodynamic code and the ASAP spectral synthesis code. Assuming that
the coronal heating of the loop is entirely due to microflares, a random
energy function with a gaussian spatial distribution is applied at the
top of the coronal loop. We investigate the hydrodynamic plasma response
to such a stocastic heating for a set of loop models; we explore the
solution parameter space, varying the loop length, its initial coronal
pressure, the typical repetition time of the energy event distribution,
while maintaining the same energy rate averaged in time. With the ASAP
code we synthesize the emission in some optically thin lines formed in
the transition region for all the loop models of the set considered,
in order to allow a direct comparison of the model results with line
intensities and velocities observed by SUMER.

---------------------------------------------------------
Title: Coronal Heating and Structuring
Authors: Peres, G.
1999ESASP.446...43P Altcode: 1999soho....8...43P
No abstract at ADS

---------------------------------------------------------
Title: Non-equilibrium ionization and the interpretation of Yohkoh/SXT
data during solar dynamic events
Authors: Orlando, S.; Bocchino, F.; Peres, G.
1999A&A...346.1003O Altcode:
Yohkoh has been dedicated to study the structure and evolution of the
solar corona, especially during dynamic events. During such events,
however, the collisional equilibrium ionization - which is invariably
assumed when deriving the plasma parameters from the observations - may
not apply. We explore the influence of the non-equilibrium ionization
effects (NEI) on the values of temperature and emission measure derived
from the data collected with the Solar X-ray Telescope (SXT) on board
the Yohkoh satellite during solar dynamic events. To this end, we have
simulated such dynamic events, assuming that the plasma temperature
rises instantaneously from the value T_0 to the value T_1, and then it
remains constant. We have considered two different temperature jumps:
the first one from 10(6.3) K to 10(7.3) K to simulate a flare and
the other, from 10(5.7) K to 10(6.5) K, to simulate a microflare. We
show that the temperature determination with the SXT wide band filter
ratio is, to some extent, affected by non-equilibrium ionization:
the maximum fractional error on T_1 amounts to ~ 0.7 for the flare
and ~ 0.5 for the microflare we have simulated. On the other hand,
the emission measure determination can be considerably affected by
such departures: the maximum fractional error on EM amounts to ~ 2
for the flare and ~ 7 for the microflare we have simulated. Therefore
NEI effects can be important on the temperature and emission measure
determination during fast evolving phenomena.

---------------------------------------------------------
Title: Effects on UV lines observations of stationary plasma flows
confined in coronal loops
Authors: Orlando, S.; Peres, G.
1999PCEC...24..401O Altcode:
We describe a method which uses a rather detailed model of coronal loop
hosting a siphon flow as a diagnostic tool to interpret solar UV and
X-ray observations in selected bands and lines. We apply the method
to investigate the deviations from ionization equilibrium induced by
stationary plasma flows confined in coronal loops and their effects
on the UV and EUV emission lines observed by the instruments on board
SOHO. We present results on the detailed synthesis of loop emission
in a set of selected emission lines observed by CDS and SUMER, taking
into account the non-equilibrium of ionization effects.

---------------------------------------------------------
Title: Analysis and comparison of loop structures imaged with NIXT
and Yohkoh/SXT
Authors: Di Matteo, V.; Reale, F.; Peres, G.; Golub, L.
1999A&A...342..563D Altcode:
We analyze and compare five coronal regions simultaneously observed
by NIXT and Yohkoh/SXT on April 12, 1993. The compact loop structures
(length ~ 10(9) cm) imaged in three regions with NIXT and with SXT
have a good general morphological correspondence. A large scale (~
1.7 x 10(10) cm) and an intermediate scale (~ 5 x 10(9) cm) structure
observed in the NIXT image have no obvious counterpart in the SXT
image. The pressure of the loop plasma detected by NIXT is derived
from the brightness profile along the loops by applying a method based
on loop models. The pressure of the loop plasma in the SXT band has
also been derived from loop models on the basis of the temperature
obtained from the standard Yohkoh data analysis. NIXT pressures are
systematically lower than those found with SXT. By comparing the
measured total loop luminosity to that expected on the basis of loop
models, we constrain quantitatively the volume filling factor of the
plasma emitting respectively in the NIXT and SXT bands. The filling
factors obtained in the NIXT band for the compact and intermediate
loops are very low (10(-3) - 10(-2) ), but they are of the order of
unity for the large structure. The low filling factors suggest a strong
loop filamentation. The filling factors for the compact structures are
instead of the order of unity in the SXT band. We discuss our results
and propose a scenario for their interpretation.

---------------------------------------------------------
Title: On the Stability of Siphon Flows Confined in Coronal Loops
Authors: Betta, Rita; Orlando, Salvatore; Peres, Giovanni; Serio,
Salvatore
1999SSRv...87..133B Altcode:
We use a time-dependent hydrodynamic model to study the dynamics of
siphon flows triggered by differences of pressure or heat deposition
asymmetries between the two footpoints of a coronal loop. We show that
steady pressure driven flows in a uniformly heated loop are unstable,
whereas those driven by asymmetries in the heating function may be
stable. We also show that, in these cases, relatively cool loops might
be filled far above their static pressure scale height.

---------------------------------------------------------
Title: The Solar-Stellar Connection in X-rays: How to Take Advantage
of the YOHKOH data
Authors: Peres, G.; Orlando, S.; Reale, F.; Rosner, R.; Hudson, H.
1999ASPC..158..391P Altcode: 1999ssa..conf..391P
No abstract at ADS

---------------------------------------------------------
Title: Loop modeling of coronal X-ray spectra. V. One- and two-loop
model fitting of G-type star ROSAT/PSPC spectra
Authors: Ventura, R.; Maggio, A.; Peres, G.
1998A&A...334..188V Altcode:
As part of a systematic study devoted to the diagnostic of solar-like
coronal structures in late-type stars, we have analyzed ROSAT/PSPC
X-ray spectra of eight main sequence G-type stars in the solar
neighborhood. We have fitted the X-ray spectra adopting two different
classes of models: the usual two-component isothermal models and the
more physically meaningful hydrostatic loop models. The two-component
isothermal models yield fairly acceptable chi (2) values, however they
allow limited physical insight on the stellar coronal structures; on the
other hand, the one-loop model fitting provides unreliable long loops,
namely orders of magnitude larger than the stellar radius, in the
majority of the cases studied. A more realistic physical description
of the observed coronae comes from the two-loop modeling approach:
two distinct classes of loops with different characteristics seem to
dominate the X-ray emission of our sample of G-type stars: relatively
cool loops (T_max =~ 1.5 - 5 x 10(6) K) with pressures ranging from
relatively low (p_0 =~ 2 - 10 dyn cm(-2) ) to high values (p_0 =~ 100
dyn cm(-2) ) and hot loops (T_max =~ 1 - 3 x 10(7) ) with very high
base pressures (p_0 =~ 10(2) - 2.4 x 10(4) dyn cm(-2) ). We compare the
results and the diagnostic power coming from the different approaches
and discuss their physical implications.

---------------------------------------------------------
Title: X-ray spectroscopy of the active giant beta Ceti: the SAX
LECS view
Authors: Maggio, A.; Favata, F.; Peres, G.; Sciortino, S.
1998A&A...330..139M Altcode:
We present an X-ray observation of the active giant beta Cet,
performed with the Low Energy Concentrator Spectrometer (LECS)
on-board the SAX satellite. The resulting X-ray spectrum is well fit
by an optically-thin plasma model with two discrete temperatures,
and the inferred coronal metallicity is compatible both with the
abundances derived from a re-analysis of the ASCA SIS spectrum of
the same object, adopting the same plasma emission code (mekal),
and with accurate photospheric abundances recently reported in the
literature. The remarkable similarity between the X-ray spectrum of beta
Cet and the one of the active binary Capella, also observed by SAX,
rises again the issue of how some putative He-burning clump giants,
like beta Cet, manage to retain (or regain) a relatively high X-ray
activity level. Some implications of the new Hipparcos parallaxes,
relevant for the coronal properties of beta Cet and the Hyades giants,
are also discussed.

---------------------------------------------------------
Title: Momentum deposition by a spectrum of Alfvén waves in fast
solar wind: effects on the emission lines observed by SOHO/UVCS
Authors: Orlando, S.; Ventura, R.; Peres, G.; Spadaro, D.
1998MmSAI..69..777O Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Solar-Stellar Connection: Relevance of YOHKOH Data
Authors: Orlando, S.; Peres, G.; Reale, F.; Rosner, R.; Hudson, H.
1998ASPC..154.1130O Altcode: 1998csss...10.1130O
The similarity of late-type stars to the Sun is often assumed
when studying the physical conditions in their coronae. In order
to explore better such a “solar-stellar” connection we use the
Yohkoh/SXT X-ray images to generate the distribution of the emission
measure vs. temperature of the Sun and, from that, the expected
emission, as it would be observed by non-solar X-ray telescopes
such as ROSAT/PSPC and ASCA/SIS. We discuss the role of the various
solar structures in determining the total distribution of the emission
measure vs. temperature and in determining the stellar-like synthesized
X-ray spectra.

---------------------------------------------------------
Title: Simulating the Source Parameter Recovery Capability from
Coronal X-ray Spectra: the SAX/LECS and the ASCA/SIS Cases
Authors: Favata, F.; Maggio, A.; Peres, G.; Sciortino, S.
1998ASPC..154.1027F Altcode: 1998csss...10.1027F
We have studied the performance of global chi^2 fitting applied to low
resolution X-ray spectroscopy, focusing on the retrieval of source
parameters, with emphasis on the coronal metallicity. The study has
been conducted by fitting large numbers of simulated spectra with
known characteristics, and studying the distribution of the best-fit
parameters, using the responses of the LECS detector onboard the SAX
satellite and of the SIS detector onboard the ASCA satellite. The
simulations have been done under the hypothesis that the intrinsic
source spectrum can be described by two discrete temperatures. The
performance of the fitting process has been studied both in the case
in which just the global metallicity is left free to vary and in the
case in which the abundance of each element is independently left free
to vary in the fit process.

---------------------------------------------------------
Title: Plasma Thermal Structure and Metallicity of Stellar Coronae
Observed with XMM
Authors: Maggio, A.; Orlando, S.; Favata, F.; Micela, G.; Peres, G.;
Sciortino, S.
1998sxmm.confE..70M Altcode:
No abstract at ADS

---------------------------------------------------------
Title: X-ray Flare Light Curves and Dimensions of the Flaring Regions
Authors: Serio, S.; Reale, F.; Betta, R.; Peres, G.; McTiernan, J.
1998ASPC..154.1161S Altcode: 1998csss...10.1161S
The decay of flare X-ray light curves provides information on the
dimensions of the flaring region. We show that a reliable estimate of
such dimensions must take into account the temperature evolution as
tracer of the presence of heating during the decay. Many solar flares
appear sustained by prolonged heating episodes. We describe a method to
estimate their spatial extension based on the analysis of the decay of
their X-ray light curve and temperature, and show that the results are
in good agreement with their dimensions measured on Yohkoh/SXT images.

---------------------------------------------------------
Title: The Sun as an X-Ray Star: Overview of the Method
Authors: Peres, G.; Orlando, S.; Reale, F.; Rosner, R.; Hudson, H.
1998ASSL..229...29P Altcode: 1998opaf.conf...29P
No abstract at ADS

---------------------------------------------------------
Title: Hydrodynamics of the plasma confined inside coronal loops:
flare and microflare models
Authors: Betta, R.; Peres, G.; Reale, F.; Serio, S.
1998cee..workE..23B Altcode:
The plasma contained in coronal loops behaves macroscopically like a
fluid and its dynamics and evolution may be described by hydrodynamics
provided mass, momentum and energy transport occurs only along
magnetic field lines. In fact, coronal loops are very often observed
not to change their geometry during a flare, and this suggests that
the magnetic field structure may basically act to confine the plasma
while chromospheric plasma evaporation and temperature increase cause
the increase in brightness. In other words, though the source of the
energy release in loops may be of magnetic origin, the subsequent
loops response may be adequately described by hydrodynamics in those
instances in which the global magnetic field does not change. We have
developed such a hydrodynamic model (Peres et al. 1982), which takes
into account the main physical effects such as gravity, viscosity,
ionization, radiative losses and thermal conduction and which is
capable of giving a correct description of the steep and dynamic
transition region between the chromosphere and the corona (Betta et
al. 1997). Here we show how a plasma confined inside coronal loops
responds when it is subject to impulsive heating. We simulate flares
by creating a sudden energy release in a localized position along
the loop (although the plasma dynamics does not depend crucially on
the position of energy release). The initial configuration consists
of a loop in hydrostatic equilibrium and steady-state energy balance
(i.e.,in which there is an average heating which balances radiation
losses and thermal conduction). The hydrodynamic calculations show the
formation of an evaporation front propagating from the chromosphere
to the corona, while the temperature increases in the loop from
the top towards the footpoints anchored in the photosphere and the
transition region moves progressively downwards. When the heating
is switched off the plasma cools slowly during the decay phase
of the flare until a thermal instability occurs (due to the large
amount of plasma evaporated in the corona which causes an increased
emission): as a consequence, the plasma temperature drops to almost
chromosperic values. Continuing the simulations for a few hours, since
the steady-state energy balance term is included in the equations,
the atmosphere returns to the initial configuration. In the microflare
heating model we assume that on average, the loop is maintained close
to the coronal conditions predicted by steady-state models. Some
preliminary results of microflare simulations may be found in Peres,
Reale and Serio (1993). A sequence of short heating pulses is released
inside the loop. The time interval between two heating events can be
either random following a Poisson distribution, or periodic. The pulse
has a different random duration but the total power given during each
heating event is such that the integral over time and space equals the
total amount of energy needed to balance losses in the hydrostatic
equations and mantain a steady-state. We have studied the dynamics
using heating pulses of different frequencies in order to find possible
diagnostics of random heating in coronal loops.

---------------------------------------------------------
Title: The Sun as an X-ray star: comparing Yohkoh/SXT data with
stellar X-ray data
Authors: Peres, G.
1998MmSAI..69..693P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: X-ray Variability in ROSAT Observations of M Stars
Authors: Marino, A.; Micela, G.; Peres, G.
1998ASPC..154.1116M Altcode: 1998csss...10.1116M
We present preliminary results of a systematic analysis of X-ray light
curves of the PSPC pointed observations of M stars contained in the
CNS3 catalog. A large fraction of the sample stars show significant
variations, regardless of the quiescent activity level. Variability
is observed at all time scales we have explored, and the variations in
X-ray luminosity are well related to variations in the hardness ratio
of the spectra. We discuss the possible effect of variability on the
spread observed in the X-ray luminosity function of M stars.

---------------------------------------------------------
Title: Coronal Loop Modeling of ROSAT/PSPC X-ray Spectra from
Solar-Type Stars
Authors: Ventura, R.; Maggio, A.; Peres, G.
1998ASPC..154.1183V Altcode: 1998csss...10.1183V
In the context of a wide project devoted to the diagnostic of structures
in stellar coronae, we have analyzed ROSAT/PSPC X-ray spectra of a
selected sample of 8 G-type stars in the solar neighborhood (d&lt;
25 pc). We fitted the X-ray spectra with a few models: the usual
two-temperature isothermal model and the more physically detailed
one- and two-loop models, in order to compare the diagnostic power
coming from the different approaches. The results point out that the
one-loop fitting provides acceptable chi^2 values, but it yields, in
the majority of the cases, unacceptable long loops, namely, orders of
magnitude larger than the stellar radius. A better physical insight
on the observed coronae comes from the two-loops model fitting. Two
classes of loops with different characteristics seem to dominate the
X-ray emission of our sample of solar-like stars: relatively cool loops
(T ~= 1.5 - 5 x 10^6 K) with pressures ranging from low (p_0 ~= 2 -
10 dyn cm^{-2}) to relatively high values (p_0 ~= 100 dyn cm^{-2})
and hot loops (T ~= 1 - 3 x 10^7 K) with very high base pressures
(p_0 ~= 10^2 - 2.4 x 10^4 dyn cm^{-2}). We discuss these results,
their physical implications and possible interpretations.

---------------------------------------------------------
Title: Alfvénic fluctuations in fast and slow solar winds
Authors: Orlando, S.; Lou, Y. -Q.; Peres, G.; Rosner, R.
1997JGR...10224139O Altcode:
We compute properties of Alfvénic
fluctuations within a wide frequency range (i.e.,
10-6s-1&lt;~f&lt;~10-2s-1)
in both high- and low-speed magnetized radial solar winds. In
particular, the radial and frequency variations of the
normalized cross helicity σc, the Alfvén ratio
&lt;script&gt;R&lt;/script&gt;A, and other relevant
quantities associated with Alfvénic fluctuations are presented and
analyzed. Because of the existence of the Alfvén critical radius
rA and a characteristic frequency fc for a given
magnetized solar wind profile, continuous reflection effects are clearly
manifest at frequencies lower than several times fc. Since
outward propagating fluctuations which many authors consider Alfvénic
fluctuations have been detected in the high-latitude solar wind
(which is fast and steady), such continuous reflection effects in
the low-frequency end of the fluctuation spectrum may be found in
the Ulysses data. In order to strengthen our case, comparisons are
carried out between our theoretical results and previous data analyses
of observations from the Helios 1 and 2 spacecraft in the radial
range of 0.29AU&lt;~r&lt;~1.0AU. Stronger evidence for continuous
reflection effects of low-frequency Alfvénic fluctuations is found
in the high-speed solar wind close to the Sun than in the low-speed
solar wind in general.

---------------------------------------------------------
Title: On coronal abundances derived with the SAX/LECS and ASCA/SIS
detectors.
Authors: Favata, F.; Maggio, A.; Peres, G.; Sciortino, S.
1997A&A...326.1013F Altcode: 1997astro.ph..7058F
We have studied the performance of global χ^2^ fitting of
low-resolution X-ray spectra in retrieving intrinsic source
parameters, with emphasis on the coronal metallicity. The study has
been conducted by fitting large numbers of simulated spectra with
known characteristics, and studying the distribution of best-fit
parameters. We have studied the behavior of the LECS detector on board
the SAX satellite and the SIS detector on board the ASCA satellite. The
fitted source spectra have either two discrete temperature components
or a power-law temperature distribution, with metallicity variations
modeled by a single global abundance parameter. The model used for
the fitting has always been a two-temperature one, with global varying
abundance, to explore the influence of the a priori ignorance of the
actual temperature stratification in the source being observed. The
simulations performed explore the influence of varying statistics
in the observed spectrum (spanning a realistic range of values)
as well as the effect of varying the intrinsic source metallicity,
with values in the range 0.15-1.0 times the solar value. We find
that the source metallicity can be retrieved to within a few tens
of percent from ASCA/SIS spectra of typical signal to noise ratio,
and within few percent from SAX/LECS spectra at the same signal
to noise ratio. However, relatively small uncertainties in the
detector calibrations and in the plasma emission codes are likely to
potentially cause large systematic off-sets in the value of the best-fit
parameters. Similar systematic off-sets may derive from assuming too
simplistic a temperature distribution for the source plasma. In addition
we have re-analyzed the ASCA/SIS spectra of the active giants β Cet
and Capella with the same set of assumptions used in the simulations,
showing how the best-fit metallicity in these two real cases depends
on the details of the fitting process, and in particular on the chosen
energy range.

---------------------------------------------------------
Title: Determination of the length of coronal loops from the decay
of X-ray flares I. Solar flares observed with YOHKOH SXT.
Authors: Reale, F.; Betta, R.; Peres, G.; Serio, S.; McTiernan, J.
1997A&A...325..782R Altcode:
We show how to estimate the size of spatially unresolved solar
and stellar coronal flaring regions from the X-ray light curve and
time-resolved temperature and emission measure values during the flare
decay. By means of extensive hydrodynamic modeling of decaying flaring
loops, we propose and test a relationship between the decay time of the
light curve in the band of a specific instrument and the slope of the
trajectory in the density-temperature diagram. From this relationship,
we obtain an expression of the loop length as a function of the decay
time, the slope and the flare maximum temperature. The novelty of this
approach is that it takes into proper quantitative account, and allows
us to estimate, the effect of a prolonged heating during the decay. In
view of its application to non-imaged solar flares and to stellar
flares, we have tested our relationship on spatially resolved solar
flares observed with Yohkoh SXT. The comparison of the predictions
to the morphology of the structures in the SXT images proves the
reliability of our approach under a wide range of conditions.

---------------------------------------------------------
Title: Loop modeling of coronal X-ray spectra. IV. One- and two-loop
model fitting of ROSAT PSPC spectra: three test cases.
Authors: Maggio, A.; Peres, G.
1997A&A...325..237M Altcode:
We present a detailed application of coronal hydrostatic loop models
to fit ROSAT/PSPC X-ray spectra, using three late-type stars, Procyon
(F5 IV - V), ι Vir (F6 III), and HR 3625 (F9 V), selected as test
cases showing different activity levels. We present results of the
spectral analysis with models of coronae made of one class of loops
or two classes of loops, and compare them with the results obtained
with two-component isothermal models. The loop model analysis is
performed with a χ^2^ fitting method as accurate as in the case of the
two-temperature modeling, including evaluation of statistical confidence
regions in the model parameter space. We show that the observed spectra
of Procyon and ι Vir can be successfully fitted with one-loop models,
while two-loop models are required to fit the spectrum of HR 3625. In
the case of Procyon, for which EUVE spectra are also available, we
find that a coronal density estimate - based on emission line ratios -
is consistent with the results of our one-loop model fitting. The case
of HR 3625 is adopted to illustrate how the confidence regions in the
two-loop model parameter space can be explored and interpreted. Our
final results indicate different coronal conditions for the three
selected stars: Procyon emission is dominated by relatively short
(L~10^9^cm), low pressure (p_0_~2dyn/cm^2^) and low temperature
(T_max_=1.7x10^6^K) loops, covering few tens percent of the stellar
surface; the corona of ι Vir appears composed of relatively higher
pressure (p_0_=2-10dyn/cm^2^), higher temperature (T_max_=1-2x10^7^K)
loops with height (L=1-6x10^10^cm) comparable with the pressure scale
height; finally, the coronal emission of HR 3625 can be modelled with
low-temperature (T_max_=1-2x10^6^K), short (L&lt;5x10^8^cm) loops, with
base pressure p_0_&gt;6dyn/cm^2^, possibly covering a large fraction of
the stellar surface, plus higher temperature (T_max_ &gt; 7 x 10^6^ K)
loops which may range from compact, very active region loops (L &lt;
10^9^ cm, p_0_ &gt; 10^2^ dyn cm^-2^) covering less than 0.5% of the
surface, to larger, relatively less active region loops (L~10^10^cm,
p_0_=5-10dyn/cm^2^) covering at most 10% of the surface.

---------------------------------------------------------
Title: The Sun as AN X-Ray Star: Overview of the Method
Authors: Peres, G.; Orlando, S.; Reale, F.; Rosner, R.; Hudson, H.
1997SoPh..172..239P Altcode: 1997ESPM....8..239P
We present a method to study the solar-stellar connection, i.e., the
close similarity of the physical phenomena occurring on the Sun and
on late-type active stars, by taking advantage of Yohkoh/SXT X-ray
images. From such images, we first generate distribution functions
of the whole disk differential emission measure, and then synthesize
from these spectra analogous to those collected by X-ray telescope
instruments aimed at stars other than the Sun. Here we illustrate the
application of this method to the ROSAT/PSPC and ASCA/SIS, and discuss
test cases as well as future applications.

---------------------------------------------------------
Title: An adaptive grid code for high resolution 1-D hydrodynamics
of the solar and stellar transition region and corona
Authors: Betta, R.; Peres, G.; Reale, F.; Serio, S.
1997A&AS..122..585B Altcode:
We present a new version of the Palermo-Harvard hydrodynamic model
of plasma confined in coronal loops, based on a regridding algorithm
which ensures limited variations of thermodynamical variables across
grid points. The code provides an accurate description of the dynamics
of the chromosphere-corona transition region, in spite of the very
steep temperature and density gradients, even during flare-like
transients. The new model can be used to synthesize, together with X-ray
emission, the optically thin emission originating from the transition
region, mainly in the EUV band. Here we show sample results.

---------------------------------------------------------
Title: Loop modeling of coronal X-ray spectra. III. Fitting loop
spectra with one- and two-component thermal models.
Authors: Ciaravella, A.; Maggio, A.; Peres, G.
1997A&A...320..945C Altcode:
In this work we study coronal loops vs. one- and two-component thermal
models, the first being the physically realistic description of the
X-ray emitting, magnetically-confined solar corona, the others the
standard analysis tool of X-ray spectra from stellar coronae. The
scope of this work is to compare directly these two paradigms of
coronal physics, also to find a possible relation between the loop
characteristics and the findings of the thermal components fitting. We
simulate observations of coronal spectra using a static loop model,
including the effects of stellar gravity and of possible non-uniform
loop cross-section. We evaluate the one- and two-temperature fitting
results through extensive simulations, varying the loop parameters,
the photon counting statistics, and considering two instruments: the
ROSAT/PSPC and the ASCA/SIS. We find that one-temperature models do
not fit adequately loop spectra with 10^3^ total counts or more, for
any of the cases explored. Two-temperature models provide a good fit to
single loop spectra in many of the cases explored, with the implication
that the two temperatures found when fitting real observations may
not necessarily indicate the presence of two classes of dominating
loops in the observed corona, but rather may be explained with only
one class of loops, as long as the ratio of the emission measures of
the hot vs. cool component is larger than unity. The goodness of the
fit becomes worse with increasing photon statistics and/or resolving
power, especially for loops with relatively intense plasma pressure
(p_0_&gt;10dyn/cm^2^) and as high as the pressure scale height. In such
cases the two-temperature fitting and the loop modeling are therefore
not equivalent, implying that, for such kind of observations, detailed
loop fitting should be attempted. We comment on our findings and draw
possible guidelines to interpret observations.

---------------------------------------------------------
Title: Radiatively driven downdrafts and redshifts in transition
region lines. II. Exploring the parameter space.
Authors: Reale, F.; Peres, G.; Serio, S.
1997A&A...318..506R Altcode:
We address the hypothesis that downdrafts driven by radiatively-cooling
condensations in the solar transition region are able to produce
significant redshifts in UV lines as frequently observed on the
solar disk and, more recently, on other stars. In a first paper,
significant redshifted line components at several km/s have been
found from modeling the evolution of an isobaric perturbation twice as
dense as the unperturbed atmosphere, almost as large as the thickness
of the transition region of an active region loop, and with central
temperature higher than the formation temperature of the UV lines. In
the present work we show the results of an extensive exploration of
the space of the important parameters controlling the evolution of
isobaric perturbations: density contrast (δ), dimensions, and ambient
pressure. The center of most of the perturbations is placed where the
temperature of the unperturbed medium is T_0_=4.5x10^5^K, but higher
temperatures do not lead to substantially different results. From
the hydrodynamic evolution we synthesize the line flux and effective
speed along the line of sight, and examine the distribution of the
most intense Doppler-shifted components. In a wide region of the
parameter space (δ&gt;0.5, dimensions of the order of the thickness
of the transition region) we find redshifted components at speeds of
several km/s for ambient pressure values ranging from those typical
of quiet Sun to active regions. The assumption of isotropic thermal
conduction, or, alternatively, of 1-D hydrodynamics, i.e. mimicking
the effect of strong magnetic fields, lead to qualitatively similar
results. Our calculations suggest also that redshifts may occur more
easily in the higher pressure plasma, typical of active regions,
in general consistency to observations.

---------------------------------------------------------
Title: A major optical flare on the recently discovered X-ray active
dMe star G 102-21.
Authors: Pagano, I.; Ventura, R.; Rodono, M.; Peres, G.; Micela, G.
1997A&A...318..467P Altcode:
During the course of UBV photoelectric measurements made at the Catania
Astrophysical Observatory we have observed an unusually intense optical
flare on the nearby M dwarf G 102-21, the optical counterpart of a
strong X-ray source recently detected by ROSAT. The amplitude of the
magnitude increase ({DELTA}U~7.3mag) and the total energy release in
the UBV bands (E_UBV_~(1.4+/-0.3)x10^35^erg) put this event among the
strongest flares ever detected on UV Ceti-type stars. The U-B and B-V
colours of the flare emission did not significantly change during most
of the flare development. While the B-V is quite a typical colour for
stellar flares, the U-B is much bluer when compared with typical values
given in literature. This unusually strong ultraviolet component of
the flare emission, together with the very fast flare decay, suggest
a possible flare site close to the limb. We find that the observed
amount of flare energy could be explained by the fast release of energy
stored in a magnetic filament located in between the two G 102-21
stellar components, with a surface magnetic field &gt;=3600G. The
characteristics of the quiescent optical emission are also discussed.

---------------------------------------------------------
Title: SOHO observations of the north polar solar wind
Authors: Peres, G.; Ciaravella, A.; Betta, R.; Orlando, S.; Reale,
F.; Kohl, J.; Noci, G.; Fineschi, S.; Romoli, M.; Brekke, P.; Fludra,
A.; Gurman, J. B.; Lemaire, P.; Schuhle, U.
1997ESASP.404..587P Altcode: 1997cswn.conf..587P
No abstract at ADS

---------------------------------------------------------
Title: Flows of Mass, Momentum and Energy in the Solar Atmosphere:
A SOHO-Oriented View of Cold Loops
Authors: Peres, G.
1997ESASP.404...55P Altcode: 1997cswn.conf...55P; 1997soho....5...55P
No abstract at ADS

---------------------------------------------------------
Title: The Sun as an X-ray Star: Overview of the Method
Authors: Peres, G.; Orlando, S.; Reale, F.; Rosner, R.; Hudson, H.
1997IAUJD..19E..37P Altcode:
We present a method to study the solar-stellar connection, i.e., the
close similarity of the physical phenomena occurring on the Sun and
on late-type active stars, by taking advantage of Yohkoh/SXT X-ray
images. From such images, we first generate distribution functions
of the whole disk differential emission measure, and then synthesize
from these spectra analogous to those collected by X-ray telescope
instruments aimed at stars other than the Sun. Here we illustrate
the application of this method to the ROSAT/PSPC and ASCA/SIS,
and discuss test cases as well as future applications. For a more
detailed discussion, please refer to a paper, by the same authors in
"OBSERVATIONAL PLASMA ASTROPHYSICS: FIVE YEARS OF YOHKOH AND BEYOND",
T. Watanabe, T. Kosugi, and A. C. Sterling, eds., Kluwer Academic
Publishers, the Proceedings of the Yohkoh 5th Anniversary Symposium,
held in November 1996, in Yoyogi, Tokyo, Japan.

---------------------------------------------------------
Title: Stellar Flaring Loop
Authors: Reale, F.; Micela, G.; Peres, G.; Betta, R.; Serio, S.
1997MmSAI..68.1103R Altcode:
We propose and test a procedure tailored to analyse extensively and
systematically stellar X-ray flares, such as those observed in open
clusters. The method yields a reliable estimate of the dimensions of
the coronal flaring loops and of the presence of a significant heating
during the flare decay, it is based on the decay of the light curve
and of temperature indicators, and it has been developed and tested
successfully on spatially-resolved solar flares. We illustrate the
application on flares observed with ROSAT/PSPC.

---------------------------------------------------------
Title: Loop modeling of X-ray coronae: solar-type stars in young
clusters and associations.
Authors: Maggio, A.; Micela, G.; Peres, G.
1997MmSAI..68.1095M Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Radiatively-driven downdrafts and redshifts in transition
region lines. I. Reference model.
Authors: Reale, F.; Peres, G.; Serio, S.
1996A&A...316..215R Altcode:
We show that downdrafts driven by radiatively-cooling subarcsecond
perturbations in the solar transition region can produce significant
UV line redshifts consistent with those frequently observed on the
solar disk, and, by extension, with those more recently observed
on stars. We model in detail the thermo-hydrodynamic evolution of
a non-linear isobaric spherical condensation inside an initially
hydrostatic atmosphere, stratified both in density and temperature,
and with initial conditions typical of a solar active region. We study
the evolution of the atmosphere both neglecting the effect of magnetic
fields and taking it into account by forcing conduction and motions to
be only along the vertical direction. The synthesis of emitted flux and
effective speed in four UV lines from model results shows the presence
of intense redshifted components at speeds between 5 and 10km/s.

---------------------------------------------------------
Title: Propagation of three-dimensional Alfvén waves in a stratified,
thermally conducting solar wind
Authors: Orlando, S.; Lou, Y. -Q.; Rosner, R.; Peres, G.
1996JGR...10124443O Altcode: 1996JGR...10124433O
We model the propagation of three-dimensional, adiabatic, linear Alfvén
waves in the solar atmosphere and wind, taking into account relevant
physical effects, including gravity stratification, thermal conduction,
radiative losses, and heating (via a phenomenological term). Our
magnetohydrodynamic solar wind model also accounts for the momentum
deposition by a spectrum of non-WKB Alfvén waves. The transmission
and reflection of such waves has been previously studied by a variety
of techniques, including calculations based on the computation of the
ratio between the wavelength and the scale length of the Alfvén speed
change and based on a globally-computed transmission coefficient. In
this paper we discuss both techniques and show how they are related. We
also discuss the physics underlying the reflection process and the
possible role wave reflection might play in the acceleration of the
solar wind and the winds from other stars.

---------------------------------------------------------
Title: EPIC system onboard the ESA XMM
Authors: Villa, Gabriele E.; Abbey, Anthony F.; Arnaud, M.; Balasini,
M.; Bignami, Giovanni F.; Boer, M.; Brauninger, H.; Butler, I.;
Cafagna, G.; Cara, C.; Chabaud, C.; Chiappetti, L.; Cole, Richard
E.; Conte, M.; Dowson, J.; Dhez, Pierre; Duc, R.; Di Cocco, Guido;
Ferrando, P.; Goodall, C. V.; Hippman, H.; Holland, Andrew D.;
Kendziorra, E.; Labeque, A.; Meidinger, Norbert; Moriggio, C.; La
Palombara, N.; Musso, C.; Poindron, E.; Peres, G.; Pferrerman, E.;
Pigot, Claude; Pye, John P.; Reppin, C.; Schmitt, D.; Sciortino,
Salvatore; Serio, Salvatore; Spragg, J.; Stephen, J. B.; Struder,
L.; Trifoglio, M.; Tua, P.; Turner, M. J.; Whitehead, S.
1996SPIE.2808..402V Altcode:
The European photon imaging camera (EPIC) is one of the two main
instruments onboard the ESA X-Ray Cornerstone Mission XMM. It is
devoted to performing imaging and spectroscopy of the x-ray sky in
the domain 0.1 10 keV with a peak sensitivity in 10(superscript 5)
seconds of 2 multiplied by 10(superscript -15) erg/cm(superscript
-2). The x-ray instrumentation is complemented by a radiation monitor
which will measure the particle background. The spectral resolution is
approximately 140 eV at 6.4 keV and 60 eV at 1 keV. The instrumentation
consists of three separate focal plane cameras at the focus of the three
XMM telescopes, containing CCDs passively cooled to typically minus
100 degrees via radiators pointing toward the anti-Sun direction. The
two cameras with the field of view partially occulted by the RGS
grating boxes will have MOS technology CCDs while the third camera,
with full field of view, will be based on p-n technology. The CCDs
in the focal plane of the cameras will cover the entire 30 foot by
30 foot field of view of the telescope while the pixel size (40 by
40 (mu) for the MOS camera and 150 multiplied by 150 (mu) for the
p-n) will be adequate to sample the approximately 20' PSF of the
mirrors. In order to cope with a wide range of sky background and
source luminosity in the visible/UV band, a filter wheel with six
positions has been implemented in each camera. The six positions
correspond to: open position, closed position, one thin filter (1600
angstrom of plastic support and 400 angstrom of Al), one medium
filter (1600 angstrom of plastic support and 800 angstrom of Al)
and one thick filter (approximately 3000 angstrom of plastic support,
approximately 1000 angstrom of Al and 300 Angstrom of Sn). The final
position will be a redundant filter of type still to be decided. A
set of radioactive sources in each camera will allow the calibration
of the CCDs in any of the operating modes and with any filter wheel
position. Vacuum doors and valves operated will allow the operation
of other camera heads on the ground, in a vacuum chamber and/or in a
controlled atmosphere, and will protect the CCDs from contamination
until the spacecraft is safely in orbit. The MOS camera will have 7
CCDs, each of 600 by 600 pixels arranged in a hexagonal pattern with
one central and six peripheral. The p-n camera head will have 12 CCDs,
each with 200 multiplied by 64 pixels, in a rectangular arrangement,
4 quadrants of 3 CCDs each. The radiation monitor is based on two
separate detectors to monitor the low (electrons greater than 30 keV)
and the high (electrons greater than 200 keV and protons greater than
10 MeV) energy particles impinging on the telescope along its orbit.

---------------------------------------------------------
Title: Loop modeling of coronal X-ray spectra. II. Parameter
constraints and diagnostics.
Authors: Maggio, A.; Peres, G.
1996A&A...306..563M Altcode:
We have investigated some aspects of the fitting of stellar X-ray
spectra by means of hydrostatic models of coronal loops. We have
performed an extensive set of simulations in which we have synthesized
and fitted loop model spectra, considering the instrument responses of
the Einstein Observatory Imaging Proportional Counter, of the ROSAT
Position Sensitive Proportional Counter, and of the ASCA Solid-state
Imaging Spectrometer, and we have derived theoretical confidence regions
for the determination of the input loop parameters. We have found
that our ability to constrain the loop model parameters depends on the
distribution of the differential emission measure. More specifically,
we have found that, for loops significantly below the pressure scale
height, the spectral fitting allows to constrain well the plasma
peak temperature and hence the product of the base pressure with the
loop semi-length, but not each of the two separately, due to scale
invariance of the equations describing the hydrostatic loop model of
uniform pressure. For input loops with lengths of the order of the
pressure scale height or larger, the spectral fitting yields closed
confidence regions in the parameter space, allowing to constrain all the
loop parameters, with different levels of discrimination depending on
photon counting statistics and instrument energy response. Loops much
longer than the star's radius and with relatively high base pressure
behave like the short loops, because the pressure decrease along the
loop is moderated by the local increase of the pressure scale height.

---------------------------------------------------------
Title: Loop modeling of coronal X-ray spectra. I. General properties.
Authors: Ciaravella, A.; Peres, G.; Maggio, A.; Serio, S.
1996A&A...306..553C Altcode:
This work is motivated by the widely accepted paradigm that the
structure and features of stellar coronae are determined by plasma
magnetically confined in coronal loops. In particular we address the
question whether it is possible to devise diagnostics amenable to
reveal some physical and geometric characteristics of static coronal
loops from X-ray observations. We show that single-loop coronal
models may give spectra differing in several respects from one- and
two-temperature models, the differences being probably observable
already with ASCA/SIS. We find that the spectra of our loop models
are essentially determined by the plasma below one pressure scale
height; therefore we do not need to consider deviations from Spitzer's
conductivity which occur in the very diluted upper atmosphere. We also
find that widening of cross section of the loops may give observable
effects depending on loop parameters and that even a small fraction of
the star's surface covered by compact coronal loops with dense plasma
can easily dominate the emission from a corona and mask the emission
from long and tenuous loops. Finally we present an example of fitting
a real ROSAT/PSPC observation with loop-emitted spectra.

---------------------------------------------------------
Title: Radiatively cooling downdrafts as the origin of redshifts in
transition region lines
Authors: Reale, F.; Peres, G.; Serio, S.
1996ASPC..109..149R Altcode: 1996csss....9..149R
No abstract at ADS

---------------------------------------------------------
Title: Loop modeling of ROSAT/PSPC spectra of Hyades and field
F-type stars
Authors: Maggio, A.; Peres, G.; Pye, J. P.; Hodgkin, S. T.; Morley,
J. E.
1996ASPC..109..269M Altcode: 1996csss....9..269M
No abstract at ADS

---------------------------------------------------------
Title: Structure and stability of EUV loops originating from sunspots
Authors: Peres, G.; Orlando, S.
1996ASPC..109...93P Altcode: 1996csss....9...93P
No abstract at ADS

---------------------------------------------------------
Title: Diagnostics of Solar Flares from Hydrodynamic Modeling:
Using the Decay Phase to Derive the Loop Dimensions
Authors: Reale, F.; Peres, G.; Betta, R.
1996ASPC..111..232R Altcode: 1997ASPC..111..232R
The authors describe a method to derive the size of solar and stellar
flaring loops from the decay time of the light curve and the slope
of the decay trajectory in the density-temperature diagram of the
flare. They have calibrated the method with extensive numerical
hydrodynamic modeling. The method is designed for observations without
angular resolution, and, in view of its application to stellar flares,
is tested on solar flares observed by the Yohkoh SXT, which provides
light curves and temperature diagnostics, together with a direct
measure of the loop length.

---------------------------------------------------------
Title: A Regridding Algorithm for High Resolution Hydrodynamics of
Flaring Coronal Loops
Authors: Betta, R. M.; Peres, G.; Serio, S.; Reale, F.
1996mpsa.conf..473B Altcode: 1996IAUCo.153..473B
No abstract at ADS

---------------------------------------------------------
Title: Detailed fitting of coronal X-ray spectra with loop models:
model characteristics and simulations
Authors: Ciaravella, A.; Maggio, A.; Peres, G.; Serio, S.
1996ASPC..109..257C Altcode: 1996csss....9..257C
No abstract at ADS

---------------------------------------------------------
Title: Hydrodynamic Modeling of Flares Well-Observed by Yohkoh/SXT
Authors: Reale, F.; Peres, G.; Hudson, H.
1996mpsa.conf..311R Altcode: 1996IAUCo.153..311R
No abstract at ADS

---------------------------------------------------------
Title: Observation of small flares and a long-lasting pre-flare
dip. on V 1054 Ophiuchi
Authors: Ventura, R.; Peres, G.; Pagano, I.; Rodono, M.
1995A&A...303..509V Altcode:
We report on the observations and analysis of two small flares and of an
exceptionally long pre-flare dip observed on V 1054 Oph by a twin-beam
UBV photometer that permit to reach high photometric precision. The
monitoring of V 1054 Oph has been carried out as part of a project
mainly devoted to the detection of low amplitude optical variability
and microflares in very active red dwarf flare stars. We discuss our
results in the framework of present theories on stellar flares and
pre-flare dips, including the particularly severe constraints that
the present results - and previous observations of pre-flare dips on
FF And - pose on the most relevant flare models.

---------------------------------------------------------
Title: Effects of flows and non-equilibrium ionization on some C IV
line ratios on the Sun.
Authors: Spadaro, D.; Orlando, S.; Peres, G.; Leto, P.
1995A&A...302..285S Altcode:
We investigate the deviations from ionization equilibrium occurring in
coronal loops hosting steady-state siphon flows, and their effects on
the EUV emission line ratios of C IV recently considered by Keenan et
al. (1992) and found discrepant with observations of highly dynamics
events. We use the same electron impact excitation rates adopted by
Keenan et al. and two independent siphon flow models. The C IV line
ratios predicted by our models do not differ appreciably between
equilibrium and non-equilibrium conditions, and therefore indicate
that the discrepancy found by Keenan et al. (1992) cannot be solved
by steady flows. We discuss alternative solutions, also in view of
forthcoming observations by CDS/SOHO.

---------------------------------------------------------
Title: Models of stationary siphon flows in stratified, thermally
conducting coronal loops. II. Shocked solutions.
Authors: Orlando, S.; Peres, G.; Serio, S.
1995A&A...300..549O Altcode:
We use a detailed hydrodynamic model which takes into account the
effects of gravity, of radiative losses, of thermal conduction and of a
parametrized heating term to explore the conditions for the presence of
stationary shocks in critical and supersonic siphon flows in coronal
loops. We consider separately isothermal and adiabatic shocks. The
exploration of the parameter space shows that in every solution,
as characterized by loop length, base dynamical pressure and base
velocity, the shock position depends on the volumetric heating rate of
the loop. In addition, with the exception of the critical solutions
with isothermal shocks, there exists a range of volumetric heating
rates for which there are two alternative positions where the shock
can form. Scaling laws for supersonic solutions are derived.

---------------------------------------------------------
Title: Solar flare X-ray imaging: coronal loop hydrodynamics and
diagnostics of the rising phase.
Authors: Reale, F.; Peres, G.
1995A&A...299..225R Altcode:
We discuss the flare diagnostics possible with the images of flaring
loops obtained with the Soft X-ray Telescope (SXT) on board Yohkoh. We
use hydrodynamic models of individual flaring loops to synthesize in
detail the distribution of surface brightness, and its evolution,
in the pass-band of the SXT during the rising thermal phase. The
point is to use our synthesized sequences as a guide to interpret
data from Yohkoh SXT. On the basis of synthesized images (and the
insight on hydrodynamics), we recognize four phases of the flare:
the conduction phase (practically unobservable, whenever present),
the chromospheric evaporation, the brightening at the loop apex and
the peak phase. For each phase we discuss the possible diagnostics and,
when possible, we discuss its connection with the heating mechanisms. A
common feature of our results for observations through the Be filter is
the initial propagation of brightness fronts upwards from the base to
the top of the loop, in fact associated with chromospheric evaporation
fronts: only the details of this propagation can depend on the flare
heating mechanism and on its location. Bright evaporation fronts are
preferentially produced by a localized thermal heating (especially at
the loop footpoints); heating by electron beams instead produce a more
gradual emission evolution. All our models predict a brightening of the
top of the loop after the evaporation phase, independently of the site
and mechanism of energy release. We find that the hardest SXT filter
(the Be filter) should access a wider variety of emission structures and
therefore provide a richer diagnostics, while the other filters should
preferentially show bright loop feet, especially during hot (~2x10^7^K)
flares in relatively long loops (e.g. 6x10^9^cm). We show that, for a
correct analysis of the images, it is crucial to evaluate accurately
loop length and the plasma temperature; for instance, some observable
features can be properly interpreted only knowing the loop temperature.

---------------------------------------------------------
Title: Models of stationary siphon flows in stratified, thermally
conducting coronal loops. I. Regular solutions.
Authors: Orlando, S.; Peres, G.; Serio, S.
1995A&A...294..861O Altcode:
We present a detailed hydrodynamic model of stationary siphon flows
in a semicircular solar coronal loop of constant cross section,
which takes into account simultaneously gravity, thermal conduction,
radiative losses and a phenomenological heating term. The model has
been designed to provide a tool for insight into coronal loop physics
and to help interpretation of the observations of new UV and X-ray
instruments. In order to achieve high reliability of the calculations
even close to critical points, we have used two very accurate and
independent algorithms, and compared their results. Extensive analytical
derivations have supplemented numerical calculations to study the
characteristics of solutions. We distinguish four classes of solutions:
subsonic, critical, supersonic and unphysical. The characteristics of
the solutions space depend significantly on the boundary conditions,
for instance supersonic solutions exist only for negative heat flux
at the base of the loop. We have explored thoroughly the space of
solutions, and have derived scaling laws for subsonic solutions which
relate the dynamic pressure p+ρv^2^, the maximum loop temperature, the
loop semilength and the volumetric heating; such scaling laws extend
those of Rosner Tucker &amp; Vaiana (1978) and of Serio et al. (1981)
to the siphon flows. Additional laws and conditions for various classes
of siphon flows are derived.

---------------------------------------------------------
Title: A Long-Duration Pre-flare Dip in the Light Curve of V1054 OPH
Authors: Ventura, R.; Pagano, L.; Peres, G.; Rodòn, M.
1995LNP...454..108V Altcode: 1995flfl.conf..108V; 1995IAUCo.151..108V
No abstract at ADS

---------------------------------------------------------
Title: A Major Optical Flare on the Recently Discovered X-ray Active
dMe Star G102-21
Authors: Pagano, I.; Ventura, R.; Rodonò, M.; Peres, G.; Micela, G.
1995LNP...454...95P Altcode: 1995flfl.conf...95P; 1995IAUCo.151...95P
No abstract at ADS

---------------------------------------------------------
Title: Thermally Unstable Perturbations in Stratified Conducting
Atmospheres
Authors: Reale, Fabio; Serio, Salvatore; Peres, Giovanni
1994ApJ...433..811R Altcode:
We investigate the thermal stability of isobaric perturbations in a
stratified isothermal background atmosphere with solar abundances,
as resulting from the competition of optically thin plasma radiative
cooling and of heating conducted from the surrounding atmosphere. We
have analyzed the threshold line between stable and unstable
perturbations, in the plane of the two important control parameters: the
initial size of the perturbation and the temperature of the unperturbed
medium; this line changes with the pressure of the unperturbed
atmosphere. We have extended the results of linear perturbation analysis
by means of numerical calculations of the evolution of spherical
isobaric perturbations, using a two-dimensional hydrodynamic code
including Spitzer heat conduction. We explore a wide range of the
parameters appropriate to the solar and stellar upper atmospheres:
the background uniform temperature is between 105 K and
107 K, the initial pressure betweeen 0.1 and 10 dyn/sq cm,
and the perturbation size between 105 and 1010
cm. The numerical results are in substantial agreement with the linear
analysis. We discuss possible implications of our results also in
terms of observable effects, especially concerning plasma downflows,
and propose thermal instability as a possible candidate to explain
the observed redshifts in solar and stellar transition region lines.

---------------------------------------------------------
Title: ASAP: A systematic approach to plasma spectral synthesis
Authors: Reale, F.; Maggio, A.; Ciaravella, A.; Peres, G.
1994SSRv...70..211R Altcode:
ASAP (Analysis System for Astrophysical Plasmas), developed at the
Osservatorio Astronomico di Palermo, is a package of procedures based
on IDL. It is aimed at the detailed presentation of theoretical models
of astrophysical plasmas and the accurate comparison with observational
data, up to fittings of specific observations. It is also useful for
the prediction of specific observations, and for the simulation of the
expected performances of forthcoming instruments, for instance those
on board SOHO.

---------------------------------------------------------
Title: Stationary flows in coronal loops
Authors: Orlando, S.; Serio, S.; Peres, G.
1994SSRv...70..203O Altcode:
We present a study of stationary flows in closed solar coronal
loops. The hydrodynamic differential equations of plasma flow and
energy balance are integrated with algorithms which achieve high
reliability. We present here results on the detailed synthesis of
loop emission in specific bands and lines, taking into account also
non-equilibrium ionization.

---------------------------------------------------------
Title: Energy Transport and Dynamics
Authors: Schmieder, B.; Peres, G.; Enome, S.; Falciani, R.; Heinzel,
P.; Hénoux, J. C.; Mariska, J.; Reale, F.; Rilee, M. L.; Rompolt, B.;
Shibasaki, K.; Stepanov, A. V.; Wülser, J. P.; Zarro, D.; Zharkova, V.
1994SoPh..153...55S Altcode:
We report findings concerning energy transport and dynamics in flares
during the impulsive and gradual phases based on new ground-based and
space observations (notably fromYohkoh). A preheating sometimes occurs
during the impulsive phase. CaXIX line shifts are confirmed to be
good tracers of bulk plasma motions, although strong blue shifts are
not as frequent as previously claimed. They often appear correlated
with hard X-rays but, forsome events, the concept that electron beams
provide the whole energy input to the thermal component seems not
to apply. Theory now yields: new diagnostics of low-energy proton
and electron beams; accurate hydrodynamical modeling of pulse beam
heating of the atmosphere; possible diagnostics of microflares (based
on X-ray line ratio or on loop variability); and simulated images of
chromospheric evaporation fronts. For the gradual phase, the continual
reorganization of magnetic field lines over active regions determines
where and when magnetic reconnection, the mechanism favoured for energy
release, will occur. Spatial and temporal fragmentation of the energy
release, observed at different wavelengths, is considered to be a
factor as well in energy transport and plasma dynamics.

---------------------------------------------------------
Title: The analysis system for astrophysical plasmas (ASAP) of the
Osservatorio Astronomico di Palermo
Authors: Maggio, A.; Reale, F.; Peres, G.; Ciaravella, A.
1994CoPhC..81..105M Altcode:
We present the software package ASAP, mainly devoted to the presentation
and analysis of fluid models of astrophysical plasmas. ASAP allows
to generate optically thin X-ray and UV spectra emitted by plasma of
given density, temperature and velocity, by means of well established
spectral synthesis codes; the emission can then be easily folded
with the instrumental response of interest and compared directly
with measurements, or can be used to predict the performance of
planned instruments. As examples of typical applications of the
ASAP package, we show high resolution emission spectra derived from
1D static models of coronal loops, detailed fitting with coronal loop
models of observations made with the ROSAT/PSPC instrument, hydrodynamic
simulations of a solar flaring loop as seen by the Yohkoh satellite,
and the evolution of line emission from a dense blob of interstellar
plasma subject to the passage of a shock. The package, designed
to be highly flexible, modular, and easily expandable, is expected to
evolve and grow rapidly in response to new needs and required tasks. It
operates within the IDL environment, and it is under consideration
for being included in the SOHO Archive System.

---------------------------------------------------------
Title: Accurate Period Determination of an Eclipsing Binary X-Ray
Source in M33
Authors: Schulman, Eric; Bregman, Joel N.; Collura, Alfonso; Reale,
Fabio; Peres, Giovanni
1994ApJ...426L..55S Altcode:
In the Letter "Accurate Period Determination of an Eclipsing Binary
X- Ray Source in M33" by Eric Schulman, Joel N. Bregman, Alfonso
Collura, Fabio Reale, and Giovanni Peres (ApJ, 418, L67 [1993]),
the time between the ROSAT and Einstein Observatory observations was
incorrectly calculated, leading to an incorrect period determination
of P_orb_ = 154286 s (1.78572^d^). The correct period is P_orb_ =
154298 s (1.78586^d^) and the center of the first eclipse observed by
Einstein occurred at J.D. 2,444,087.6061. The figures are unchanged,
except that the times shown in Figures 1d and 1e are too large by
4.32 x 10^5^s. The correct stop time of the ROSAT observation section
RH600020b is 00^h^32^m^52^s^ (Table 1).

---------------------------------------------------------
Title: Loop Models of Low Coronal Structures Observed by the Normal
Incidence X-Ray Telescope (NIXT)
Authors: Peres, G.; Reale, F.; Golub, L.
1994ApJ...422..412P Altcode:
The X-ray pictures obtained with the Normal Incidence X-Ray Telescope
(NIXT), apart from the ubiquitous coronal loops well known from
previous X-ray observations, show a new and peculiar morphology: in
many active regions there are wide and apparently low-lying areas of
intense emission which resemble H alpha plages. By means of hydrostatic
models of coronal arches, we analyze the distribution of temperature,
density, emission measure, and plasma emissivity in the spectral band to
which NIXT is sensitive, and we show that the above morphology can be
explained by the characteristics of high pressure loops having a thin
region of high surface brightness at the base. We therefore propose
that this finding might help to identify high-pressure X-ray emitting
coronal regions in NIXT images, and it is in principle applicable to
any imaging instrument which has high sensitivity to 104 -
106 K plasma within a narrow coronal-temperature passband. As
a more general result of this study, we propose that the comparison
of NIXT observations with models of stationary loops might provide
a new diagnostic: the determination of the loop plasma pressure from
measurements of brightness distribution along the loop.

---------------------------------------------------------
Title: 15-30 arcsec resolution replica x-ray optics for AXAF-S
Authors: Stella, Luigi; Chincarini, G.; Citterio, Oberto; Conconi,
Paolo; Maccacaro, T.; Tagliaferri, G.; Trinchieri, G.; Wolter, A.;
Bignami, Giovanni F.; Bocchino, F.; Maggio, A.; Micela, G.; Sciortino,
Salvatore; Serio, Salvatore; Collura, Alfonso; Giommi, P.; Maraschi,
L.; Pallavicini, R.; Pellegrini, S.; Peres, G.
1994SPIE.2011..149S Altcode:
This document describes a proposal for a replica X-ray optics to be
developed in Italy for NASA's X-ray spectroscopy mission AXAF-S. The
program is based on state of the art technology for the production of
replica X-ray optics. On the basis of the experience with the Jet-X
mirror shells (to be flown on the Spectrum X-(gamma) satellite),
a spatial resolution of 15 - 30 arcsec half power diameter (HPD)
can be achieved for the AXAF-S optics. The characteristics of the
proposed optical system are described and its performances evaluated
by using the current baseline configuration for the array of X-ray
calorimeters in the focal plane. The impact of the proposed replica
X-ray optics is briefly outlined and a comparison with foil optics
(1 - 3 arcmin HPD resolution) is carried out.

---------------------------------------------------------
Title: Siphon Flows in Stratified Coronal Loops
Authors: Orlando, S.; Peres, G.; Serio, S.
1994scs..conf..185O Altcode: 1994IAUCo.144..185O
The authors have developed a detailed siphon flow model for coronal
loops. They find scaling laws relating the characteristic parameters
of the loop, explore systematically the space of solutions and show
that supersonic flows are impossible for realistic values of heat flux
at the base of the upflowing leg.

---------------------------------------------------------
Title: Impulsive Heating of Coronal Loops
Authors: Reale, F.; Peres, G.; Serio, S.
1994scs..conf..215R Altcode: 1994IAUCo.144..215R
With the aid of hydrodynamic modeling of coronal plasma, the authors
have investigated the signatures of a heating composed by a sequence of
time-limited episodes (microflares) in possible continuous observations
of loops by several instruments, like Yohkoh/SXT and the forthcoming
SOHO/EIT.

---------------------------------------------------------
Title: Pressure diagnostics of coronal loops observed by NIXT
Authors: Peres, G.; Reale, F.; Golub, L.
1994LNP...432..179P Altcode: 1994LNPM...11..179P
The Normal Incidence X-ray Telescope (NIXT sounding rocket payload —
a set of multilayer telescopes of novel design — provides images of
the corona at sub-arcsec angular resolution in narrow X-ray spectral
bands centered at the wavelengths of particular coronal emission
lines. The NIXT 63.5 angstrom coronal images show the well-known
and ubiquitous coronal loops but also, mostly in active regions,
shallow and bright areas of intense emission not resembling loops. We
have explained such areas within the traditional physics of coronal
loop models as intense emission in the NIXT band coming from a narrow
region at the base of high pressure loops; the particular nature of the
NIXT temperature sensitivity, with its bimodal temperature response,
is the key to detecting such a feature. We discuss the implications
of this finding and, in particular, we show the possibility of new
diagnostics of plasma pressure independent of the traditional one based
on the determination of emission measure. In addition, we show that
there is very little dependence of the spatial distribution of the
X-ray emissivity in the NIXT passband on the details of the spatial
distribution of the heating function.

---------------------------------------------------------
Title: Accurate Period Determination of an Eclipsing Binary X-Ray
Source in M33
Authors: Schulman, Eric; Bregman, Joel N.; Collura, Alfonso; Reale,
Fabio; Peres, Giovanni
1993ApJ...418L..67S Altcode:
We have analyzed the time variability of one of the X-ray sources in
M33 observed by both the ROSAT and Einstein Observatory telescopes. The
light curve of M33 X-7 exhibits a variability pattern of high and low
states, suggesting an eclipsing binary X-ray source. The data suggest
a binary period P = 1.78572 days (very close to that of Her X-1) and
an eclipse duration of ~0.4 days. The low phase lasts about one-fourth
of the period as in Cen X-3.

---------------------------------------------------------
Title: Low amplitude variability and transient periodicity in FF
Andromedae and other active stars.
Authors: Peres, G.; Ventura, R.; Pagano, I.; Rodono, M.
1993A&A...278..179P Altcode:
This work presents the first results of a project mainly dedicated to
the detection of low amplitude variability in stars with the URSULA
double channel photometer fed by the 91 cm telescope of Catania
Astrophysical Observatory. URSULA simultaneously observes the target
star and a nearby comparison star with two identical photometers and
achieves a relative photometric precision down to a few thousandths
of magnitude because rapid fluctuations of sky transparency equally
affect both stars and are easily removed. We can, therefore, use short
integration times and attain high time resolution. The analysis of
our observations is based on the methods of periodograms, according to
the formulation for unequally spaced time sequences devised by Scargle
(1982). We have observed FF And, BY Dra, V 1054 Oph and V 1396 Cyg and
detected, at a high level of confidence, several cases of small scale
variability, particularly evident in the data of FF And and of V1054
Oph. From FF And we have detected small, but highly significant, dips
in the light curve, approximately 25 minutes before a large flare. We
compare this finding with the known phenomenology of pre-flare dips and,
among other interpretations, we propose them as possible signatures
of pre-flare atmospheric activation. We also discuss the data set
pertaining to all observed stars.

---------------------------------------------------------
Title: Simulations of the CA XIX Spectral Emission from a Flaring
Solar Coronal Loop. II. Impulsive Heating by Accelerated Electrons
Authors: Antonucci, E.; Dodero, M. A.; Martin, R.; Peres, G.; Reale,
F.; Serio, S.
1993ApJ...413..786A Altcode:
In this paper we analyze the response of the plasma inside a coronal
loop to the passage of a beam of accelerated electrons and study it
systematically as a function of the parameters and temporal profile
of the beam, as well as of the initial conditions of the corona. Our
approach consists in computing the spectral emission from an impulsively
heated loop in the Ca XIX line complex in the range 3.165-3.231 A and
in analyzing the simulated spectra with the same technique used for
the observations. This allows a direct comparison of the simulations
with the results of the spectral observations of the Bent Crystal
Spectrometer on board the SMM. While in some cases the values and
evolution of average plasma properties, as derived from the analysis
of spectra, are reproduced with good accuracy, the detailed shape of
the Ca XIX lines cannot be accurately reproduced by this kind of model
of flares heated by nonthermal electron beams.

---------------------------------------------------------
Title: Detectability of chromospheric evaporation fronts in solar
flares
Authors: Peres, G.; Reale, F.
1993A&A...275L..13P Altcode:
We compute the surface brightness distribution of the chromospheric
evaporation front during the flare impulsive phase in the pass-band of
the imaging Soft X-ray Telescope (SXT) on board Yohkoh. Hydrodynamic
simulations of the flaring plasma allow us to derive the evolution
of density and temperature distribution and therefore of emission
distribution across the evaporation fronts. We show that SXT - as
well as imaging X-ray telescopes with equivalent or better spatial
and temporal resolution - should be able to detect clearly the front
and follow its evolution along the loop.

---------------------------------------------------------
Title: Dynamics of the decay of confined stellar X-ray flares
Authors: Reale, F.; Serio, S.; Peres, G.
1993A&A...272..486R Altcode:
We extend and generalize to stellar X-ray flares the analysis of
the decay phase of flares inside solar coronal loops of constant
cross-section, and investigate related diagnostics from X-ray
observations. The work is based on hydrodynamic simulations of compact
stellar flares for different values of stellar surface gravity,
loop length and peak temperature, and characteristics of loop heating
during the decay phase. We discuss the validity of the thermodynamic
scaling law of Serio et al. (1991) in the wider context of stellar
flares, and find that a generalized version of this law holds if the
loop half-length L is smaller than the peak pressure scale height
h. In the light of the diagnostics which can be derived from the
density-temperature (n-T) diagram, already tested for solar flares, we
consider how the ratio ζ of the density decay time to the temperature
decay time depends on gravity. We find that ζ, as computed at the
top of the loop, has a maximum value ζ ∼ 3 for very small L/h and
decreases as L/h increases. Alternatively we consider the average flare
density and temperature as they would be derived from observations
with the ROSAT PSPC or other moderate energy resolution detectors,
we obtain a flatter curve, with a maximum value around 2. We discuss
also how energy deposition during the flare decay phase influences ζ,
and propose a combined diagnostic approach on stellar flares based
on the joint use of the n-T diagrams and of the fitting of the light
curve with hydrodynamic models. As a first practical result we can
safely exclude the presence of sustained heating during the decay of
an X-ray flare observed on Prox Cen by the EINSTEIN/IPC instrument.

---------------------------------------------------------
Title: Thermal Conduction and Modeling of Static Stellar Coronal Loops
Authors: Ciaravella, A.; Peres, G.; Serio, S.
1993SoPh..145...45C Altcode:
We have modeled stellar coronal loops in static conditions for a wide
range of loop length, plasma pressure at the base of the loop and
stellar surface gravity, so as to describe physical conditions that
can occur in coronae of stars ranging from low mass dwarfs to giants
as well as on a significant fraction of the Main-Sequence stars.

---------------------------------------------------------
Title: Stellar XUV Spectra from Coronal Loop Models
Authors: Maggio, A.; Serio, S.; Peres, G.
1993AAS...182.4126M Altcode: 1993BAAS...25..864M
Following the analogy with the solar corona, we describe the
observed EUV and X-ray emission from late-type stars as originating
from optically thin plasma magnetically confined in closed loops,
according to the model by Serio et al. (1981). This model takes into
account optically thin radiative losses, thermal conduction, and
heating terms in the energy balance equation, as well as gravity in
the hydrostatic equilibrium equation, thus providing a description
physically more appropriate than that obtained by simple one-
or two-temperature thermal models. We generate loop models with a
variety of characteristic parameters (such as loop base pressure,
loop length, or stellar surface gravity), to synthesize XUV spectra
from the emission measure vs. temperature model distribution, and
to fold them through the relevant instrumental response, in order to
allow a direct comparison with observational data. In this paper, we
show some examples of this approach pertaining to ROSAT and EUVE data.

---------------------------------------------------------
Title: Sky simulations for X-ray telescopes
Authors: Peres, G.; Micela, G.; Barbera, M.; Sciortino, S.; Serio, S.
1993MmSAI..64..716P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Palermo analysis system for astrophysical plasmas (ASAP):
rationale, development, and applications
Authors: Maggio, A.; Reale, F.; Peres, G.
1993MmSAI..64.1023M Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The importance of plasma viscosity on X-ray line diagnostics
of solar flares
Authors: Peres, G.; Reale, F.
1993A&A...267..566P Altcode:
We discuss the importance of compressional viscosity for a correct
description of flare plasma dynamics and for its diagnostics with high
resolution soft X-ray spectroscopy. We have separated the influence
of viscosity from that of other physical effects at work during flares
by comparing hydrodynamic simulations differing only for the presence
or absence of viscosity. We show that the influence is significant on
the flaring plasma dynamics as well as on the relevant spectra, when
the flare heating is mostly delivered either at the loop apex or by
means of low-energy-cutoff nonthermal electron beams. For base-heated
loop flares, instead, the effect of viscosity is significant only with
very intense heating. We also discuss possible markers of shocks in
the observed spectra of Ca XIX (3.18 A) and Fe XXV (1.85 A) and show
that viscosity influences time-integrated spectra and, a fortiori,
spectra with one second-time resolution, as available with the Bragg
Crystal Spectrometer on board YOHKOH.

---------------------------------------------------------
Title: Thermodynamic decay of stellar coronal flares
Authors: Reale, F.; Serio, S.; Peres, G.
1993MmSAI..64..555R Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Hydrodynamics and Diagnostics of Coronal Loops Subject to
Dynamic Heating
Authors: Peres, G.; Reale, F.; Serio, S.
1993ASSL..183..151P Altcode: 1993pssc.symp..151P
No abstract at ADS

---------------------------------------------------------
Title: Scaling Laws for the Decay Phase of Stellar Loop Flares
Authors: Reale, F.; Serio, S.; Peres, G.
1993ASSL..183..599R Altcode: 1993pssc.symp..599R
No abstract at ADS

---------------------------------------------------------
Title: Soft X-ray Line Shifts as Signature of the Flare Heating
Process
Authors: Antonucci, E.; Dodero, M. A.; Peres, G.; Reale, F.; Serio,
S.; Somov, B. V.
1993ASSL..183..159A Altcode: 1993pssc.symp..159A
No abstract at ADS

---------------------------------------------------------
Title: The G. S. Vaiana X-ray Astronomy Calibration and Testing
(XACT) Facility
Authors: Collura, A.; Barbera, M.; Inzerillo, G.; Maggio, A.; Micela,
G.; Mirabello, F.; Sciortino, S.; Serio, S.; Peres, G.
1993ASSL..183..275C Altcode: 1993pssc.symp..275C
No abstract at ADS

---------------------------------------------------------
Title: Thermal Transport in Static Stellar Coronal Loops
Authors: Ciaravella, A.; Peres, G.; Serio, S.
1993ASSL..183..595C Altcode: 1993pssc.symp..595C
No abstract at ADS

---------------------------------------------------------
Title: Observations of Low Level Optical Variability in Active Stars
Authors: Ventura, R.; Pagano, I.; Peres, G.; Rodonò, M.
1993ASSL..183..463V Altcode: 1993pssc.symp..463V
No abstract at ADS

---------------------------------------------------------
Title: A tool for X-Ray sky simulations.
Authors: Peres, G.; Micela, G.; Sciortino, S.; Serio, S.
1993MmSAI..64..385P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Siphon Flow Models of Coronal Loops
Authors: Orlando, S.; Serio, S.; Peres, G.
1993ASSL..183..163O Altcode: 1993pssc.symp..163O
No abstract at ADS

---------------------------------------------------------
Title: Steady Siphon Flows in Closed Coronal Structures: Comparison
with Extreme-Ultraviolet Observations
Authors: Peres, G.; Spadaro, D.; Noci, G.
1992ApJ...389..777P Altcode:
Models of steady siphon flow in solar coronal loops are computed and the
intensities of some EUV transition region emission lines synthesized
from these models are compared with representative obsevations of
typical solar regions. It is found that siphon flow models of active
region loops are in better agreement with observations of EUV transition
region lines, while for large loops interconnecting different active
regions, static models work slightly better. The role of siphon flows
in determining the structure of the outer solar atmosphere is discussed
based on these results.

---------------------------------------------------------
Title: The effect of viscosity on hydrodynamics of coronal flares
Authors: Reale, F.; Peres, G.
1992AIPC..267..140R Altcode: 1992ecsa.work..140R
We investigate the effect of plasma viscosity in the hydrodynamics
of coronal flares. To this end we compute two otherwise identical
models of a typical coronal compact flare, one including and the other
neglecting viscosity terms from the relevant hydrodynamic equations. We
find significant differences which may affect the diagnosis of observed
high resolution X-ray spectra.

---------------------------------------------------------
Title: Dynamics of flaring loops. II - Flare evolution in the
density-temperature diagram
Authors: Jakimiec, J.; Sylwester, B.; Sylwester, J.; Serio, S.; Peres,
G.; Reale, F.
1992A&A...253..269J Altcode:
The evolution of basic thermodynamic parameters of a single flaring
solar loop has been investigated in terms of density-temperature (N-T)
diagram. A grid of hydrodynamic models has been calculated for this
purpose, using the Palermo-Harvard code. The calculated models differ
in their initial conditions and the form of the energy imput (i.e., the
heating rate value, the heating duration, the assumed time profile). The
consequences of variation of these model parameters on the evolutionary
paths in the density-temperature diagrams is considered; over a
substantial duration, the decay occurs along a T varies as N-squared
trajectory when the impulsive flare heating function is switched-off
abruptly. The results obtained can be very useful as diagnostics of
the flare heating process, based on soft X-ray observations.

---------------------------------------------------------
Title: Thermodynamic decay scaling laws in solar loop flares
Authors: Serio, S.; Reale, F.; Peres, G.; Jakimiec, J.; Sylwester,
B.; Sylwester, J.
1992MmSAI..63..763S Altcode:
Results of numerical calculations are used, together with analytical
considerations, as a guide to set up diagnostic tools for the flare
decay phase in terms of the temperature-density (n-T) diagram. The
decaying loop was modeled as a hydrodynamic process in a rigid
semicircular tube. It is shown that the trajectory on the n-T plane
has a slope of about 2 for a wide set of peak model flare conditions.

---------------------------------------------------------
Title: Modelling of open and closed coronal structures - Comparison
with detailed EUV observations
Authors: Peres, G.; Spadaro, D.
1992sws..coll...87P Altcode:
We consider the modelling and EUV diagnostics of plasma in steady
state motion both within closed coronal structures (siphon flows) and
outflowing from coronal holes toward interplanetary space. We take into
account nonequilibrium ionization in the synthesis of emission lines
originating from the modelled closed structures and compare the computed
line intensities with detailed EUV observations, in order to constrain
significantly the model. We evaluate the importance of nonequilibrium
ionizations effects for some published coronal hole models.

---------------------------------------------------------
Title: Hydrostatic models of X-ray coronal loops observed by NIXT
Authors: Peres, G.; Reale, F.; Golub, Leon
1992AIPC..267..136P Altcode: 1992ecsa.work..136P
Observations made with the Normal Incidence X-ray Telescope (NIXT)
have shown that some X-ray emitting structures observed with NIXT
resemble very closely the corresponding Hα plages. We have used
hydrostatic models of coronal loops to explain such observations as
strong emission from the lower section of high-presssure coronal arches.

---------------------------------------------------------
Title: Flare Evolution in the Density - Temperature Diagram
Authors: Serio, S.; Reale, F.; Peres, G.; Jakimiec, J.; Sylwester,
B.; Sylwester, J.
1992LNP...399..135S Altcode: 1992esf..coll..135S; 1992IAUCo.133..135S
No abstract at ADS

---------------------------------------------------------
Title: Non-local Thermal Conduction in Solar and Stellar Coronal Loops
Authors: Ciaravella, A.; Peres, G.; Serio, S.
1992LNP...397..261C Altcode: 1992sils.conf..261C
No abstract at ADS

---------------------------------------------------------
Title: Numerical simulations of thermal instabilities in stratified
gases. II - Exploration of the parameter space
Authors: Reale, F.; Rosner, R.; Malagoli, A.; Peres, G.; Serio, S.
1991MNRAS.251..379R Altcode:
The temporal evolution of density perturbations in an initially
hydrostatic isothermal atmosphere consisting of an optically thin
radiating compressible plasma is studied. Numerical techniques are
used to describe the nonlinear evolution of the perturbations, and
the relative equilibrium between dynamic and thermal instabilities as
governed by three independent control parameters are examined, namely,
the initial density contrast of the perturbation, the ratio of the
local buoyancy oscillation period to the local radiative cooling time,
and the ratio of the perturbation radius to the local scaleheight. Four
orders of magnitude of initial density contrasts and ratios of buoyancy
and cooling times, and one order of magnitude of the bubble dimensions
are explored. Well-defined oscillations were found to occur in a limited
parameter range, and thermal instability to occur even within secondary
condensations deriving from the bubble fragmentation.

---------------------------------------------------------
Title: Non-local heat transport in static solar coronal loops
Authors: Ciaravella, A.; Peres, G.; Serio, S.
1991SoPh..132..279C Altcode:
We investigate the limits of applicability of the Spitzer-Harm
thermal conductivity in solar coronal loops and show that the ratio
λ0/LTof electron mean-free path to temperature
scale height in large-scale structures can approach the limits of
the Spitzer-Harm theory. We use a non-local formulation of heat
transport to compute a grid of loop models: the effects of non-local
transport on the distribution of differential emission measure are
particularly important in the coronal part of loops longer than the
pressure scale height sp.We derive a scaling law for
λ0/LTin the corona, showing that it grows
exponentially with L/sp, and discuss effects of non-local
heat transport in the transition region.

---------------------------------------------------------
Title: European Photon Imaging Camera (EPIC) for X-ray astronomy.
Authors: Bignami, G. F.; Villa, G. E.; Boella, G.; Bonelli, G.;
Caraveo, P.; Chiappetti, L.; Quadrini, M. E.; Di Cocco, G.; Trifoglio,
M.; Ubertini, P.; Peres, G.; Sciortino, S.; Serio, S.; Vaiana, G.;
Rothenflug, R.; Vigroux, L.; Koch, L.; Rio, Y.; Pigot, C.; Cretolle,
J.; Gabriel, A.; Foing, B.; Atteia, J. L.; Roques, J. P.; Bräuninger,
H.; Pietsch, W.; Predehl, P.; Reppin, C.; Struder, L.; Trümper, J.;
Lutz, G.; Kendziorra, E.; Staubert, R.; Holland, A. D.; Cole, R. E.;
Wells, A.; Pounds, K.; Lumb, D. A.; Pye, J.; Turner, M. J. L.; Goodall,
C. V.; Ponman, T. J.; Skinner, G. K.; Willmore, A. P.
1990SPIE.1344..144B Altcode: 1990exrg.conf..144B
ESA has selected the final payload for its "Cornerstone" mission
in X-Ray astronomy with multiple mirrors (XMM), to be flown in the
late nineties in the context of the "Horizon 2000" long term science
plan. EPIC represents the main instrument of the mission, to include
three CCD arrays in the focal planes of the three telescopes of the
spacecraft. They will be dedicated to source imaging, photometry,
spectroscopy and timing. The goals of EPIC are described.

---------------------------------------------------------
Title: A two-dimensional hydrodynamic code for astrophysical flows.
Authors: Reale, F.; Peres, G.; Serio, S.
1990NCimB.105.1235R Altcode:
The authors present a two-dimensional hydrodynamic code suited to study
astrophysical flows in many different environments. The code solves the
hydrodynamic equations in conservative form in the most used coordinate
systems and is based on an explicit fully two-dimensional flux corrected
transport (FCT) technique, which ensures an accurate description of
steep gradient regions and shocks, a relatively ample flexibility
to include a variety of physical effects, and a good efficiency for
speed on vector or array processors. Extensive testing has allowed
an accurate "tuning" of the FCT numerical parameters. This code is
among the best FCT codes and performs well in a whole set of demanding
strongly nonlinear hydrodynamic tests, getting close to performances
of more complex codes, while remaining less computationally expensive
and more than sufficient for most astrophysical applications.

---------------------------------------------------------
Title: One-dimensional hydrodynamic modeling of coronal plasmas on
transputer arrays
Authors: Reale, F.; Brugé, F.; Peres, G.; Fornili, S. L.; Martorana,
V.; Serio, S.
1990CoPhC..60..201R Altcode:
We describe a concurrent implementation of the Palermo-Harvard
hydrodynamic code on cost-effective and modularity expandable
transputer arrays. We have tested the effectiveness of our approach
by simulating an already well-studied compact solar-flare model on
different transputer configurations and compared their performances
with those of other machines. We have found that the speed of the
concurrent program on a 16-T800 transputers array is ~1/9 of that of
the equivalent code optimized for a CRAY X-MP/48. This work clearly
shows that transputer-based arrays provide locally available high
computing-power tools to extend the investigation of compact solar
flares and similar astrophysical problems.

---------------------------------------------------------
Title: Two Variable X-Ray Sources in M31
Authors: Collura, A.; Reale, F.; Peres, G.
1990ApJ...356..119C Altcode:
We have analyzed the entire set of Einstein observations of M31,
consisting of five imaging proportional counter and nine high resolution
imager observations, for variability of the X-ray sources. The
time scales explored range up to ~ 10^5^ s, the maximum time span of
individual observations. We detect variability above the adopted 99.73%
significance level in two sources and derive constraints on their
possible periodicity. The observed X-ray luminosities, the pattern of
variability, and the optical identifications suggest the possibility
that one of them is a low- mass X-ray binary and the other one a compact
massive X-ray binary. We also discuss the analysis of the nonvariable
sources and derive upper limits on the detectable variability of the
various sources. The number of variable sources detected is in agreement
with the expectations based on the features of intense Galactic X-ray
sources and on the characteristics of the observations.

---------------------------------------------------------
Title: X-ray spectral synthesis in hydrodynamic flare models
Authors: Serio, S.; Antonucci, E.; Dodero, M. A.; Peres, G.; Reale, F.
1990hrxr.conf..126S Altcode: 1990IAUCo.115..126S
Compact solar flares are triggered by sudden energy release in
magnetically confined plasma. This class of flares is well suited
to be studied with numerical hydrodynamic models. In particular,
it is possible to compare the evolution of observed and synthetic
X-ray spectra, computed under various assumptions for the mechanism
of impulsive energy deposition, to constrain theoretical models and
their parameter space. Recent results on solar flares along this line,
nonthermal to models of energy depositions by relativistic electron
beams are discussed. Possible applications of X-ray spectral synthesis
to stellar flares are also discussed.

---------------------------------------------------------
Title: X-ray observations, scaling laws and magnetic fields
Authors: Peres, G.; Vaiana, G. S.
1990MmSAI..61..401P Altcode:
Sounding-rocket, satellite, and Skylab X-ray observations of the sun,
obtained at high spatial resolution using grazing-incidence-telescopes
during the period 1963-1975, are reviewed, with an emphasis on their
astrophysical implications. The history of the observation programs is
discussed; sample images and tables of numerical data are presented;
and it is shown how the X-ray data revealed the magnetic confinement
of the hot coronal plasma and permitted the formulation of scaling
laws, which relate the loop length to the maximum plasma temperature
and pressure at the base of the corona and account for gravitational
effects and magnetic-field heating. More recent Einstein Observatory,
Exosat, and Rosat X-ray observations of stellar coronae are briefly
described, and the applicability of the solar coronal-loop models to
late-type stars is considered.

---------------------------------------------------------
Title: Variability of the X-Ray Sources in M33 and M31
Authors: Peres, G.; Reale, F.; Collura, A.; Fabbiano, G.
1990ixra.conf..267P Altcode: 1990ixra.symp..267P
Contents: Introduction. M33: an eclipsing binary and an "X-ray only"
AGN. M31: two binaries? AXAF prospects.

---------------------------------------------------------
Title: Variability of the X-Ray Sources in M31
Authors: Collura, A.; Reale, F.; Peres, G.
1989ESASP.296..359C Altcode: 1989ttxa.symp..359C
The Einstein observations of M31, looking for variability of
the X-ray sources up to time scales of approximately 100,000 are
analyzed. Two sources are significantly variable. The characteristics
of their variability are discussed and constraints on their possible
periodicity are derived. The observed X-ray luminosities, the pattern
of variability, and the optical identifications suggest the possibility
that one of them is low mass X-ray binary and the other one a compact
massive X-ray binary.

---------------------------------------------------------
Title: Hydrodynamic Models of Solar and Stellar Flares
Authors: Peres, Giovanni
1989SoPh..121..289P Altcode: 1989IAUCo.104..289P
This paper discusses the hydrodynamic modeling of flaring plasma
confined in magnetic loops and its objectives within the broader
scope of flare physics. In particular, the Palermo-Harvard model is
discussed along with its applications to the detailed fitting of X-ray
light curves of solar flares and to the simulation of high-resolution
Ca xix spectra in the impulsive phase. These two approaches provide
complementary constraints on the relevant features of solar flares. The
extension to the stellar case, with the fitting of the light curve of
an X-ray flare which occurred on Proxima Centauri, demonstrates the
feasibility of using this kind of model for stars too. Although the
stellar observations do not provide the wealth of details available for
the Sun, and, therefore, constrain the model more loosely, there are
strong motivations to pursue this line of research: the wider range of
physical parameters in stellar flares and the possibility of studying
further the solar-stellar connection.

---------------------------------------------------------
Title: Time Variability of the X-Ray Sources in M33
Authors: Peres, G.; Reale, F.; Collura, A.; Fabbiano, G.
1989ApJ...336..140P Altcode:
We have analyzed the variability of the X-ray sources in M33 observed
by the Einstein Observatory telescope. Our analysis has been based
on statistical techniques sensitive to variability over several time
scales and has shown that two of the 15 known sources are variable
above the 99.73% confidence level. The light curve of one of these
sources, M33 X-7, exhibits a variability pattern of high and low states,
suggesting an eclipsing binary X-ray source. The result would be the
first identification of a close accreting binary system with an X-ray
source in an external galaxy other than the Magellanic Clouds. The
data suggest a binary period P = 1.7857 days and an eclipse duration
of ~0.4 days. The nuclear source M33 X-8 varies only in the softest
part of the spectrum (kT &lt;= 1.2 keV). The observations suggest a
rapid variability (t ~ 3000 s) and show a rapid flare with a rise time
shorter than 3 days together with longer time scale variability.

---------------------------------------------------------
Title: Variability of the X-ray sources in M 33.
Authors: Peres, G.; Reale, F.; Collura, A.; Fabbiano, G.
1989MmSAI..60..221P Altcode: 1989MSAIt..60..221P
Variability analysis of the X-ray sources in M33 observed with the
Einstein telescope is presented. Two of the fifteen sources of M33 have
been found to be variable beyond 99.77 percent significance level: M33
X-7 and M33 X-8. M33 X-7 showed a very strong variability on many time
scales ranging from months to less than hours. This result signifies
the first observation of a close accreting binary system with an X-ray
source in an external galaxy other than the Magellanic Clouds. The
nuclear source M33 X-8 is variable on a six-month time scale in the
High Resolution Imager (HRI), the Imaging Proportional Counter (IPC),
and the nonimaging Monitor Proportional Counter (MPC). The spectral
characteristics of the X-ray emission of this source and the similarity
with other low luminosity nuclei favor the possibility of galactic
nuclear activity.

---------------------------------------------------------
Title: Numerical simulations of thermal instabilities in galactic
and cluster halos
Authors: Peres, G.; Rosner, R.; Bodo, G.
1989MmSAI..60..139P Altcode:
X-ray observations have shown the presence of diffuse hot gas in
many clusters of galaxies and around some elliptical galaxies. It is
typically expected that radiative cooling in the central region of
these halos drives significant subsonic inflow of gas to the center
of the system. Steady infall models of such flows typically predict
that the gas will 'drop out' of the flow, so that the mass accretion
rate is a function of radius. The plausible mechanism that removes
some fraction of the gas from the flow at various radii is thermal
instability in the cooling flow itself. Here, results are reported of
a fully nonlinear analysis of the monotonic radial modes in a cooling
flow which uses physical parameters appropriate to the cooling flow
of M87. A case of spontaneous formation of the radiative instability
at the center of the halo is described and the relevance of thermal
conductivity in determining the stability is shown.

---------------------------------------------------------
Title: Flare energetics.
Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.;
Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner,
M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.;
Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres,
G.; Serio, S.; Sylwester, J.; Veck, N. J.
1989epos.conf..377W Altcode:
The authors have sought to establish a comprehensive and self-consistent
picture of the sources and transport of energy within a flare. To
achieve this goal, they chose five flares in 1980 that were well
observed with instruments on the Solar Maximum Mission, and with other
space-borne and ground-based instruments. The events were chosen to
represent various types of flares. Details of the observations available
for them and the corresponding physical parameters derived from these
data are presented. The flares were studied from two perspectives,
the impulsive and gradual phases, and then the results were compared
to obtain the overall picture of the energetics of these flares. The
authors also discuss the role that modeling can play in estimating the
total energy of a flare when the observationally determined parameters
are used as the input to a numerical model. Finally, a critique of
our current understanding of flare energetics and the methods used to
determine various energetics terms is outlined, and possible future
directions of research in this area are suggested.

---------------------------------------------------------
Title: Intercomparison of numerical models of flaring coronal loops.
Authors: Kopp, R. A.; Fisher, G. H.; MacNiece, P.; McWhirter, R. W. P.;
Peres, G.
1989epos.conf..597K Altcode:
The numerical modelling group was concerned with computations relevant
to the problem of the hydrodynamic and radiative response of a single
magnetic flux tube to a sudden release of energy in it. The group
initially considered a simple "benchmark model" in which the physics of
real loops - radiation, thermal conduction, compressible hydrodynamics,
gravity and nonthermal heating - could be incorporated with some degree
of realism. The primary goal of the benchmark model was to intercompare
code calculations on a standardized, although hypothetical, problem,
rather than to establish the best possible physical model.

---------------------------------------------------------
Title: Hydrodynamic simulations of stellar flares in various magnetic
field geometries
Authors: Reale, F.; Peres, G.; Serio, S.
1989MmSAI..60..203R Altcode:
Hydrodynamic simulations of coronal flares from the solar to the stellar
environment have recently been extended to a flare that occurred on
Proxima Centauri and that was observed by the imaging proportional
counter (IPC) instrument on the Einstein X-ray satellite. Numerical
simulations of this flare have provided information about the dimensions
of the flaring region. The moderate spectral resolution of the IPC
also made it possible to set constraints on the energetics of the
flare. A new flare model was considered for an X-ray flare observed
by the Exosat on the RS CVn stellar system Sigma CrB on September 29,
1983 at 4:07 UT, with the medium energy detector in the 2-8 keV energy
band, which is considerably harder than the Einstein-IPC energy band
of 0.2-4.0 keV. Preliminary results on the simulation of the initial
phase of the flare are presented.

---------------------------------------------------------
Title: Two-dimensional hydrodynamics of astrophysical plasmas
Authors: Reale, F.; Peres, G.; Serio, S.
1989MmSAI..60..207R Altcode:
A preliminary version of a fully two-dimensional flux corrected
transport (CFT) code which can solve hydrodynamic equations of mass
and momentum conservation is presented. A set of tests is being carried
out to verify stability, accuracy, and efficiency of the code. Results
of a properly two-dimensional test describing the rigid clockwise
revolution of a fluid cylinder around the center of a spatial grid
(100 x 100 points) are reported.

---------------------------------------------------------
Title: Variability of sources in the M33 and M31 Galaxies
Authors: Peres, G.; Collura, A.; Fabbiano, G.
1988feta.conf...23P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Morphology and Spectral Characteristics of the X-Ray Emission
of M33: Erratum
Authors: Trinchieri, G.; Fabbiano, G.; Peres, G.
1988ApJ...329.1037T Altcode:
In regard to the paper "Morphological and Spectral Characteristics of
the X-ray Emission of M33" by G. Trinchieri, G. Fabbiano, and G. Peres
(Ap. J., 325, 531 [1988]), several additional remarks should be made.

---------------------------------------------------------
Title: Hydrodynamic Modeling of an X-Ray Flare on Proxima Centauri
Observed by the Einstein Telescope
Authors: Reale, F.; Peres, G.; Serio, S.; Rosner, R.; Schmitt,
J. H. M. M.
1988ApJ...328..256R Altcode:
Hydrodynamic numerical calculations of a flare which occurred on Proxima
Centauri and was observed by the Einstein satellite on August 20, 1980
at 12:50 UT are presented. The highlights of the hydrodynamic code
are reviewed, and the physical and geometrical parameters necessary
for the calculations are derived and compared with observations. The
results are consistent with the stellar flare being caused by the
rapid dissipation of 5.9 x 10 to the 31st ergs, within a magnetic loop
structure whose semilength is 7 x 10 to the 9th cm and cross-sectional
radius is 7.3 x 10 to the 8th cm. The results provide evidence that
flares on late-type stars can be described by a hydrodynamic model
with a relatively simple geometry, similar to solar compact flares.

---------------------------------------------------------
Title: Morphology and Spectral Characteristics of the X-Ray Emission
of M33
Authors: Trinchieri, G.; Fabbiano, G.; Peres, G.
1988ApJ...325..531T Altcode:
We have extended previous analysis of the X-ray data on M33 to include
a detailed study of the morphology and spectral characteristics of
its X-ray emission. In addition to the bright nuclear source and to
several bright pointlike sources in the disk and arms of this galaxy,
which had already been reported, we find a lower surface brightness,
diffuse emission from the plane of M33. This extended emission is
further separable in a hard (&gt;2 keV) and in a soft (&lt;1 keV)
component, similarly distributed in the plane of the galaxy. We argue
that the hard component is most likely the result of the integrated
contribution of several lower luminosity discrete sources, similar to
the ones observed in the Milky Way, e.g., compact accreting systems
and young SNRs; the soft component is most likely due to the integrated
emission of stellar coronae and old SNRs, although diffuse hot gas in
the disk of M33 could also contribute. The brightest source in M33 is
in the nuclear region, and its average luminosity (L_x_~ 4 x 10^39^
ergs s^-1^) is &gt;=10 times higher than the next brightest source in
the plane. The nature of this source remains undetermined. The spectral
results indicate a soft and intrinsically absorbed spectrum, consistent
with those of low-luminosity active galactic nuclei, although the lack
of signs of activity at frequencies other than the X-ray ones would make
this source a unique example of " X-ray-only" activity. Alternatively
the source could be explained as a new type of X-ray binary system.

---------------------------------------------------------
Title: Two-Dimensional Numerical Hydrodynamics
Authors: Reale, F.; Peres, G.; Serio, S.
1988sca..conf...41R Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The theoretical and computational activity at the Osservatorio
DI Palermo
Authors: Peres, Giovanni
1988MmSAI..59..389P Altcode:
The construction of theoretical models of solar flares and
magnetically confined stellar coronal plasmas and the use of
these models in the analysis of observational data are discussed,
reviewing the results of recent investigations at the Osservatorio di
Palermo. The governing equations for the Palermo-Harvard one-dimensional
hydrodynamic/thermodynamic plasma model (Peres et al., 1982; Pallavicini
et al., 1983) are presented; the numerical implementation of the model
is described; computed and observed X-ray light curves of solar flares
are compared; and the treatment of nonlocal thermal conduction is
considered. The steps in a typical simulation project are outlined, and
the potential advantages of transputers for astrophysical simulations
are indicated.

---------------------------------------------------------
Title: Simulations of the CA XIX Spectral Emission from a Flaring
Solar Crornal Loop. I. Thermal Case
Authors: Antonucci, E.; Dodero, M. A.; Peres, G.; Serio, S.; Rosner, R.
1987ApJ...322..522A Altcode:
Spectral profiles synthesized from the results of numerical simulations
of solar coronal loop plasma are compared with the observed line
profiles, and the sensitivity of this comparison to variations in
the parameter values is tested. The comparison allows the procedures
commonly adopted for obtaining temperatures and velocities directly
from the bent crystal spectrometer observations, as well as the
interpretation of parameters obtained from hydrodynamical calculations,
to be independently validated. The characteristic spectral signatures
of different models for the deposition of impulsive thermal energy
in the loop are derived assuming two distinct spatial distributions
of thermal heating: in one case, the heating function peaks near the
apex of the loop, while in the second, energy is deposited directly
in the lower atmosphere.

---------------------------------------------------------
Title: Hydrodynamics of an X-Ray Flare on Proxima Centauri
Authors: Reale, F.; Peres, G.; Serio, S.; Rosner, R.; Schmitt,
J. H. M. M.
1987LNP...291..179R Altcode: 1987csss....5..179R; 1987LNP87.291..179R
We apply the Palermo-Harvard hydrodynamic numerical code to compute
the evolution of temperature, density, pressure and velocity in
a semicircular symmetric rigid loop to reproduce the Einstein IPC
observations of the 20 August 1980 flare on Proxima Centauri.

---------------------------------------------------------
Title: Explosive chromospheric instability in hydrodynamic loopflare
models:the problem and its cure.
Authors: Peres, G.; Serio, S.; Rosner, R.
1987NCimB..99...15P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Nonlocal thermal conduction in hydrodynamic loop flare models.
Authors: Peres, G.; Rosner, R.; Serio, S.
1987NCimB..99...29P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: LEBAN diagnostic for basic flaring loop parameters.
Authors: Sylwester, B.; Sylwester, J.; Jakimiec, J.; Fludra, A.;
Peres, G.; Serio, S.
1987PAICz..66..229S Altcode: 1987eram....1..229S
The authors describe the diagnostic procedure called LEBAN which may
be helpful in deriving basic geometrical loop parameters. "Palermo
Code" hydrodynamic flare model calculations have been used to test
reliability of the procedure.

---------------------------------------------------------
Title: High-Temperature Plasma Diagnostics of Solar Flares and
Comparison with Model Calculations
Authors: Jakimiec, J.; Sylwester, B.; Sylwester, J.; Lemen, J. R.;
Mewe, R.; Bentley, R. D.; Peres, G.; Serio, S.; Schrijver, J.
1987sman.work...91J Altcode:
The present state of flare diagnostics from X-ray spectra is briefly
outlined. The authors discuss how improved diagnostic results can be
used in flare heating process investigations.

---------------------------------------------------------
Title: Hydrodynamic Flare Modeling: Comparison of Numerical
Calculations with SMM Observations of the 1980 November 12 17:00
UT Flare
Authors: Peres, G.; Reale, F.; Serio, S.; Pallavicini, R.
1987ApJ...312..895P Altcode:
The results of calculations of the evolution of flaring plasma
in a simple coronal loop are compared with SMM observations of a
well-studied compact flare. Calculations assuming different models of
impulsive heating are performed, all starting from the same initial
static loop configuration. A mechanism of local heat deposition and
a mechanism of flare heating by energetic electron beams are tested
with a variety of parameters. It is found that the evolution of the
soft X-ray compact flare is mainly dominated by the thermodynamic and
hydrodynamic properties of the confined plasma. In order to obtain
a close match between calculated and observed X-ray polychromator
light curves, a sizable fraction of impulsive energy must be deposited
directly in the corona. Local Fe XXV brightenings in the flaring loop
do not provide a good diagnostic of the site of energy deposition in
solar flares. The total amount of impulsive energy necessary to match
the calculated light curves with the observations depends strongly on
the effective deposition depth.

---------------------------------------------------------
Title: Characterization of the Total Flare Energy
Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.;
Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner,
M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.;
Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres,
G.; Serio, S.; Sylwester, J.; Veek, N. J.
1986epos.conf.5.41W Altcode: 1986epos.confE..41W
No abstract at ADS

---------------------------------------------------------
Title: Energetics of the Impulsive Phase
Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.;
Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner,
M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.;
Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres,
G.; Serio, S.; Sylwester, J.; Veek, N. J.
1986epos.conf..5.5W Altcode: 1986epos.confE...5W
No abstract at ADS

---------------------------------------------------------
Title: Energetics of the Gradual Phase
Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.;
Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner,
M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.;
Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres,
G.; Serio, S.; Sylwester, J.; Veek, N. J.
1986epos.conf.5.20W Altcode: 1986epos.confE..20W
No abstract at ADS

---------------------------------------------------------
Title: Review of Impulsive Phase Phenomena
Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.;
Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner,
M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.;
Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres,
G.; Serio, S.; Sylwester, J.; Veek, N. J.
1986epos.conf.5.60W Altcode: 1986epos.confE..60W
No abstract at ADS

---------------------------------------------------------
Title: Intercomparison of numerical models of flaring coronal loops
Authors: Kopp, R. A.; Fisher, G. H.; MacNeice, P.; McWhirter, R. W. P.;
Peres, G.
1986epos.conf..7.1K Altcode: 1986epos.confG...1K
The proposed Benchmark Problem consists of an infinitesimal magnetic
flux tube containing a low-beta plasma. The field strength is assumed
to be so large that the plasma can move only along the flux tube,
whose shape remains invariant with time (i.e., the fluid motion is
essentially one-dimensional). The flux tube cross section is taken to
be constant over its entire length. In planar view the flux tube has a
semi-circular shape, symmetric about its midpoint s = smax
and intersecting the chromosphere-corona interface (CCI) perpendicularly
at each foot point. The arc length from the loop apex to the CCI is
10,000 km. The flux tube extends an additional 2000 km below the CCI
to include the chromosphere, which initially has a uniform temperature
of 8000 K. The temperature at the top of the loop was fixed initially
at 2 X 1 million K. The plasma is assumed to be a perfect gas (gamma
= 5/3), consisting of pure hydrogen which is considered to be fully
ionized at all temperatures. For simplicity, moreover, the electron
and ion temperatures are taken to be everywhere equal at all times
(corresponding to an artificially enhanced electron-ion collisional
coupling). While there was more-or-less unanimous agreement as to
certain global properties of the system behavior (peak temperature
reached, thermal-wave time scales, etc.), no two groups could claim
satisfactory accord when a more detailed comparison of solutions
was attempted.

---------------------------------------------------------
Title: Flares Chosen for Energetics Study
Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.;
Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner,
M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.;
Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres,
G.; Serio, S.; Sylwester, J.; Veek, N. J.
1986epos.conf.5.47W Altcode: 1986epos.confE..47W
No abstract at ADS

---------------------------------------------------------
Title: Relationships among the Phases
Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.;
Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner,
M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.;
Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres,
G.; Serio, S.; Sylwester, J.; Veek, N. J.
1986epos.conf.5.39W Altcode: 1986epos.confE..39W
No abstract at ADS

---------------------------------------------------------
Title: The X-Ray Morphology and Spectrum of M33
Authors: Trinchieri, G.; Fabbiano, G.; Peres, G.
1986BAAS...18..998T Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Investigation of flare heating based on X-ray observations
Authors: Jakimiec, J.; Sylwester, B.; Sylwester, J.; Mewe, R.; Peres,
G.; Serio, S.; Schrijver, J.
1986CoSka..15..123J Altcode:
Using X-ray data recorded by the Solar Maximum Mission Hard X-ray
Imaging Spectrometer the authors have investigated flare evolution
in a (Tm, N)-diagram, where Tm is the maximum
temperature and N is the mean density in the flare volume. This
flare diagnostics allows one to show that most large flares achieve a
quasi-steady-state during their decay, which means that the cooling
is then so slow that a flare evolves along the line of steady-state
loops in the (Tm, N)-diagram. The diagnostics allows one
to determine the time evolution of the flare heating function, which
gives the rate of thermal energy release, per unit volume.

---------------------------------------------------------
Title: Investigation of flare heating based on X-ray observations
Authors: Jakiemiec, J.; Sylwester, B.; Sylwester, J.; Mewe, R.; Peres,
G.; Serio, S.; Schrijver, J.
1986AdSpR...6f.237J Altcode: 1986AdSpR...6..237J
Using X-ray data recorded by the Solar Maximum Mission Hard X-ray
Imaging Spectrometer we have investigated flare evolution in a
(Tm, N)-diagram, where Tm is the maximum
temperature and N is the mean density in the flare volume. It is
important that the behaviour of a flare in such a diagram does not
depend significantly on details of the flare geometry and therefore
can be effectively compared with simplified model calculations of
flare loops. This flare diagnostics allows us to show that most large
flares achieve a quasi-steady-state during their decay, which means
that the cooling is then so slow that a flare evolves along the line of
steady-state loops in the (Tm, N)-diagram. The diagnostics
allows us to determine the time evolution of the flare heating function,
EH(t), which gives the rate of thermal energy release,
per unit volume. For the flares which achieve the quasi-steady-state
branch it gives a new valuable method of estimation of the electron
density in the flare loops.

---------------------------------------------------------
Title: Hydrodynamics of coronal loops: A comparison of observed and
simulated spectra emitted from flaring coronal loops
Authors: Antonucci, E.; Dodero, M. A.; Peres, G.; Serio, S.
1986AdSpR...6f.151A Altcode: 1986AdSpR...6..151A
The data obtained at high temporal and spectral resolution with
the Bent Crystal Spectrometer of the Solar Maximum Mission have
shown that the soft x-ray spectra emitted by highly ionized heavy
ions are characterised during the impulsive phase of flares by
blue-shifted components. The blue emission has been interpreted as
due to convective plasma flows from the chromosphere, which in flare
conditions is heated to coronal temperatures. In the present study,
the properties of the Ca XIX solar spectra are compared with the
results of numerical calculations simulating the behaviour of coronal
plasma subject to impulsive heating. The plasma hydrodynamic response
to transient heating is modeled in the assumption of a fluid confined
in a semicircular loop of uniform cross-section. Two different forms
of heating are considered: direct thermal heating, and heating due to
injection in the loop of non-thermal electrons.

---------------------------------------------------------
Title: Flare energetics.
Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.;
Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner,
M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.;
Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres,
G.; Serio, S.; Sylwester, J.; Veck, N. J.
1986NASCP2439....5W Altcode:
In this investigation of flare energetics, the authors establish a
comprehensive and self-consistent picture of the sources and transport
of energy within a flare. They chose five flares in 1980 that were
well observed with instruments on the SMM, and with other space-borne
and ground-based instruments. Details of the observations available
for them and the corresponding physical parameters derived from these
data are presented. The flares were studied from two perspectives,
the impulsive and gradual phases, and then the results were compared
to obtain the overall picture of the energetics of these flares. The
authors also discuss the role that modeling can play in estimating the
total energy of a flare when the observationally determined parameters
are used as the input to a numerical model.

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Title: Intercomparison of numerical models of flaring coronal loops.
Authors: Kopp, R. A.; Fisher, G. H.; MacNiece, P.; McWhirter, R. W. P.;
Peres, G.
1986NASCP2439....7K Altcode:
The authors were concerned with computations relevant to the problem
of the hydrodynamic and radiative response of a single magnetic flux
tube to a sudden release of energy in it. They considered a simple
"Benchmark Model" in which the physics of real loops - radiation,
thermal conduction, compressible hydrodynamics, gravity and nonthermal
heating could be incorporated with some degree of realism. The primary
goal of the Benchmark Model was to intercompare code calculations on a
standardized, although hypothetical, problem, rather than to establish
the best possible physical model.

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Title: Solar X-ray spectrum simulations for flaring loop models with
emphasis on transient ionization effects during the impulsive phase
Authors: Mewe, R.; Lemen, J. R.; Peres, G.; Schrijver, J.; Serio, S.
1985A&A...152..229M Altcode:
X-ray spectra are simulated for observations with the Bent Crystal
Spectrometer (BCS) on the Solar Maximum Mission (SMM) spacecraft
(around 1.9 A and 3 A) using results of a time-dependent numerical
code for a dynamic flaring loop model. The spectra are integrated
over the whole loop and fully take into account the effects of
deviations from ionization equilibrium and Doppler shifts resulting
from plasma movements. The effect of transient ionization on the
spectra emitted during the initial impulsive phase is emphasized. In
all the cases considered a strong depletion (relative to the continuum)
of high-ionization spectral lines takes place during about the first
minute of the flare for electron densities below about 10 to the 11th/cu
cm. A preliminary comparison to observations made with the BCS show
some indications for such effects in a few strong flares, but more
sensitive instruments will be needed in the future to exploit such
transient ionization effects as a possible valid density diagnostic
for hot solar flares.

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Title: More on momentum deposition by electron beams in solar flares
Authors: Reale, F.; Peres, G.; Serio, S.
1985A&A...152L...5R Altcode:
Numerical experiments were carried out to determine if the momentum
deposited by electron beams is significant in the evolution of a solar
flare. The Palermo-Harvard numerical model, with its hydrodynamic
differential equations of conservation of mass, momentum and energy for
a solar magnetic loop, was modified to include thick target heating
by nonthermal electrons. The model was used to simulate flares
forming 20,000 km loops, both with and without inclusion of beam
momentum transfer. The results of the simulation were compared to Ca
XIX spectral data for a solar flare event. Inclusion of the momemtum
transfer factor caused significant changes in the simulated impulsive
phase and in the associated spectral signature. Downward pressure from
the electron beam reduced the upward moving velocity by as much as 50
km/sec. The results underscore the necessity of considering electron
beam momentum transfer effects in modeling of solar flare events.

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Title: Closed coronal structures. VI. Far-ultraviolet and X-ray
emission from active late-type stars and the applicability of coronal
loop models.
Authors: Giampapa, M. S.; Golub, L.; Peres, G.; Serio, S.; Vaiana,
G. S.
1985ApJ...289..203G Altcode:
We present far-ultraviolet line fluxes of prominent transition region
emission lines, as obtained with the International Ultraviolet
Explorer satellite, for a sample of solar-type stars. We combine
the ultraviolet observations with existing soft X-ray measurements
obtained by the Einstein Observatory (HEAO 2). We utilize the resulting
data set and a new coronal loop model numerical code developed at the
Harvard-Smithsonian Center for Astrophysics to perform a preliminary
investigation of the applicability of coronal loop models to solar-type
stars. In a few cases, reasonable agreement between the predictions of
single-component, coronal loop model atmospheres and the observational
data is achieved for a relatively well-defined, plausible range
of values in the pressure-filling factor (p, f) plane. In general,
however, we find that the addition of non- simultaneous ultraviolet
observations to a previously acquired soft X-ray data set does
not provide a sufficient constraint on the range of possible loop
filling factors and pressures for loop model atmospheres that may
be producing the observed X-ray and transition region emissions. We
discuss the origins of the discrepancies between the model results
and the observations within the context of (1) stellar variability,
(2) multiple coronal components, and (3) the presence of relatively
low temperature loops that give rise to far-ultraviolet emission
but not to coronal X-ray emission. We suggest on the basis of the
results presented in this investigation that in order to verify the
applicability of coronal loop models to solar-type stars, simultaneous
far-ultraviolet and moderate spectral resolution X-ray observations
will eventually have to be obtained.

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Title: The X-ray corona of Procyon.
Authors: Schmitt, J. H. M. M.; Harnden, F. R., Jr.; Rosner, R.; Peres,
G.; Serio, S.
1985ApJ...288..751S Altcode:
X-ray emission from the nearby system Procyon A/B (F5 IV + DF) was
detected, using the IPC (Imaging Proportional Counter) on board the
Einstein Observatory. Analysis of the X-ray pulse height spectrum
suggests that the observed X-ray emission originates in Procyon A
rather than in the white dwarf companion Procyon B, since the derived
X-ray temperature, log T = 6.2, agrees well with temperatures found
for quiescent solar X-ray emission. Modeling Procyon's corona with
loops characterized by some apex temperature Tmax and emission
length scale L, it is found that Tmax is well constrained, but L,
and consequently the filling factor of the X-ray emitting gas, are
essentially unconstrained even when EUV emission from the transition
region is included in the analysis.

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Title: Momentum deposition by electron beams in solar flares
Authors: Reale, F.; Peres, G.; Serio, S.
1985MmSAI..56..801R Altcode:
In a study of coronal plasma dynamics, to include electron beam
interactions with the ambient plasma, the effects of electron beam
momentum deposition have been computed in detail. The differences
in the evolution of a model flare when this effect is not taken into
account is studied. Important differences are detectable from velocity
profiles at advanced times (greater than 100 sec). Large differences in
bulk plasma velocity, which manifest over extended portions of the loop
and for a long time, suggest a corresponding discrepancy between the
predicted line shapes. Ongoing work aims to delineate the observable
differences between the spectra predicted in the two cases.

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Title: Dynamic heating of stationary coronal loops
Authors: Reale, F.; Peres, G.; Serio, S.
1985MmSAI..56..805R Altcode:
The influence of dynamic heating of constant average intensity
on apparently stationary loop features is studied. The intent is
to investigate the possibility that the 'steady state' coronal
heating might be due, at least partially, to the superposition of
frequent coronal microflares of hard X-rays (events such as were
reported by Lin et al., 1984). A few cases of coronal loops heated
by periodic pulses of electron beams were simulated by means of the
Palermo-Harvard hydrodynamic code. It is shown that if the solar corona
is entirely heated by microflares, the X-ray emission would fluctuate,
as expected. Detecting such fluctuations is not straightforward, but
chances of detecting them if they exist would be higher if observations
are made with high angular resolution instruments such as the recently
proposed normal incidence X-ray telescope.

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Title: Active late-type stars and the applicability of coronal
loop models.
Authors: Giampapa, M. S.; Golub, L.; Peres, G.; Serio, S.; Vaiana,
G. S.
1984NASCP2349..454G Altcode: 1984fiue.rept..454G; 1984IUE84......454G
Far ultraviolet IUE observations of a sample of solar type stars
were combined with existing soft X-ray measurements obtained by HEAO
B. The resulting data set was utilized and a new coronal loop model
numerical code was developed to perform a preliminary investigation of
the applicability of coronal loop models to solar-type stars. Reasonable
agreement was found to exist between the predictions of single-component
coronal loop model atmospheres. It was demonstrated that semi-empirical,
coronal loop models can be applied to account for observed stellar
transition region and coronal emission. This result is corroborative
evidence for the presence of magnetic field structures analogous to
solar coronal loops on the surfaces of solar-type stars. It is suggested
that stellar transition region emission arises predominantly from the
base of quiescent coronal loop configurations.

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Title: Solar flare hydrodynamics
Authors: Peres, G.; Serio, S.
1984MmSAI..55..749P Altcode:
The Palermo-Harvard numerical code for confined solar loop atmospheres
is presented, evidencing general problems and advantages of the
numerical modelling of flares, and discussing recent results of this
code and their comparison with SMM observations. Future improvements
and probable evolutions of this kind of models are discussed.

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Title: Closed coronal structures. V - Gasdynamic models of flaring
loops and comparison with SMM observations
Authors: Pallavicini, R.; Peres, G.; Serio, S.; Vaiana, G.; Acton,
L.; Leibacher, J.; Rosner, R.
1983ApJ...270..270P Altcode:
The hydrodynamic response of confined magnetic structures to strong
heating perturbations is investigated by means of a time-dependent
one-dimensional colde which incorporates the energy, momentum, and
mass conservation equations. The entire atmospheric structure from the
chromosphere to the corona is taken into account. Models with different
energy input, heating time dependence, preflare conditions and heating
location have been examined in the numerical simulations. The
result of model calculations are compared with observations of flares
obtained with the X-ray Polychromator experiment on the Solar Maximum
Mission. These include light curves of spectral lines formed over a
wide range of coronal flare temperatures, as well as determinations
of Doppler shifts for the high temperature plasma. Several examples
are used to illustrate the range of the observational variation. It is shown that the predictions of the numerical simulations
are in good overall agreement with the observed evolution of the
flare coronal plasma. The model reproduces correctly the temporal
profile of X-ray spectral lines and -- to first order at least --
their relative intensities. The upflow velocities predicted by
model calculations are in agreement with the observed blueshifts,
supporting the interpretation of the blueshifts as due to evaporation
of chromospheric material. The possibility of using the comparison
of model predictions with observations to derive information on the
processes of energy release and transfer in solar flares is discussed.

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Title: Hydrodynamics of Flaring Loops - SMM Observations and Numerical
Simulations
Authors: Pallavicini, R.; Peres, G.
1983SoPh...86..147P Altcode:
The hydrodynamic response of confined magnetic structures to strong
heating perturbations is investigated by means of a timedependent
one-dimensional code which incorporates the energy, momentum and
mass conservation equations. The entire atmospheric structure from
the chromosphere to the corona is taken into account. The results of
model calculations are compared with observations of flares obtained
with the X-Ray Polychromator experiment on the Solar Maximum Mission.

---------------------------------------------------------
Title: The Palermo-Harvard numerical code for the dynamics of confined
coronal plasma.
Authors: Peres, G.; Serio, S.; Vaiana, G. S.; Rosner, R.
1983pig..conf...96P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Temporal evolution of soft X-ray emitting plasma in solar
flares.
Authors: Pallavicini, R.; Peres, G.
1982BAAS...14..776P Altcode:
No abstract at ADS

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Title: Coronal closed structures. IV - Hydrodynamical stability and
response to heating perturbations
Authors: Peres, G.; Serio, S.; Vaiana, G. S.; Rosner, R.
1982ApJ...252..791P Altcode:
The response of magnetically confined atmospheres to perturbations in
the temperature and density distribution, and the local heating rate
by means of a one-dimensional time-dependent hydrodynamical code,
which incorporates the full energy, momentum and mass conservation
equations is studied. These studies extend the linear instability
analysis of Habbal and Rosner (1979) into the finite-amplitude regime,
and generalize the confined atmosphere models of Serio et al., to the
time-dependent domain. The results show that closed coronal atmospheres
are stable against finite-amplitude perturbations if the chromospheric
response is taken into account; and observed correlated increases in
coronal density and temperature can only be achieved under quiescent
conditions by increasing the heat deposition rate relatively more in
the chromosphere than in the corona.

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Title: Il sistema di trattamento di immagini presso l'Osservatorio
Astronomico di Palermo.
Authors: Sciortino, S.; Peres, G.; Serio, S.; Vaiana, G. S.
1982MmSAI..53..115S Altcode:
No abstract at ADS

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Title: X-Ray Flare Spectroscopy: SMM Observations and Loop Modeling.
Authors: Acton, L.; Pallavicini, R.; Peres, G.; Vaiana, G.
1982uxsa.coll....1A Altcode: 1982IAUCo..73....1A
No abstract at ADS

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Title: Closed coronal structures. III - Comparison of static models
with X-ray, EUV, and radio observations
Authors: Pallavicini, R.; Peres, G.; Serio, S.; Vaiana, G. S.; Golub,
L.; Rosner, R.
1981ApJ...247..692P Altcode:
Numerical models of static coronal loops in energy balance are compared
with high spatial resolution observations of extreme ultraviolet lines,
broad-band X-ray emission, and interferometric observations at 2.8 cm
of a solar active region. Difficulties of using scaling laws to test
static models of coronal loops are reviewed. The theoretical model
used for the comparison is summarized; the detailed X-ray, EUV, and
microwave observations of the selected active region are presented;
and the comparison of the model with the observations is performed. It
is shown that simple static models with conductive flux vanishing at the
loop base reproduce satisfactorily the observed properties in the upper
portion of loop structures from compact, high-pressure loops in the
core of the region to more extended, fainter loops and to large-scale
loops interconnecting different active regions. Effects of changing
loop parameters are investigated, and it is argued, that in contrast
to the present approach, scaling laws cannot be used to discriminate
between different static energy balance models. Some discrepancy is
found between model predictions and observations for the lower sections
of loop structures. Possible causes of the discrepancy are discussed.

---------------------------------------------------------
Title: Closed coronal structures. II - Generalized hydrostatic model
Authors: Serio, S.; Peres, G.; Vaiana, G. S.; Golub, L.; Rosner, R.
1981ApJ...243..288S Altcode:
Numerical computations of stationary solar coronal loop atmospheres
are used to extend earlier analytical work. Two classes of loops are
examined, namely symmetric loops with a temperature maximum at the
top but now having a length greater than the pressure scale height and
loops which have a local temperature minimum at the top. For the first
class, new scaling laws are found which relate the base pressure and
loop length to the base heating, the heating deposition scale height,
and the pressure scale height. It is found that loops for which the
length is greater than about two to three times the pressure scale
height do not have stable solutions unless they have a temperature
minimum at the top. Computed models with a temperature inversion at the
top are permitted in a wider range of heating deposition scale height
values than are loops with a temperature maximum at the top. These
results are discussed in relation to observations showing a dependence
of prominence formation and stability on the state of evolution of
magnetic structures, and a general scenario is suggested for the
understanding of loop evolution from emergence in active regions
through the large-scale structure phase to opening in coronal holes.

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Title: Transition Region and Corona in Solar Active Regions:
Observations and Numerical Modeling
Authors: Golub, L.; Pallavicini, R.; Peres, G.; Rosner, R.; Serio,
S.; Vaiana, G. S.
1980BAAS...12..908G Altcode:
No abstract at ADS

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Title: Transienti nella corona solare: determinazione dei parametri
fisici del plasma e leggi di scala.
Authors: Peres, G.; Sciortino, S.; Serio, S.; Vaiana, G. S.
1979RSAI...22..165P Altcode:
No abstract at ADS

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Title: Rivelazione quantica di immagini ad alta risoluzione
spazio-temporale: il progetto Photicon.
Authors: Peres, G.; Sciortino, S.; Serio, S.; Vaiana, G. S.
1979RSAI...22..237P Altcode:
No abstract at ADS

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Title: Sistema analisi dati per fotometria nel lontano infrarosso
della collaborazione Palermo-Harvard. Risultati preliminari per la
regione intorno a W42.
Authors: Peres, G.; Sciortino, S.; Serio, S.; Vaiana, G. S.
1979RSAI...22..190P Altcode:
No abstract at ADS

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Title: The Palermo four-color far infrared program
Authors: Peres, G.; Serio, S.; Vaiana, G. S.; Cali, C.; Daneu, V.;
Maxson, C.; Steier, M.
1978MmSAI..49...95P Altcode:
A four-channel far-IR photometer designed for use with a balloon-borne
102-cm telescope is described. The data analysis system for this
photometer is discussed, and a typical observational program is
outlined. Possible objects that may be observed with the four-color
far-IR photometer include molecular clouds, the galactic-center region,
H II regions, quasars, Seyfert galaxies, and BL Lacertae objects.

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Title: Four Color Infrared Bolometer System for One-Meter Telescope
Authors: Daneu, V.; Maxson, C.; Peres, G.; Serio, S.; Vaiana, G.
1978ASIC...38..335D Altcode: 1978infa.proc..335D
A far-infrared photometer has been developed as a focal plane instrument
for a balloon-borne 102-cm telescope. The four gallium-doped Ge
bolometers adopted for the system operate at 1.8 K in a liquid-helium
dewar. Wavelength bands of the four-bolometer system are a narrow
spectrum between 18 and 22 microns, and bands centered at 42, 70,
and 140 microns. The responsivity and Johnson noise of the detectors
limit the performance of the instrument.

---------------------------------------------------------
Title: Solar X-ray transients in magnetically confined plasma:
observational data and hydrodynamic model.
Authors: Peres, G.; Serio, S.; Vaiana, G. S.; Rosner, R.
1978spre.conf..341P Altcode: 1978spre.proc..341P
No abstract at ADS

---------------------------------------------------------
Title: The Palermo four-color far infrared program.
Authors: Peres, G.; Serio, S.; Vaiana, G. S.; Cali', C.; Daneu, V.;
Maxson, C.; Steier, M.
1978ASIC...38...95P Altcode:
No abstract at ADS