1,114 research outputs found
On the Frequency of Field Galactic Be Stars
Since Be stars belong to the high velocity tail of a single B star velocity
distribution, the appearance of the Be phenomenon must be independent of the
stellar mass. In the present paper we show that the shape of the distribution
of the number fraction N(Be)/N(Be+B) against the spectral type can be explained
in terms of the Balmer line emission efficiency as a function of the effective
temperature.Comment: Societe Francaise d'Astronomie et d'Astrophysique, France (2005
ZAMS rotational velocities of Be/Oe stars and LGRBs progenitors in the Magellanic Clouds
The Large and Small Magellanic Clouds are priviledged environments to perform
tests of theoretical predictions at low metallicity on rotational velocities
and stellar evolution. According to theoretical predictions, the rotational
velocities of B-type stars are expected to be higher in low metallicity
(LMC/SMC) than in high metallicity (MW) environments. To verify the models, we
observed with the VLT-FLAMES 523 B and Be stars, which form, at the moment, the
largest observed sample of these kind of objects in the MCs. We first
determined the stellar fundamental parameters and we found that B and Be stars
rotate faster in the MCs than in the MW. We also determined the first
distribution of the average ZAMS rotational velocities versus the mass of Be
stars. These results indicate that the appearance of Be stars is mass-,
metallicity-, stellar evolution-, and star-formation regions-dependant.
Moreover, the recent models of Long Gamma Ray Bursts progenitors foresee
possible LGRBs progenitors at the SMC's metallicity. We confront these models
with the observed (ZAMS rotational velocities, masses) distributions of the
fastest rotators (Be and Oe stars) in our sample. Furthermore, we compare the
corresponding predicted rates from our study with observed rates of LGRBs.Comment: poster IAUS25
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
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
Can massive Be/Oe stars be progenitors of long gamma ray bursts?
Context: The identification of long-gamma-ray-bursts (LGRBs) is still
uncertain, although the collapsar engine of fast-rotating massive stars is
gaining a strong consensus. Aims: We propose that low-metallicity Be and Oe
stars, which are massive fast rotators, as potential LGRBs progenitors.
Methods: We checked this hypothesis by 1) testing the global specific angular
momentum of Oe/Be stars in the ZAMS with the SMC metallicity, 2) comparing the
ZAMS () parameters of these stars with the
area predicted theoretically for progenitors with metallicity , and 3)
calculating the expected rate of LGRBs/year/galaxy and comparing them with the
observed ones. To this end, we determined the ZAMS linear and angular
rotational velocities for SMC Be and Oe stars using the observed vsini
parameters, corrected from the underestimation induced by the gravitational
darkening effect. Results: The angular velocities of SMC Oe/Be stars are on
average in the ZAMS. These velocities are in the
area theoretically predicted for the LGRBs progenitors. We estimated the yearly
rate per galaxy of LGRBs and the number of LGRBs produced in the local Universe
up to z=0.2. We have considered that the mass range of LGRB progenitors
corresponds to stars hotter than spectral types B0-B1 and used individual
beaming angles from 5 to 15\degr. We thus obtain to LGRBs/year/galaxy, which represents on
average 2 to 14 LGRB predicted events in the local Universe during the past 11
years. The predicted rates could widely surpass the observed ones
[(0.2-3) LGRBs/year/galaxy; 8 LGRBs observed in the local
Universe during the last 11 years] if the stellar counts were made from the
spectral type B1-B2, in accordance with the expected apparent spectral types of
the appropriate massive fast rotators. Conclusion: We conclude that the massive
Be/Oe stars with SMC metallicity could be LGRBs progenitors. Nevertheless,
other SMC O/B stars without emission lines, which have high enough specific
angular momentum, can enhance the predicted rate
The spectrum of the B[e] star BAL224
We present optical spectroscopy of the emission line star BAL 224 (V=17.3,
B-V=0.46). This star also named KWBBE 485, [MA93]906 is located at the
periphery of the young SMC cluster NGC 330; it is known as a photometric
variable with a possible period around 1 day (Balona 1992). Furthermore it was
reported as the optical counterpart of the prominent mid-infrared source (MIR1)
by Kucinskas et al. (2000), indicating the presence of a dust shell. The star
was included in a sample of B-type stars observed using the ESO VLT-FLAMES
facilities. The presence of emission lines such as Fe II,[Fe II], [S II] make
this object like a B[e] star. The H, H and H lines show
an asymmetrical double-peaked emission profile suggesting the presence of an
accretion disk. Moreover the MACHO and OGLE light curves were analyzed; in
addition to a long-term variability ( 2300d), a short period very close
to 1 day has been detected using different methods, confirming the variability
previously reported by Balona (1992). Finally the nature of this object is
reconsidered.Comment: 4 page
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
Fast Rotation vs. Metallicity
Fast rotation seems to be the major factor to trigger the Be phenomenon.
Surface fast rotation can be favored by initial formation conditions such as
metal abundance. Models of fast rotating atmospheres and evolutionary tracks
are used to determine the stellar fundamental parameters of 120 Be stars
situated in spatially well-separated regions to imply there is between them
some gradient of metallicity. We study the effects of the incidence of this
gradient on the nature of the studied stars as fast rotators.Comment: Societe Francaise d'Astronomie et d'Astrophysique, France (2005
Properties of Regions Forming the FeII Emission Lines in Be Stars
We study FeII and Balmer hydrogen emission lines observed simultaneously of
18 southern Be stars. We use the self-absorption-curve method (SAC) to
determine the optical depth regime of FeII emission lines and to derive first
insights on the physical properties of their forming regions.Comment: Societe Francaise d'Astronomie et d'Astrophysique, France (2005
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