383 research outputs found
Candidate exoplanet host HD131399A: a nascent Am star
Direct imaging suggests that there is a Jovian exoplanet around the primary
A-star in the triple-star system HD131399. We investigate a high-quality
spectrum of the primary component HD131399A obtained with FEROS on the ESO/MPG
2.2m telescope, aiming to characterise the star's atmospheric and fundamental
parameters, and to determine elemental abundances at high precision and
accuracy. The aim is to constrain the chemical composition of the birth cloud
of the system and therefore the bulk composition of the putative planet. A
hybrid non-local thermal equilibrium (non-LTE) model atmosphere technique is
adopted for the quantitative spectral analysis. Comparison with the most recent
stellar evolution models yields the fundamental parameters. The atmospheric and
fundamental stellar parameters of HD131399A are constrained to Teff=9200+-100
K, log g=4.37+-0.10, M=1.95+0.08-0.06 Msun, R=1.51+0.13-0.10 Rsun, and log
L/Lsun=1.17+-0.07, locating the star on the zero-age main sequence. Non-LTE
effects on the derived metal abundances are often smaller than 0.1dex, but can
reach up to ~0.8dex for individual lines. The observed lighter elements up to
calcium are overall consistent with present-day cosmic abundances, with a C/O
ratio of 0.450.07 by number, while the heavier elements show mild
overabundances. We conclude that the birth cloud of the system had a standard
chemical composition, but we witness the onset of the Am phenomenon in the
slowly rotating star. We furthermore show that non-LTE analyses have the
potential to solve the remaining discrepancies between observed abundances and
predictions by diffusion models for Am stars. Moreover, the present case allows
mass loss, not turbulent mixing, to be identified as the main transport process
competing with diffusion in very young Am stars.Comment: 5 pages + 3 pages appendix, 3 figures. Accepted for publication in
A&
The Puzzling Spectrum of HD 94509
The spectral features of HD 94509 are highly unusual, adding an extreme to
the zoo of Be and shell stars. The shell dominates the spectrum, showing lines
typical for spectral types mid-A to early-F, while the presence of a late/mid
B-type central star is indicated by photospheric hydrogen line wings and helium
lines. Numerous metallic absorption lines have broad wings but taper to narrow
cores. They cannot be fit by Voigt profiles.
We aim to describe and illustrate unusual spectral features of this star, and
make rough calculations to estimate physical conditions and abundances in the
shell. Furthermore, the central star is characterized.
We assume mean conditions for the shell. An electron density estimate is made
from the Inglis-Teller formula. Excitation temperatures and column densities
for Fe I and Fe II are derived from curves of growth. The neutral H column
density is estimated from high Paschen members. The column densities are
compared with calculations made with the photoionization code Cloudy.
Atmospheric parameters of the central star are constrained employing non-LTE
spectrum synthesis.
Overall chemical abundances are close to solar. Column densities of the
dominant ions of several elements, as well as excitation temperatures and the
mean electron density are well accounted for by a simple model. Several
features, including the degree of ionization, are less well described.
HD 94509 is a Be star with a stable shell, close to the terminal-age main
sequence. The dynamical state of the shell and the unusually shaped, but
symmetric line profiles, require a separate study.Comment: 10 pages, 9 tables, 13 figures; accepted for publication by Astronomy
and Astrophysic
Testing common classical LTE and NLTE model atmosphere and line-formation codes for quantitative spectroscopy of early-type stars
It is generally accepted that the atmospheres of cool/lukewarm stars of
spectral types A and later are described well by LTE model atmospheres, while
the O-type stars require a detailed treatment of NLTE effects. Here model
atmosphere structures, spectral energy distributions and synthetic spectra
computed with ATLAS9/SYNTHE and TLUSTY/SYNSPEC, and results from a hybrid
method combining LTE atmospheres and NLTE line-formation with DETAIL/SURFACE
are compared. Their ability to reproduce observations for effective
temperatures between 15000 and 35000 K are verified. Strengths and weaknesses
of the different approaches are identified. Recommendations are made as to how
to improve the models in order to derive unbiased stellar parameters and
chemical abundances in future applications, with special emphasis on Gaia
science.Comment: 12 pages, 8 figures; accepted for publication in Journal of Physics:
Conference Series, GREAT-ESF Workshop: Stellar Atmospheres in the Gaia Er
The chemical composition of the Orion star forming region: stars, gas and dust
We present a summary of main results from the studies performed in the series
of papers "The chemical composition of the Orion star forming region". We
reinvestigate the chemical composition of B-type stars in the Orion OB1
association by means of state-of-the-art stellar atmosphere codes, atomic
models and techniques, and compare the resulting abundances with those obtained
from the emission line spectra of the Orion nebula (M42), and recent
determinations of the Solar chemical composition.Comment: 5 pages, 4 figures, 2 tables. Poster contribution to the proceedings
of the LIAC2010 conference "The multi-wavelength view of hot, massive stars
Nitrogen Enrichment in Atmospheres of A- and F- Type Supergiants
Using new accurate fundamental parameters of 30 Galactic A and F supergiants,
namely their effective temperatures Teff and surface gravities log g, we
implemented a non-LTE analysis of the nitrogen abundance in their atmospheres.
It is shown that the non-LTE corrections to the N abundances increase with
Teff. The nitrogen overabundance as a general feature of this type of stars is
confirmed. A majority of the stars has a nitrogen excess [N/Fe] between 0.2 and
0.9 dex with the maximum position of the star's distribution on [N/Fe] between
0.4 and 0.7 dex. The N excesses are discussed in light of predictions for
B-type main sequence (MS) stars with rotationally induced mixing and for their
next evolutionary phase, i.e. A- and F-type supergiants that have experienced
the first dredge-up. Rotationally induced mixing in the MS progenitors of the
supergiants may be a significant cause of the nitrogen excesses. When comparing
our results with predictions of the theory developed for stars with the mixing,
we find that the bulk of the supergiants (28 of 30) show the N enrichment that
can be expected (i) either after the MS phase for stars with the initial
rotational velocities v0 = 200-400 km s-1, (ii) or after the first dredge-up
for stars with v0 = 50-400 km s-1. The latter possibility is preferred on
account of the longer lifetime for stars on red-blue loops following the first
dredge-up. Two supergiants without a discernible N enrichment, namely HR 825
and HR 7876, may be post-MS objects with the relatively low initial rotational
velocity of about 100 km s-1. The suggested range for v0 is approximately
consistent with inferences from the observed projected rotational velocities of
B-type MS stars, progenitors of A and F supergiants.Comment: 14 pages, 13 figure
Extreme helium stars: non-LTE matters Helium and hydrogen spectra of the unique objects V652 Her and HD144941
Quantitative analyses of low-mass hydrogen-deficient (super-)giant stars -
so-called extreme helium stars - to date face two major difficulties. First,
theory fails to reproduce the observed helium lines in their entirety, wings
and line cores. Second, a general mismatch exists for effective temperatures
derived from ionization equilibria and from spectral energy distributions.
Here, we demonstrate how the issue can be resolved using state-of-the-art
non-LTE line-formation for these chemically peculiar objects. Two unique
high-gravity B-type objects are discussed in detail, the pulsating variable
V652 Her and the metal-poor star HD144941. In the first case atmospheric
parameters from published LTE analyses are largely recovered, in the other a
systematic offset is found. Hydrogen abundances are systematically smaller than
previously reported, by up to a factor ~2. Extreme helium stars turn out to be
important testbeds for non-LTE model atoms for helium. Improved non-LTE
computations show that analyses assuming LTE or based on older non-LTE model
atoms can predict equivalent widths, for the HeI 10830A transition in
particular, in error by up to a factor ~3.Comment: 4 pages, 5 figures; accepted for publication in A&
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