151 research outputs found
Non-LTE Abundances of Magnesium, Aluminum and Sulfur in OB Stars Near the Solar Circle
Non-LTE abundances of magnesium, aluminum and sulfur are derived for a sample
of 23 low-v \sin i stars belonging to six northern OB associations of the
Galactic disk within 1 kpc of the Sun. The abundances are obtained from the
fitting of synthetic line profiles to high resolution spectra. A comparison of
our results with HII region abundances indicates good agreement for sulfur
while the cepheid abundances are higher. The derived abundances of Mg show good
overlap with the cepheid results. The aluminum abundances for OB stars are
significantly below the cepheid values. But, the OB star results show a
dependence with effective temperature and need further investigation. The high
Al abundances in the cepheids could be the result of mixing. A discussion of
the oxygen abundance in objects near the solar circle suggests that the current
mean galactic oxygen abundance in this region is 8.6-8.7 and in agreement with
the recently revised oxygen abundance in the solar photosphere. Meaningful
comparisons of the absolute S, Al and Mg abundances in OB stars with the Sun
must await a reinvestigation of these elements, as well as the meteoritic
reference element Si, with 3D hydrodynamical model atmospheres for the Sun. No
abundance gradients are found within the limited range in galactocentric
distances in the present study. Such variations would be expected only if there
were large metallicity gradients in the disk.Comment: 3 figures, accepted for publication in A&A, needs aa.st
Surface abundances of light elements for a large sample of early B-type stars - IV. The magnesium abundance in 52 stars - a test of metallicity
From high-resolution spectra a non-LTE analysis of the MgII 4481.2 A feature
is implemented for 52 early and medium local B stars on the main sequence (MS).
The influence of the neighbouring line AlIII 4479.9 A is considered. The
magnesium abundance is determined; it is found that log e(Mg) = 7.67 +- 0.21 on
average. It is shown that uncertainties in the microturbulent parameter Vt are
the main source of errors in log e(Mg). When using 36 stars with the most
reliable Vt values derived from OII and NII lines, we obtain the mean abundance
log e(Mg) = 7.59 +- 0.15. The latter value is precisely confirmed for several
hot B stars from an analysis of the MgII 7877 A weak line. The derived
abundance log e(Mg) = 7.59 +- 0.15 is in excellent agreement with the solar
magnesium abundance log e_sun(Mg) = 7.55 +- 0.02, as well as with the proto-Sun
abundance log e_ps(Mg) = 7.62 +- 0.02. Thus, it is confirmed that the Sun and
the B-type MS stars in our neighbourhood have the same metallicity.Comment: 9 pages, 6 figures. Has been accepted for publication at MNRA
Projected Rotational Velocities and Stellar Characterization of 350 B Stars in the Nearby Galactic Disk
Projected rotational velocities (vsini) are presented for a sample of 350
early B-type main sequence stars in the nearby Galactic disk. The stars are
located within ~1.5 kpc from the Sun, and the great majority within 700 pc. The
analysis is based on high-resolution spectra obtained with the MIKE
spectrograph on the Magellan Clay 6.5-m telescope at the Las Campanas
Observatory in Chile.Spectral types were estimated based on relative
intensities of some key line absorption ratios and comparisons to synthetic
spectra. Effective temperatures were estimated from the reddening-free Q index,
and projected rotational velocities were then determined via interpolation on a
published grid that correlates the synthetic full width at half maximum of the
He I lines at 4026, 4388 and 4471 A with vsini. As the sample has been selected
solely on the basis of spectral types it contains an selection of B stars in
the field, in clusters, and in OB associations. The vsini distribution obtained
for the entire sample is found to be essentially flat for vsini values between
0-150 km/s, with only a modest peak at low projected rotational velocities.
Considering subsamples of stars, there appears to be a gradation in the vsini
distribution with the field stars presenting a larger fraction of the slow
rotators and the cluster stars distribution showing an excess of stars with
vsini between 70 and 130 km/s. Furthermore, for a subsample of potential
runaway stars we find that the vsini distribution resembles the distribution
seen in denser environments, which could suggest that these runaway stars have
been subject to dynamical ejection mechanisms.Comment: 38 pages, 11 figures. Complete sample table. AJ accepte
Radial velocity measurements of B stars in the Scorpius-Centaurus association
We derive single-epoch radial velocities for a sample of 56 B-type stars
members of the subgroups Upper Scorpius, Upper Centaurus Lupus and Lower
Centaurus Crux of the nearby Sco-Cen OB association. The radial velocity
measurements were obtained by means of high-resolution echelle spectra via
analysis of individual lines. The internal accuracy obtained in the
measurements is estimated to be typically 2-3 km/s, but depends on the
projected rotational velocity of the target. Radial velocity measurements taken
for 2-3 epochs for the targets HD120307, HD142990 and HD139365 are variable and
confirm that they are spectroscopic binaries, as previously identified in the
literature. Spectral lines from two stellar components are resolved in the
observed spectra of target stars HD133242, HD133955 and HD143018, identifying
them as spectroscopic binaries.Comment: accepted for publication in A&
Chemical abundances of fast-rotating massive stars. I. Description of the methods and individual results
Aims: Recent observations have challenged our understanding of rotational
mixing in massive stars by revealing a population of fast-rotating objects with
apparently normal surface nitrogen abundances. However, several questions have
arisen because of a number of issues, which have rendered a reinvestigation
necessary; these issues include the presence of numerous upper limits for the
nitrogen abundance, unknown multiplicity status, and a mix of stars with
different physical properties, such as their mass and evolutionary state, which
are known to control the amount of rotational mixing. Methods: We have
carefully selected a large sample of bright, fast-rotating early-type stars of
our Galaxy (40 objects with spectral types between B0.5 and O4). Their
high-quality, high-resolution optical spectra were then analysed with the
stellar atmosphere modelling codes DETAIL/SURFACE or CMFGEN, depending on the
temperature of the target. Several internal and external checks were performed
to validate our methods; notably, we compared our results with literature data
for some well-known objects, studied the effect of gravity darkening, or
confronted the results provided by the two codes for stars amenable to both
analyses. Furthermore, we studied the radial velocities of the stars to assess
their binarity. Results: This first part of our study presents our methods and
provides the derived stellar parameters, He, CNO abundances, and the
multiplicity status of every star of the sample. It is the first time that He
and CNO abundances of such a large number of Galactic massive fast rotators are
determined in a homogeneous way.Comment: accepted for publication by A&
Radial abundance gradients in the outer Galactic disk as traced by main-sequence OB stars
Using a sample of 31 main-sequence OB stars located between galactocentric
distances 8.4 - 15.6 kpc, we aim to probe the present-day radial abundance
gradients of the Galactic disk. The analysis is based on high-resolution
spectra obtained with the MIKE spectrograph on the Magellan Clay 6.5-m
telescope on Las Campanas. We used a non-NLTE analysis in a self-consistent
semi-automatic routine based on TLUSTY and SYNSPEC to determine atmospheric
parameters and chemical abundances. Stellar parameters (effective temperature,
surface gravity, projected rotational velocity, microturbulence, and
macroturbulence) and silicon and oxygen abundances are presented for 28 stars
located beyond 9 kpc from the Galactic centre plus three stars in the solar
neighborhood. The stars of our sample are mostly on the main-sequence, with
effective temperatures between 20800 - 31300 K, and surface gravities between
3.23 - 4.45 dex. The radial oxygen and silicon abundance gradients are negative
and have slopes of -0.07 dex/kpc and -0.09 dex/kpc, respectively, in the region
\,kpc. The obtained gradients are compatible with the
present-day oxygen and silicon abundances measured in the solar neighborhood
and are consistent with radial metallicity gradients predicted by
chemodynamical models of Galaxy Evolution for a subsample of young stars
located close to the Galactic plane.Comment: Accepted for publication in the A&
Pristine CNO abundances from Magellanic Cloud B stars II. Fast rotators in the LMC cluster NGC 2004
We present spectroscopic abundance analyses of three main-sequence B stars in
the young Large Magellanic Cloud cluster NGC 2004. All three targets have
projected rotational velocities around 130 km/s. Techniques are presented that
allow the derivation of stellar parameters and chemical abundances in spite of
these high v sin i values. Together with previous analyses of stars in this
cluster, we find no evidence among the main-sequence stars for effects due to
rotational mixing up to v sin i around 130 km/s. Unless the equatorial
rotational velocities are significantly larger than the v sin i values, this
finding is probably in line with theoretical expectations. NGC 2004/B30, a star
of uncertain evolutionary status located in the Blue Hertzsprung Gap, clearly
shows signs of mixing in its atmosphere. To verify the effects due to
rotational mixing will therefore require homogeneous analysis of statistically
significant samples of low-metallicity main-sequence B stars over a wide range
of rotational velocities.Comment: 12 pages, 5 figures, 2 tables; accepted for publication in ApJ (vol.
633, p. 899
The Projected Rotational Velocity Distribution of a Sample of OB stars from a Calibration based on Synthetic He I lines
We derive projected rotational velocities (vsini) for a sample of 156
Galactic OB star members of 35 clusters, HII regions, and associations. The HeI
lines at 4026, 4388, and 4471A were analyzed in order to define
a calibration of the synthetic HeI full-widths at half maximum versus stellar
vsini. A grid of synthetic spectra of HeI line profiles was calculated in
non-LTE using an extensive helium model atom and updated atomic data. The
vsini's for all stars were derived using the He I FWHM calibrations but also,
for those target stars with relatively sharp lines, vsini values were obtained
from best fit synthetic spectra of up to 40 lines of CII, NII, OII, AlIII,
MgII, SiIII, and SIII. This calibration is a useful and efficient tool for
estimating the projected rotational velocities of O9-B5 main-sequence stars.
The distribution of vsini for an unbiased sample of early B stars in the
unbound association Cep OB2 is consistent with the distribution reported
elsewhere for other unbound associations.Comment: Accepted for publication in The Astronomical Journa
The first chemical abundance analysis of K giants in the inner Galactic disc
The elemental abundance structure of the Galactic disc has been extensively
studied in the solar neighbourhood using long-lived stars such as F and G
dwarfs or K and M giants. These are stars whose atmospheres preserve the
chemical composition of their natal gas clouds, and are hence excellent tracers
of the chemical evolution of the Galaxy. As far as we are aware, there are no
such studies of the inner Galactic disc, which hampers our ability to constrain
and trace the origin and evolution of the Milky Way. Therefore, we aim in this
study to establish the elemental abundance trend(s) of the disc(s) in the inner
regions of the Galaxy. Based on equivalent width measurements in
high-resolution spectra obtained with the MIKE spectrograph on the Magellan II
telescope on Las Campanas in Chile, we determine elemental abundances for 44
K-type red giant stars in the inner Galactic disc, located at Galactocentric
distances of 4-7\,kpc. The analysis method is identical to the one recently
used on red giant stars in the Galactic bulge and in the nearby thin and thick
discs, enabling us to perform a truly differential comparison of the different
stellar populations. We present the first detailed elemental abundance study of
a significant number of red giant stars in the inner Galactic disc. We find
that these inner disc stars show the same type of chemical and kinematical
dichotomy as the thin and thick discs show in the solar neighbourhood. The
abundance trends of the inner disc agree very well with those of the nearby
thick disc, and also to those of the Bulge. The chemical similarities between
the Bulge and the Galactic thick disc stellar populations indicate that they
have similar chemical histories, and any model trying to understand the
formation and evolution of either of the two should preferably incorporate both
of them.Comment: A&A Letters, accepte
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