22 research outputs found
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&
Field Blue Stragglers and Related Mass Transfer Issues
This chapter contains my impressions and perspectives about the current state
of knowledge about field blue stragglers (FBS) stars, drawn from an extensive
literature that I searched. I conclude my review of issues that attend FBS and
mass transfer, by a brief enumeration of a few mildly disquieting observational
facts.Comment: Chapter 4, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G.
Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe
Multi-wavelength polarimetric study towards the open cluster NGC 1893
We present multi-wavelength linear polarimetric observations for 44 stars of
the NGC 1893 young open cluster region along with V-band polarimetric
observations of stars of other four open clusters located between l ~160 to
~175 degree. We found evidence for the presence of two dust layers located at a
distance of ~170 pc and ~360 pc. The dust layers produce a polarization Pv
~2.2%. It is evident from the clusters studied in the present work that, in the
Galactic longitude range l ~160 to 175 degree and within the Galactic plane
(|b| < 2 degree), the polarization angles remain almost constant, with a mean
~163 degree and a dispersion of 6 degree. The small dispersion in polarization
angle could be due to the presence of uniform dust layer beyond 1 kpc. Present
observations reveal that in case of NGC 1893, the foreground two dust layers,
in addition to the intracluster medium, seems to be responsible for the
polarization effects. It is also found that towards the direction of NGC 1893,
the dust layer that exists between 2-3 kpc has a negligible contribution
towards the total observed polarization. The weighted mean for percentage of
polarization (Pmax) and the wavelength at maximum polarization ({\lambda}max)
are found to be 2.59 \pm 0.02% and 0.55 \pm 0.01 \mum respectively. The
estimated mean value of {\lambda}max indicates that the average size of the
dust grains within the cluster is similar to that in the general interstellar
medium. The spatial variation of the polarization is found to decrease towards
the outer region of the cluster. In the present work, we support the notion, as
already has been shown in previous studies, that polarimetry, in combination
with (U-B)/(B-V) colour-colour diagram, is a useful tool for identifying
non-members in a cluster.Comment: 36 pages, 12 figures, 10 tables, accepted for the publication in
MNRA
Projected Rotational Velocities of 136 Early B-type Stars in the Outer Galactic Disk
We have determined projected rotational velocities, v sin i, from Magellan/MIKE echelle spectra for a sample of 136 early B-type stars having large Galactocentric distances. The target selection was done independently of their possible membership in clusters, associations or field stars. We subsequently examined the literature and assigned each star as Field, Association, or Cluster. Our v sin i results are consistent with a difference in aggregate v sin i with stellar density. We fit bimodal Maxwellian distributions to the Field, Association, and Cluster subsamples representing sharp-lined and broad-lined components. The first two distributions, in particular, for the Field and Association are consistent with strong bimodality in v sin i. Radial velocities are also presented, which are useful for further studies of binarity in B-type stars, and we also identify a sample of possible new double-lined spectroscopic binaries. In addition, we find 18 candidate Be stars showing emission at Ha
A Spectroscopic Study of OB Stars Located in the Outer Galactic Disk
<p>Poster to be presented on IAU Symposium "<em>New WIndows on Massive Stars</em>".</p>
<p>Â </p>
<p><strong>Abstract</strong></p>
<p>Abundance gradients are generally observed in all galaxies, with their metallicities decreasing outwards from the galactic centers. The abundance slopes for different elements may vary depending on the galactic morphology. In our Galaxy, the radial abundance gradients remain somewhat uncertain on the outer parts of the Galactic disk, due to the small number of studied stars located beyond the Perseus Arm. The aim of this study is to provide a large abundance database of distant stars in order to better constrain the Galactic gradient in the Outer Disk. We have obtained high-resolution, echelle spectra for a sample of 137 OB stars located towards the Galactic anticenter using the 6.5m Magellan Clay telescope + MIKE spectrograph. A subsample of 50 single stars presenting sharp absorption lines has been selected to carry out the abundance analysis. Stellar parameters and silicon abundances are derived consistently based on non-LTE synthesis of hydrogen, helium, and silicon lines, requiring ionization balance of SiII/III/IV and O I/II/III. In this work, we present preliminary abundance results of our analysis.</p>
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<p><strong>Acknowledgements</strong></p>
<p>We acknowledge financial support of National Science Foundation, Coordenação de Aperfeiçoamento de Pessoal de NÃvel Superior</p>
<p> and the International Astronomomic Union. This research has made use of the SIMBAD database,<br>operated at CDS, Strasbourg, France, and of Astropy, a community-developed core Python package for Astronomy (Astropy Collaboration, 2013).</p