900 research outputs found
On the abundance of Lithium in T Coronae Borealis
We have obtained high resolution echelle spectroscopy of the recurrent nova T
CrB. We find that the surface lithium abundance in T CrB is signifcantly
enhanced compared to field M giants, where it is not detectable. We offer
possible explanations for this in terms of either a delay in the onset of
convection in the giant star, enhanced coronal activity due to star-spots or
the enhancement of Li resulting from the nova explosion(s).Comment: 3 pages, 1 figure (a and b), accepted by MNRA
Atmospheric Analysis of the M/L- and M/T-Dwarf Binary Systems LHS 102 and Gliese 229
We present 0.9-2.5um spectroscopy with R~800 and 1.12-1.22um spectroscopy
with R~5800 for the M dwarfs Gl 229A and LHS 102A, and for the L dwarf LHS
102B. We also report IZJHKL' photometry for both components of the LHS 102
system, and L' photometry for Gl 229A. The data are combined with previously
published spectroscopy and photometry to produce flux distributions for each
component of the kinematically old disk M/L-dwarf binary system LHS 102 and the
kinematically young disk M/T-dwarf binary system Gliese 229. The data are
analyzed using synthetic spectra generated by the latest "AMES-dusty" and
"AMES-cond" models by Allard & Hauschildt. Although the models are not able to
reproduce the overall slope of the infrared flux distribution of the L dwarf,
most likely due to the treatment of dust in the photosphere, the data for the M
dwarfs and the T dwarf are well matched. We find that the Gl 229 system is
metal-poor despite having kinematics of the young disk, and that the LHS 102
system has solar metallicity. The observed luminosities and derived
temperatures and gravities are consistent with evolutionary model predictions
if the Gl 229 system is very young (age ~30 Myr) with masses (A,B) of
(0.38,>0.007)M(sun), and the LHS 102 system is older, aged 1-10 Gyr with masses
(A,B) of (0.19,0.07)M(sun).Comment: 29 pages incl. 13 figures and 5 tables;; accepted for publication in
MNRA
Spectral Types of Planetary Host Star Candidates: Two New Transiting Planets?
Recently, 46 low-luminosity object transits were reported from the Optical
Gravitational Lensing Experiment. Our follow-up spectroscopy of the 16 most
promising candidates provides a spectral classification of the primary.
Together with the radius ratio from the transit measurements, we derived the
radii of the low-luminosity companions. This allows to examine the possible
sub-stellar nature of these objects. Fourteen of them can be clearly identified
as low-mass stars. Two objects, OGLE-TR-03 and OGLE-TR-10 have companions with
radii of 0.15 R_sun which is very similar to the radius of the transiting
planet HD209458B. The planetary nature of these two objects should therefore be
confirmed by dynamical mass determinations.Comment: 4 pages, 3 figures, accepted for publication by A&A Letter
Spectral Energy Distributions for Disk and Halo M--Dwarfs
We have obtained infrared (1 to 2.5 micron) spectroscopy for 42 halo and disk
dwarfs with spectral type M1 to M6.5. These data are compared to synthetic
spectra generated by the latest model atmospheres of Allard & Hauschildt.
Photospheric parameters metallicity, effective temperature and radius are
determined for the sample. We find good agreement between observation and
theory except for known problems due to incomplete molecular data for metal
hydrides and water. The metal-poor M subdwarfs are well matched by the models
as oxide opacity sources are less important in this case. The derived effective
temperatures for the sample range from 3600K to 2600K; at these temperatures
grain formation and extinction are not significant in the photosphere. The
derived metallicities range from solar to one-tenth solar. The radii and
effective temperatures derived agree well with recent models of low mass stars.Comment: 24 pages including 13 figures, 4 Tables; accepted by Ap
Broad-band photometric colors and effective temperature calibrations for late-type giants. II. Z<0.02
(Abridged) We investigate the effects of metallicity on the broad-band
photometric colors of late-type giants, and make a comparison of synthetic
colors with observed photometric properties of late-type giants over a wide
range of effective temperatures (T_eff=3500-4800 K) and gravities (log
g=0.0-2.5), at [M/H]=-1.0 and -2.0. The influence of metallicity on the
synthetic photometric colors is generally small at effective temperatures above
\~3800 K, but the effects grow larger at lower T_eff, due to the changing
efficiency of molecule formation which reduces molecular opacities at lower
[M/H]. To make a detailed comparison of the synthetic and observed photometric
colors of late type giants in the T_eff--color and color--color planes, we
derive a set of new T_eff--log g--color relations based on synthetic
photometric colors, at [M/H]=-0.5, -1.0, -1.5, and -2.0. While differences
between the new T_eff--color relations and those available from the literature
are typically well within ~100 K, effective temperatures predicted by the
scales based on synthetic colors tend to be slightly higher than those
resulting from the T_eff--color relations based on observations, with the
offsets up to ~100 K. This is clearly seen both at [M/H]=-1.0 and -2.0,
especially in the T_eff--(B-V) and T_eff--(V-K) planes. The consistency between
T_eff--log g--color scales based on synthetic colors calculated with different
stellar atmosphere codes is very good, with typical differences being well
within \Delta T_eff~70 K at [M/H]=-1.0 and \Delta T_eff~40 K at [M/H]=-2.0.Comment: 20 pages, 11 figures, A&A accepte
Photometric colors of late-type giants: theory versus observations
To assess the current status in the theoretical modeling of the spectral
properties of late-type giants, we provide a comparison of synthetic
photometric colors of late-type giants (calculated with PHOENIX, MARCS and
ATLAS model atmospheres) with observations, at [M/H]=0.0 and -2.0. Overall,
there is a good agreement between observed and synthetic colors, and synthetic
colors and published Teff-color relations, both at [M/H]=0.0 and -2.0.
Deviations from the observed trends in Teff-color planes are generally within
\pm 150K (or less) in the effective temperature range Teff=3500-4800K.
Synthetic colors calculated with different stellar atmosphere models typically
agree to ~100K, within a large range of effective temperatures and gravities.
Some discrepancies are seen in the Teff-(B-V) plane below Teff~3800K at
[M/H]=0.0, due to difficulties in reproducing the 'turn-off' to the bluer
colors which is seen in the observed data at Teff~3600K. Note that at
[M/H]=-2.0 effective temperatures given by the scale of Alonso et al. (1999)
are generally lower than those resulting from other Teff-color relations based
both on observed and synthetic colors.Comment: 2 pages, 1 figure. Proceedings of the IAU Symposium 232 "The
Scientific Requirements for Extremely Large Telescopes", eds. P. Whitelock,
B. Leibundgut, and M. Dennefel
Broad-band photometric colors and effective temperature calibrations for late-type giants. I. Z=0.02
We present new synthetic broad-band photometric colors for late-type giants
based on synthetic spectra calculated with the PHOENIX model atmosphere code.
The grid covers effective temperatures T_eff=3000...5000K, gravities log
g=-0.5...+3.5, and metallicities [M/H]=+0.5...-4.0. We show that individual
broad-band photometric colors are strongly affected by model parameters such as
molecular opacities, gravity, microturbulent velocity, and stellar mass. Our
exploratory 3D modeling of a prototypical late-type giant shows that convection
has a noticeable effect on the photometric colors too, as it alters
significantly both the vertical and horizontal thermal structures in the outer
atmosphere. The differences between colors calculated with full 3D
hydrodynamical and 1D model atmospheres are significant (e.g., \Delta(V-K)~0.2
mag), translating into offsets in effective temperature of up to ~70K. For a
sample of 74 late-type giants in the Solar neighborhood, with interferometric
effective temperatures and broad-band photometry available in the literature,
we compare observed colors with a new PHOENIX grid of synthetic photometric
colors, as well as with photometric colors calculated with the MARCS and ATLAS
model atmosphere codes. (abridged)Comment: 30 pages, 21 figures, A&A in press. Table 2 can be obtained from the
CDS or directly from the author
Comparison of cloud models for Brown Dwarfs
A test case comparison is presented for different dust cloud model approaches
applied in brown dwarfs and giant gas planets. We aim to achieve more
transparency in evaluating the uncertainty inherent to theoretical modelling.
We show in how far model results for characteristic dust quantities vary due to
different assumptions. We also demonstrate differences in the spectral energy
distributions resulting from our individual cloud modelling in 1D substellar
atmosphere simulationsComment: 5 pages, Proceeding to "Exoplantes: Detection, Formation, Dynamics",
eds. Ferraz-Mello et
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