491 research outputs found
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
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
Open clusters: III. Fundamental parameters of B stars in NGC 6087, NGC 6250, NGC 6383 and NGC 6530. B type stars with circumstellar envelopes
Context. Stellar physical properties of star clusters are poorly known. Aims.
Our goals are to perform a spectrophotometric study of the B star population in
open clusters to derive accurate stellar parameters, search for the presence of
circumstellar envelopes, and discuss the characteristics of these stars.
Methods. The BCD spectrophotometric system is a powerful tool to obtain
fundamental parameters and infer the main properties of open clusters: distance
modulus, color excess, and age. We inspected the Balmer discontinuity to seek
circumstellar disks and identify Be-star candidates. High resolution spectra in
the H region are used to confirm the Be nature. Results. We provide
Teff , log g, Mv , Mbol and spectral types for a sample of 68 stars in the
field of the open clusters NGC 6087, NGC 6250, NGC 6383, and NGC 6530, as well
as the cluster distances, ages and reddening. Then, based on a sample of 230 B
stars in the direction of the 11 open clusters studied along this series of
three papers, we report 6 new Be stars, 4 blue straggler candidates, and 15
B-type stars (called Bdd) with a double Balmer discontinuity. We also find that
the majority of the Be stars are dwarfs and present a maximum at the spectral
type B2-B4 in young and intermediate-age open clusters. Another maximum of Be
stars is observed at the spectral type B6-B8 in open clusters older than 40
Myr, where the population of Bdd stars also becomes relevant. Conclusions. Our
results support previous statements that the Be phenomenon is present along the
whole main sequence band and occurs in very different evolutionary states. We
find clear evidence of an increase of stars with circumstellar envelopes with
cluster age. The Be phenomenon reaches its maximum in clusters of intermediate
age and the number of B stars with circumstellar envelopes (Be+Bdd stars) is
also high for the older clusters
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
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
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
Rotational velocities of A-type stars II. Measurement of vsini in the northern hemisphere
This work is the second part of the set of measurements of vsini for A-type
stars, begun by Royer et al. (2002). Spectra of 249 B8 to F2-type stars
brighter than V=7 have been collected at Observatoire de Haute-Provence (OHP).
Fourier transforms of several line profiles in the range 4200--4600 A are used
to derive vsini from the frequency of the first zero. Statistical analysis of
the sample indicates that measurement error mainly depends on vsini and this
relative error of the rotational velocity is found to be about 5% on average.
The systematic shift with respect to standard values from Slettebak et al.
(1975), previously found in the first paper, is here confirmed. Comparisons
with data from the literature agree with our findings: vsini values from
Slettebak et al. are underestimated and the relation between both scales
follows a linear law: vsini(new) = 1.03 vsini(old) + 7.7. Finally, these data
are combined with those from the previous paper (Royer et al. 2002), together
with the catalogue of Abt & Morrell (1995). The resulting sample includes some
2150 stars with homogenized rotational velocities.Comment: 16 pages, includes 13 figures, accepted in A&
On the evolutionary status of Be stars. I. Field Be stars near the Sun
A sample of 97 galactic field Be stars were studied by taking into account
the effects induced by the fast rotation on their fundamental parameters. All
program stars were observed in the BCD spectrophotometric system in order to
minimize the perturbations produced by the circumstellar environment on the
spectral photospheric signatures. This is one of the first attempts at
determining stellar masses and ages by simultaneously using model atmospheres
and evolutionary tracks, both calculated for rotating objects. The stellar ages
() normalized to the respective inferred time that each rotating star can
spend in the main sequence phase () reveal a mass-dependent
trend. This trend shows that: a) there are Be stars spread over the whole
interval 0 \la \tau/\tau\_{\rm MS} \la 1 of the main sequence evolutionary
phase; b) the distribution of points in the () diagram indicates that in massive stars (M \ga
12M\_{\odot}) the Be phenomenon is present at smaller
age ratios than for less massive stars (M \la 12M\_{\odot}). This
distribution can be due to: ) higher mass-loss rates in massive objets,
which can act to reduce the surface fast rotation; ) circulation time
scales to transport angular momentum from the core to the surface, which are
longer the lower the stellar mass.Comment: 18 pages, 6 figures, A&A, in pres
Physical parameters of IPHAS-selected classical Be stars. (I. Determination procedure and evaluation of the results.)
Reproduced with permission from Astronomy & Astrophysics, © 2016 ESOWe present a semi-automatic procedure to obtain fundamental physical parameters and distances of classical Be (CBe) stars, based on the Barbier-Chalonge-Divan (BCD) spectrophotometric system. Our aim is to apply this procedure to a large sample of CBe stars detected by the IPHAS photometric survey, to determine their fundamental physical parameters and to explore their suitability as galactic structure tracers. In this paper we describe the methodology used and the validation of the procedure by comparing our results with those obtained from different independent astrophysical techniques for subsamples of stars in common with other studies. We also present a test case study of the galactic structure in the direction of the Perseus Galactic Arm, in order to compare our results with others recently obtained with different techniques and the same sample of stars. We did not find any significant clustering of stars at the expected positions of the Perseus and Outer Galactic Arms, in agreement with previous studies in the same area that we used for verification.Peer reviewedFinal Published versio
Fundamental parameters of Be stars located in the seismology fields of COROT
In preparation for the COROT space mission, we determined the fundamental
parameters (spectral type, temperature, gravity, vsini) of the Be stars
observable by COROT in its seismology fields (64 Be stars). We applied a
careful and detailed modeling of the stellar spectra, taking into account the
veiling caused by the envelope, as well as the gravitational darkening and
stellar flattening due to rapid rotation. Evolutionary tracks for fast rotators
were used to derive stellar masses and ages. The derived parameters will be
used to select Be stars as secondary targets (i.e. observed for 5 consecutive
months) and short-run targets of the COROT mission. Furthermore, we note that
the main part of our stellar sample is falling in the second half of the main
sequence life time, and that in most cases the luminosity class of Be stars is
inaccurate in characterizing their evolutionary status.Comment: 25 pages, 9 figures, Accepted for publication in A&
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