17 research outputs found
First Stellar Abundances in the Dwarf Irregular Galaxy Sextans A
We present the abundance analyses of three isolated A-type supergiant stars
in the dwarf irregular galaxy Sextans A from high-resolution spectra the UVES
spectrograph at the VLT. Detailed model atmosphere analyses have been used to
determine the stellar atmospheric parameters and the elemental abundances of
the stars. The mean iron group abundance was determined from these three stars
to be [(FeII,CrII)/H]=-0.99+/-0.04+/-0.06. This is the first determination of
the present-day iron group abundances in Sextans A. These three stars now
represent the most metal-poor massive stars for which detailed abundance
analyses have been carried out. The mean stellar alpha element abundance was
determined from the alpha element magnesium as
[alpha(MgI)/H]=-1.09+/-0.02+/-0.19. This is in excellent agreement with the
nebular alpha element abundances as determined from oxygen in the H II regions.
These results are consistent from star-to-star with no significant spatial
variations over a length of 0.8 kpc in Sextans A. This supports the nebular
abundance studies of dwarf irregular galaxies, where homogeneous oxygen
abundances are found throughout, and argues against in situ enrichment. The
alpha/Fe abundance ratio is [alpha(MgI)/FeII,CrII]=-0.11+/-0.02+/-0.10, which
is consistent with the solar ratio. This is consistent with the results from
A-supergiant analyses in other Local Group dwarf irregular galaxies but in
stark contrast with the high [alpha/Fe] results from metal-poor stars in the
Galaxy, and is most clearly seen from these three stars in Sextans A because of
their lower metallicities. The low [alpha/Fe] ratios are consistent with the
slow chemical evolution expected for dwarf galaxies from analyses of their
stellar populations.Comment: 40 pages, 8 figures, accepted for publication in A
Abundance analysis of two late A-type stars HD 32115 and HD 37594
We have performed abundance analysis of two slowly rotating, late A-type
stars, HD 32115 (HR 1613) and HD 37594 (HR 1940), based on obtained echelle
spectra covering the spectral range 4000-9850 AAngstrom. These spectra allowed
us to identify an extensive line list for 31 chemical elements, the most
complete to date for A-type stars. Two approaches to abundance analysis were
used, namely a ``manual'' (interactive) and a semi-automatic procedure for
comparison of synthetic and observed spectra and equivalent widths. For some
elements non-LTE (NLTE) calculations were carried out and the corresponding
corrections have been applied. The abundance pattern of HD 32115 was found to
be very close to the solar abundance pattern, and thus may be used as an
abundance standard for chemical composition studies in middle and late A stars.
Further, its H-alpha line profile shows no core-to-wing anomaly like that found
for cool Ap stars and therefore also may be used as a standard in comparative
studies of the atmospheric structures of cool, slowly rotating Ap stars. HD
37594 shows a metal deficiency at the level of -0.3 dex for most elements and
triangle-like cores of spectral lines. This star most probably belongs to the
Delta Scuti group.Comment: 10 pages, 4 figure
Abundance analysis of Am binaries and search for tidally driven abundance anomalies - III. HD116657, HD138213, HD155375, HD159560, HD196544 and HD204188
We continue here the systematic abundance analysis of a sample of Am binaries
in order to search for possible abundance anomalies driven by tidal interaction
in these binary systems. New CCD observations in two spectral regions
(6400-6500, 6660-6760 AA) of HD116657, HD138213, HD155375, HD159560, HD196544
and HD204188 were obtained. Synthetic spectrum analysis was carried out and
basic stellar properties, effective temperatures, gravities, projected
rotational velocities, masses, ages and abundances of several elements were
determined. We conclude that all six stars are Am stars. These stars were put
into the context of other Am binaries with 10 < Porb < 200 days and their
abundance anomalies discussed in the context of possible tidal effects. There
is clear anti-correlation of the Am peculiarities with v sin i. However, there
seems to be also a correlation with the eccentricity and may be with the
orbital period. The dependence on the temperature, age, mass, and
microturbulence was studied as well. The projected rotational velocities
obtained by us were compared to those of Royer et al. (2002) and Abt & Morrell
(1995).Comment: 11 pages, 3 tables, 12 figures. Accepted for publication in Monthly
Notices of the Royal Astronomical Societ
Do dusty A stars exhibit accretion signatures in their photospheres?
We determined abundances of O, Ca, Fe, Ba and Y for a sample of dusty and
dust-free A stars, taken from the list of Cheng et al. (1992). Five of the
stars have an infrared-excess due to circumstellar dust. Ongoing accretion from
their circumstellar surroundings might have modified the abundances in the
photospheres of these stars, but our results clearly show, that there is no
difference in the photospheric composition of the dusty and dust-free stars.
Instead all of them show the typical diffusion pattern which diminishes towards
larger rotational velocities.Comment: 8 pages, 3 figures, accepted for publication in A&
The solar photospheric nitrogen abundance. Analysis of atomic transitions with 3D and 1D model atmospheres
CONTEXT: In recent years, the solar chemical abundances have been studied in
considerable detail because of discrepant values of solar metallicity inferred
from different indicators, i.e., on the one hand, the "sub-solar" photospheric
abundances resulting from spectroscopic chemical composition analyses with the
aid of 3D hydrodynamical models of the solar atmosphere, and, on the other
hand, the high metallicity inferred by helioseismology. AIMS: After
investigating the solar oxygen abundance using a CO5BOLD 3D hydrodynamical
solar model in previous work, we undertake a similar approach studying the
solar abundance of nitrogen, since this element accounts for a significant
fraction of the overall solar metallicity, Z. METHOD: We used a selection of
atomic spectral lines to determine the solar nitrogen abundance, relying mainly
on equivalent width measurements in the literature. We investigate the
influence on the abundance analysis, of both deviations from local
thermodynamic equilibrium ("NLTE effects") and photospheric inhomogeneities
("granulation effects"). RESULTS: We recommend use of a solar nitrogen
abundance of A(N)=7.86+-0.12 whose error bar reflects the line-to-line scatter.
CONCLUSION: The solar metallicity implied by the CO5BOLD-based nitrogen and
oxygen abundances is in the range 0.0145<= Z <= 0.0167. This result is a step
towards reconciling photospheric abundances with helioseismic constraints on Z.
Our most suitable estimates are Z=0.0156 and Z/X=0.0213.Comment: To be published on A&
The solar-like CoRoT target HD 170987: spectroscopic and seismic observations
The CoRoT mission is in its third year of observation and the data from the
second long run in the galactic centre direction are being analysed. The
solar-like oscillating stars that have been observed up to now have given some
interesting results, specially concerning the amplitudes that are lower than
predicted. We present here the results from the analysis of the star HD
170987.The goal of this research work is to characterise the global parameters
of HD 170987. We look for global seismic parameters such as the mean large
separation, maximum amplitude of the modes, and surface rotation because the
signal-to-noise ratio in the observations do not allow us to measure individual
modes. We also want to retrieve the stellar parameters of the star and its
chemical composition.We have studied the chemical composition of the star using
ground-based observations performed with the NARVAL spectrograph. We have used
several methods to calculate the global parameters from the acoustic
oscillations based on CoRoT data. The light curve of the star has been
interpolated using inpainting algorithms to reduce the effect of data gaps. We
find power excess related to p modes in the range [400 - 1200]muHz with a mean
large separation of 55.2+-0.8muHz with a probability above 95% that increases
to 55.9 +-0.2muHz in a higher frequency range [500 - 1250] muHz and a rejection
level of 1%. A hint of the variation of this quantity with frequency is also
found. The rotation period of the star is estimated to be around 4.3 days with
an inclination axis of i=50 deg +20/-13. We measure a bolometric amplitude per
radial mode in a range [2.4 - 2.9] ppm around 1000 muHz. Finally, using a grid
of models, we estimate the stellar mass, M=1.43+-0.05 Msun, the radius,
R=1.96+-0.046 Rsun, and the age ~2.4 Gyr.Comment: 12 pages, 15 figures, accepted for publication in A&
DETECTION OF SOLAR-LIKE OSCILLATIONS, OBSERVATIONAL CONSTRAINTS, AND STELLAR MODELS FOR θ CYG, THE BRIGHTEST STAR OBSERVED BY THE KEPLER MISSION
θ Cygni is an F3 spectral type magnitude V = 4.48 main-sequence star that was the brightest star observed by the original Kepler spacecraft mission. Short-cadence (58.8 s) photometric data using a custom aperture were first obtained during Quarter 6 (2010 June–September) and subsequently in Quarters 8 and 12–17. We present analyses of solar-like oscillations based on Q6 and Q8 data, identifying angular degree l = 0, 1, and 2 modes with frequencies of 1000–2700 μHz, a large frequency separation of 83.9 ± 0.4 μHz, and maximum oscillation amplitude at frequency νmax = 1829 ± 54 μHz. We also present analyses of new ground-based spectroscopic
observations, which, combined with interferometric angular diameter measurements, give Teff = 6697 ± 78 K, radius 1.49 ± 0.03 Re, [Fe/H] = −0.02 ± 0.06 dex, and log g = 4.23 ± 0.03. We calculate stellar models matching these constraints using the Yale Rotating Evolution Code and the Asteroseismic Modeling Portal. The best-fit models have masses of 1.35–1.39 Me and ages of 1.0–1.6 Gyr. θ Cyg’s Teff and log g place it cooler than the red edge of the γ Doradus instability region established from pre-Kepler ground-based observations, but just at the red edge derived from pulsation modeling. The pulsation models show γ Dor gravity modes driven by the
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convective blocking mechanism, with frequencies of 1–3 cycles per day (11 to 33 μHz). However, gravity modes were not seen in Kepler data; one signal at 1.776 cycles per day (20.56 μHz) may be attributable to a faint, possibly background, binary