explanation blue bibcodes open ADS page with paths to full text
Author name code: crivellari
ADS astronomy entries on 2022-09-14
author:"Crivellari, Lucio"
---------------------------------------------------------
Title: AGB Stars and Their Circumstellar Envelopes: An Operative
Approach to Computing Their Atmospheres
Authors: Crivellari, Lucio; Cristallo, Sergio; Piersanti, Luciano
2021Univ....7..340C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Alternative Strategies to Solve the Stellar Atmosphere Problem
Authors: Crivellari, L.
2021POBeo.100...15C Altcode:
At the heart of the computation of model atmospheres there is
the so-called Stellar Atmosphere Problem, which consists of the
self-consistent solution of the radiative transfer equations under
specific constraints. The amazing progresses achieved in the field since
the 1970s are due to both the dramatic increase of the computational
facilities and the development of effective numerical algorithms. The
purpose of this review is to draw attention to some methods, alternative
to those that are mostly used nowadays such as the ALI methods. The
improvement of the latter has been brought about by mathematical
refinement, whereas the former are the result of a careful analysis
of the physics of the problem. Rather than attempting an exhaustive
presentation of these novel methods, which would be out of place here,
the prime aim of this article is to sketch the main guidelines and
to stress that it is always the physics itself that dictated the most
effective algorithm.
---------------------------------------------------------
Title: AGB Stars and Their Circumstellar Envelopes. I. the VULCAN Code
Authors: Cristallo, Sergio; Piersanti, Luciano; Gobrecht, David;
Crivellari, Lucio; Nanni, Ambra
2021Univ....7...80C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Radiative Transfer in Stellar and Planetary Atmospheres
Authors: Crivellari, Lucio; Simón-Díaz, Sergio; Arévalo, María
Jesús
2019rtsp.book.....C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Stellar Atmosphere Physical System II. An Operative
Sequential Algorithm to Solve the Stellar Atmosphere Problem
Authors: Crivellari, L.
2019SerAJ.198....1C Altcode: 2019SerAJ.tmp....1C
In this paper, the second and the last of the series, we present a
sequential algorithm to solve the stellar atmosphere problem that may
serve as a paradigm for the solution of more general non-linear and
non-local problems. The Iteration Factors (IF) Method is applied to
achieve a solution of the radiative transfer equations, consistent
with the radiative equilibrium constraint.
---------------------------------------------------------
Title: The Stellar Atmosphere Physical System I. Phenomenological
Definition and Representation of a Stellar Atmosphere
Authors: Crivellari, Lucio
2018SerAJ.196....1C Altcode:
This paper is the first in a series of two that deals with the
physical and numerical grounds of stellar atmospheres modelling. After
a phenomenological definition of a star and a stellar atmosphere, the
physics that shapes the stellar atmosphere physical system is discussed
and three alternative pictures are considered for its representation.
---------------------------------------------------------
Title: An improved version of the implicit integral method to solving
radiative transfer problems
Authors: Simonneau, E.; Cardona, O.; Crivellari, L.
2012Ap.....55..110S Altcode: 2012Ap....tmp...12S; 2011arXiv1110.2018S
Radiative transfer (RT) problems in which the source function
includes a scattering-like integral are typical two-points boundary
problems. Their solution via differential equations implies making
hypotheses on the solution itself, namely the specific intensity I
(τ; n) of the radiation field. On the contrary, integral methods
require making hypotheses on the source function S(τ). It seems of
course more reasonable to make hypotheses on the latter because one can
expect that the run of S(τ) with depth is smoother than that of I (τ;
n). In previous works we assumed a piecewise parabolic approximation
for the source function, which warrants the continuity of S(τ) and its
first derivative at each depth point. Here we impose the continuity
of the second derivative S”(τ). In other words, we adopt a cubic
spline representation to the source function, which highly stabilizes
the numerical processes.
---------------------------------------------------------
Title: Thermodynamic Coefficients for Stellar Atmospheres and Plasma
Spectroscopy
Authors: Cardona, O.; Simonneau, E.; Crivellari, L.
2009ApJ...695..855C Altcode:
A new method to obtain the thermodynamic coefficients in an analytic
and exact form for applications in radiative and convective transport in
thermodynamic equilibrium for stellar atmosphere and plasma spectroscopy
is developed. The resulting exact expressions are formed by sums of
the degrees of ionization of the components of the system. Therefore,
they are easy to calculate and are numerically stable. The method is
developed initially for two elements: hydrogen and helium, with constant
partition functions, to show the simplicity of the procedure. The
method is very easy to generalize to any number of elements and for
partition functions dependent on temperature and pressure as well as
for the first negative ions. The thermodynamic coefficients derived
are the adiabatic gradient, the specific heats for constant volume
and constant pressure, the dilatation coefficient, and the velocity
of sound in the given medium. The derivation is based on the perfect
gas and Saha ionization equations. Therefore, the results are valid
for the regime where these equations are valid. These results will
be of value to astrophysics in stellar structure and atmospheres,
in geophysics, in planetary atmospheres, and in plasma physics in
spectroscopic analysis and diagnostics.
---------------------------------------------------------
Title: A Novel Precise Method for Correcting the Temperature in
Stellar Atmosphere Models
Authors: Cardona, O.; Crivellari, L.; Simonneau, E.
2009ASSP....7..231C Altcode: 2009nqsa.conf..231C
A mayor problem that arises in the computation of stellar atmosphere
models is the self consistent determination of the temperature
distribution via the constraint of energy conservation. The energy
balance includes the gains due to the absorption of radiation:
int χ(ν) J(ν) dν, and the losses due to emission: int χ(ν)
S(ν) dν . It is well known that, within each one of the two above
integrals, the part corresponding to spectral ranges whose opacity
χ(ν) is huge can overcome by many orders of magnitude the part that
corresponds to the remaining frequencies. On the other hand, at those
frequencies where χ(ν) is very large, the mean intensity J(ν) of
the radiation field shall be equal, up to many significant digits,
to the source function S(ν) and consequently to the Planck function
B(ν,T). Then their net share to the energy balance shall be null,
albeit separately their contributions to the gain and loss integrals
are the most important numerically. Thus, the spectral range whose
physical contribution to the overall balance is null will dominate
numerically both sides of the energy balance equation, and consequently
the errors on the determination of J(ν) and S(ν) at these frequencies
will falsify the balance. It is possible to circumvent the numerical
problem brought about by the foregoing circumstances by solving the
radiative transfer equation for the variable I(n,ν) - S(ν), instead
of the customary intensity I(n,ν). We present here a novel iterative
algorithm, based on iteration factors already employed by us with
success, which makes it possible a fast correction of the temperature
by computing directly the difference between the radiative losses and
gains at each step of the iterations.
---------------------------------------------------------
Title: Method for Calculating the Opacity of the Atomic Lines in
Stellar Atmospheres
Authors: Cardona, O.; Simonneau, E.; Crivellari, L.
2009ApJ...690.1378C Altcode:
A new approximate method for calculating the opacity of the
atomic lines in the computation of stellar model atmospheres is
presented. Transforming the sums in the equation of the opacity of
the lines into integrals and considering the Kramers equation for the
oscillator strengths for level transitions in the hydrogenic atoms as
a continuous function of frequency, we can apply the mean value theorem
for integrals in order to describe the variation of the global opacity
of the lines of all the chemical elements. The high-lying levels in
nonhydrogenic atoms as well as their hydrogen-like transitions can also
benefit from this method. For the case of stellar atmospheres not in
local thermodynamic equilibrium, we can apply our method directly. The
compact analytic expressions obtained are easy to use. The application
of the method to the hydrogen lines shows the classical line-blanketing
effects.
---------------------------------------------------------
Title: A Novel Precise Method for Correcting the Temperature in
Stellar Atmosphere Models
Authors: Cardona, Octavio; Crivellari, Lucio; Simonneau, Eduardo
2007arXiv0711.0026C Altcode:
A mayor problem that arises in the computation of stellar atmosphere
models is the self consistent determination of the temperature
distribution via the constraint of energy conservation. The energy
balance includes the gains due to the absorption of radiation and the
losses due to emission. It is well known that within each one of the
two above integrals the part corresponding to spectral ranges whose
opacity X(nu) is huge can overcome by many orders of magnitude the part
that corresponds to the remaining frequencies. On the other hand, at
those frequencies where X(nu) is very large, the mean intensity J(nu)
of the radiation field shall be equal, up to many significant digits,
to the source function S(nu) and consequently to the Planck function
B(nu,T). Then their net share to the energy balance shall be null,
albeit separately their contributions to the gain and loss integrals
are the most important numerically. Thus the spectral range whose
physical contribution to the overall balance is null will dominate
numerically both sides of the energy balance equation, and consequently
the errors on the determination of J(nu) and S(nu) at these frequencies
will falsify the balance. It is possible to circumvent the numerical
problem brought about by the foregoing circumstances by solving the
radiative transfer equation for the variable I(n,nu) - S(nu), instead
of the customary intensity I(n,nu). We present here a novel iterative
algorithm, based on iteration factors already employed by us with
success, which makes it possible a fast correction of the temperature
by computing directly the difference between the radiative losses and
gains at each step of the iterations.
---------------------------------------------------------
Title: Physical requirements for modeling stellar atmospheres
according to the different spectral features observed
Authors: Crivellari, L.; Cardona, O.; Simonneau, E.
2007IAUS..241...91C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Método integral implícito para resolver problemas de
transporte radiativo en condiciones típicas de las atmósferas
estelares
---------------------------------------------------------
Title: Método integral implícito para resolver problemas
de transporte radiativo en condiciones típicas de las atmósferas
estelares
---------------------------------------------------------
Title: An implicit integral method to solve radiative
transfer problems under conditions typcal of stellar atmospheres;
Authors: Crivellari, Lucio
2004PhDT.......297C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Structure and components of an LTE stellar atmosphere model
Authors: Crivellari, L.; Simonneau, E.; Cardona, O.
2004MmSAI..75..154C Altcode:
We summarize in this paper the principles for the algorithmic
representation of the structure of a stellar atmosphere. The physical
analysis of the problem leads to the classification of the elementary
blocks that constitute a stellar atmosphere model and suggest the
numerical algorithms required for its computation. Compatibly with the
space allotted, we review our progress in the treatment of the basic
components of an LTE stellar atmosphere model, and show a significant
preliminary result.
---------------------------------------------------------
Title: A numerical laboratory for the diagnostics of stellar
properties
Authors: Crivellari, L.; Simonneau, E.; Cardona, O.
2003MSAIS...3...97C Altcode:
In a recent paper (Crivellari et al., 2003) we summarized the
principles of the algorithmic representation of the structure of a
stellar atmosphere. We mentioned there our recent progresses in the
numerical treatment of both the global method and the basic components
of the corresponding models. As the natural sequel to that paper,
we wish to outline here the iterative sequential procedure that we
have designed for the computation of such models, and briefly comment
about the possibilities that it offers.
---------------------------------------------------------
Title: Multilevel Line Transfer with the Implicit Integral Method
Authors: Crivellari, L.; Cardona, O.; Simonneau, E.
2002Ap.....45..480C Altcode:
Once the need for an iterative procedure in order to solve the problem
of the formation of spectral lines in the case of a model atom with
many energy levels, the sequel is to seek for the most effective form
of such an iterative scheme. It is an almost universal is assumed
within all the iterative methods for the solution of those radiative
transfer problems, in which the transfer equations are coupled to the
state of the matter, to take as the input of each step of iterations
the values of the opacity coefficients obtained as a result of the
previous one. This is, for instance, the procedure used to correct
the temperature in the computation of stellar atmosphere models, or
that to build the -operator (either the exact or the approximated one)
within the Accelerated Lambda Iteration methods. Yet, if we assume,
in order to solve the multilevel line transfer problem, that at each
step of iterations the line opacities are known, we can express via
the statistical equilibrium equations the populations of the energy
levels - and consequently the source functions of the relevant spectral
lines - as a linear function of the full set of the corresponding
mean intensities of the radiation field. Once such linear forms
for the source functions, we are able to solve without the need of
any further approximation the radiative transfer equations for are
obtained lines, now linearly coupled through the above linear forms
of the statistical equilibrium equations. This is achieved by means
of the Implicit Integral Method that we already presented in a series
of previous papers.
---------------------------------------------------------
Title: Some Iterative Methods for Radiative Transfer Problems
Authors: Simonneau, E.; Crivellari, L.
2002EAS.....5...31S Altcode:
In the mathematical computation of many physical problems we have
to work with the numerical solution of non-linear problems. Even
in case that a direct solution does in principle exist, its
feasibility is however actually restricted to a very limited
number of instances. Therefore, in this study we will disregard this
possibility. Consequently, we assume that an iterative procedure shall
be necessary in order to tackle, in practice, all kind of non-linear
problems.
---------------------------------------------------------
Title: A precise new method to correcting temperature in stellar
atmospheres
Authors: Cardona, O.; Crivellari, L.; Simonnneau, E.
2002ASSL..274...29C Altcode:
Calculations of energy balance in model stellar atmospheres for cold
and moderately cold stars show that the overall energy gained or
lost at the Lyman frequencies (lines plus continuum) can be up to 20
orders of magnitude higher than in the rest of the spectrum. Due to the
great optical depth at those frequencies, however, monochromatic mean
intensity of the radiation and the corresponding source function are
numerically equal with many significant figures, and give therefore
a vanishing net contribution to the energy balance. As all methods
of temperature correction compute both integrals independently, this
can lead to a biased temperature correction to the model output. We
present here a method to calculate the correction of the temperature
in stellar atmospheres that overcomes the aforesaid difficulty.
---------------------------------------------------------
Title: On the temperature determination in theoretical modelling of
stellar atmospheres
Authors: Giammanco, C.; Crivellari, L.; Caccin, B.
1998MmSAI..69..663G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Angular resolution of the classical redistribution functions
Authors: Crivellari, L.
1998MmSAI..69..643C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: An Implicit Integral Method to Solve Selected Radiative
Transfer Problems. IV. The Case of Spherical Geometry
Authors: Gros, M.; Crivellari, L.; Simonneau, E.
1997ApJ...489..331G Altcode:
In the previous papers of this series, we introduced the implicit
integral method (IIM) to solve those radiative transfer (RT) problems
in which the source function depends on an integral of the specific
intensity of the radiation field over directions and frequencies. The
IIM rests upon a forward-elimination, back-substitution scheme naturally
based on the physics of the RT process, and does not require any
matricial algorithm. <P />Customary methods to solve RT problems,
in which the source function depends on the aforesaid integral,
rest upon matrix algorithms. In spherical geometry, due to the strong
anisotropy of the radiation field brought about by the limb curvature,
the so-called peaking effect, the number of directions necessary to
describe this anisotropy is exceedingly high, and consequently the
relevant matrices are hard to handle. <P />The present paper deals
with the application of the IIM to RT problems in spherical geometry,
where the distinctive nonmatricial character of the method can be
fully exploited, given the intrinsic high dimensionality of the problem.
---------------------------------------------------------
Title: A Forth-and-Back Implicit Λ-Iteration
Authors: Atanacković-Vukmanović, O.; Crivellari, L.; Simonneau, E.
1997ApJ...487..735A Altcode:
In many radiative transfer (RT) problems, the sources contain a
scattering term that couples all the specific RT equations, one for
each frequency and direction, so that solving the problem means solving
the system formed by these equations. Each of them is a first-order
linear differential equation with its own initial condition assigned
at a different point of the medium, which makes the solution of the
system extraordinarily difficult. <P />One simple way to achieve a
solution is with the so-called Λ-iteration: sources and sinks given as
a first approximation --> computation of the specific intensities
from their own RT equations --> computation of the scattering
terms --> recomputation of the sources and sinks. This scheme is
straightforward, but unfortunately in practice its convergence rate
is too slow to be of value in the case of optically thick systems. <P
/>The aim of this paper is to show that a forth-and-back approach
(the natural approach to describing sequentially the two intensities
propagating along the two directions of a straight line), together with
an implicit representation of the source function in the computation
of the intensities within the above iterative scheme, can dramatically
accelerate the convergence of the iterative process while retaining
the straightforwardness of ordinary Λ-iteration.
---------------------------------------------------------
Title: Iteration Profiles in Radiative Transfer Problems. I. From
Vectorial to Scalar Coupling
Authors: Crivellari, L.; Simonneau, E.
1995ApJ...451..328C Altcode:
We have recently introduced a new algorithm, the implicit integral
method (IIM), for solving radiative transfer problems in which
the specific source functions (for each frequency and direction)
depend linearly on the radiation field via a single quantity which
is independent of both frequency and direction. We define this
kind of relationship as scalar coupling. The fact that our method
turned out to be fast, robust, and highly reliable leads us to seek
its extension to include those problems where the above, necessary
condition is not fulfilled. In these problems, the specific source
functions depend on the radiation field through a nonfactorable
redistribution operator. In our definition, these are cases of vectorial
coupling. <P />In this paper we present the successful application of
the IIM, through an iterative procedure, to two specific instances of
vectorial coupling. The first is the determination of the temperature
distribution, self-consistent with the energy conservation constraint,
within a LTE stellar atmosphere model. Here the physical processes other
than radiative transfer require an iterative procedure for the global
solution of the problem. Thus we take advantage of this circumstance to
solve iteratively the radiative transfer part as well. <P />The second
is the case of the non-LTE two-level-atom line formation problem in
which partial redistribution is taken into account in the presence
of a background continuum. This problem allows a direct solution,
but at the cost of using algorithms that necessarily require the
storage and inversion of very high order matrices. On the contrary,
we show that a solution based on the iterative application of the IIM,
thanks to the outstanding features of the latter, is not only fast,
but above all much more reliable in numerical terms.
---------------------------------------------------------
Title: An Implicit Integral Method to Solve Selected Radiative
Transfer Problems. II. LTE Stellar Atmosphere Models
Authors: Crivellari, L.; Simonneau, E.
1994ApJ...429..331C Altcode:
The implicit intergral method that we have already introduced in
a previous paper, together with its application to the non-Local
Thermal Equilibrium (non-LTE) line formation problem, is considered
here for another paradigm problem, namely, the self-consistent
temperature correction when computing Local Thermal Equilibrium (LTE)
stellar atmosphere models. The specific form of the source function,
required by the algorithm, is obtained through a linear expansion of
the Planck function around a trial temperature distribution. The correct
solution is quickly obtained via an iterative correction procedure. The
nonmatricial structure, which is the distinctive feature of the implicit
intergral method, offers the possibility to take into account a very
large number of frequency, angle, and depth discrete points, without
any penalty in terms of numerical or computational problems.
---------------------------------------------------------
Title: An Implicit Integral Method to Solve Selected Radiative
Transfer Problems. I. Non-LTE Line Formation
Authors: Simonneau, E.; Crivellari, L.
1993ApJ...409..830S Altcode:
In this paper we present a new method to solve those radiative transfer
problems where the scattering term in the source function, i.e.,
the frequency-integrated mean intensity J(phi), is independent of both
frequencies and directions. This particular form of the source function,
together with an implicit description of the evolution of the specific
intensities incoming to an individual layer from the neighboring ones,
allows one to solve implicitly the radiative transfer equation layer by
layer. Consequently, J(phi) can be expressed as an explicit function
of the (as yet unknown) specific intensities without any need to
solve numerically a system of equations or to invert matrices. In this
way, the global problem is reduced to a series of one-layer two-point
boundary problems. The resulting algorithm is the representation of the
actual physical process. This, together with the fact that it does not
require a matricial formalism, brings about self-evident advantages in
terms of reliability and numerical accuracy, as well as computational
time and memory storage. As an application, the instance of the spectral
line formation in a two-level atomic model is considered.
---------------------------------------------------------
Title: Structural algorithms to solve radiative transfer problems
Authors: Simonneau, Eduardo; Crivellari, Lucio
1993sats.book.....S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Chromospheres of late-type active and quiscent
dwarfs. III. Variability of CA II H emission profiles.
Authors: Garcia Lopez, R. J.; Crivellari, L.; Beckman, J. E.;
Rebolo, R.
1992A&A...262..195G Altcode:
We have used high-resolution spectra of the Ca II H resonance line
in late-type dwarfs, obtained with high S:N ratios, over a period of
four years to widen our understanding of the dynamical behavior of the
Ca II emission cores. All of the stars dealt with in this article,
which are chromospherically active, show variability both in core
emission flux and line width. They also show significant wavelength
shifts with time of order hundreds of meters per second in the mean
core wavelength, and with lower amplitude in the H3 self-absorption,
compared to the photospheric rest wavelength of Ca II H. Comparing
the emission core shifts with those observed in the H3 features, we
find, for the first time, direct prima facie evidence for vertical
chromospheric velocity fields, which show stability in sense over
periods of years in a given star, with notable modulation in gradient,
and which differ in gradient from star to star. We present evidence
to show that the observed effects are almost certainly not due
to projected rotational modulation, and offer new prospects, given
spectral measurements closely sampled in time, for investigating the
vertical velocity structures of chromospheres.
---------------------------------------------------------
Title: Book-Review - Stellar Atmospheres Beyond Classical Models
Authors: Crivellari, L.; Hubeny, I.; Hummer, D. G.
1991JBAA..101..364C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Red-shifted chromospheric emission in 70 OPH A
Authors: Beckman, J. E.; Garcia Lopez, R. J.; Rebolo, R.; Crivellari,
L.
1991Obs...111..314B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Use of Iteration Factors for Temperature Correction in
a Stellar Atmosphere When Convective Transport Is Present
Authors: Crivellari, L.; Simonneau, E.
1991ApJ...367..612C Altcode:
In a previous work (Paper I of this series), we presented a procedure
for computing the temperature distribution in a stellar atmosphere
under the constraint of radiative equilibrium. This scheme, which
answers the major requirements of simplicity and computational speed,
is based on the choice of proper iteration factors that allow a quick
convergence to the correct solution. <P />In this paper, we extended
the method to accounting for a stellar atmosphere where convective
transport is present. This is achieved by introducing a new iteration
factor: the ratio of convective to radiative flux. The new iteration
factor is introduced into a new loop of iterations, where the current
temperature is doubly corrected by making use of both the convective
and the radiative transport equations. <P />Thanks to the use of the
radiative iteration factors, as defined in Paper I, also inside the
new iteration loop, the computational time of each single iteration
is kept small. The introduction of the new factor reduces the number
of iterations within the loop to very small proportions.
---------------------------------------------------------
Title: Stellar atmospheres. Beyond classical models
Authors: Crivellari, Lucio; Hubeny, I.; Hummer, D. G.
1991ASIC..341.....C Altcode: 1991QB809.N38......; 1991sabc.conf.....C
No abstract at ADS
---------------------------------------------------------
Title: Ca II H high-resolution spectral monitoring of active
late-type dwarfs
Authors: Beckman, J. E.; Garcia López, R. J.; Rebolo, R.; Crivellari,
L.
1991LNP...380..463B Altcode: 1991sacs.coll..463B; 1991IAUCo.130..463B
We have monitored Ca II H with a spectral resolution of 8 x 104,
in a sample of late-type MS stars over a four-year period. The high
resolution enables us to add information on velocity fields to the
usual flux monitoring. We detect changes in wavelength of different
parts of the Ca II H feature, which can be interpreted as velocity
fields in the lower chromosphere, with downflow and upflow of order
0.5 km s-1, depending on the star. Flux variations in Ca II H emission
can be ascribed, via velocity tagging, to long-term change in plage
cover rather than short-term modulation by (incompletely sampled)
rotation cycles.
---------------------------------------------------------
Title: Chromospheric variability in late-type dwarfs from
high-resolution spectra of Ca ii H
Authors: García López, R. J.; Rebolo, R.; Crivellari, L.; Beckman,
J. E.
1990Ap&SS.170..109G Altcode:
We present an analysis of high-resolution (λ/δλ≈8×10<SUP>4</SUP>)
high signal-to-noise-ratio spectra of late-type chromospherically
active dwarfs in Ca ii H (λ3968.47 Å), obtained with the Coudé
Auxiliary Telescope plus the Coudé Echelle Spectrometer at ESO,
La Silla. Observational parameters employed are the total emission
flux attributable to the chromosphere, the asymmetry of the emission
component and its bisector, the ratio of the intensities of the blue and
red H<SUB>2</SUB> emission peaks, and the absolute wavelengths of the
H<SUB>3</SUB> minimum and the mean emission feature. Given time-spaced
sets of spectra for each object, which are as yet relatively sparse,
we explore the potential use of these observational parameters to
deepen our understanding of the chromospheric activity.
---------------------------------------------------------
Title: Diagnostics of chromospheric velocity fields
Authors: Crivellari, Lucio
1990MmSAI..61..527C Altcode:
This paper examines the possibility of recovering quantitative
information on chromospheric velocity fields from high resolution
(lambda/Delta lambda of about 100,000) spectrograms of the Ca II H and
K resonance lines. A qualitative discussion of the observed features
is given in terms of a schematic theory of line formation in the case
of collisionally dominated resonance lines.
---------------------------------------------------------
Title: Chromospheres of late-type active and quiescent dwarfs. I. an
atlas ofhigh resolution CA II H profiles.
Authors: Rebolo, R.; Garcia Lopez, R.; Beckman, J. E.; Vladilo, G.;
Foing, B. H.; Crivellari, L.
1989A&AS...80..135R Altcode:
As a result of a major program of spectral observations using the
CAT 1.4 m telescope and Coude Echelle Spectrograph at ESO, La Silla,
68 high resolution profiles of the 3968.5 A Ca H resonance line have
been obtained for 16 late type stars (dwarfs and subgiants, including
the sun) ranging from F9 to K5, selected to be representative of a
wide span of chromospheric activity. The methods, and results of a
set of reduction and analysis techniques applied to these spectra are
presented. Each wavelength scale is converted to a frame at rest with
respect to the photosphere of the star, and the flux scale is finally
transformed into units of absolute flux. A major effort was applied to
quantify the effects of the tilted baselines of the recorded spectra
on these absolute flux calibrations. The calibrated profiles are used
to extract and tabulate measured chromospheric H emission fluxes and
the velocity shifts with respect to the photosphere of the emission
cores and of the H3 self-reversals.
---------------------------------------------------------
Title: Chromospheres of late-type active and quiescent dwarfs. II. an
activity index derived from profiles of the CA II lambda 8498 A and
lambda 8542 A triplet lines.
Authors: Foing, B. H.; Crivellari, L.; Vladilo, G.; Rebolo, R.;
Beckman, J. E.
1989A&AS...80..189F Altcode:
High resolution, high signal to noise spectral profiles of the
8498 and 8542 A lines of the Ca II infrared triplet (IRT) have been
obtained for a sample of late type dwarfs, with a wide range of levels
of chromospheric activity, in the spectral range F9 to K4/5. These
profiles, and the difference profiles for selected active and quiescent
objects of corresponding spectral type are presented. After absolute
calibration in flux, advantage is taken of the high resolution and S/N
ratios of the spectra to define several IRT indices of chromospheric
activity, for a set of bandwidths between 0.2 A and 1 A about the line
centers. Excellent correlations for active stars between the IRT and
the Ca H indices and between the respective line widths are shown,
which pertain despite the different depths of formation of the lines
in the stellar chromospheres.
---------------------------------------------------------
Title: A spectroscopic analysis of the G8V star tau Cet.
Authors: Arribas, S.; Crivellari, L.
1989A&A...210..211A Altcode:
The physical parameters of Tau Cet are analyzed based on new and
high-quality data. The effective temperature is determined to be
5250 + or - 50 K. The discrepancy of the abundance inferred from the
Fe I lines with the lowest excitation cannot be explained by errors
either in atomic or stellar parameters and implies the need for models
incorporating non-LTE and/or a more reliable UV opacity. The obtained
abundances for the alpha-particle elements exceed that of the Fe group
by 0.2 dex, in complete agreement with previous results for metal-poor
dwarfs of the Galactic disk population. The microturbulence value is
found to be 1.1 + or - 0.1 km/s. The surface gravity value is determined
to be 1.0 + or - 0.3 solar mass.
---------------------------------------------------------
Title: Abundances of light metals and NI in a sample of disc stars.
Authors: Abia, C.; Rebolo, R.; Beckman, J. E.; Crivellari, L.
1988A&A...206..100A Altcode:
High resolution, high S/N spectra obtained using RETICON and CCD
detectors are used to determine the abundances of Al, Si, Ca, Ni
and Fe in 23 disc stars in the metallicity range -1.2 ⪉ [Fe/H] ⪉
+0.3. The authors have found overabundances, with respect to iron for Al
(≡0.4), Si (≡0.3), and Ca (≡0.2) in stars between -1.2 ⪉ [Fe/H]
⪉ -0.5. The results for Ca and Si (α-elements) are in excellent
agreement with previous observational studies. The Al overabundances
reported here show somewhat larger values close to [Fe/H] = -1.0 than
those previously found in the literature. The abundance of Ni seems
to follow that of Fe over the whole metallicity range studied.
---------------------------------------------------------
Title: On the Application of Iteration Factors for Temperature
Correction in Stellar Atmospheres
Authors: Simonneau, E.; Crivellari, L.
1988ApJ...330..415S Altcode:
The authors present an improved iterative method for deriving
temperature corrections in stellar atmospheres. At each step of
iteration, the procedure computes, from the current values of the
radiation field, a set of depth-dependent factors, averaged over angles
and frequencies. Even a crude initial estimate of the radiation field
leads to a fairly accurate determination of these factors on the first
iteration: they are quasi-invariant in subsequent iterations. For
a range of effective temperatures the procedure quickly converges
to the correct solution of the radiative transfer equation. At each
iteration, the temperature distribution is directly derived via the
energy conservation constraint.
---------------------------------------------------------
Title: Improved Data Reduction Techniques for the ESO CES Plus
RETICON Spectra
Authors: Foing, B. H.; Crivellari, L.
1988IAUS..132..355F Altcode:
The authors developed routines for intensity, equivalent width, and
radial velocity measurements on CES plus Reticon spectra obtained with
the ESO 1.4 m telescope. In order to achieve the optimal recovery of
the signal, the noise has to be minimized by removing any parasitical
effect. Special care has been devoted to the correction for remanence
effects in the Reticon dark counts. Typical results are presented
and discussed.
---------------------------------------------------------
Title: A Method for Calibrating, in Absolute Flux Units, CA II H
Profiles of Late Type Stars Observed at ESO
Authors: Castelli, F.; Gouttebroze, P.; Beckman, J.; Crivellari, L.;
Foing, B.
1988IAUS..132..153C Altcode:
The authors have applied to the Sun a method for calibrating, in
absolute flux units, Ca II H profiles of late-type stars. After
comparing, in the region 3948 - 3882 Å, an LTE synthetic spectrum
with the data of the solar flux atlas by Kurucz et al. (1984), they
have defined the wavelength ranges where observations agree with
computations, based on specific radiative equilibrium models and
collisional broadening parameters. By fitting in these regions the
spectrum of the moon observed at ESO with the corresponding synthetic
spectrum, the authors derived a calibration factor that enables them
to calibrate, in absolute flux units, the whole observed range.
---------------------------------------------------------
Title: Abundances of Light Metals in Field Stars with Metallicity
Range - 1.2< [Fe/H]< + 0.3
Authors: Abia, C.; Rebolo, R.; Beckman, J. E.; Crivellari, L.; Vila, B.
1988IAUS..132..421A Altcode:
High resolution, high S:N spectra are used to determine the abundances
of Fe, Ni, Ca, Al and Si in 25 field dwarfs with -1.2 < [Fe/H]
< +0.3. The authors find overabundances for Al, Ca and Si in stars
with -1.2 < [Fe/H] < -0.5 and solar [Ni/Fe] over the whole
studied range.
---------------------------------------------------------
Title: Search for Circumstellar Envelopes Around Late-Type Binary
Systems with Lism-Free MGII Emission Lines
Authors: Crivellari, L.
1988iue..prop.3271C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Chromospheric MG II H and K emissions free of interstellar
contamination : velocity structure in late-type dwarfs and giants.
Authors: Vladilo, G.; Molaro, P.; Crivellari, L.; Foing, B. H.;
Beckman, J. E.; Genova, R.
1987A&A...185..233V Altcode:
The authors have used high resolution IUE spectra from their own studies
and from the archive to examine the Mg II h and k chromospheric emission
cores of a sample of late-type dwarfs and giants. Sharp photospheric
absorptions were used to provide a velocity rest-frame with respect
to each stellar photosphere with the IUE-limited precision of ±4
km s<SUP>-1</SUP>. The knowledge of the kinematics of the local
interstellar medium (LISM) could then be used to identify cases
where either the cores or the wings, or in best circumstances both
features of the chromospheric lines were uncontaminated by LISM
absorption. The authors derive, using only LISM-free emission wings,
accurate Wilson-Bappu relations for both the h and k line, characterized
by a slope higher than in previous determinations.
---------------------------------------------------------
Title: Detections of diffuse interstellar bands toward the SN 1987A
in the Large Magellanic Cloud.
Authors: Vladilo, G.; Crivellari, L.; Molaro, P.; Beckman, J. E.
1987A&A...182L..59V Altcode:
The authors report the results of high resolution and high S/N
observations of diffuse interstellar bands (DIBs) towards SN 1987A
in the LMC. For the first time they clearly detect the LMC component
of the 5780, 5797 and 6284 Å DIBs, and possibly that of 5778 and
6269 Å. Moreover, they observe the Galactic components of the 5778,
5780, 5797, 6283, 6376 and 6379 Å bands, along a line of sight with
very low reddening [E(B-V) ≅ 0.07]. The LMC components are generally
weaker than the Galactic ones, while the opposite is observed for ISNa
I and K I. The band strengths are compared with the total color excess
towards the supernova.
---------------------------------------------------------
Title: CA II H emission line cores of late-type dwarfs : variability
measurements and velocity field diagnostics.
Authors: Crivellari, L.; Beckman, J. E.; Foing, B. H.; Vladilo, G.
1987A&A...174..127C Altcode:
High-resolution spectra of Alpha Cen B, Xi Boo A, 70 Oph A, and
Epsilon Ind obtained using the Coude Echelle Spectrometer and the
1.4-m Coude Auxiliary Telescope of ESO are analyzed. Variations in
the normalized fluxes for the four dwarfs are described. Position
and intensity measurements of the Ca II H features are examined. The
use of the spectra for chromospheric velocity field diagnostics is
discussed. The data reveal that it is possible to separate the effects
of plages from those due to vertical velocity fields; in some stars
the H core samples net upflow and in other net downflow; the layer
sampled by the H3 feature shows a velocity close to that defined by
the photospheric rest frame; and the H2V/H2R asymmetry is useful as
a linear measure of the chromospheric vertical velocity gradient.
---------------------------------------------------------
Title: Search for Circumstellar Envelopes Around Late Type Binary
Systems with LISM-Free Mg II Emission Lines
Authors: Crivellari, L.
1987iue..prop.2952C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: High Resolution Profiles of the Ca II Infrared Triplet Lines
in Late Type Active and Quiescent Dwarfs
Authors: Foing, B. H.; Crivellari, L.; Beckman, J. E.; Castelli, F.;
Vladilo, G.
1987LNP...291..158F Altcode: 1987LNP87.291..158F; 1987csss....5..158F
In the framework of a long term programme of chromospheric modelling
of late-type dwarfs, high resolution high signal-to-noise spectral
profiles of the 8498 and 8542 A lines of the Ca II infrared triplet
(IRT) have been obtained for a sample of quiescent and active dwarfs
from spectral types F8 to K5. Line core intensity indices are related
to the stellar spectral type, effective temperature, gravity and
rotation. The Ca II IRT spectra have been calibrated to estimate the
excess chromospheric fluxes related to the activity. A relation is
derived between the fluxes measured in the line cores and the Rossby
number for the sample stars with known rotational periods. Line widths,
core and wing intensities, and the differences between active and
quiescent stars of the same spectral type are discussed.
---------------------------------------------------------
Title: Effects of interstellar obsorptions of MgII chromospheric
emissions
Authors: Vladilo, G.; Crivellari, L.; Molaro, P.; Beckman, J. E.;
Genova, R.
1986ESASP.263..233V Altcode: 1986NIA86......233V; 1986niia.conf..233V
The Mg II chromospheric emissions in cool stars were analyzed using
high resolution IUE archive spectra. The ways in which interstellar
(IS) absorptions can affect the shapes of the chromospheric profiles
are discussed. It is possible to indicate regions of the sky where
the Mg II IS absorption can be neglected, up to distances of a few
tens of parsecs. When the absorption is present, radial velocity can
be estimated, to predict the kind of distortion introduced onto the
line profile. Once the IS contamination is taken into account, the
chromospheric profiles of the stars tend to have a homogeneous behavior.
---------------------------------------------------------
Title: Lithium abundances and 7Li/6Li ratios in late-type population
I field dwarfs.
Authors: Rebolo, R.; Crivellari, L.; Castelli, F.; Foing, B.; Beckman,
J. E.
1986A&A...166..195R Altcode:
Using the Coudé Echelle Spectrograph fed by the 1.4 m Coudé Auxiliary
Telescope at ESO, La Silla, the authors have obtained a sequence of
fourteen high resolution (λ/Δλ = 10<SUP>5</SUP>) spectra at high
signal to noise ratio, of main sequence and near main sequence objects
in the wavelength range containing the 6708 Å resonance doublet
of lithium. Using Kurucz thermal equilibrium model atmospheres, and
relying on the excellent signal to noise ratios in the spectra, they
were able to assess not only the <SUP>7</SUP>Li components, but also
the weaker <SUP>6</SUP>Li components of the blended doublet.
---------------------------------------------------------
Title: Book-Review - Progress in Stellar Spectral Line Formation
Theory
Authors: Beckman, J. E.; Crivellari, L.; Dworetsky, M. M.
1986Obs...106Q.123B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Book-Review - Progress in Stellar Spectral Line Formation
Theory
Authors: Beckman, J. E.; Crivellari, L.
1986JBAA...96Q.187B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: High Resolution Observations of Chromospheric Lines in
Late-Type Dwarfs
Authors: Foing, B. H.; Beckman, J.; Crivellari, L.; Galleguillos, D.
1986RMxAA..12..214F Altcode:
We obtained at the Coude Echelle Spectrometer (CES) attached to the 1.4m
CAT telescope of the European Southern Observatory ,high resolution
(R=1O0000),high signal to noise (S/N=30-20O) spectra of the Ca II H
and k,H alpha and Ca II infrared triplet lines on a sample of southern
late-type dwarfs also observed with lUE for the Mg II h and k lines
.These profiles are compared for progressive spectral types from
F8V to k5V,and for pairs of active-low active stars of the same type
and we show the variety of the core emissions and asymmetries. The
differential emission in the cores of is discussed in terms of
chromospheric heating and of coverage of these stars with active
regions.The spectral characteristics of the activity signature are
compared with observations and modelling of the profiles for solar
"plages" and active components.Some activity indicators obtained
from the Ca infrared triplet and H alpha lines are plotted versus the
spectral type,showing a lower envelope in the activity corresponding
to the quiescent stars,and the range of variation of the activity at
a given spectral type. These high resolution profiles are used as a
constraint on multi-component models of the stellar chromospheres,which
are being developed to estimate the temperature structure,the radiative
losses and the heating processes in these late-type dwarfs.
---------------------------------------------------------
Title: Circumstellar Envelopes Around Late-Type Binary Systems
Authors: Crivellari, L.
1986iue..prop.2705C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Spectroscopic Variability Associated with Chromospheric
Activity: The α Cen System
Authors: Foing, B.; Beckman, J.; Crivellari, L.; Vladilo, G.; Char, S.
1986LNP...254..488F Altcode: 1986csss....4..488F
No abstract at ADS
---------------------------------------------------------
Title: Book-Review - Progress in Stellar Spectral Line Formation
Theory
Authors: Beckman, J. E.; Crivellari, L.; Rutten, R. J.
1986SSRv...43Q.384B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Book-Review - Progress in Stellar Spectral Line Formation
Theory
Authors: Beckman, J. E.; Crivellari, L.
1985Sci...230..835B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Book-Review - Progress in Stellar Spectral Line Formation
Theory
Authors: Beckman, J. E.; Crivellari, L.
1985JBAA...95..277B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Chromospheric modelling in late-type dwarfs. 2. CES (Coudé
Echelle Spectrograph) observations of active and quiescent stars.
Authors: Foing, B. H.; Beckman, J.; Crivellari, L.; Galleguillos, D.
1985Msngr..41...18F Altcode:
The characteristics of the chromospheres of late-type dwarf stars
were studied using the CoudeEchelle Spectrograph (CES) at ESO and the
IUE. The study focused on the h and k Mg II lines of F and G dwarfs. The
stability of the IUE spectra permitted calculations of averaged
absorption features and the subtraction of the Mg II lines to leave
intrinsic chromospheric and photospheric line characteristics. Stras
with Mg emission cores are very similar in their chromospheres,
while stars with Ca II cores vary greatly and exhibit more intense
emissions. The higher activity has been linked to magnetic forces which
produce spicules with diameters in the range of 1000 km. CES spectra
on the Ca II core H lines in Epsilon Eri and Alpha Cen B, active and
passive stars, respectively, show a 40 percent lower intensity in the
quiescnet star than in the active star, which has an asymmetric core
emission and a sharp central absorption with Ca IR triplet lines. The
emission features in the active stars are similar to those of the
active regions, photospheric spots, chromospheric plages and coronal
structures observed in the sun.
---------------------------------------------------------
Title: The distribution of the local interstellar medium derived
from MG II column densities towards seven cool stars.
Authors: Vladilo, G.; Beckman, J. E.; Crivellari, L.; Franco, M. L.;
Molaro, P.
1985A&A...144...81V Altcode:
High resolution spectra containing the Mg II h and k resonance doublet
have been obtained using the long wavelength spectrometer of IUE, for
7 late-type stars (F0 to G8). In one of these stars, β Hyi, two sharp
absorption features were detected against the chromospheric emission
cores. Using the study of Crutcher (1982) the authors can employ the
velocity of one of these features to identify it as arising from the
local interstellar medium (LISM), while the other feature appears
due to chromospheric self-absorption. Although the other stars do
not show separate intrinsic and LISM features it is possible to use
evidence both from wavelength shifts and line half-widths to identify
the interstellar absorption components. This gives the Mg II column
densities along the lines of sight to five of these stars and upper
limits towards the other two.
---------------------------------------------------------
Title: Progress in stellar spectral line formation theory
Authors: Beckman, J. E.; Crivellari, L.
1985ASIC..152.....B Altcode: 1985pssl.proc.....B
Papers on stellar spectral line formation theory are presented. The
general topics considered include: frequency redistribution problems
in line formation theory, methods in line radiative transfer, and
observational and theoretical aspects of spectral line formation in
astrophysical and laboratory environments. Some individual subjects
discussed include: kinetic aspects of redistribution in spectral lines,
computational methods in redistribution functions, numerical methods
in radiative transfer, partial versus complete linearization, line
formation in laboratory plasmas, observational problems in spectral
line formation, line transfer in expanding atmospheres, partial
redistribution in the wind of red giants, transfer of Lyman-alpha
radiation in solar coronal loops, pressure broadening and the solar
limb effect, and line formation in molecular clouds.
---------------------------------------------------------
Title: The spectra of late-type dwarfs and sub-dwarfs in the near
ultraviolet. III. an atlas of MG II H and K profiles.
Authors: Franco, M. L.; Crivellari, L.; Molaro, P.; Vladilo, G.;
Ramella, M.; Morossi, C.; Allocchio, C.; Beckman, J. E.
1984A&AS...58..693F Altcode:
Using a radically new suite of data extraction routines, IUEARM, high
resolution IUE spectra of six F and G stars, taken at regular intervals
since 1978 are analysed. The salient features of the new routines are:
reduction of interorder scattering via two-dimensional deconvolution of
the echelle spectrograph image plane, with high frequency filtering,
to reduce the effects of the instrument function; enhanced spectral
resolution by data extraction using a small pseudo-slit; improved
"ripple correction" yielding mutually consistent inter-order overlap
regions; and an improved absolute calibration. Rigorous limits, twice as
precise as any previously available, are set on weak spectral features,
and on possible variability of the h and k chromospheric emission cores
in the G stars. The authors show how the interstellar Mg II components
of the stars can be abstracted and used to examine a sector of the
local interstellar medium.
---------------------------------------------------------
Title: Chromospheric Modelling in Late-Type Dwarfs - Part One -
Quiescent Objects
Authors: Beckman, J.; Crivellari, L.; Foing, B.
1984Msngr..38...24B Altcode:
Attention is given to problems which arise when an attempt is made to
use observational material to clarify the mechanisms responsible for
the heating of the chromosphere in a late-type star, in order to derive
a clearer physical view of the nature of a chromosphere, its relation
to the underlying photosphere, and the overlying corona. Problems
arise in the prediction of line shapes by means of simple models,
in chromospheric inhomogeneities, in two-stream modeling, and in the
coexistence of two temperature structures. It is deemed necessary to
take the dominant role played by inhomogeneities into account, and to
ensure that any interstellar effects in the line profile data are well
and truly eliminated.
---------------------------------------------------------
Title: MG II Spectra of Late Type Stars Used to Probe the LISM
Authors: Beckman, J. E.; Crivellari, L.; Franco, M.; Molaro, P.;
Vladilo, G.
1984NASCP2345...67B Altcode: 1984lism.rept...67B; 1984IAUCo..81...67B
IUE spectra of Mg II h and k in late type dwarfs and giants were used
to detect and measure absorption components due to the LISM. This
technique gives a method of probing the awkward range from d = 3 pc to
d = 80 pc from the Sun. In spite of interpretational uncertainties the
HI component of the LISM can be plotted well enough to confirm it as a
cloud some 20 to 30 pc in extent, peaking sharply in density towards
l(II)-25 deg., moving towards the Sun from l(II)-25 deg, b(II) = +
10 deg., at 28 Km/sec. The hole towards l(II) = 150 deg is confirmed,
suggesting a solar position close to the cloud's edge in this direction.
---------------------------------------------------------
Title: Use of high resolution IUE spectra of Mg II in cool stars to
probe the local interstellar medium.
Authors: Molaro, P.; Beckman, J. E.; Crivellari, L.; Franco, M. L.;
Vladilo, G.
1984ESASP.218..139M Altcode: 1984iue..conf..139M
It is shown that, given suitable radial velocity values for the stars
and the local interstellar medium (LISM) interstellar absorption
can be distinguished from chromospheric self absorption within the
MgII emissions from late type stars. Beta Hyi (G2IV) offers a clear
example. Zeta Tuc (G2V) and delta Pav (G8V) which are very close to
beta Hyi show similar narrow MgII absorption features, which are also
interpretable as interstellar. Narrow MgII absorptions in the rapidly
rotating F-type stars beta TrA and alpha Hyi are reported. The absence
of IS Mg II in the stars tau Cet and 24UMa confirms the presence of
a hole in the LISM near galactic longitude 150 deg.
---------------------------------------------------------
Title: The spectra of late type dwarfs and sub-dwarfs in the near
ultraviolet II. Limits to variability in MgII emission from IUE
spectrophotometry.
Authors: Crivellari, L.; Franco, M. L.; Molaro, P.; Vladilo, G.;
Beckman, J. E.
1983A&AS...52..135C Altcode:
The chromospheric h and k emission cores of the G dwarfs and subdwarfs
Beta Hyi (14), Zeta Tuc (5), Delta Pav (5), and Tau Cet (5) have been
monitored over a period of four years using the IUE satellite long
wavelength camera. A detailed statistical examination of the data has
shown that no variation in h and k fluxes of less than 15 percent could
have been detected by means of this technique. This limit applies,
even after removing the effects of saturation, variable gain, and
scattered light between echelle orders, for any astrophysical use of
comparable IUE data. No flux excursions larger than this value were
detected in any of the objects presently considered.
---------------------------------------------------------
Title: Observed and computed UV spectral distribution of A and
F stars.
Authors: Malagnini, M. L.; Faraggiana, R.; Morossi, C.; Crivellari, L.
1982A&A...114..170M Altcode:
An automatic and fast procedure was implemented to determine
T<SUB>e</SUB> and log g from the comparison of the UV S 2/68 spectra
with the Kurucz models. The method has been applied to all A and F stars
with (B-V)0 greater than or equal to 0 and luminosity classes III, IV,
IV-V, and V, included in the Ultraviolet Bright-Star Spectrophotometric
Catalogue. From the analysis, it appears that the models match the
observed fluxes fairly well and that the effective temperatures derived
from the UV data agree with those derived from Stromgren photometric
indices within 200 K.
---------------------------------------------------------
Title: On the MG II and FE II resonance lines in Herbig AE stars :
preliminary results.
Authors: Talavera, A.; Catala, C.; Crivellari, L.; Czarny, J.;
Felenbok, P.; Praderie, F.
1982ESASP.176...99T Altcode: 1982iue..conf...99T; 1982IUE3r.......99T
The IUE observed AB Aur, HD250550, and BD + 46 deg 3471 at high
resolution in the range 2000 to 32000 A. The MgII resonance line
profiles all show a P Cygni structure, with an intense emission,
and a shortward wing velocity in the absorption component of 300 to
600 km/sec. In HD 250550, the Fe II line spectrum exhibits a shell
phenomenon: all lines are blue displaced by - 290 km/sec; other lines
of singly ionized species show the same effect. The mass loss rate
in AB Aur estimated from an analysis of both MgII resonance lines and
unsaturated Fe II multiplet 1 lines is smaller than previously reported.
---------------------------------------------------------
Title: A search for MG II and K emission core variability in six
main sequence stars.
Authors: Beckman, J. E.; Franco, M. L.; Molaro, P.; Uladilo, G.;
Crivellari, L.
1982ESASP.176..149B Altcode: 1982iue..conf..149B; 1982IUE3r......149B
Spectra of six main sequence stars with spectral types from F0 to
G8 were obtained with the IUE long wavelength spectrograph at high
resolution. Four or more spectra of each object were taken over a
period of 3.5 years, supplemented by archive spectra from IUE of the
same type. Quantitative analysis of the Mg II h and k emission cores
from these spectra was performed to search for sporadic or periodic
variability. No variations the + or - 10% level set by the statistical
fluctuations within each set of spectra occur for any object. Even
the limited photometric accuracy claimed here is not guaranteed among
spectra which vary in exposure time by more than 15%. This situation
for all IUE LWR spectra can be improved only by solving the problem
of background subtraction.
---------------------------------------------------------
Title: On the search for transition zone lines in late A type stars.
Authors: Crivellari, L.; Praderie, F.
1982A&A...107...75C Altcode:
Observations of the 1400-1800 A region in late A type stars are
presented, which were obtained using the IUE satellite at low and
high resolution. The calibrated energy distribution is derived and
presents a large variation with wavelength in the spectral range due
to Si I photoionization discontinuities. Emission lines formed in the
transition region do not appear, although upper limits on the emission
in these lines are established. A schematic model is proposed for the
outer layers in A type stars, which suggests that the chromosphere
in solar type stars is likely absent in A stars, while the NLTE
photosphere could be immediately connected to the transition region,
joining the upper photosphere to the corona. In addition, the amount
of matter in the transition zone is found to be small.
---------------------------------------------------------
Title: An alternative procedure for extracting IUE low resolution
spectra.
Authors: Crivellari, L.; Morossi, C.
1982A&A...106..332C Altcode:
This paper presents an alternative procedure which extracts spectra
from IUE low resolution images. The -by-line image is scanned
perpendicularly to the direction of dispersion . The resulting
cross-cuts are then fitted with a known-form analytical function,
which consists of a linear base-line describing the background plus
as many bands as there are spectra in the image under study. At each
wavelength point the value of the spectrum is assumed to be the area
of the corresponding band. A proper choice of the band profile allows
us to recover the light scattered perpendicularly to the direction of
dispersion, due to the "halation" effect in the on-board UV-to-visible
image converter. In such a way a more correct extracting procedure
is achieved. It furnishes noticeably better results than the standard
VILSPA extracting routine in the case of highly exposed images. As a
by-product, some properties of the Point Spread Function perpendicular
to the direction of dispersion are determined.
---------------------------------------------------------
Title: The spectra of late-type dwarfs in the near
ultraviolet. I. Line identifications.
Authors: Beckman, J. E.; Crivellari, L.; Selvelli, P. L.
1982A&AS...47..295B Altcode:
Spectra from IUE of 6 main sequence and near main sequence stars from
classes A5 to G8, between 2700 A and 2900 A wavelength are described. In
toto, some 250 absorption lines are identified. Observed differences
in those atomic and ionic multiplets present in each spectrum are
shown to be due either to the regular progression in spectral class,
or to the smearing effect of stellar rotation.
---------------------------------------------------------
Title: The Visible Spectra of Three B-Stars - Pi-Ceti 36-LYNCIS
134-TAURII
Authors: Crivellari, L.; Flora, U.; Mercanti, M.; Morossi, C.; Rusconi,
L.; Sedmak, G.
1981Ap&SS..80..425C Altcode:
We present an identification list for the visible spectra of
three B stars: π Cet (B7V), 36 Lyn B8 IIIp (?)),and 134 Tau (B9
IV). Equivalent widths have been measured on medium dispersion plates
(7.0 and 12.4 Å mm<SUP>-1</SUP>) taken with the 152 cm coudé reflector
at the Observatoire de Haute Provence (France). These results are also
presented. The photographic plates were digitized by using a PDS 1010A
microdensitometer. The spectroscopic data were reduced by means of a
dedicated software package and an attempt was made to compute equivalent
widths in a homogeneous way. T <SUB>eff</SUB> and logg parameters
were estimated by using the computer to compare automatically the
spectroscopic data with the value of theoretical models.
---------------------------------------------------------
Title: A statistical analysis of Mg II h and k emission cores for
variability in beta Hydri (G2 IV).
Authors: Beckman, J. E.; Crivellari, L.; Morossi, C.; Ramella, M.;
Vladilo, G.
1980idr..conf..207B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Theoretical Approach to the Estimate of Scattered Light
into IUE Low Resolution Short Wavelength Spectra
Authors: Crivellari, L.; Morossi, C.; Ramella, M.
1980idr..conf..185C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: An automatic procedure for a determination of the effective
temperature and gravity. Application to 100 O-type stars.
Authors: Morossi, C.; Crivellari, L.
1980A&AS...41..299M Altcode:
We have developed an automatic procedure to determine T<SUB>eff</SUB>
and log g from the comparison of observational data with their
theoretical values. After a description of the method we compare our
results for a sample of 100 O-type stars with the classifications of
Conti and Walborn and with the determinations of Morrison by photometric
indices. The agreement is fair and we present a HR diagram for O-type
stars based on our data.
---------------------------------------------------------
Title: Parasite light effects in International Ultraviolet Explorer
(IUE) low resolution spectra
Authors: Crivellari, L.; Morossi, C.
1980ESASP.157...99C Altcode: 1980iue..conf...99C
Procedures were developed for extracting low resolution spectra
while correcting for halation effects and scattered light within the
instrument. To correct for halation, the spectra were extracted from the
line-by-line images. Perpendicular scans of the image were fitted using
an iterative least squares procedure with a known form function. The
fitting function was a linear combination of a Gaussian with a Lorentz
function. Correction for light scattered from the gratings employed
a guess and test method. A priori choices of schematic spectral
distributions were used to compute different sets of scattered light
coefficients. Satisfactory results are shown for both methods.
---------------------------------------------------------
Title: Correction of Halation Effects in Low-Resolution IUE Spectra
Authors: Crivellari, L.
1980IUEEN...5...34C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Outer Layers of Dwarf A Type Stars.
Authors: Praderie, F.; Crivellari, L.
1979BAAS...11..776P Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The visible spectrum of beta Ori, B8 Ia.
Authors: Crivellari, L.; Flora, U.; Rusconi, L.; Sedmak, G.
1979A&AS...36...73C Altcode:
An exhaustive description of the visible spectrum of Beta Ori from the
Balmer discontinuity to about 9000 A is reported. Equivalent widths and
radial-velocity values for the most significant lines in the spectral
range studied are presented. Moreover, an automatic data processing
technique is described.
---------------------------------------------------------
Title: A criterion for the measurement of equivalent widths.
Authors: Crivellari, L.; Mardirossian, F.; Morossi, C.
1979A&A....72..256C Altcode:
A simple standard criterion for computer measurement of the equivalent
widths of digitized blended spectral lines and for drawing a local
continuum as defined by Ardeberg (1975) is outlined. A procedure is
developed with which the user need only select the spectral interval
enclosing the line blend and the spectral features themselves determine
the position of the local continuum as well as the parameters of
the mathematical function that fits the lines. The errors associated
with the procedure are evaluated. Some examples involving the Si III,
C I, and S III features at 1192-1194 A in the spectra of Alpha Cam,
Delta Ori A, and Iota Ori are provided. It is noted that the program
used to solve the line blends is particularly suitable for interactive
small computers.
---------------------------------------------------------
Title: Analysis of UV spectrophotometric observations for O-type
stars.
Authors: Morossi, C.; Stalio, R.; Crivellari, L.
1979IAUS...83..109M Altcode:
Spectral indices (delta m<SUB>2100</SUB> and delta m<SUB>1500)</SUB>
for 32 O-type stars with known spectral class and reddening are plotted
versus log of effective temperature. The delta m<SUB>2100</SUB> indices
are considered to be almost luminosity-independent, while evidence of
a well-established relationship between observed spectral indices and
empirical temperatures appears. Also graphed are equivalent widths
of selected photospheric lines versus temperature. Theoretical line
strengths computed with a code that solves the LTE line transfer
problem from the model atmosphere input of Kurucz, et al. (1974)
indicate that the general behavior of the silicon lines, but not the
P V resonance line, appears consistent with the computations.
---------------------------------------------------------
Title: Lo spettro visibilie di beta Ori (B8 Ia).
Authors: Crivellari, L.; Flora, U.
1979RSAI...22..194C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A software package for the presentation and evaluation of
digitized images
Authors: Crivellari, L.; Giuricin, G.; Mardirossian, F.; Mezzetti,
M.; Pucillo, M.; Santin, P.
1978MmSAI..49..741C Altcode:
A brief description is given of the IMAGE-1 software package for the
presentation and evaluation of digitized images at the Osservatorio
Astronomico di Trieste. The package makes it possible to draw isophotes
in an interactive way and to have an assonometric display of the
image studied.
---------------------------------------------------------
Title: Processing of stellar spectrograms digitized by means of the
PDS 1010A microdensitometer - The ELSPEC/1 software package
Authors: Crivellari, L.; Santin, P.; Sedmak, G.; Rusconi, L.
1978MmSAI..49..717C Altcode:
The software package ELSPEC/1 written at Trieste Astronomical
Observatory for the automatic reduction of spectrograms digitized by a
PDS 1010A microdensitometer is described and an example of the results
obtained is shown. The package accepts spectrograms in the format of the
Observatoire de Hauts Provence and operates on a HP 2100A minicomputer.
---------------------------------------------------------
Title: a Criterion for the Measurement of Equivalent Widths
Authors: Crivellari, L.; Mardirossian, F.; Morossi, C.
1978hrs..conf..625C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Spectral energy distribution and line blanketing in beta Ori.
Authors: Crivellari, L.; Stalio, R.
1977MmSAI..48..773C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Line blocking and reddening of beta Orionis. A new
determination of the empirical effective temperature.
Authors: Stalio, R.; Selvelli, P. L.; Crivellari, L.
1977A&A....60..109S Altcode:
Summary. The ultraviolet spectral energy distribution of the B8 Ia
star fi Orionis, derived from OAO-2 observations, has been corrected
for the line-blocking fraction (LBF) measured from Copernicus scans
with 0.4 A resolution. The resulting continuum appears to be affected
by a not negligible color excess; from the size of the characteristic
interstellar extinction bump at 2175 one may estimate a value of EB -
V =0.04. The empirical effective temperature determined from the angular
diameter and from the total absolute flux, corrected for reddening,
is 12070 0K. This value is in good agreement with the temperature
obtained by comparing the observed continuum flux, corrected for LBF
and reddening, with LTE, plane-parallel model atmospheres predictions
in the region A. At shorter wavelengths, however, the continuum flux
appears deficient, thus indicating a lower effective temperature,
while in the visual it is in excess. This effect may be ascribed to
line- blanketing in the UV which forces a flux redistribution and
maintains the region between 2000 and 3000 A invariant as a result of
a balance between the fluxes in the two neighbouring regions. There
are indications of a higher value ofthe effective temperature from the
line spectrum. The LBF curve in the region A, compared with theoretical
LBFs, indicates a Teff 13000 0K, while the ionization ratios of Fe
and Si indicate an even greater value. Key words: Orionis - stellar
atmospheres effective temperature - line blocking reddening
---------------------------------------------------------
Title: The ultraviolet spectrum of beta Ori, B8 Ia.
Authors: Selvelli, P. L.; Crivellari, L.; Stalio, R.
1977A&AS...27....1S Altcode:
A Copernicus spectrum of the early-type supergiant Beta Ori, extending
from 1010 to 1440 A with 0.2-A resolution and from 1710 to 3215
A with 0.4-A resolution, is described. Some 1137 absorption lines
are identified, and equivalent widths and profiles are computed for
selected features. It is found that: (1) the Ly-alpha line is the
major feature over the entire spectrum; (2) 'metals' are mostly in
the first and second state of ionization; and (3) only the resonance
lines of Si III and Si IV are markedly blue-shifted by about 180 km/s,
but there is no evidence of P Cyg profiles.
---------------------------------------------------------
Title: ELSPEC/1. Un software package per l'elaborazione automatizzata
di spettrogrammi stellari. I. Struttura.
Authors: Rusconi, L.; Sedmak, G.; Crivellari, L.
1977TriP..544.....R Altcode:
No abstract at ADS
