Author name code: semel
ADS astronomy entries on 2022-09-14
author:"Semel, Meir"
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Title: Magnetic fields and differential rotation on the pre-main
sequence - III. The early-G star HD 106506
Authors: Waite, I. A.; Marsden, S. C.; Carter, B. D.; Hart, R.;
Donati, J. -F.; Ramírez Vélez, J. C.; Semel, M.; Dunstone, N.
Bibcode: 2011MNRAS.413.1949W
Altcode: 2011arXiv1102.3475W; 2011MNRAS.tmp..486W
We present the photometry and spectropolarimetry of the
pre-main-sequence star HD 106506. A photometric rotational period of
∼1.416 ± 0.133 d has been derived using observations at Mount Kent
Observatory (MKO). Spectropolarimetric data obtained with the 3.9-m
Anglo-Australian Telescope (AAT) were used to derive spot occupancy
and magnetic maps of the star through the technique of Zeeman Doppler
imaging (ZDI). The resulting brightness maps indicate that HD 106506
displays photospheric spots at all latitudes including a predominant
polar spot. Azimuthal and radial magnetic images of this star have
been derived, and a significant azimuthal magnetic field is indicated,
in line with other active young stars. A solar-like differential
rotation law was incorporated into the imaging process. Using Stokes I
information the equatorial rotation rate, Ωeq, was found to
be 4.54 ± 0.01 rad d-1, with a photospheric shear δΩ of
0.21+0.02-0.03 rad d-1. This equates to
an equatorial rotation period of ∼1.39 ± 0.01 d, with the equatorial
region lapping the poles every ∼ 30+5-3 d. Using
the magnetic features, the equatorial rotation rate, Ωeq,
was found to be 4.51 ± 0.01 rad d-1, with a photospheric
shear δΩ of 0.24 ± 0.03 rad d-1. This differential
rotation is approximately four times that observed on the Sun.
Title: Magnetic fields and differential rotation on the pre-main
sequence - I. The early-G star HD 141943 - brightness and magnetic
topologies
Authors: Marsden, S. C.; Jardine, M. M.; Ramírez Vélez, J. C.;
Alecian, E.; Brown, C. J.; Carter, B. D.; Donati, J. -F.; Dunstone,
N.; Hart, R.; Semel, M.; Waite, I. A.
Bibcode: 2011MNRAS.413.1922M
Altcode: 2011arXiv1101.5859M; 2011MNRAS.tmp..487M
Spectroscopic and spectropolarimetric observations of the pre-main
sequence early-G star HD 141943 were obtained at four observing
epochs (in 2006, 2007, 2009 and 2010). The observations were
undertaken at the 3.9-m Anglo-Australian Telescope using the UCLES
echelle spectrograph and the SEMPOL spectropolarimeter visitor
instrument. Brightness and surface magnetic field topologies were
reconstructed for the star using the technique of least-squares
deconvolution to increase the signal-to-noise ratio of the data.
The reconstructed brightness maps show that HD 141943 had a weak
polar spot and a significant amount of low-latitude features, with
little change in the latitude distribution of the spots over the 4 yr
of observations. The surface magnetic field was reconstructed at three
of the epochs from a high-order (l≤ 30) spherical harmonic expansion
of the spectropolarimetric observations. The reconstructed magnetic
topologies show that in 2007 and 2010 the surface magnetic field was
reasonably balanced between poloidal and toroidal components. However,
we find tentative evidence of a change in the poloidal/toroidal ratio
in 2009 with the poloidal component becoming more dominant. At all
epochs the radial magnetic field is predominantly non-axisymmetric
while the azimuthal field is predominantly axisymmetric with a ring
of positive azimuthal field around the pole similar to that seen on
other active stars.
Title: Magnetic fields and differential rotation on the pre-main
sequence - II. The early-G star HD 141943 - coronal magnetic field,
Hα emission and differential rotation
Authors: Marsden, S. C.; Jardine, M. M.; Ramírez Vélez, J. C.;
Alecian, E.; Brown, C. J.; Carter, B. D.; Donati, J. -F.; Dunstone,
N.; Hart, R.; Semel, M.; Waite, I. A.
Bibcode: 2011MNRAS.413.1939M
Altcode: 2011arXiv1101.5866M; 2011MNRAS.tmp..485M
Spectropolarimetric observations of the pre-main sequence early-G star
HD 141943 were made at three observing epochs (2007, 2009 and 2010). The
observations were made using the 3.9-m Anglo-Australian Telescope with
the UCLES echelle spectrograph and the SEMPOL spectropolarimeter visitor
instrument. The brightness and surface magnetic field topologies (given
in Paper I) were used to determine the star's surface differential
rotation and reconstruct the coronal magnetic field of the star. The coronal magnetic field at the three epochs shows on the largest
scales that the field structure is dominated by the dipole component
with possible evidence for the tilt of the dipole axis shifting between
observations. We find very high levels of differential rotation on HD
141943 (∼8 times the solar value for the magnetic features and ∼5
times solar for the brightness features), similar to that evidenced by
another young early-G star, HD 171488. These results indicate that a
significant increase in the level of differential rotation occurs for
young stars around a spectral type of early-G. We also find for the
2010 observations that there is a large difference in the differential
rotation measured from the brightness and magnetic features, similar
to that seen on early-K stars, but with the difference being much
larger. We find only tentative evidence for temporal evolution in the
differential rotation of HD 141943.
Title: Eigenpolarimetry: An Algebraic Approach to Polarization-free
Telescopes and Instruments
Authors: López Ariste, A.; Semel, M.
Bibcode: 2011ASPC..437..403L
Altcode:
We have developed new algebraic tools based upon the symmetries of
Mueller matrices that allow us to circumvent those problems while
keeping the adjective polarization-free for our telescopes and not
requiring expensive and delicate calibrations. The basic concept is
to modulate entrance polarization into the eigenvectors of the system
so that polarization information arrives unscathed at the analyzing
beamsplitters, placed at the instrument or detector level. I will
describe the basic concept, its application to THEMIS and the
propositions for EST.
Title: Spectropolarimetric multi line analysis of stellar magnetic
fields
Authors: Ramírez Vélez, J. C.; Semel, M.; Stift, M.; Martínez
González, M. J.; Petit, P.; Dunstone, N.
Bibcode: 2010A&A...512A...6R
Altcode: 2010arXiv1001.1003R
Aims: In this paper we study the feasibility of inferring the
magnetic field from polarized multi line spectra using two methods:
The pseudo line approach and The PCA-ZDI approach.
Methods:
We use multi line techniques, meaning that all the lines of a stellar
spectrum contribute to obtain a polarization signature. The use of
multiple lines dramatically increases the signal-to-noise-ratio of
these polarizations signatures. Using one technique, the pseudo line
approach, we construct the pseudo line as the mean profile of all the
individual lines. The other technique, the PCA-ZDI approach proposed
recently by Semel et al. (2006, ASPC, 358, 355) for the detection
of polarized signals, combines principle components analysis (PCA)
and the Zeeman Doppler imaging technique (ZDI). This new method has
a main advantage: the polarized signature is extracted using cross
correlations between the stellar spectra and functions containing
the polarization properties of each line. is extracted using cross
correlations between the stellar spectra and functions containing the
polarization properties of each line." These functions are the principal
components of a database of synthetic spectra. The synthesis of the
spectra of the database are obtained using the radiative transfer
equations in LTE. The profiles built with the PCA-ZDI technique are
called multi Zeeman signatures.
Results: The construction of the
pseudo line as well as the multi Zeeman signatures is a powerful tool
in the study of stellar and solar magnetic fields. The information of
the physical parameters that governs the line formation is contained
in the final polarized profiles. We have shown in particular using
inversion codes that the magnetic field vector can be properly inferred
with both approaches despite the magnetic field regime.
Title: Multiline Zeeman signatures through line addition
Authors: Semel, M.; Ramírez Vélez, J. C.; Martínez González,
M. J.; Asensio Ramos, A.; Stift, M. J.; López Ariste, A.; Leone, F.
Bibcode: 2009A&A...504.1003S
Altcode: 2009arXiv0906.4226S
Context: To obtain a significant Zeeman signature in the polarised
spectra of a magnetic star, we usually “add” the contributions
of numerous spectral lines; the ultimate goal is to recover the
spectropolarimetric prints of the magnetic field in these line
additions.
Aims: Here we want to clarify the meaning of these
techniques of line addition; in particular, we try to interpret the
meaning of the “pseudo-line” formed during this process and to
find out why and how its Zeeman signature is still meaningful.
Methods: We create a synthetic case of line addition and apply well
tested standard solar methods routinely used in research on magnetism
in the Sun.
Results: The results are convincing and the Zeeman
signatures well detected; Solar methods are found to be quite efficient
for stellar observations. We statistically compare line addition
with least-squares deconvolution and demonstrate that they both give
very similar results, as a consequence of the special statistical
properties of the weights.
Conclusions: The Zeeman signatures
are unequivocally detected in this multiline approach. We suggest
that magnetic field detection is reliable well beyond the weak-field
approximation. Linear polarisation in the spectra of solar type stars
can be detected when the spectral resolution is sufficiently high.
Title: Surface magnetic fields on two accreting TTauri stars: CVCha
and CRCha
Authors: Hussain, G. A. J.; Collier Cameron, A.; Jardine, M. M.;
Dunstone, N.; Ramirez Velez, J.; Stempels, H. C.; Donati, J. -F.;
Semel, M.; Aulanier, G.; Harries, T.; Bouvier, J.; Dougados, C.;
Ferreira, J.; Carter, B. D.; Lawson, W. A.
Bibcode: 2009MNRAS.398..189H
Altcode: 2009arXiv0905.0914H; 2009MNRAS.tmp..997H
We have produced brightness and magnetic field maps of the surfaces of
CVCha and CRCha: two actively accreting G- and K-type TTauri stars in
the ChamaeleonI star-forming cloud with ages of 3-5Myr. Our magnetic
field maps show evidence for strong, complex multipolar fields
similar to those obtained for young rapidly rotating main-sequence
stars. Brightness maps indicate the presence of dark polar caps and
low-latitude spots - these brightness maps are very similar to those
obtained for other pre-main-sequence and rapidly rotating main-sequence
stars. Only two other classical TTauri stars have been studied
using similar techniques so far: V2129Oph and BPTau. CVCha and CRCha
show magnetic field patterns that are significantly more complex than
those recovered for BPTau, a fully convective TTauri star. We
discuss possible reasons for this difference and suggest that the
complexity of the stellar magnetic field is related to the convection
zone; with more complex fields being found in TTauri stars with
radiative cores (V2129Oph, CVCha and CRCha). However, it is clearly
necessary to conduct magnetic field studies of TTauri star systems,
exploring a wide range of stellar parameters in order to establish how
they affect magnetic field generation, and thus how these magnetic
fields are likely to affect the evolution of TTauri star systems as
they approach the main sequence.
Title: Stellar Magnetic Fields in Swollen Convection Zones
Authors: Marsden, S. C.; Donati, J. -F.; Petit, P.; Dunstone, N. J.;
Jardine, M.; Carter, B. D.; Waite, I. A.; Semel, M.; Ramirez Velez, J.
Bibcode: 2009ASPC..405..531M
Altcode:
Solar magnetic activity is generated through dynamo action operating at
the base of the solar convection zone. However, for rapidly rotating
solar-type stars this might not be the case with magnetic images
showing regions of near-surface azimuthal field indicating that
the operation of dynamo may in fact be distributed throughout the
entire convection zone. Here we present the first magnetic images of
a pre-main sequence star with both components having swollen outer
convection zones. These results are part of an international study
to understand how the generation of magnetic fields is affected by
basic stellar parameters such as mass, rotation rate, the depth of
the stellar convection zone, and binarity. The magnetic images were
obtained by observing the star in circularly polarised light and using
the technique of Zeeman Doppler imaging.
Title: Multiline Zeeman signatures as demonstrated through the
Pseudo-line
Authors: Semel, M.; Ramirez Velez, J. C.; Stift, M. J.; Martinez
Gonzalez, M. J.; Lopez Ariste, A.; Leone, F.
Bibcode: 2008arXiv0810.3543S
Altcode:
In order to get a significant Zeeman signature in the polarised spectra
of a magnetic star, we usually 'add' the contributions of numerous
spectral lines; the ultimate goal is to recover the spectropolarimetric
prints of the magnetic field in these line additions. Here we want to
clarify the meaning of these techniques of line addition; in particular,
we try to interpret the meaning of the 'pseudo-line' formed during
this process and to find out why and how its Zeeman signature is still
meaningful. We create a synthetic case of lines addition and apply
well tested standard solar methods routinely used in the research
on magnetism in our nearest star. The results are convincing and the
Zeeman signatures well detected; Solar methods are found to be quite
efficient also for stellar observations. The Zeeman signatures are
unequivocally detected in this multiline approach. We may anticipate
the outcome magnetic fields to be reliable well beyond the weak-field
approximation. Linear polarisation in the spectra of solar type stars
can be detected when the spectral resolution is sufficiently high.
Title: PCA detection and denoising of Zeeman signatures in polarised
stellar spectra
Authors: Martínez González, M. J.; Asensio Ramos, A.; Carroll,
T. A.; Kopf, M.; Ramírez Vélez, J. C.; Semel, M.
Bibcode: 2008A&A...486..637M
Altcode: 2008arXiv0804.4692M
Aims: Our main objective is to develop a denoising strategy to
increase the signal to noise ratio of individual spectral lines
of stellar spectropolarimetric observations.
Methods: We
use a multivariate statistics technique called Principal Component
Analysis. The cross-product matrix of the observations is diagonalized
to obtain the eigenvectors in which the original observations can be
developed. This basis is such that the first eigenvectors contain
the greatest variance. Assuming that the noise is uncorrelated a
denoising is possible by reconstructing the data with a truncated
basis. We propose a method to identify the number of eigenvectors for
an efficient noise filtering.
Results: Numerical simulations are
used to demonstrate that an important increase of the signal to noise
ratio per spectral line is possible using PCA denoising techniques. It
can be also applied for detection of magnetic fields in stellar
atmospheres. We analyze the relation between PCA and commonly used
techniques like line addition and least-squares deconvolution. Moreover,
PCA is very robust and easy to compute.
Title: Strength distribution of solar magnetic fields in photospheric
quiet Sun regions
Authors: Ramírez Vélez, J. C.; López Ariste, A.; Semel, M.
Bibcode: 2008A&A...487..731R
Altcode: 2008arXiv0806.1326R
Context: The magnetic topology of the solar photosphere in its quietest
regions is hidden by the difficulties to disentangle magnetic flux
through the resolution element from the field strength of unresolved
structures. The observation of spectral lines with strong coupling with
hyperfine structure, like the observed Mn i line at 553.7 nm, allows
such differentiation.
Aims: To analyse the distribution of field
strengths in the network and intranetwork of the solar photosphere
through inversion of the Mn i line at 553.7 nm.
Methods: An
inversion code for the magnetic field using the principal component
analysis (PCA) has been developed. Statistical tests are run on
the code to validate it. The code has to draw information from the
small-amplitude spectral feature appearing in the core of the Stokes
V profile of the observed line for field strengths below a certain
threshold, coinciding with lower limit of the Paschen-Back effect in
the fine structure of the involved atomic levels.
Results: The
inversion of the observed profiles, using the circular polarisation (V)
and the intensity (I), shows the presence of magnetic fields strengths
in a range from 0 to 2 kG, with predominant weak strength values. Mixed
regions with mean strength field values of 1130 and 435 Gauss are
found associated with the network and intranetwork, respectively.
Conclusions: The Mn i line at 553 nm probes the field strength
distribution in the quiet sun and shows the predominance of weak,
hectoGauss fields in the intranetwork, and strong, kiloGauss fields in
the network. It also shows that both network and intranetwork are to be
understood at our present spatial resolutions as field distributions,
of which we hint at the mean properties.
Title: First polarimetric observations and modeling of the FeH
F4Δ-X4Δ system
Authors: Afram, N.; Berdyugina, S. V.; Fluri, D. M.; Semel, M.;
Bianda, M.; Ramelli, R.
Bibcode: 2007A&A...473L...1A
Altcode: 2007arXiv0708.0298A
Context: Lines of diatomic molecules are typically much more
temperature and pressure sensitive than atomic lines, which makes
them ideal, complementary tools for studying cool stellar atmospheres
as well as the internal structure of sunspots and starspots. The
FeH F^4Δ-X^4Δ system represents such an example that exhibits in
addition a large magnetic field sensitivity. However, the current
theoretical descriptions of these transitions including the molecular
constants involved are only based on intensity measurements because
polarimetric observations have not been available so far, which limits
their diagnostic value. Furthermore, the theory was optimized to
reproduce energy levels and line strengths without taking the magnetic
sensitivities into account.
Aims: We present for the first
time spectropolarimetric observations of the FeH F^4Δ-X^4Δ system
measured in sunspots to investigate their diagnostic capabilities for
probing solar and stellar magnetic fields. In particular, we investigate
whether the current theoretical model of FeH can reproduce the observed
Stokes profiles including their magnetic properties.
Methods: The
polarimetric observations of the FeH F^4Δ-X^4Δ system in Stokes I and
V are compared with synthetic Stokes profiles modeled with radiative
transfer calculations. This allows us to infer the temperature and
the magnetic field strength of the observed sunspots.
Results:
We find that the current theory successfully reproduces the magnetic
properties of a large number of lines in the FeH F^4Δ-X^4Δ
system. In a few cases the observations indicate a larger Zeeman
splitting than predicted by the theory. There, our observations have
provided additional constraints, which allowed us to determine empirical
molecular constants.
Conclusions: The FeH F^4Δ-X^4Δ system is
found to be a very sensitive magnetic diagnostic tool. Polarimetric
data of these lines, in contrast to intensity measurements, provide us
with more direct and detailed information to study the coolest parts
of sunspot and starspot umbrae, and cool active dwarfs.
Title: The photospheric magnetic field and coronal structure of
HD 171488
Authors: Marsden, S. C.; Jardine, M.; Donati, J. -F.; Petit, P.;
Semel, M.; Carter, B. D.
Bibcode: 2007MmSAI..78..307M
Altcode:
From spectropolarimetric observations of the young, single early
G-dwarf HD 171488, we have used Zeeman Doppler imaging to reconstruct
brightness and magnetic maps of the star. The inclusion of a solar-like
differential rotation law into the imaging process has enabled the
measurement of the star's surface differential rotation from the
brightness features. In addition, we have created a coronal structure
for HD 171488 based on the surface magnetic field distribution.
Title: Quiet-Sun Magnetism Seen with a Mn Line: Km-Sized Magnetic
Structures
Authors: López Ariste, A.; Ramírez Vélez, J. C.; Tomczyk, S.;
Casini, R.; Semel, M.
Bibcode: 2006ASPC..358...54L
Altcode:
We observed Manganese lines with large hyperfine structure and used them
to disentangle strength from flux in the measurement of photospheric
magnetic fields. In observations of the quiet sun with both ASP and
THEMIS, we measure flux from the amplitude of Stokes V in Fe lines, and
the Mn line, crudely analyzed, places the field strength either above
or below a threshold of 600 G, which is set by the atomic structure. In
the case of THEMIS observations, having determined magnetic flux and
field strength for every pixel, one can estimate filling factors of
the magnetic field and determine characteristic scales. Structures at
scales smaller than 50 km are revealed.
Title: Multi-Line Spectro-Polarimetry of Stellar Magnetic Fields
Using Principal Components Analysis
Authors: Semel, M.; Rees, D. E.; Ramírez Vélez, J. C.; Stift, M. J.;
Leone, F.
Bibcode: 2006ASPC..358..355S
Altcode:
Zeeman-Doppler-Imaging (ZDI) is a technique for the diagnosis of stellar
magnetic fields from observations of circularly polarized light, based
on the weak-field approximation. This approach has proved adequate
in numerous studies of the magnetic fields of solar-type stars. To
improve the signal-to-noise ratio, one can add the contributions of
different lines to the polarization signature. However, this multi-line
extension has been limited by a number of assumptions (e.g., the
weak-field approximation). Here we present a novel approach. We created
a database of synthetic polarized spectra for different combinations
of magnetic-field intensities and orientations. Principal Components
Analysis was used to compress the database, so that only few Principal
Components are needed to represent all the calculated spectra.
Title: The Magnetic Activity of Young Suns: Magnetic Topologies and
Differential Rotation of F- and G-Dwarfs
Authors: Marsden, S. C.; Mengel, M. W.; Donati, F.; Carter, B. D.;
Semel, M.; Petit, P.
Bibcode: 2006ASPC..358..401M
Altcode:
Maps of magnetic field topologies for rapidly rotating, solar-type stars
have been produced for the past decade, and offer a unique insight into
the operation of the stellar dynamo. The creation of these maps uses
the combined techniques of Least-Squares Deconvolution (LSD) to increase
the signal-to-noise of the Stokes-V profiles, and Zeeman Doppler Imaging
(ZDI) to create the magnetic maps from the Stokes-V profiles. However,
for solar-type stars almost all of the targets imaged to date have
been lower-mass (early-K) stars, limiting the range of operating
parameters of the stellar dynamo. We present magnetic maps of two
higher-mass, solar-type stars, the early G-dwarf HD 171488 (v sin i =
38 km s-1) and the late F-dwarf HR 1817 (v sin i = 52 km
s-1). Both stars have been observed in left- and right-hand
circularly polarized light using the SEMPOL spectro-polarimeter with
the Anglo-Australian telescope. Both LSD and ZDI have then been used to
infer topologies of the photospheric magnetic fields of the two stars,
thus expanding the mass range of solar-type stars imaged using these
techniques. In addition, the surface differential rotation of the
two stars has been determined from the data, with both stars showing
significantly higher differential rotation than that of lower-mass
stars.
Title: PCA Technique Applied to the Detection of ZDI Circular
Polarization in Solar-Type Stars
Authors: Rámirez Vélez, J. C.; Semel, M.; Stift, M. J.; Leone, F.
Bibcode: 2006ASPC..358..405R
Altcode:
We present a new detection method of ZDI circular polarization applied
to observations of solar-type stars. To extract the polarization signal
we analyze the observed spectra over intervals of about 2500 Å, and
we take into account the contribution of all spectral lines to the
resulting polarization signal. We employ a PCA technique to analyze the
synthetic database of profiles. We apply singular value decomposition
to the synthetic data over the same spectral range as for the observed
data. The principal parameters entering the magnetic spectral synthesis
code COSSAM are the effective temperature Teff and the
magnetic field strength B. We find that the first 10 eigenvectors
are sufficient to approximate the contribution of all the lines to
the resultant averaged circular polarization signal. The principle of
this new method can also be employed to detect linear polarization,
and it is of course not restricted to solar type stars.
Title: Multisite, Multiwavelength Studies of the Active Cool Binary
CC Eri
Authors: Budding, E.; Bembrick, C.; Carter, B. D.; Erkan, N.; Jardine,
M.; Marsden, S. C.; Osten, R.; Petit, P.; Semel, M.; Slee, O. B.;
Waite, I.
Bibcode: 2006Ap&SS.304...13B
Altcode: 2006Ap&SS.tmp...94B
New data acquired on the active, cool binary CC Eri ranged
across the spectrum from Chandra X-ray to broadband photometry and
microwave observations using the VLA and ATCA. Also, high-dispersion
spectropolarimetry using the AAT enabled Zeeman-Doppler imaging to be
performed. Our interpretations infer strong localised concentrations
of the stellar magnetic field, manifested by surface activity and
related large coronal plasma structures. Comprehensive matching of the
modelling parameters awaits more detailed investigation. This brief
interim review includes consideration of the ATCA data. Microwave radio
emission is usually low level (‘quiescent’), but occasionally flares
of several mJy peak intensity are observed. We associate the emission,
generally, with wave-like mechanisms, expanding through the outer
atmosphere. Related characteristics of this emission are discussed.
Title: Inversion of solar magnetic fields in photospheric quiet
regions.
Authors: Ramírez Vélez, J. C.; López Ariste, A.; Semel, M.
Bibcode: 2006IAUJD...3E..13R
Altcode:
Spectropolarymetric observational data and the correspondent
magnetic field inversion of the solar photosphere in quiet regions are
presented. The presence of atomic coupling between the nuclei momentum
(I) and total electron momentum ( J[LS]), known as hyperfine structure
atomic regime where the total momentum F = I + J, was founded in the
observed data for the atom MnI at 553.7 nm. The signature of these
hyperfine structure is clearly detected in circular polarisation in form
of protuberance in the central part of the profile. We have developed
an inversion code of the magnetic field using the Principal Component
Analysis (PCA). An statistical test is applied to the code obtaining
satisfactory inversion results. The inversion of the observed profiles,
using the circular polarisation (V) and the intensity (I), shows mixed
regions with values of |B| higher than 800 Gauss and lower than 600
Gauss associated with the network and internetwork respectively.
Title: Surface differential rotation and photospheric magnetic field
of the young solar-type star HD 171488 (V889 Her)
Authors: Marsden, S. C.; Donati, J. -F.; Semel, M.; Petit, P.; Carter,
B. D.
Bibcode: 2006MNRAS.370..468M
Altcode: 2006MNRAS.tmp..643M
We present spectropolarimetric observations of the young, single early
G-dwarf HD 171488. These observations were obtained over a five-night
period in 2004 September at the 3.9-m Anglo-Australian Telescope using
the SEMPOL spectropolarimeter visitor instrument. Using the technique
of least-squares deconvolution to increase the signal-to-noise ratio
of the data, we have applied Zeeman Doppler imaging to reconstruct
brightness and magnetic surface topologies of the star. The brightness
image shows a large polar spot with weaker low- to mid-latitude
features, confirming an earlier Doppler imaging observation. The
reconstruction of the surface magnetic field shows regions of radial
field at all latitudes (except near the pole) and regions of azimuthal
field predominantly at high latitudes (60°-70°), with the azimuthal
field almost forming a ring around the polar regions. We have
incorporated a solar-like differential rotation law into the imaging
process to determine the surface differential rotation of cool spots on
HD 171488. This gives an equatorial rotation rate of 1.313 +/- 0.004 d
and a surface shear of dΩ = 0.402 +/- 0.044 radd-1. This
means that the equator of HD 171488 laps the poles every ~16 +/-
2 d and that HD 171488 has a photospheric shear approximately seven
times the solar value. This is the largest measurement of surface
differential rotation yet obtained using the Doppler imaging method
and is over twice the value of previously observed early G-dwarfs.
Title: A Sun in the Spectroscopic Binary IM Pegasi, the Guide Star
for the Gravity Probe B Mission
Authors: Marsden, S. C.; Berdyugina, S. V.; Donati, J. -F.; Eaton,
J. A.; Williamson, M. H.; Ilyin, I.; Fischer, D. A.; Muñoz, M.;
Isaacson, H.; Ratner, M. I.; Semel, M.; Petit, P.; Carter, B. D.
Bibcode: 2005ApJ...634L.173M
Altcode:
We present the first detection of the secondary of the spectroscopic
binary system IM Pegasi (HR 8703), the guide star for the NASA-Stanford
relativity gyroscope mission Gravity Probe B. In support of this
mission, high-resolution echelle spectra of IM Peg have been obtained
on an almost nightly basis. Applying the technique of least-squares
deconvolution, we achieve very high signal-to-noise ratio line profiles
and detect the orbit of the secondary of the system. Combining almost
700 new radial velocity measurements of both the primary and secondary
of the system with previous measurements, we derive improved orbital
parameters of the IM Peg system. Using these estimates along with
the previously determined range of orbital inclination angles for
the system, we find that the primary of IM Peg is a giant of mass
1.8+/-0.2 Msolar, while the secondary is a dwarf of mass
1.0+/-0.1 Msolar.
Title: Full Stokes Spectropolarimetry of Hα in Prominences
Authors: López Ariste, A.; Casini, R.; Paletou, F.; Tomczyk, S.;
Lites, B. W.; Semel, M.; Landi Degl'Innocenti, E.; Trujillo Bueno,
J.; Balasubramaniam, K. S.
Bibcode: 2005ApJ...621L.145L
Altcode:
We report on spectropolarimetric observations of Hα in prominences
made with the Télescope Héliographique pour l'Etude du Magnétisme et
des Instabilités Solaires and the High Altitude Observatory/Advanced
Stokes Polarimeter. Stokes Q and U show the expected profile shape from
resonance scattering polarization and the Hanle effect. In contrast,
most of the time, Stokes V does not show the antisymmetric profile shape
typical of the Zeeman effect but a profile that indicates the presence
of strong atomic orientation in the hydrogen levels, to an extent that
cannot be explained by invoking the alignment-to-orientation transfer
mechanism induced by the prominence magnetic field. We found that the
largest signal amplitudes of Stokes V (comparable to that of Stokes
Q and U) could be produced by a process of selective absorption of
circularly polarized radiation from the photosphere, which requires
that the prominence be in the vicinity of an active region. Although
recent observations of active region filaments indicate such a
selective absorption mechanism as a plausible explanation of the
anomalous signals observed, the particular set of conditions that
must be met suggest that a different explanation may be required
to explain the almost ubiquitous symmetric V signal observed in Hα
prominences. Therefore, we speculate that an alternative mechanism
inducing strong atomic orientation at the observed level could be due
to the presence of electric fields inducing an electric Hanle effect
on Hα. Although we are still working toward a careful modeling of
this effect, including both electric and magnetic fields, we present
some preliminary considerations that seem to support this possibility.
Title: τ Boo b: Not so bright, but just as heavy
Authors: Collier-Cameron, Andrew; Horne, Keith; James, David; Penny,
Alan; Semel, Meir
Bibcode: 2004IAUS..202...75C
Altcode:
No abstract at ADS
Title: Dynamo processes and activity cycles of the active stars AB
Doradus, LQ Hydrae and HR 1099
Authors: Donati, J. -F.; Collier Cameron, A.; Semel, M.; Hussain,
G. A. J.; Petit, P.; Carter, B. D.; Marsden, S. C.; Mengel, M.;
López Ariste, A.; Jeffers, S. V.; Rees, D. E.
Bibcode: 2003MNRAS.345.1145D
Altcode:
In this paper, we present new brightness and magnetic surface images of
the young K0 dwarfs AB Doradus and LQ Hydrae, and of the K1 subgiant
of the RS CVn system HR 1099 (=V711 Tauri), reconstructed from
Zeeman-Doppler imaging spectropolarimetric observations collected
at the Anglo-Australian Telescope during five observing campaigns
(totalling 50 nights), from 1998 January to 2002 January. Along with
the older images of the same stars (published in previous papers),
our complete data set represents the first long-term series on temporal
fluctuations of magnetic topologies of very active stars. All of
the magnetic images presented here indicate that large regions with
predominantly azimuthal magnetic fields are continuously present at
the surfaces of these stars. We take this as further evidence that the
underlying dynamo processes that produce them are probably distributed
throughout the entire convective zone (and not confined at its base,
as in the Sun). We speculate that the radial and azimuthal field
maps that we recover correspond, respectively, to the poloidal and
toroidal components of the large-scale dynamo field. We find, in
particular, that some signatures, for instance the relative fraction
of magnetic energy stored in the large-scale poloidal and toroidal
field components, and the polarity of the axisymmetric component of
the field, are variable with time, and provide potentially fruitful
diagnostics for investigating magnetic cycles in active stars other
than the Sun. We report here the detection of partial polarity switches
in some of the axisymmetric field components of two of our programme
stars (AB Dor and LQ Hya), suggesting that the dynamo operating in
these stars may be cyclic.
Title: Spectropolarimetry and polarization-dependent fringes
Authors: Semel, M.
Bibcode: 2003A&A...401....1S
Altcode:
An account is given of the formation of polarization-dependent
fringes, the ``enemies" of spectropolarimetry, present practically
in all polarization devices. Typically, multiple reflections
give rise to secondary beams coherent with the main beam, but with
wavelength-dependent phase differences. Polarized fringes may appear in
any particular Stokes parameter or their combinations, and may (or may
not) be seen in the intensity. The mechanism of formation of fringes in
polarization is demonstrated. Fringes of analyzers, beam-splitter and
retarders are evaluated. Flat-fielding techniques are discussed and
ways for the removal of polarized fringes are suggested. Techniques
that are efficient in removing fringes in intensity may be inadequate
or insufficient for polarization. Anti-reflection coating, for
instance, may reduce fringes, but not below 10-3 of
the intensity. This level would often be acceptable for intensity
measurements but not necessarily for polarization, where one may often
need to evaluate signals down to the level of 10-5, as in
the observation of resonance line polarization and the second solar
spectrum. Flat-fielding, by observing disk center, may be satisfactory
to remove fringes for solar observations but this is not appropriate
for stellar observations. The understanding of how these fringes are
formed is essential both for the design of polarimeters and for their
proper use. Appendix C is only available in electronic form at
http://www.edpsciences.org
Title: Polarimeter for the study of magnetic fields in prominences
Authors: Lopez Ariste, Arturo; Tomczyk, Steven; Semel, Meir;
Darnell, A.
Bibcode: 2003SPIE.4853..235L
Altcode:
We report on a new spectropolarimeter for the measurement of vector
magnetic fields on prominences using the HeD3 line in the
Evans Solar Facility at the Sacramento Peak Observatory.
Title: The magnetic field and wind confinement of θ1
Orionis C
Authors: Donati, J. -F.; Babel, J.; Harries, T. J.; Howarth, I. D.;
Petit, P.; Semel, M.
Bibcode: 2002MNRAS.333...55D
Altcode:
We report the detection, through spectropolarimetric observations,
of a strong dipolar magnetic field of presumably fossil origin at
the surface of the very young O star θ1 Ori C. The Stokes
V signatures we detect are variable with time, the variations being
consistent with rotational modulation. A detailed modelling of our
observations indicates that this dipole field has an intensity of
1.1+/-0.1kG and is inclined at 42°+/-6° with respect to the rotation
axis (assumed to be inclined at 45° to the line of sight). We find,
in particular, that the positive magnetic pole comes closest to the
observer when the variable Hα emission component observed on this star
reaches maximum strength. This discovery represents the first definite
detection of a magnetic field in an O star, as well as the first
detection of a fossil field in a very young star. We also investigate
in this paper the magnetic confinement of the radiatively driven wind
of θ1 Ori C in the context of the magnetically confined
wind-shock model of Babel & Montmerle. In the case of θ1
Ori C, this model predicts the formation of a large magnetosphere
(extending as far as 2-3R*), consisting of a very hot post-shock
region (with temperatures in excess of 10MK and densities of about
1010-1011cm-3) generated by the strong
collision of the wind streams from both stellar magnetic hemispheres, as
well as a dense cooling disc forming in the magnetospheric equator. We
find that this model includes most of the physics required to obtain a
satisfactory level of agreement with the extensive data sets available
for θ1 Ori C in the literature (and, in particular, with
the recent X-ray data and the phase-resolved spectroscopic observations
of ultraviolet and optical wind lines) provided that the mass-loss
rate of θ1 Ori C is at least 5 times smaller than that
predicted by radiatively driven wind models. We finally show how new
observations with the XMM or Chandra spacecraft could help us constrain
this model much more tightly and thus obtain a clear picture of how
magnetic fields can influence the winds of hot stars.
Title: Stellar differential rotation from direct star-spot tracking
Authors: Collier Cameron, A.; Donati, J. -F.; Semel, M.
Bibcode: 2002MNRAS.330..699C
Altcode: 2001astro.ph.11050C
On the Sun, the rotation periods of individual sunspots not only trace
the latitude-dependence of the surface rotation rate, but also provide
clues as to the amount of subsurface fluid shear. In this paper we
present the first measurements of stellar differential rotation made by
tracking the rotation of individual star-spots with sizes comparable
to the largest sunspots. To achieve this we re-analyse four sequences
of densely sampled, high signal-to-noise ratio echelle spectra of AB
Doradus spanning several stellar rotations in 1996 December. Using
spectral subtraction, least-squares deconvolution and matched-filter
analysis, we demonstrate that it is possible to measure directly
the velocity amplitudes and rotation periods of large numbers of
individual star-spots at low to intermediate latitude. We derive values
for the equatorial rotation rate and the magnitude of the surface
differential rotation, both of which are in excellent agreement with
those obtained by Donati & Collier Cameron from cross-correlation
of Doppler images derived a year earlier in 1995 December, and with a
re-analysis of the 1996 data by the χ2 landscape method. The
differences between the rotation rates of individual spots and the
fitted differential rotation law are substantially greater than the
observational errors. The smaller spots show a greater scatter about
the mean relation than the larger ones, which suggests that buffeting
by turbulent supergranular flows could be responsible.
Title: Full-Stokes spectropolarimetry of solar prominences
Authors: Paletou, F.; López Ariste, A.; Bommier, V.; Semel, M.
Bibcode: 2001A&A...375L..39P
Altcode:
We report on first spectropolarimetric observations of solar prominences
made at THEMIS. Hereafter we shall emphasis on the preliminary analysis
of He I D3 full-Stokes observations. Our measurements
clearly show manifestations of both the Hanle and Zeeman effects. More
generally, we demonstrate the very favourable performances of the THEMIS
multi-line spectropolarimetric mode for further new investigations of
the magnetic field in solar prominences. Based on observations made at
THEMIS, operated on the Island of Tenerife by CNRS-CNR in the Spanish
Observatorio del Teide of the Instituto de Astrofísica de Canarias.
Title: Current Ground-Based Stellar Spectropolarimeters and their
Use for Zeeman-Doppler Imaging
Authors: Semel, M.; López Ariste, A.
Bibcode: 2001ASPC..248..575S
Altcode: 2001mfah.conf..575S
No abstract at ADS
Title: Scattering Polarization Observations with the Tenerife Gregory
Coudé Telescope
Authors: Dittmann, O.; Trujillo Bueno, J.; Semel, M.; López Ariste, A.
Bibcode: 2001ASPC..236..125D
Altcode: 2001aspt.conf..125D
No abstract at ADS
Title: tau Boo b: Not so bright, but just as heavy
Authors: Collier Cameron, Andrew; Horne, Keith; James, David; Penny,
Alan; Semel, Meir
Bibcode: 2000astro.ph.12186C
Altcode:
We present new results derived from high-resolution optical spectra
of the tau Boo system, secured in March-May 2000. The results do
not show the same feature reported by Cameron et al (1999) as a
candidate reflected-light signature from the planet. Together with
earlier results from the 1998 and 1999 seasons, the new data yield
a 99.9 percent upper limit on the opposition planet/star flux ratio
epsilon<3.5E-5 between 387.4 and 586.3 nm, a factor 3 deeper than the
upper limit of Charbonneau et al (1999). For an assumed planet radius
Rp= 1.2 RJup, the upper limit on the mean geometric albedo is p <
0.22, 40 percent that of Jupiter. We find new evidence that the star's
rotation is synchronised with the planet's orbital motion. Using a
Monte Carlo analysis we infer that the planet's mass must lie in the
range 5.5 to 10 times the mass of Jupiter.
Title: Fibre Feeding the UES
Authors: Walton, N. A.; Collier-Cameron, A.; Semel, M.
Bibcode: 2000INGN....3...29W
Altcode:
This report summarises the recent successful deployment of a fibre
feed to the Utrecht Echelle Spectrograph (UES). The main advantage
of this feed, as opposed to the conventional Nasmyth direct feed, is
that the starlight is spread along the slit, redu cing sensitivity to
flat field errors, and allowing increased integration times on bright
objects. The prototype setup, which uses a 33-m fibre and is optimised
for wavelengths around 5500 Å, gives throughput at least as good as
conventional slit spectrosc opy with the more efficient of the UES
de-rotators between 5000 and 6000 Å.
Title: Fast inversion of spectral lines using principal component
analysis. I. Fundamentals
Authors: Rees, D. E.; López Ariste, A.; Thatcher, J.; Semel, M.
Bibcode: 2000A&A...355..759R
Altcode:
This paper presents PCA inversion, a novel application of Principal
Component Analysis to the problem of spectral line inversion,
ie. solar/stellar atmospheric model parameter estimation from spectral
lines. For a given type of spectral line we compute a database of
synthetic spectral profiles using a large number of models. Inversion
of an observed profile to obtain an atmospheric model is equivalent
to a problem in pattern recognition, finding the nearest profile in
the synthetic profile database. To reduce dimensionality we use the
synthetic data as a PCA training set to decompose each synthetic (and
observed) profile into a sum of a small number of principal components,
or eigenprofiles. The coefficients of this decomposition can be regarded
as elements of a low-dimensional eigenfeature vector. The eigenfeatures
are smooth functions of model parameters, indicating that eigenfeatures
for parameters not in the training set could be easily estimated by
interpolation. Search for the nearest profile is fast because it is
done in the eigenfeature vector space. We illustrate the method using
several types of synthetic spectra: unpolarised intensity profiles of
a line formed in a Milne-Eddington model atmosphere; unpolarised Hα
flux profiles of a line formed in non-Local Thermodynamic Equilibrium
in the chromosphere of a cool star; and polarised Stokes parameter
profiles of a line split by the Zeeman effect in the presence of a
magnetic field. We also apply PCA to a set of Stokes data observed
in a sunspot region by the High Altitude Observatory Advanced Stokes
Polarimeter. PCA inversion is proposed as a fast alternative to
non-linear least squares inversion commonly used for solar magnetic
field measurements based on such Stokes data.
Title: First results from THEMIS spectropolarimetric mode
Authors: López Ariste, A.; Rayrole, J.; Semel, M.
Bibcode: 2000A&AS..142..137L
Altcode:
We present here the very first spectropolarimetric results obtained with
the multiline spectroscopy mode (MTR) of THEMIS. The principal problems
found during the data analysis are exposed. A first characterisation
of the quality of this observing mode is given. Based on observations
made with THEMIS operated on the island of Tenerife by CNRS-CNR in
the Spanish Observatorio del Teide of the Instituto de Astrofísica
de Canarias.
Title: DIAGONAL: A numerical solution of the Stokes transfer equation
Authors: López Ariste, A.; Semel, M.
Bibcode: 1999A&AS..139..417L
Altcode:
In a previous paper \cite[(Semel & López Ariste 1999]{art1},
hereafter referred to as Paper I) the authors found a new solution for
the integration of the radiative transfer equation for polarized light
(RTE). The present paper is a continuation of that work, and shows how
this solution has been incorporated into a new numerical code for the
integration of the RTE. This code, called DIAGONAL, is characterized
by an analytical handling of most model atmospheres, differing in that
from other codes. It gives exact solutions in new, non-trivial cases. In
the general case when no complete analytical solution is available,
the code resorts to a multi-layer technique, thus providing a solution
that combines an approximated analytical model and a minimized residual
treated linearly. The algorithm is shown both to be very stable and
to reach a high precision even for a small number of integration
layers. This results in a very fast code. A comparison with DELO has
been carried out. DIAGONAL is available, upon request to the authors.
Title: Analytical solution of the radiative transfer equation for
polarized light
Authors: López Ariste, A.; Semel, M.
Bibcode: 1999A&A...350.1089L
Altcode: 1999astro.ph..9232A; 1999astro.ph..9232L
A new formalism is introduced for the transfer of polarized
radiation. Stokes parameters are shown to be four-vectors in a
Minkowski-like space and, most strikingly, the radiative transfer
equation (RTE) turns out to be an infinitesimal transformation under
the Poincaré (plus dilatations) group. A solution to the transfer
equation as a finite element of this group is proposed.
Title: Integration of the radiative transfer equation for polarized
light: the exponential solution
Authors: Semel, M.; López Ariste, A.
Bibcode: 1999A&A...342..201S
Altcode: 1998astro.ph.11153S
The radiative transfer equation (RTE) for polarized light accepts a
convenient exponential solution when the absorption matrix commutes
with its integral. We characterize some of the matrix depth variations
which are compatible with the commutation condition. Eventually
the vector solution may be diagonalized and one may obtain four
independent scalar solutions with four optical depths, complex in
general. When the commutation condition is not satisfied, one must
resort to a determination of an appropriate evolution operator, which
is shown to be well determined mathematically, but whose explicit
form is, in general, not easy to apply in a numerical code. However,
we propose here an approach to solve a general case not satisfying
the commutation condition.
Title: Exponential solutions to the radiative transfer equation for
polarized light
Authors: López Ariste, A.; Semel, M.
Bibcode: 1999ASSL..243..157L
Altcode: 1999sopo.conf..157L
No abstract at ADS
Title: Magnetic topology and prominence patterns on AB Doradus
Authors: Donati, J. -F.; Collier Cameron, A.; Hussain, G. A. J.;
Semel, M.
Bibcode: 1999MNRAS.302..437D
Altcode:
We report new Zeeman--Doppler imaging observations of the rapidly
rotating young K0 dwarf AB Doradus, obtained with the Anglo-Australian
Telescope in 1996 December. From such observations, simultaneous
brightness and magnetic images of the stellar photosphere of AB Dor
were reconstructed at three different epochs. The magnetic topology
of AB Dor is found to be very complex, with at least 12 different
radial field regions of opposite polarities located all around the
star. Significant azimuthal field fluxes are also detected in the
form of one negative polarity region close to the equator, a series
of positive polarity patches at intermediate latitudes and an almost
complete ring of negative polarity encircling the rotational pole at
high latitudes. In particular, the azimuthal polarities we reconstruct
are in very good agreement with those obtained by Donati &
Cameron, confirming that this field component is directly related
to the dynamo-generated large-scale toroidal magnetic structure. The
triple polarity latitudinal pattern observed for this structure in the
upper hemisphere of AB Dor indicates that the degree of the underlying
large-scale poloidal structure in an axisymmetric spherical harmonics
expansion is equal to or greater than five. It also strengthens the
idea that the dynamo processes operating in AB Dor feature a non-solar
component distributed throughout the convective zone. From the subtle
distortion of successive brightness images, we can also confirm the
surface differential rotation first measured on this star by Donati
& Cameron in both sense and magnitude, with a pole rotating more
slowly than the equator by about one part in 220. Finally, the rotation
periods we measure for four prominences (from the recurrence rate of
their spectral signatures in Balmer lines) confirm the presumption
that such clouds are anchored at intermediate to high latitudes. The
intrinsic variability of these prominences is not associated with any
abrupt changes in the photospheric brightness or magnetic distributions,
implying that they essentially result from the reorganization of
coronal field lines.
Title: An ambiguity-free determination of J_Z in solar active regions
Authors: Semel, M.; Skumanich, A.
Bibcode: 1998A&A...331..383S
Altcode:
We propose a way to derive a radial electric current density from
vector magnetograms that is free of any particular choice of the two
possible azimuths for the plane of the sky field. The method is applied
to active region data obtained with the Advanced Stokes Polarimeter
(ASP). A comparison of this current density with that derived for
two particular sets of possible azimuth assignments allows one to
evaluate the appropriateness of each set. In addition one may verify
the detection of currents in general and the occurrence of current
sheets, i.e. field discontinuities, in particular.
Title: Differential Rotation and Magnetic Polarity Patterns on
AB Doradus
Authors: Donati, J. -F.; Collier Cameron, A.; Hussain, G. A. J.;
Semel, M.
Bibcode: 1998ASPC..154.1966D
Altcode: 1998csss...10.1966D
We report new Zeeman-Doppler imaging observations of the rapidly
rotating young K0 dwarf AB Doradus, obtained with the Anglo-Australian
Telescope in 1996 December. We show simultaneous brightness and
magnetic images of the stellar photosphere, reconstructed at
three different epochs over the course of a seven-night observing
run. Latitude-by-latitude cross-correlation of the resulting images
confirms the form and amplitude of the surface differential pattern
found in the previous year's observations by Donati & Cameron
(1997), with the pole rotating slower than the equator by about one
part in 220. We also present dynamic spectra showing the distribution
of H alpha-absorbing clouds in the stellar corona at the same epoch.
Title: Boundary Conditions and the Extrapolation of Magnetic Fields
(Invited Review)
Authors: Semel, M.
Bibcode: 1998ASPC..155..423S
Altcode: 1998sasp.conf..423S
No abstract at ADS
Title: Spectropolarimetric observations of active stars
Authors: Donati, J. -F.; Semel, M.; Carter, B. D.; Rees, D. E.;
Collier Cameron, A.
Bibcode: 1997MNRAS.291..658D
Altcode:
This paper reports the results of five years (five runs, 23 nights)
of spectropolarimetric observations of active stars with the UCL
Echelle Spectrograph of the Anglo-Australian Telescope. 225 circularly
(and four linearly) polarized spectra were recorded on 28 objects
(21 active stars and seven calibration standards) using the new
technique of Zeeman-Doppler imaging. To extract polarization echelle
spectra from raw frames, we developed a new dedicated automatic
software package (called ESprit, which utilizes optimal extraction
techniques) whose detailed description is given in the paper. For
each recorded spectrum, we extract `mean' polarized and unpolarized
profiles using `least-squares deconvolution', a technique similar
to cross-correlation, which can enhance enormously the sensitivity
of Zeeman-Doppler imaging, by up to 7.5 mag in flux with respect
to a single average line analysis or by 4.5 mag compared with the
older technique of Donati et al. in the particular case of a K1
star. Magnetic field is detected unambiguously on 14 objects, namely
the weak-line T Tauri star V410 Tau, the pre-main-sequence binary
HD 155555, the ZAMS stars AB Dor and LQ Hya, the dwarf flare star
CC Eri, the RS CVn systems HR 1099, EI Eri, TY Pyx, CF Tuc, SZ Psc,
II Peg, IM Peg and IL Hya, and the FK Com star YY Men. Marginal field
detections are also obtained for the weak-line T Tauri star HD 283572
and the Herbig Ae star HD 104237. Except on HR 1099 and II Peg, our
results represent the first direct field detections ever reported on
these objects, and in particular the first direct field detection on
as young a star as V410 Tau. Most of the magnetic signatures we detect
on cool stars show several sign reversals throughout the line profile,
indicating that the parent field structure is rather complex and must
feature (as expected) many small-scale magnetic regions of different
polarities. For all stars on which Zeeman detections are recorded
with sufficient accuracy (namely LQ Hya, CC Eri, HR 1099, EI Eri,
II Peg, IL Hya and YY Men), differential least-squares deconvolution
from both the blue and the red parts of the spectral domain indicates
that the magnetic regions we detect are mostly 500 to 1000 K cooler
than, and sometimes at the same temperature as, but never warmer than
the surrounding photosphere. Serendipitous results include the first
detection (i) of small-amplitude radial velocity variations (1.3kms^-1
peak to peak) of the Herbig Ae star HD 104237 with small enough a period
(37.5+/-1 min) that they must be due to stellar pulsations and (ii)
of the solar-like secondary component of the RS CVn system IL Hya.
Title: Differential rotation and magnetic polarity patterns on AB Dor.
Authors: Collier Cameron, A.; Donati, J. -F.; Semel, M.
Bibcode: 1997AAONw..83....8C
Altcode:
No abstract at ADS
Title: On The Detection of Currents and Parker's Objection
Authors: Skumanich, A.; Semel, M.
Bibcode: 1997SPD....28.1605S
Altcode: 1997BAAS...29..920S
We propose a way to derive a radial electric current density
(hereafter current) from vector magnetograms that is free of any
particular choice of the two possible azimuths for the plane of
the sky (POS) field. The method is applied to active region data
obtained with the Advanced Stokes Polarimeter. A comparison of this
current density with that derived for two particular sets of possible
azimuth assignments allows one to evaluate the appropriateness of
each set and to detect the occurrence of current sheets, i.e. field
discontinuities. A minimum current constructed from ambiguity-free
POS and line-of-sight currents allows one to verify the detection of
currents in general. Parker's scenario leads to a current distribution
that is statistically distributed about zero, looks like noise and is
also discontinuous. We observe such noise as well as continuous and
significant currents. We conclude that the unresolved magnetic field
is not necessarily always in the form of individual flux tubes.
Title: Azimuthal structures in the wind and chromosphere of the
Herbig AE star AB Aurigae. Results from the MUSICOS 1992 campaign.
Authors: Boehm, T.; Catala, C.; Donati, J. -F.; Welty, A.; Baudrand,
J.; Butler, C. J.; Carter, B.; Collier-Cameron, A.; Czarny, J.; Foing,
B.; Ghosh, K.; Hao, J.; Houdebine, E.; Huang, L.; Jiang, S.; Neff,
J. E.; Rees, D.; Semel, M.; Simon, T.; Talavera, A.; Zhai, D.; Zhao, F.
Bibcode: 1996A&AS..120..431B
Altcode:
The observations of the MUSICOS 1992 campaign concerned three scientific
programs, among which was the study of azimuthal structures in the wind
and chromosphere of the pre-main sequence Herbig Ae star AB Aur. The He
I 5876A line of AB Aur, which is formed in the expanding chromosphere
of this star, most probably in the innermost parts of its wind, was
continuously monitored at a spectral resolution of 30000 for about 4
days. The line was discovered to be variable in a spectacular way, the
profile changing from pure emission to a composite profile including a
deep absorption component in the course of a few hours. In this paper we
present strong clues that suggest a rotational modulation of the line,
with a period corresponding to the stars rotation period of 32hrs. We
confirmed the covariation of several non-photospheric spectral lines,
formed at very different radial distances of the star, which strengthens
the idea that the variability is the signature of azimuthal structures
in the wind of AB Aur, most probably due to magnetically confined
streams or loops emanating from the stellar surface. We present the
data collected during the MUSICOS 1992 campaign, but also a data set of
the He I D3 line obtained with FOE at KPNO during the years 1991-1994
and a series of IUE spectra containing the Mg II h & k UV lines
obtained one month prior to the campaign. A thorough discussion of
the possible interpretations of the spectacular variations of the He
I 5876A line is presented.
Title: Zeeman Doppler imaging of stars with the AAT
Authors: Carter, B.; Brown, S.; Donati, J. -F.; Rees, D.; Semel, M.
Bibcode: 1996PASA...13..150C
Altcode:
Zeeman Doppler Imaging (ZDI) is a recent technique for measuring
magnetic fields on rapidly rotating, active stars. ZDI employs
spectropolarimetry taken at different rotational phases, to derive
information on the magnetic field distribution over the stellar
surface. The Zeeman effect is used to identify the presence of a
magnetic field, and variations in Doppler wavelength shifts across
the rapidly rotating star allow fields to be resolved on different
parts of the visible disc. Analysis of the spectra can be used
to produce both thermal and surface magnetic images. ZDI requires
very high S/N spectra to be acquired, within a time interval short
compared to the stellar rotation period. As a result, a large aperture
telescope is needed. Since an initial successful test in 1989, the
3.9m Anglo-Australian Telescope has been used to obtain ZDI spectra
of active stars of different evolutionary stages. The observations
have concentrated on the K subgiant in the RS CVn system HR 1099,
to monitor changes on this bright and active star. With the advent
in 1991 of ZDI spectropolarimetry with the AAT 'echelle spectrograph,
it has become possible to co-add the polarisation signature from the
many magnetically sensitive lines recorded simultaneously. As a result,
stellar magnetic field detections of unprecedented quality have been
obtained. The aims of this paper are to briefly outline the principles
of ZDI, describe the instrumental setup at the AAT and present some
preliminary results from recent observations.
Title: Field Azimuth Disambiguation Using Ambiguity-Free Currents
Authors: Skumanich, A.; Semel, M.
Bibcode: 1996SoPh..164..291S
Altcode:
Using the ambiguity-free vertical current defined by Semel and Skumanich
(1995) we derive a minimum-current azimuth disambiguation for the
observed magnetic field in the active region NOAA 7201. A comparison of
such a minimum-current azimuth resolution with those from other extant
methods indicates that the resulting resolution, even though found
to be limited by noise, is a useful first approximation. A comparison
of our minimum current distribution with the currents we derive from
an extant disambiguation (Lites et al., 1995) indicates the presence
of current discontinuities in the form of linear features near the
magnetic neutral line of the associated δ-spot.
Title: Zeeman-Doppler Imaging of Solar-Type Stars: Multi Line
Technique
Authors: Semel, M.; Li, J.
Bibcode: 1996SoPh..164..417S
Altcode:
In this work, a multi-line spectropolarimetric detection using an
Echelle spectrograph is described. The polarization of Zeeman effect
is detected by the use of more than 200 lines observed in the solar
type star, HR1099. Using the statistics analysis in a sample of 200
lines, we found on the average a polarization signal of about 3 ×
10−4.
Title: The magnetohydrodynamic equilibrium of coronal helmet
streamers.
Authors: Cuperman, S.; Bruma, C.; Dryer, M.; Semel, M.
Bibcode: 1995A&A...299..389C
Altcode:
The equilibrium of coronal helmet streamers is investigated within
the framework of an axisymmetric magnetohydrodynamic polytropic
model. The main features of this work are: (i) The eight MHD vector
equations are reduced - via analytical manipulations - to three scalar
equations involving the magnetic flux function, the current density
and temperature; (ii) The reduced system of equations is solved by
the aid of a computational algorithm including both relaxation and
iterative techniques, and is continued until the maximum local relative
change with time becomes smaller than 10^-4^; (iii) At the Sun, only
three physical quantities are fixed, namely, the radial component of
the magnetic field, the temperature and the particle density. The
other quantities - the θ-magnetic field, radial and θ-components
of the streaming velocity - are determined in a self-consistent way;
(iv) The coronal helmet streamer structure is obtained by: specifying
the (observed) latitude extension on the sun of the region of closed
magnetic field lines, requiring the field lines in the meridian plane
to become purely radial at 10 solar radii, and imposing the continuity
condition on the field line separating the region of closed field
lines from that of open field lines. The algorithm used in this work
enables the separate analysis of the following problems of interest:
(i) the linear problem - plasma acceleration in a prescribed helmet
streamer-like configuration; (ii) The quasi-linear problem - the
feedback effect of the plasma (solar wind) flow on the vacuum magnetic
configuration; and (iii) the non-linear problem - the consistent
equilibrium state of the magnetic field and plasma flow in coronal
helmet streamers. The quantitative results obtained in these studies
are presented, compared and discussed.
Title: Zeeman Doppler Imaging
Authors: Semel, M.
Bibcode: 1995ASPC...71..340S
Altcode: 1995tosm.conf..340S; 1995IAUCo.149..340S
No abstract at ADS
Title: Azimuthal Structures in the Wind and Chromosphere of the
Herbig Ae-Star Ab-Aurigae - Preliminary Results from the MUSICOS-1992
Campaign
Authors: Catala, C.; Bohm, T.; Donati, J. F.; Simon, T.; Welty, A.;
Houdebine, E.; Huang, L.; Jiang, S.; Zhai, D.; Neff, J.; Foing, B. H.;
Ghosh, K.; Butler, J.; Collier-Cameron, A.; Baudrand, J.; Czarny, J.;
Zhao, F.; Talavera, A.; Kennelly, T.; Carter, B.; Rees, D.; Semel,
M.; Cutispoto, G.; Rodono, M.
Bibcode: 1994SoPh..155..185C
Altcode:
MUSICOS (for MUlti-SIte COntinuous Spectroscopy) is an international
project to facilitate and organize world-wide multi-site campaigns
in high resolution spectroscopy, in view of obtaining a complete time
coverage of various types of variable stellar phenomena.
Title: THEMIS polarimetry
Authors: Semel, M.
Bibcode: 1994ASIC..433..509S
Altcode:
No abstract at ADS
Title: Polarization analyzer installed at the Pic du Midi Observatory.
Authors: Mouradian, Z.; Scholiers, W.; Semel, M.
Bibcode: 1994imfm.conf..110M
Altcode:
No abstract at ADS
Title: On the removal of the 180 deg sign ambiguity in vector
magnetograph measurements: the divergence-free method (divergence
of B identical to 0)
Authors: Li, J.; Cuperman, S.; Semel, M.
Bibcode: 1993A&A...279..214L
Altcode:
A systematic numerical study concerning the applicability of a
new criterion for the removal of the 180 deg ambiguity in sign of
vector magnetograph measurements (Wu & Ai 1990) is presented. This
criterion based on the condition divergence of B identical to 0 alone,
is free of any assumption concerning the divergence (convergence) of the
magnetic field with the height. Some twenty analytical (force-free field
and potential field) models covering a variety of possible magnetic
configurations are used as test cases for our study. The limitations
of the method are also indicated.
Title: Circular polarization and variability in the spectra of
herbig Ae/Be stars. I. The Fe II 5018 A and He I 5876 A lines of AV
OnB Aurigae.
Authors: Catala, C.; Bohm, T.; Donati, J. -F.; Semel, M.
Bibcode: 1993A&A...278..187C
Altcode:
We present high resolution spectroscopic and spectro-polarimetric
observations of the Fe II 5018 and He I 5876 A lines in the pre-main
sequence Herbig Ae star AB Aur. No signal was detected in the circular
polarization (Stokes V) profile of the magnetically sensitive Fe
II 5018 A line, yielding upper limits of the order of 1 kG for the
photospheric magnetic field. This upper limit is still much higher than
the equipartition magnetic field at the photosphere, estimated to be 60
G. Both lines are spectacularly variable on a night-to-night basis. In
particular, the Fe II 5018 A line appears as a photospheric absorption
line in one of our spectra, but as a variable emission line, usually
composed of a flat-topped broad emission and a roughly triangular
emission, in the other spectra. The He I 5876 A line often includes a
red absorption component and a blue emission component, but also often
appears entirely in emission with a varible asymmetry. We estimate the
regions of formation of the two lines, with a very simplified treatment
of ionization and excitation processes, and within the framework
of existing models of the wind and chromosphere of AB Aur. We find
that the Fe II 5018 A line is formed in the bulk of the expanding
chromosphere, while the He I 5876 A line is formed at the very base
of the wind/chromosphere complex. Although the data presented here
are not sufficient to fully interpret such a complex variability,
we suggest the idea that it could be linked to the co-rotation of
structures in the wind and chromosphere of AB Aur. The photospheric
appearance of the Fe II 5018 A line in one of our spectra may be
due to a temporary change in the chromospheric physical conditions,
like an increase of the temperature, followed by a return to 'normal'
conditions, with a time scale shorter than one day.
Title: Zeeman-Doppler imaging of active stars. III. Instrumental
and technical considerations.
Authors: Semel, M.; Donati, J. -F.; Rees, D. E.
Bibcode: 1993A&A...278..231S
Altcode:
An account is given of the instrumental setup and observational
procedure that led to the qualifying of Zeeman-Doppler imaging
and to new detections of magnetic fields on four stars HR 1099,
sigma2 CrB, II Peg and UX Ari.
Title: Identification and elimination of the residual ambiguity in
the sign of observed photospheric magnetic fields
Authors: Cuperman, S.; Li, J.; Semel, M.
Bibcode: 1993A&A...278..279C
Altcode:
The existing methods for the removal of the 180 degree ambiguity in
the sign of the observed transverse photospheric magnetic field present
some inherent-computational and/or intrinsic limitations. In this work
we propose and demonstrate suitable procedures for the identification
and elimination of the residual 180 degree ambiguity in the sign of
Bperpendicular observed fields, that is of the ambiguity
which is not removed by the various methods in use. These procedures
are tested on the case of force-free magnetic field configurations
for which exact analytical solutions to compare with exist.
Title: Reconstruction of coronal magnetic configurations - The case
of strongly nonlinear force-free fields
Authors: Cuperman, S.; Bruma, C.; Zoler, D.; Semel, M.
Bibcode: 1993A&A...270..480C
Altcode:
A method for the reconstruction of force-free field (FFF) coronal
magnetic configurations above active regions is proposed. It is based on
the mapping of the entire space surrounding the sun onto a rectangular
box of dimensions (1,2,27 pi), inside which the FFF Maxwell's equations
are solved. The boundary conditions used are the field components
observed at the photosphere, their vanishing at infinity, and finiteness
of the magnetic potential components along the poles. The solution is
based on relaxation techniques and is illustrated for the 2 1/2D case,
that is, the case in which all three field components B(r), B(theta),
and B(phi) are finite and depend only on the polar coordinates r and
theta. The equilibrium state is considered to be reached when the mean
relative change in any of the field components, from one time step to
another, is smaller than a prescribed infinitesimal number.
Title: Alternative method for the removal of the 180 ambiguity in
the sign of the observed transverse photospheric magnetic field
Authors: Cuperman, S.; Li, J.; Semel, M.
Bibcode: 1993A&A...268..749C
Altcode:
A relatively simple method for the removal of the 180 deg ambiguity
in the azimuth of the observed transverse photospheric magnetic
field is proposed; it applies to both center of disk and off-disk
center observations. The method consists in selecting that sign which
corresponds to a decrease with height of the heliographic magnetic
field. For this, the observed field components at a point have
to be first transformed into heliographic components and then the
selection rule must be applied. After application of the method to
simulated observations, heliographic analytical solutions were used
as a benchmark test. Both center-of-disk and off-disk-center cases
were considered; in the last case, all latitude angles between 0 deg
and 90 deg were treated. The results were expressed as percentage of
success in determining the correct sign of the observed transverse
component as a function of the complexity of the heliographic magnetic
field configuration as well as of the value of the latitude angle. In
the center of disk case, the percentage of success varies between
100 percent and a minimum of about 90 percent, as depending on the
complexity of the magnetic configuration; the performance of the method
can be further improved by increasing the number of grid points. In
the off disk-center case, the method is limited to relatively low
latitude angles.
Title: Circularly polarized spectroscopic observations of RS CVn
systems.
Authors: Donati, J. -F.; Semel, M.; Rees, D. E.
Bibcode: 1992A&A...265..669D
Altcode:
We applied the new technique of Zeeman-Doppler Imaging on nine active
RS CVn binary systems. In four systems (HR 1099, sigma 2 CrB, II Peg
and UX Ari), we detected a magnetic field on the cool active component;
in each of them, the corresponding Stokes V signatures is found to vary
with rotational phase. Moreover, on HR 1099, we have observational
evidence that the magnetic structure varied significantly within
a period of 1.3 yr. By contrast, magnetic field is repeatedly not
detected on other stars, such as the hotter component of sigma 2 CrB,
although these are also chromospherically very active. Our results
suggest that the horizontal size scale of magnetic topologies may
differ significantly among the observed stars. Large monopolar regions
are likely present on those for which Zeeman signatures are detected;
a more complex ordering of field lines with smaller monopolar regions
and close bipolar groups may define the others.
Title: Magnetic shear-based removal of the 180-deg ambiguity in
the observed transverse photospheric magnetic field - Feasibility
and limitations
Authors: Cuperman, S.; Li, J.; Semel, M.
Bibcode: 1992A&A...265..296C
Altcode:
Results of a systematic investigation of the reliability of the magnetic
shear-based method for the removal of the 180-deg ambiguity in the
observed transverse photospheric magnetic field are presented. The
method consists of the evaluation of the azimuth angle between the
observed transverse field component and that of a potential field,
calculated by using the line-of-sight component as the boundary
condition for the field normal to the image plane. A variety of the
force-free field (FFF) magnetic configurations is considered. This
includes the cases of relatively weakly and strongly space-dependent
fields, relatively small and large maximal values of the FFF-function
alpha(r), and same-sign and space-changing sign alpha(r) functions. The
'logical' and 'global' success of the method in removing the 180-deg
ambiguity for the 20 cases studied is presented and discussed.
Title: Photospheric imaging of the RS CVn system HR 1099.
Authors: Donati, J. -F.; Brown, S. F.; Semel, M.; Rees, D. E.; Dempsey,
R. C.; Matthews, J. M.; Henry, G. W.; Hall, D. S.
Bibcode: 1992A&A...265..682D
Altcode:
Spectropolarimetric, spectroscopic, and photometric observations
of the RS CVn binary system HR 1099 = V711 Tau (K1IV+G5V) were made
from Oct. 1988 to Jan. 1991. From Doppler imaging of the unpolarized
spectra and the corresponding data, two maximum entropy images of the
temperature distribution of the active K1 subgiant at epochs 1988.9
and 1990.9 were derived. Zeeman-Doppler imaging of the circularly
polarized spectra has led to the first magnetic maps of a star other
than the sun. The fragmentary observations made at epoch 1989.6 suggest
that the star's magnetic field lines were emerging radially and/or
poloidally from an equatorial warm region at that time, quite likely
one of these reconstructed in the 1988.9 temperature image.
Title: Determination of force-free magnetic fields above the
photosphere using three-component boundary conditions. II - Analysis
and minimization of scale-related growing modes and of computational
induced singularities
Authors: Demoulin, P.; Cuperman, S.; Semel, M.
Bibcode: 1992A&A...263..351D
Altcode:
The problem of the extrapolation of the observed three-component
photospheric magnetic fields within is studied within the framework
of the nonlinear force-free field model. The possible vertically
growing modes and their relationship to the horizontal and vertical
chosen scales is discussed, and the mathematical singularities arising
along neutral field lines and leading to nondefinite alpha values are
examined. A search is made for simple procedures aimed at minimizing
these algorithm-dependent computational errors.
Title: Mapping Magnetic Fields on Rapidly Rotating Stars: Application
to the RS CVn System HR 1099
Authors: Donati, J. -F.; Brown, S. F.; Semel, M.; Rees, D. E.
Bibcode: 1992ASPC...26..353D
Altcode: 1992csss....7..353D
No abstract at ADS
Title: Zeeman-Doppler imaging of solar-type and AP stars. IV. Maximum
entropy reconstruction of 2D magnetic topologies.
Authors: Brown, S. F.; Donati, J. -F.; Rees, D. E.; Semel, M.
Bibcode: 1991A&A...250..463B
Altcode:
Recently Semel (1989) introduced the technique of Zeeman-Doppler Imaging
(ZDI) which essentially consists of Doppler Imaging using the Stokes
V (circular polarization) profile. A code which uses maximum entropy
principles to recover the distribution of brightness, abundance, and
magnetic fields across the surface of stars is presented. For brightness
and abundance mappings, the code is found to produce results comparable
to those found in the literature. The first reconstructions of magnetic
test structures from the Stokes V parameter alone show that this
quantity contains enough spatial information to recover spotted images
but not dipolar ones. According to the magnetic topologies that may be
expected in solar-type or Ap stars, it is concluded that ZDI may be well
adapted in deriving magnetic maps of active RS CVn systems, while it
is anticipated that the use of the two other Stokes parameters (linear
polarizations) are needed to derive reliable results for Ap stars.
Title: Erratum - Detection of a Magnetic Region on HR:1099
Authors: Donati, J. F.; Semel, M.; Rees, D. E.; Taylor, K.; Robinson,
R. D.
Bibcode: 1991A&A...248..337D
Altcode:
No abstract at ADS
Title: Removal of singularities in the Cauchy problem for the
extrapolation of solar force-free magnetic fields
Authors: Cuperman, S.; Demoulin, P.; Semel, M.
Bibcode: 1991A&A...245..285C
Altcode:
The singularities occurring in the Cauchy problem for the
extrapolation of solar nonlinear force-free magnetic fields at
positions of vanishing normal component, B(z), are removed. This is
based on the observation that the constancy of the quantity alpha(r),
characterizing the force-free magnetic fields, along a given magnetic
field line, implies that the singularity in Maxwell's equation is of
mathematical rather than of physical origin. Thus, requiring also the
vanishing of the numerator at P(0) leads to an undetermined form for
alpha. By using Taylor's expansions in two variables (x and y) about
P(0) for both numerator and denominator, the actual value for alpha,
namely alpha (P(0), is obtained. The procedure is tested on the case
of the analytical model proposed by Low (1982).
Title: Active regions, sunspots and their magnetic fields.
Authors: Semel, Meir; Mouradian, Zadig; Soru-Escaut, Irina; Maltby,
Per; Rees, David; Makita, Mitsugu; Sakurai, Takashi
Bibcode: 1991sia..book..844S
Altcode:
Surface magnetism is the progenitor of active regions, sunspots, and
all related phenomena. This cause and effect is reversible so that,
using well-established empirical laws, the presence and morphology of
photospheric magnetic fields can be deduced from active-region light
emission structure. In the (simplifying) case of sunspots, MHD and
thermodynamic theory find some success in the interpretation of the
interaction of magnetic fields and solar plasma. Coronal magnetic fields
also appear to be predictable by extrapolation techniques starting
from the photospheric conditions. Alternatively, surface magnetism can
be observed "directly" by means of the spectroscopic Zeeman effect
and Stokes polarimetry. Eventually these empirical, theoretical and
direct-measurement techniques must converge to identical results as
we better understand the physics of active regions.
Title: Towards Magnetic Images of Rapidly Rotating Late-Type Stars
Authors: Donati, J. -F.; Semel, M.
Bibcode: 1991LNP...380..326D
Altcode: 1991sacs.coll..326D; 1991IAUCo.130..326D
We present the first magnetic detections using Zeeman-Doppler Imaging
(ZDI) in the two bright rapidly rotating RS CVn systems HR 1099 and
2 CrB, and discuss their compatibility with various recent results on
magnetic activity in cool stars.
Title: The magnetic field of the AP star epsilon UMa.
Authors: Donati, J. -F.; Semel, M.; del Toro Iniestia, J. C.
Bibcode: 1990A&A...233L..17D
Altcode:
The magnetic field of the Ap star Epsilon UMa was measured using the
Zeeman-Doppler Imaging method. Ultrahigh S/N polarized profiles of the
4923 A magnetically sensitive Fe II line were recorded. Unambiguous
Zeeman signatures were found at phases here S/N was greater than or
equal to 1750.
Title: Zeeman-Doppler Imaging - a New Option for Magnetic Field
Study of Ap-Stars and Solar Type Stars
Authors: Donati, J. -F.; Semel, M.
Bibcode: 1990SoPh..128..227D
Altcode: 1990IAUCo.121P.227D
In the task of studying stellar magnetic fields, polarimetric methods
have been intensively used in Ap stars. But the observational material
classically used to reconstruct stellar magnetic structures (average
longitudinal magnetic field as a function of rotational phase) is not
rich enough in spatial information to derive geometries more complex
than centered or decentered dipoles.
Title: Detection of a magnetic region of HR 1099.
Authors: Donati, J. -F.; Semel, M.; Rees, D. E.; Taylor, K.; Robinson,
R. D.
Bibcode: 1990A&A...232L...1D
Altcode:
The paper reports results of a magnetic field measuring campaign on the
active K component of the RS CVn system HR 1099, using Zeeman-Doppler
imaging. A localized magnetic field is detected near quadrature
(phase 0.85). Assuming this magnetic region has a circular shape, its
longitude and latitude are, respectively, estimated to be 86 + or -
4 deg and 5 + or - 5 deg. This equatorial region is largely monopolar,
has a mean field strength of 985 + or - 270 G, covers 18 + or - 3
percent of the total stellar surface and may be colocalized with a
bright photospheric spot.
Title: Determination of force-free magnetic fields above the
photosphere using three-component boundary conditions - Moderately
non-linear case
Authors: Cuperman, S.; Ofman, L.; Semel, M.
Bibcode: 1990A&A...230..193C
Altcode:
The calculation of the magnetic field components and the tracing
of the magnetic field lines above the photosphere are considered
within the framework of the nonlinear force-free field model, upon
using three-component magnetic fields as boundary conditions. This
vertical integration represents an extrapolation in the small and is
free of any implicit or explicit assumption in the large. As a study
case, an analytical model providing magnetic field components at a
surface (representing the photosphere) and above it is used. Magnetic
field components and magnetic field lines at z greater than 0 are
obtained. Their comparison with the analytical ones provides a measure
of the calculational accuracy. The method is demonstrated for the case
of moderately nonlinear force-free forces.
Title: The absolute value and sign of the function alpha(r) in the
force-free magnetic field modelling of photospheric observations
Authors: Cuperman, S.; Ofman, L.; Semel, M.
Bibcode: 1990A&A...227..227C
Altcode:
A relatively simple method for the determination of the quantity α =
J/B characterizing the force-free magnetic fields based on photospheric
vector magnetic fields is presented. Magnetic configurations
for which the sign of α does not change are considered. The method consists of two steps, namely: (a) Expression of
force-free field equations in terms of the observed quantities
Bz, B2x, B2y
and BxBy and determination of |α|; (b) Selection
of the proper sign of α (corresponding to the magnetic configuration
under consideration) by (i) solving the FFF equations under the
assumptions α = - |α| and α = +|α|, (ii) using the two types of
solutions to calculate the surface energy integral ES(z)
= ∫ (B2x + B2y
+ B2x)ds, and (iii) retaining the sign that
leads to a decrease with height (z) of Es(z). The
proposed method is tested by considering three different FFF magnetic
configurations for which analytical solutions exist.
Title: Extrapolation of photospheric potential magnetic fields using
oblique boundary values - A simplified approach
Authors: Cuperman, S.; Ofman, L.; Semel, M.
Bibcode: 1990A&A...227..583C
Altcode:
The problem of extrapolating photospheric potential magnetic fields
is addressed using the oblique line-of-sight component B1(rs) as a
boundary condition and reducing it to that for the normal line-of-sight
component Bz(rs). The observed B1(rs) component is used along with
the direction cosines of the line-of-sight alpha, beta, and gamma
to calculate semianalytically the photospheric components Bx(rs)
and By(rs). All these values are used to find the normal photospheric
component Bx(rs). Finally, using the distribution Bz(rs) as boundary
values, the relatively simpler problem corresponding to the case
in which the normal components are known is solved. The method is
tested on the case of an analytical model configuration for which
exact solutions in the half-space above the photosphere exist.
Title: Spectropolarimetry of solar faculae - High spatial resolution
results
Authors: del Toro Iniesta, J. C.; Collados, M.; Sanchez Almeida, J.;
Semel, M.
Bibcode: 1990A&A...227..591D
Altcode:
A new method to measure the magnetic field strength of small-scale
solar magnetic concentrations is presented. It is based on the center
of gravity method (Semel, 1967), is independent of radiative transfer
calculations and only observable parameters are needed. This method
also provides parameters like filling factor (area fraction occupied by
the tubes), continuum intensity contrast between flux tubes and their
surroundings, in a two-component model scheme. The method is applied
to spectropolarimetric high spatial resolution data. Local variations
of the above parameters inside single faculae are found. This result
suggests some indications about flux tube evolution. A comparison with
low spatial resolution results is also made.
Title: Zeeman-Doppler imaging of active stars. II. Numerical
simulation and first observational results.
Authors: Donati, J. -F.; Semel, M.; Praderie, F.
Bibcode: 1989A&A...225..467D
Altcode:
A numerical study is proposed for testing the sensitivity and resolving
ability of the Zeeman-Doppler Imaging (DZI) of active stars. It is
shown that, in a star with v sin i = 32 km/s, a line at 6000 A with
a Lande factor of two can be used to detect a 1000 G single magnetic
spot covering 10 percent of the visible stellar surface, provided
S/N = 350 or greater per 80 mA pixel. It is also shown that this 1000
G magnetic threshold is lowered by a factor of two in the case of a
Doppler-resolved bipolar group, each spot of which covers 10 percent
of the visible stellar hemisphere.
Title: Zeeman-Doppler imaging of active stars. I - Basic principles.
Authors: Semel, M.
Bibcode: 1989A&A...225..456S
Altcode:
Methods are proposed for the analysis of polarimetric measurements in
the spectra of active stars. The Doppler effect of rapidly rotating
stars may help to disclose magnetic field distributions, which otherwise
may not be seen due to even distribution of positive and negative
fields. The methods developed allow one to average the measurements
of many lines.
Title: Determination of constant-alpha force-free magnetic fields
above the photosphere using three-component boundary conditions
Authors: Cuperman, S.; Ofman, L.; Semel, M.
Bibcode: 1989A&A...216..265C
Altcode:
The constant-α, force-free magnetic field equations are
numerically integrated for the case in which all three field
components are specified at the photo sphere and used as boundary
conditions. Test-cases successfully compare the numerical results with
exact analytical values.
Title: Zeeman-Doppler imaging method.
Authors: Donati, J. F.; Semel, M.; Praderie, F.
Bibcode: 1989musi.work...37D
Altcode:
No abstract at ADS
Title: Méthode d'imagerie Zeeman-Doppler.
Authors: Donati, J. -F.; Semel, M.; Praderie, F.
Bibcode: 1989JAF....34R...6D
Altcode:
No abstract at ADS
Title: Extrapolation functions for constant-alpha force-free fields -
Green's method for the oblique boundary value
Authors: Semel, M.
Bibcode: 1988A&A...198..293S
Altcode:
The Green's method approach is used to generalize the existing
solutions for the constant-alpha force-free fields to the oblique
case, in which the observed longitudinal magnetic field component is
not normal to the surface. Consideration is given to the existence
and uniqueness of the solutions. If a bounded solution exists, it is
uniquely determined in terms of one component of the magnetic field
on the observed plane surface.
Title: Polarimetry and imagery through uniaxial crystals - Application
to solar observations with high spatial resolution
Authors: Semel, M.
Bibcode: 1987A&A...178..257S
Altcode:
Huygens's principle is applied to the general case of plane waves
incident on uniaxial crystals with plane surface and the corresponding
laws of refraction in analytical form are derived. Optical aberrations
are calculated and their effects on solar observations with high
spatial resolution are discussed. Solutions for beam-splitters free
of aberrations are proposed.
Title: Observations of the Magnetic Fine Structure of a Facula
Authors: del Toro Iniesta, J. C.; Semel, M.; Collados, M.
Bibcode: 1987rfsm.conf..122D
Altcode:
No abstract at ADS
Title: Continuum intensity and magnetic flux of solar fluxtubes.
Authors: Del Toro Iniesta, J. C.; Semel, M.; Collados, M.; Sánchez
Almeida, J.
Bibcode: 1987PAICz..66..265D
Altcode: 1987eram....1..265D
The continuum contrast between fluxtubes and their quiet background,
and the magnetic flux carried by these magnetic elements, have been
determined at different points of a solar facula, in the frame of a
two-component model from spectropolarimetric observations of 1arcsec
spatial resolution. Local spatial variations of these two parameters
have been obtained.
Title: Observations of the magnetic fine structure of a facula.
Authors: Del Toro Iniesta, J. C.; Semel, M.; Collados, M.
Bibcode: 1987rfsm.conf..127D
Altcode:
Simultaneous spectropolarimetric observations of a facula have
been carried out in 10 spectral lines with a spatial resolution of
1arcsec. Local variations of the magnetic field strength and the filling
factor of fluxtubes were obtained. The analysis of the velocities inside
fluxtubes shows that positive and negative Doppler shifts are present,
at the same time, at different points of the facula.
Title: Unresolved Magnetic Structures in the Sun: Oberservational
Results from Analysis of Faculae and Network Spectra
Authors: Semel, M.
Bibcode: 1986ssmf.conf...39S
Altcode:
There are various methods to determine the field strength in the
unresolved flux tube. They vary according to the choice of spectral
lines, the weight given to the observables (intensity, profiles,
polarization...), model assumptions and physical considerations. The
author summarizes some methods.
Title: Determination of Magnetic Fields in Unresolved Features
Authors: Semel, M.
Bibcode: 1985LNP...233..178S
Altcode: 1985hrsp.proc..178S
Observations of solar faculae and networks reveal the unresolved nature
of the magnetic field configurations, with present achieved spatial
resolutions. While the apparent field is weak (from a few gauss to
several hundreds) the "true" field is likely to be much stronger
(up to kilogauss fields). There are various methods to determine
the field strength in the unresolved flux tube. They vary according
to the choice of spectral lines, the weight given to the observables
(intensity, profiles, polarisation ...), model assumptions and physical
considerations. The purpose of this paper is to give a review of these
methods and assessment of their capacities and limits.
Title: Energy Release and Energy Transport Below the Transition Zone
in Solar Flares
Authors: Hénoux, J. C.; Chambe, G.; Heristchi, D.; Semel, M.;
Woodgate, B.; Shine, R.; Beckers, J.
Bibcode: 1985spit.conf..758H
Altcode:
No abstract at ADS
Title: Measurement of Magnetic Fields - Sun and Stars
Authors: Semel, M.
Bibcode: 1984apoa.conf...37S
Altcode:
No abstract at ADS
Title: Transfert de rayonnement en lumière polarisée. Application
à la mesure des champs magnétiques par effet Zeeman.
Authors: Semel, M.
Bibcode: 1984cms..conf...79S
Altcode:
No abstract at ADS
Title: Détermination des champs magnétiques dans les structures
solaires non résolues.
Authors: Semel, M.
Bibcode: 1984cms..conf..375S
Altcode:
No abstract at ADS
Title: Conductive Heat Flux in the Chromosphere Derived from Line
Linear Polarization Observation
Authors: Henoux, J. -C.; Chambe, G.; Heristchi, D.; Semel, M.;
Woodgate, B.; Shine, D.; Beckers, J.
Bibcode: 1983SoPh...86..115H
Altcode:
Linear polarization in two chromospheric lines (Hα and SI 1437 A) was
observed in the gradual phase of solar flares. The polarized electric
vector is directed towards disk center.
Title: Impact linear polarization observed in a UV chromospheric
line during a solar flare
Authors: Henoux, J. C.; Chambe, G.; Sahal, S.; Semel, M.; Woodgate,
B.; Shine, D.; Beckers, J.; Machado, M.
Bibcode: 1983ApJ...265.1066H
Altcode:
Linear polarization was observed in the S 11437 A line in bright
flaring points during the soft X-ray emission from a flare on 1980
July 15. The degree of polarization is about 25% and is detected at a
signal-to-noise ratio of 2.9. The polarized electric vector is directed
toward disk center to within 3°. This polarization could be due
to collisional excitation of S I by energetic electrons beamed in the
vertical direction. Direct excitation by a highly energetic beam of
electrons of order 10-100 keV is doubtful. We suggest that the heat flux
in the region connecting the transition zone to the high chromosphere
during the gradual phase of a flare could lead to an anisotropic
excitation. Then the observed polarization would be due to vertical
motions of the transition zone sweeping the preexisting chromosphere.
Title: Impact linear polarization observed in a UV chromospheric
line during a solar flare
Authors: Henoux, J. C.; Chambe, G.; Semel, M.; Woodgate, B.; Shine,
R.; Beckers, J.
Bibcode: 1982AdSpR...2k.155H
Altcode: 1982AdSpR...2..155H
Linear polarization was observed in the S I 1437Å line in bright
flaring points during the soft X-ray emission. The degree of
polarization is about 25% and is detected at a signal to noise ratio
of 2.9. The polarized electric vector is directed towards disk center
to within 3°. This polarization could be due to collisional
excitationm of S I by energetic electrons beamed in the vertical
direction. We suggest that the heat flux in the region interconnecting
the transition zone to the high chromosphere during the gradual phase
of a flare could lead to an anisotropic excitation. Then the observed
polarization would be due to vertical motions of the transition zone
sweeping the preexisting chromosphere.
Title: Magnetic and Velocity Fields of Emerging Flux Regions on
the Sun
Authors: Martres, M. J.; Rayrole, J.; Semel, M.; Soru-Escaut, I.;
Tanaka, K.; Makita, M.; Moriyama, F.; Unno, W.
Bibcode: 1982PASJ...34..299M
Altcode:
Emerging Flux Regions (EFR's) on the Sun observed in the Meudon-Mitaka
Collaborative Solar Observation Program (1979) are discussed. The
rule found earlier (Martres et al. 1973) for the growth (or decay) of
the N polarity or the decay (or growth) of the S polarity magnetic field
in relation to the photospheric clockwise (or counterclockwise) vortex
motion is reconfirmed for the EFR in the present solar cycle. The rule
is emphasized as an indication of active generation of an electric
current system by a photospheric motion. The chromospheric
manifestations of EFR's seen in Hα as arch filament systems (AFS's) and
the enhanced brightness are examined with respect to the photospheric
magnetic field structure derived from the white light and magnetic
observations. An isolated EFR is associated with an AFS which is
well coaligned with the photospheric transverse field. The typical
EFR configuration may be distorted if the EFR appears in the region
of the pre-existing strong field. The observational characteristics
of such EFR's can also be interpreted in terms of the emergence of a
nearly potential field associated with the dissipation of an excess
magnetic energy. An EFR born in an older EFR shows that the flux loop
is low-lying and tends to relax into a potential field configuration.
Title: Magnetic fields observed in a sunspot and faculae using 12
lines simultaneously
Authors: Semel, M.
Bibcode: 1981A&A....97...75S
Altcode:
In a sunspot the measured magnetic fields are in agreement for all
12 lines observed, indicating the reliability of the observational
method used. On the other hand, the discrepancies between the fields
observed in 12 different lines are quite large for faculae, which is
a characteristic of unresolved structures. There is, however, a good
correlation between the measured magnetic field and the equivalent width
of the corresponding line. An analysis in terms of 'relative magnetic
curve of growth' is proposed. Application to star spots is discussed.
Title: Hydrodynamical and Electrodynamical Interactions Between
Magnetic Features in the Active Region Photosphere
Authors: Unno, W.; Tanaka, K.; Semel, M.
Bibcode: 1981PASJ...33..495U
Altcode:
Theoretical interpretation is attempted for hydrodynamical and
electrodynamical phenomena in an active region observed by various means
in the Meudon-Mitaka Solar Collaboration Observations in 1979. The
theoretical treatment assumes the vertical motions to be localized
in small regions so that these regions should behave as singular
points in a two-dimensional (horizontal) irrotational flow. (a)
Some pieces of observational evidence are quantitatively examined,
showing that hydrodynamical interactions are likely to be effective
between magnetic features in an active region. (b) There is
also evidence which shows the existence of the dynamo action in the
active-region photosphere. The law found by Martres et al. (1973)
on the growth and decay of different magnetic polarities relative to
the sense of rotation of the associated vortex motions is interpreted
also in terms of the photospheric electromotoric mechanism.
Title: A precise optical polarization analyzer
Authors: Semel, M.
Bibcode: 1980A&A....91..369S
Altcode:
An optical analyzer system suitable for the precise measurement of
polarization (down to 0.0001 or less) is described. Two states of
polarization in the solar spectrum for a wide spectral range and for
many solar points can be observed simultaneously. The analysis of
polarization is obtained by using a pair of quartz crystals ensuring
the same optical path for the two beams, whose separation is very small
(200 or 400 microns in the present experiment). Consequently, all the
effects such as optical aberrations and turbulence are practically
the same for the two beams when passing through an optical system. An
application to magnetic measurements is discussed.
Title: Observational Test for Hydrodynamical Models of Solar Faculae
Authors: Semel, M.; Ribes, E.; Rees, D.
Bibcode: 1980jfss.conf..149S
Altcode:
No abstract at ADS
Title: Small Scale Unresolved Solar Magnetic Fields
Authors: Ribes, E.; Semel, M.
Bibcode: 1980jfss.conf..129R
Altcode:
No abstract at ADS
Title: Line formation in an unresolved magnetic element: a test of
the centre of gravity method.
Authors: Rees, D. E.; Semel, M. D.
Bibcode: 1979A&A....74....1R
Altcode:
The problem of multidimensional LTE line formation in a magnetic
field is solved in a model of an unresolved magnetic element imbedded
in the solar photosphere. Horizontal and vertical variations of
the physical parameters are included. Calculations show that, in
circular polarization analysis, the centre of gravity displacement
of the line profile gives a very good measurement of the longitudinal
field averaged over the area observed. For small line weakenings the
accuracy is probably better than 10%. Also the method is not affected
by Zeeman saturation. A method based on centre of gravity measurements
is proposed for inferring a 'model-independent' value of the average
longitudinal field inside an unresolved magnetic element.
Title: Polarimetry with the coronograph of the Norikura Observatory.
Authors: Shimizu, I.; Semel, M.
Bibcode: 1979TokRe..18..696S
Altcode:
No abstract at ADS
Title: The Treatment of the Stokes Parameters and Meaurement of
Magnetic Field
Authors: Semel, M.
Bibcode: 1972lfpm.conf...89S
Altcode:
No abstract at ADS
Title: On the Broadening of the Balmer Lines during a Flare
Authors: Semel, M.; Soru-Escaut, I.
Bibcode: 1971A&A....12..340S
Altcode:
The hypothesis involved in the interpretation of flare spectra were
chosen in this work as follows: 1) the total number of ab sorb ants
per cm2, N2, in the emission layer should be practically the same
for all the members of the Balmer series; 2) the maximum intensity
in each line approximates the source function. With the help of
these hypotheses, the analysis of the observational data showed that
independently of the broadening mecanism, the first members of the
series are saturated. Consequently it was necessary to study the
line wings and to introduce a hypothesis concerning the broadening
mecanism: either Stark or Doppler. As a criterion of choice between
these two effects we have examined the ratios of the optical depths r,
corresponding to different members of the series. This method led us
to draw the conclusion that the Stark effect predominates. A coherent
solution for the whole series is proposed. Finally we discussed the
problem concerning the determination of the electron density The and
of N2 - We have suggested the possible effects of inhomogeneities of
the electron density. Key words: flare spectra - Stark broadening
Title: Measurement of Magnetic Fields
Authors: Semel, M.
Bibcode: 1971IAUS...43...37S
Altcode:
No abstract at ADS
Title: A Proposed Method of Calibrating Measurements of Linearly
Polarized Light for the Obser- vation and Analysis of Zeeman Effects
in Solar Spectral Lines
Authors: Semel, M.
Bibcode: 1970A&A.....9..356S
Altcode:
Th the proposed method the calibration depends only on the broadening
mechanism of Fraunhofer lines. Because this mechanism is expected to
be common to many lines, the use of several lines with different Land
factors may lead to reliable measurements of the magnetic field and
to the determination of the broadening parameters.
Title: The Use of the Magnetograph for the Study of the Unresolved
Solar Fine Structure (Answer to Harvey and Livingston)
Authors: Semel, M.
Bibcode: 1970A&A.....9..152S
Altcode:
The influence of the unresolved solar fine structure on magnetic
measurements is discussed. A method for the determination of such
effects by the use of a magnetograph is advanced.
Title: Evaluation of the Electric Current in a Sunspot by the Study
of the Observed Transverse Component of the Magnetic Field
Authors: Rayrole, J.; Semel, M.
Bibcode: 1970A&A.....6..288R
Altcode:
The method employed for the evaluation of electric current seems
to give significant values. The analysis of these results leads to
arguments unfavourable for a force free model. At a certain point,
a direct estimation of the Lorentz force is made. Finally we discuss
the difficulties and errors in the reduction of the data.
Title: A Proposal for a Solar Magnetograph
Authors: Semel, M.
Bibcode: 1970A&A.....5..330S
Altcode:
A proposal for a magnetograph which should solve the whole problem
of calibration is advanced. In fact calibration becomes almost banal,
and is almost independent of line formation. A standard magnetograph
could be modified for use of this method with very little difficulty.
Title: Étude Critique d'un Champ Current-Free dans l'Atmosphère
Solaire
Authors: Semel, M.; Rayrole, J.
Bibcode: 1968IAUS...35..134S
Altcode:
No abstract at ADS
Title: Contribution à létude des champs magnétiques dans les
régions actives solaires
Authors: Semel, M.
Bibcode: 1967AnAp...30..513S
Altcode:
No abstract at ADS
Title: Champs magnétiques dans un centre d'activité solaire avant
et pendant une éruption
Authors: Michard, R.; Mouradian, Z.; Semel, M.
Bibcode: 1961AnAp...24...54M
Altcode:
No abstract at ADS