630 research outputs found
Evolution and appearance of Be stars in SMC clusters
Star clusters are privileged laboratories for studying the evolution of
massive stars (OB stars). One particularly interesting question concerns the
phases, during which the classical Be stars occur, which unlike HAe/Be stars,
are not pre-main sequence objects, nor supergiants. Rather, they are extremely
rapidly rotating B-type stars with a circumstellar decretion disk formed by
episodic ejections of matter from the central star. To study the impact of
mass, metallicity, and age on the Be phase, we observed SMC open clusters with
two different techniques: 1) with the ESO-WFI in its slitless mode, which
allowed us to find the brighter Be and other emission-line stars in 84 SMC open
clusters 2) with the VLT-FLAMES multi-fiber spectrograph in order to determine
accurately the evolutionary phases of Be stars in the Be-star rich SMC open
cluster NGC 330. Based on a comparison to the Milky Way, a model of Be stellar
evolution / appearance as a function of metallicity and mass / spectral type is
developed, involving the fractional critical rotation rate as a key parameter.Comment: Proceedings of the IAUS266 of the GA200
Differential rotation in early type stars
Using 2D models of rotating stars, the interferometric measurements of alpha
Eri and its fundamental parameters corrected for gravitational darkening
effects we infer that the star might have a core rotating 2.7 times faster than
the surface. We explore the consequences on spectral lines produced by surface
differential rotation combined with the effects due to a kind of internal
differential rotation with rotational energies higher than allowed for rigid
rotation which induce geometrical deformations that do not distinguish strongly
from those carried by the rigid rotation.Comment: 3 pages ; to appear in the proceedings of the Sapporo meeting on
active OB stars ; ASP Conference Series ; eds: S. Stefl, S. Owocki and A.
Okazak
Circumstellar rings, flat and flaring discs
Emission lines formed in the circumstellar envelopes of several type of stars
can be modeled using first principles of line formation. We present simple ways
of calculating line emission profiles formed in circumstellar envelopes having
different geometrical configurations. The fit of the observed line profiles
with the calculated ones may give first order estimates of the physical
parameters characterizing the line formation regions: opacity, size, particle
density distribution, velocity fields, excitation temperature.Comment: 3 pages ; to appear in the proceedings of the Sapporo meeting on
active OB stars ; ASP Conference Series ; eds: S. Stefl, S. Owocki and A.
Okazak
Young open clusters in the Milky Way and Small Magellanic Cloud
NGC6611, Trumpler 14, Trumpler 15, Trumpler 16, Collinder 232 are very young
open clusters located in star-formation regions of the Eagle Nebula or the
Carina in the MW, and NGC346 in the SMC. With different instrumentations and
techniques, it was possible to detect and classify new Herbig Ae/Be stars,
classical Be stars and to provide new tests / comparisons about the Be stars
appearance models. Special stars (He-strong) of these star-formation regions
are also presented.Comment: Proceedings IAUS266 at the IAU-GA 200
Rotation in the ZAMS: Be and Bn stars
We show that Be stars belong to a high velocity tail of a single B-type star
rotational velocity distribution in the MS. This implies that: 1) the number
fraction N(Be)/N(Be+B) is independent of the mass; 2) Bn stars having ZAMS
rotational velocities higher than a given limit might become Be stars.Comment: 3 pages ; to appear in the proceedings of the Sapporo meeting on
active OB stars ; ASP Conference Series ; eds: S. Stefl, S. Owocki and A.
Okazak
Metallicity vs. Be phenomenon relation in the solar neighborhood
Fast rotation seems to be the mayor factor to trigger the Be phenomenon.
Surface fast rotation can be favored by initial formation conditions, such as
abundance of metals. We have observed 118 Be stars up to the apparent
magnitudes V=9 mag. Models of fast rotating atmospheres and evolutionary tracks
were used to interpret the stellar spectra and to determine the stellar
fundamental parameters. Since the studied stars are formed in regions that are
separated enough to imply some non negligible gradient of galactic metallicity,
we study the effects of possible incidence of this gradient on the nature as
rotators of the studied stars.Comment: 3 pages ; to appear in the proceedings of the Sapporo meeting on
active OB stars ; ASP Conference Series ; eds: S. Stefl, S. Owocki and A.
Okazak
Spectra disentangling applied to the Hyades binary Theta^2 Tau AB: new orbit, orbital parallax and component properties
Theta^2 Tauri is a detached and single-lined interferometric-spectroscopic
binary as well as the most massive binary system of the Hyades cluster. The
system revolves in an eccentric orbit with a periodicity of 140.7 days. The
secondary has a similar temperature but is less evolved and fainter than the
primary. It is also rotating more rapidly. Since the composite spectra are
heavily blended, the direct extraction of radial velocities over the orbit of
component B was hitherto unsuccessful. Using high-resolution spectroscopic data
recently obtained with the Elodie (OHP, France) and Hermes (ORM, La Palma,
Spain) spectrographs, and applying a spectra disentangling algorithm to three
independent data sets including spectra from the Oak Ridge Observatory (USA),
we derived an improved spectroscopic orbit and refined the solution by
performing a combined astrometric-spectroscopic analysis based on the new
spectroscopy and the long-baseline data from the Mark III optical
interferometer. As a result, the velocity amplitude of the fainter component is
obtained in a direct and objective way. Major progress based on this new
determination includes an improved computation of the orbital parallax. Our
mass ratio is in good agreement with the older estimates of Peterson et al.
(1991, 1993), but the mass of the primary is 15-25% higher than the more recent
estimates by Torres et al. (1997) and Armstrong et al. (2006). Due to the
strategic position of the components in the turnoff region of the cluster,
these new determinations imply stricter constraints for the age and the
metallicity of the Hyades cluster. The location of component B can be explained
by current evolutionary models, but the location of the more evolved component
A is not trivially explained and requires a detailed abundance analysis of its
disentangled spectrum.Comment: in press, 13 pages, 10 Postscript figures, 5 tables. Table~4 is
available as online material. Keywords: astrometry - techniques: high angular
resolution - stars: binaries: visual - stars: binaries: spectroscopic -
stars: fundamental parameter
A semi-automatic procedure for abundance determination of A- and F-type stars
A variety of physical processes leading to different types of pulsations and
chemical compositions is observed among A- and F-type stars. To investigate the
underlying mechanisms responsible for these processes in stars with similar
locations in the H-R diagram, an accurate abundance determination is needed,
among others. Here, we describe a semi-automatic procedure developed to
determine chemical abundances of various elements ranging from helium to
mercury for this type of stars. We test our procedure on synthetic spectra,
demonstrating that our procedure provides abundances consistent with the input
values, even when the stellar parameters are offset by reasonable observational
errors. For a fast-rotating star such as Vega, our analysis is consistent with
those carried out with other plane-parallel model atmospheres. Simulations show
that the offsets from the input abundances increase for stars with low
inclination angle of about 4 degrees. For this inclination angle, we also show
that the distribution of the iron abundance found in different regions is
bimodal. Furthermore, the effect of rapid rotation can be seen in the peculiar
behaviour of the H_beta line.Comment: accepted for publication in MNRAS, contains 6 tables and 8 figure
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