41 research outputs found
On the sensitivity of HeI singlet lines to the FeIV model atom in O stars
Recent calculations and analyses of O star spectra have revealed
discrepancies between theory and observations, and between different
theoretical calculations, for the strength of optical HeI singlet
transitions.We investigate the source of these discrepancies. Using a non-LTE
radiative transfer code we have undertaken detailed test calculations for a
range of O star properties. Our principal test model has parameters similar to
those of the O9V star, 10 Lac. We show that the discrepancies arise from
uncertainties in the radiation field in the HeI resonance transition near
584Angs. The radiation field at 584Angs. is influenced by model assumptions,
such as the treatment of line-blanketing and the adopted turbulent velocity,
and by the FeIV atomic data. It isshown that two FeIV transitions near 584Angs
can have a substantial influence on the strength of the HeI singlet
transitions. Because of the difficulty of modeling the HeI singlet lines,
particularly in stars with solar metalicity, the HeI triplet lines should be
preferred in spectral analyses. These lines are much less sensitive to model
assumptions.Comment: 7 pages, 9 figures, accepted for publication in A&
EC 11481-2303 - A Peculiar Subdwarf OB Star Revisited
EC 11481-2303 is a peculiar, hot, high-gravity pre-white dwarf. Previous
optical spectroscopy revealed that it is a sdOB star with an effective
temperature (Teff) of 41790 K, a surface gravity log(g)= 5.84, and He/H = 0.014
by number. We present an on-going spectral analysis by means of non-LTE
model-atmosphere techniques based on high-resolution, high-S/N optical
(VLT-UVES) and ultraviolet (FUSE, IUE) observations. We are able to reproduce
the optical and UV observations simultaneously with a chemically homogeneous
NLTE model atmosphere with a significantly higher effective temperature and
lower He abundance (Teff = 55000 K, log (g) = 5.8, and He / H = 0.0025 by
number). While C, N, and O appear less than 0.15 times solar, the iron-group
abundance is strongly enhanced by at least a factor of ten.Comment: 8 pages, 11 figure
Atmospheric NLTE-models for the spectroscopic analysis of blue stars with winds : II. Line-blanketed models
We present new or improved methods for calculating NLTE, line-blanketed model atmospheres for hot stars with winds (spectral types A to O), with particular emphasis on fast performance. These methods have been implemented into a previous, more simple version of the model atmosphere code FASTWIND (Santolaya-Rey et al. 1997) and allow us to spectroscopically analyze large samples of massive stars in a reasonable time-scale, using state-of-the-art physics. Although this updated version of the code has already been used in a number of recent investigations, the corresponding methods have not been explained in detail so far, and no rigorous comparison with results from alternative codes has been performed. This paper intends to address both topics.
In particular, we describe our (partly approximate) approach to solve the equations of statistical equilibrium for those elements that are primarily responsible for line-blocking and blanketing, as well as an approximate treatment of the line-blocking itself, which is based on a simple statistical approach using suitable means of line opacities and emissivities. Both methods are validated by specific tests. Furthermore, we comment on our implementation of a consistent temperature structure.
In the second part, we concentrate on a detailed comparison with results from two codes used in alternative spectroscopical investigations, namely CMFGEN (Hillier & Miller 1998) and WM-Basic (Pauldrach et al. 2001). All three codes predict almost identical temperature structures and fluxes for λ > 400 Å, whereas at lower wavelengths a number of discrepancies are found. Particularly in the HeII continua, where fluxes and corresponding numbers of ionizing photons react extremely sensitively to subtle differences in the models, we consider any uncritical use of these quantities (e.g., in the context of nebula diagnostics) as unreliable. Optical H/He lines as synthesized by FASTWIND are compared with results from CMFGEN, obtaining a remarkable coincidence, except for the HeI singlets in the temperature range between 36 000 to 41 000 K for dwarfs and between 31 000 to 35 000 K for supergiants, where CMFGEN predicts much weaker lines. Consequences of these discrepancies are discussed.
Finally, suggestions are presented as to adequately parameterize model-grids for hot stars with winds, with only one additional parameter compared to standard grids from plane-parallel, hydrostatic models.Facultad de Ciencias AstronĂłmicas y GeofĂsica
Spindown of massive rotating stars
Models of rapidly rotating massive stars at low metallicities show
significantly different evolution and higher metal yields compared to
non-rotating stars. We estimate the spin-down time-scale of rapid rotating
non-convective stars supporting an alpha-Omega dynamo. The magnetic dynamo
gives rise to mass loss in a magnetically controlled stellar wind and hence
stellar spin down owing to loss of angular momentum. The dynamo is maintained
by strong horizontal rotation-driven turbulence which dominates over the Parker
instability. We calculate the spin-down time-scale and find that it could be
relatively short, a small fraction of the main-sequence lifetime. The spin-down
time-scale decreases dramatically for higher surface rotations suggesting that
rapid rotators may only exhibit such high surface velocities for a short time,
only a small fraction of their main-sequence lifetime.Comment: Accepted by MNRA
Atmospheric NLTE-Models for the Spectroscopic Analysis of Blue Stars with Winds. II. Line-Blanketed Models
We present new or improved methods for calculating NLTE, line-blanketed model
atmospheres for hot stars with winds (spectral types A to O), with particular
emphasis on a fast performance. These methods have been implemented into a
previous, more simple version of the model atmosphere code FASTWIND
(Santolaya-Rey et al.1997) and allow to spectroscopically analyze rather large
samples of massive stars in a reasonable time-scale, using state-of-the-art
physics.
We describe our (partly approximate) approach to solve the equations of
statistical equilibrium for those elements which are primarily responsible for
line-blocking and blanketing, as well as an approximate treatment of the
line-blocking itself, which is based on a simple statistical approach using
suitable means for line opacities and emissivities. Furthermore, we comment on
our implementation of a consistent temperature structure.
In the second part, we concentrate on a detailed comparison with results from
those two codes which have been used in alternative spectroscopical
investigations, namely CMFGEN (Hillier & Miller 1998) and WM-Basic (Pauldrach
et al. 2001). All three codes predict almost identical temperature structures
and fluxes for lambda > 400 A, whereas at lower wavelengths a number of
discrepancies are found. Optical H/He lines as synthesized by FASTWIND are
compared with results from CMFGEN, obtaining a remarkable coincidence, except
for the HeI singlets in the temperature range between 36,000 to 41,000 K for
dwarfs and between 31,000 to 35,000 K for supergiants, where CMFGEN predicts
much weaker lines. Consequences due to these discrepancies are discussed.Comment: 30 pages incl. 20 figures, accepted by A&
On the sdOB primary of the post common-envelope binary AA Doradus (LB 3459)
AA Dor is an eclipsing, post common-envelope binary with an sdOB-type primary
and a low-mass secondary. Eleven years ago, an NLTE spectral analysis showed a
discrepancy in the surface gravity that was derived by radial-velocity and
light-curve analysis, log g = 5.21 +/- 0.1 (cm/sec^2) and log g = 5.53 +/-
0.03, respectively.
We aim to determine both the effective temperature and surface gravity of AA
Dor precisely from high-resolution, high-S/N observations taken during the
occultation of the secondary.
We calculated an extended grid of metal-line blanketed, state-of-the-art,
non-LTE model atmospheres in the parameter range of the primary of AA Dor.
Synthetic spectra calculated from this grid were compared to optical
observations.
We verify Teff = 42000 +/- 1000 K from our former analyses and determine a
higher log g = 5.46 +/- 0.05. The main reason are new Stark-broadening tables
that were used for calculating of the theoretical Balmer-line profiles.
Our result for the surface gravity agrees with the value from light-curve
analysis within the error limits, thereby solving the so-called gravity problem
in AA Dor.Comment: 6 pages, 7 figure
Spectroscopic determination of the fundamental parameters of 66 B-type stars in the field-of-view of the CoRoT satellite
We aim to determine the fundamental parameters of a sample of B stars with
apparent visual magnitudes below 8 in the field-of-view of the CoRoT space
mission, from high-resolution spectroscopy. We developed an automatic procedure
for the spectroscopic analysis of B-type stars with winds, based on an
extensive grid of FASTWIND model atmospheres. We use the equivalent widths
and/or the line profile shapes of continuum normalized hydrogen, helium and
silicon line profiles to determine the fundamental properties of these stars in
an automated way. After thorough tests, both on synthetic datasets and on very
high-quality, high-resolution spectra of B stars for which we already had
accurate values of their physical properties from alternative analyses, we
applied our method to 66 B-type stars contained in the ground-based archive of
the CoRoT space mission. We discuss the statistical properties of the sample
and compare them with those predicted by evolutionary models of B stars. Our
spectroscopic results provide a valuable starting point for any future seismic
modelling of the stars, should they be observed by CoRoT.Comment: 31 pages (including 14 pages online material), 32 figure
Low mass loss rates in O-type stars: Spectral signatures of dense clumps in the wind of two Galactic O4 stars
We have analyzed the far-UV spectrum of two Galactic O4 stars, the O4If+
supergiant HD190429A and the O4V((f)) dwarf HD96715, using archival FUSE and
IUE data. We have conducted a quantitative analysis based on the two NLTE model
atmosphere and wind codes, TLUSTY and CMFGEN. We have derived the stellar and
wind parameters and the surface composition of the two stars. The surface of
HD190429A has a composition typical of an evolved O supergiant (N-rich, C and
O-poor), while HD96715 exhibits surface N enhancement similar to the enrichment
found in SMC O dwarfs and attributed to rotationally-induced mixing. We find
that homogeneous wind models could not match the observed profile of O V1371
and require very low phosphorus abundance to fit the P V1118-1128 resonance
lines. However, we are able to match the O V and P V lines using clumped wind
models. We find that N IV1718 is also sensitive to wind clumping. For both
stars, we have calculated clumped wind models that match well all these lines
from different species and that remain consistent with Halpha data. These fits
therefore provide a coherent and thus much stronger evidence of wind clumping
in O stars than earlier claims. We find that the wind of these two stars is
highly clumped, as expressed by very small volume filling factors, namely
f=0.04 for HD190429A and f=0.02 for HD96715. In agreement with our analysis of
SMC stars, clumping starts deep in the wind, just above the sonic point. The
most crucial consequence of our analysis is that the mass loss rates of O stars
need to be revised downward significantly, by a factor of 3 and more.
Accounting for wind clumping is essential when determining the wind properties
of O stars. Our study therefore calls for a fundamental revision in our
understanding of mass loss and of O-type star winds. (abridged)Comment: To appear in Astronomy & Astrophysics; 16 pages; accepted version
after minor revisio
Atmospheric NLTE-models for the spectroscopic analysis of blue stars with winds : II. Line-blanketed models
We present new or improved methods for calculating NLTE, line-blanketed model atmospheres for hot stars with winds (spectral types A to O), with particular emphasis on fast performance. These methods have been implemented into a previous, more simple version of the model atmosphere code FASTWIND (Santolaya-Rey et al. 1997) and allow us to spectroscopically analyze large samples of massive stars in a reasonable time-scale, using state-of-the-art physics. Although this updated version of the code has already been used in a number of recent investigations, the corresponding methods have not been explained in detail so far, and no rigorous comparison with results from alternative codes has been performed. This paper intends to address both topics.
In particular, we describe our (partly approximate) approach to solve the equations of statistical equilibrium for those elements that are primarily responsible for line-blocking and blanketing, as well as an approximate treatment of the line-blocking itself, which is based on a simple statistical approach using suitable means of line opacities and emissivities. Both methods are validated by specific tests. Furthermore, we comment on our implementation of a consistent temperature structure.
In the second part, we concentrate on a detailed comparison with results from two codes used in alternative spectroscopical investigations, namely CMFGEN (Hillier & Miller 1998) and WM-Basic (Pauldrach et al. 2001). All three codes predict almost identical temperature structures and fluxes for λ > 400 Å, whereas at lower wavelengths a number of discrepancies are found. Particularly in the HeII continua, where fluxes and corresponding numbers of ionizing photons react extremely sensitively to subtle differences in the models, we consider any uncritical use of these quantities (e.g., in the context of nebula diagnostics) as unreliable. Optical H/He lines as synthesized by FASTWIND are compared with results from CMFGEN, obtaining a remarkable coincidence, except for the HeI singlets in the temperature range between 36 000 to 41 000 K for dwarfs and between 31 000 to 35 000 K for supergiants, where CMFGEN predicts much weaker lines. Consequences of these discrepancies are discussed.
Finally, suggestions are presented as to adequately parameterize model-grids for hot stars with winds, with only one additional parameter compared to standard grids from plane-parallel, hydrostatic models.Facultad de Ciencias AstronĂłmicas y GeofĂsica
Towards an understanding of the Of?p star HD 191612: optical spectroscopy
We present extensive optical spectroscopy of the early-type magnetic star HD
191612 (O6.5f?pe-O8fp). The Balmer and HeI lines show strongly variable
emission which is highly reproducible on a well-determined 538-d period. Metal
lines and HeII absorptions (including many selective emission lines but
excluding He II 4686A emission) are essentially constant in line strength, but
are variable in velocity, establishing a double-lined binary orbit with P(orb)
= 1542d, e=0.45. We conduct a model-atmosphere analysis of the primary, and
find that the system is consistent with a O8: giant with a B1: main-sequence
secondary. Since the periodic 538-d changes are unrelated to orbital motion,
rotational modulation of a magnetically constrained plasma is strongly favoured
as the most likely underlying `clock'. An upper limit on the equatorial
rotation is consistent with this hypothesis, but is too weak to provide a
strong constraint.Comment: Accepted for MNRA