199 research outputs found
Massive Oe/Be stars at low metallicity: Candidate progenitors of long GRBs?
At low metallicity the B-type stars rotate faster than at higher metallicity,
typically in the SMC. As a consequence, it was expected a larger number of fast
rotators in the SMC than in the Galaxy, in particular more Be/Oe stars. With
the ESO-WFI in its slitless mode, the SMC open clusters were examined and an
occurence of Be stars 3 to 5 times larger than in the Galaxy was found. The
evolution of the angular rotational velocity seems to be the main key on the
understanding of the specific behaviour and of the stellar evolution of such
stars at different metallicities. With the results of this WFI study and using
observational clues on the SMC WR stars and massive stars, as well as the
theoretical indications of long gamma-ray burst progenitors, we identify the
low metallicity massive Be and Oe stars as potential LGRB progenitors.
Therefore the expected rates and numbers of LGRB are calculated and compared to
the observed ones, leading to a good probability that low metallicity Be/Oe
stars are actually LGRB progenitors.Comment: poster at IAUS27
A slitless spectroscopic survey for Halpha emission-line objects in SMC clusters
This paper checks on the roles of metallicity and evolutionary age in the
appearance of the so-called Be phenomenon. Slitless CCD spectra were obtained
covering the bulk of the Small Magellanic Cloud. For Halpha line emission twice
as strong as the ambient continuum, the survey is complete to spectral type
B2/B3 on the main sequence. About 8120 spectra of 4437 stars were searched for
emission lines in 84 open clusters. 370 emission-line stars were found, among
them at least 231 near the main sequence. For 176 of them, photometry could be
found in the OGLE database. For comparison with a higher-metallicity
environment, the Galactic sample of the photometric Halpha survey by McSwain &
Gies (2005) was used. Among early spectral sub-types, Be stars are more
frequent by a factor 3-5 in the SMC than in the Galaxy. The distribution with
spectral type is similar in both galaxies, i.e. not strongly dependent on
metallicity. The fraction of Be stars does not seem to vary with local star
density. The Be phenomenon mainly sets in towards the end of the main-sequence
evolution (this trend may be more pronounced in the SMC); but some Be stars
already form with Be-star characteristics. In all probability, the fractional
critical angular rotation rate, \omc, is one of the main parameters governing
the occurrence of the Be phenomenon. If the Be character is only acquired
during the course of evolution, the key circumstance is the evolution of \omc,
which not only is dependent on metallicity but differently so for different
mass ranges.Comment: Accepted by A&A, language correction not yet done. Short version of
the abstrac
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
The WFI Halpha spectroscopic survey of the Magellanic Clouds: Be stars in SMC open clusters
At low metallicity, B-type stars show lower loss of mass and, therefore,
angular momentum so that it is expected that there are more Be stars in the
Magellanic Clouds than in the Milky Way. However, till now, searches for Be
stars were only performed in a very small number of open clusters in the
Magellanic Clouds. Using the ESO/WFI in its slitless spectroscopic mode, we
performed a Halpha survey of the Large and Small Magellanic Cloud. Eight
million low-resolution spectra centered on Halpha were obtained. For their
automatic analysis, we developed the ALBUM code. Here, we present the
observations, the method to exploit the data and first results for 84 open
clusters in the SMC. In particular, cross-correlating our catalogs with OGLE
positional and photometric data, we classified more than 4000 stars and were
able to find the B and Be stars in them. We show the evolution of the rates of
Be stars as functions of area density, metallicity, spectral type, and age.Comment: talk at IAUS25
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
Review about populations of Be stars: stellar evolution of extreme stars
Among the emission-line stars, the classical Be stars known for their extreme
properties are remarkable. The Be stars are B-type main sequence stars that
have displayed at least once in their life emission lines in their spectrum.
Beyond this phenomenological approach some progresses were made on the
understanding of this class of stars. With high-technology techniques
(interferometry, adaptive optics, multi-objects spectroscopy,
spectropolarimetry, high-resolution photometry, etc) from different instruments
and space mission such as the VLTI, CHARA, FLAMES, ESPADONS-NARVAL, COROT,
MOST, SPITZER, etc, some discoveries were performed allowing to constrain the
modelling of the Be stars stellar evolution but also their circumstellar
decretion disks. In particular, the confrontation between theory and
observations about the effects of the stellar formation and evolution on the
main sequence, the metallicity, the magnetic fields, the stellar pulsations,
the rotational velocity, and the binarity (including the X-rays binaries) on
the Be phenomenon appearance is discussed. The disks observations and the
efforts made on their modelling is mentioned. As the life of a star does not
finish at the end of the main sequence, we also mention their stellar evolution
post main sequence including the gamma-ray bursts. Finally, the different new
results and remaining questions about the main physical properties of the Be
stars are summarized and possible ways of investigations proposed. The recent
and future facilities (XSHOOTER, ALMA, E-ELT, TMT, GMT, JWST, GAIA, etc) and
their instruments that may help to improve the knowledge of Be stars are also
briefly introduced.Comment: Review about Be stars populations at the IAUS27
The WFI Halpha spectroscopic survey
This document presents the results from our spectroscopic survey of Halpha
emitters in galactic and SMC open clusters with the ESO Wide Field Imager in
its slitless spectroscopic mode. First of all, for the galactic open cluster
NGC6611, in which, the number and the nature of emission line stars is still
the object of debates, we show that the number of true circumstellar emission
line stars is small. Second, at low metallicity, typically in the Small
Magellanic Cloud, B-type stars rotate faster than in the Milky Way and thus it
is expected a larger number of Be stars. However, till now, search for Be stars
was only performed in a very small number of open clusters in the Magellanic
Clouds. Using the ESO/WFI in its slitless spectroscopic mode, we performed a
Halpha survey of the Small Magellanic Cloud. 3 million low-resolution spectra
centered on Halpha were obtained in the whole SMC. Here, we present the method
to exploit the data and first results for 84 open clusters in the SMC about the
ratios of Be stars to B stars.Comment: proceedings SF2A2008, PNPS sessio
ALBUM: a tool for the analysis of slitless spectra and its application to ESO WFI data
ALBUM is a general-purpose tool to visualize and screen large amounts of
slitless spectra. It was developed for a search for emission-line stars in SMC
and LMC clusters. The observations were obtained with ESO's Wide Field Imager
(WFI) and comprise ~8 million low-resolution spectra. The tool as well as the
results of its application to the SMC part of the database are presented. The
inferred frequency of Be stars is compared to the one in the higher-metallicity
environment of the Milky Way.Comment: The 2007 ESO instrument calibration workshop, Garching bei Muenchen :
Allemagne (2007); 4 page
Be stars in open clusters in the Small Magellanic Cloud
We report on the study of the population of B and Be stars in SMC young
clusters, performed with the Wide Field Imager in slitless spectroscopic mode
at ESO/T2.2m with a filter centered at Halpha. First, we explain the reduction
methods we used and our selection of different types of objects. Second, we
present results on the proportion of Be stars in SMC clusters, and we compare
this proportion to the one observed in the Milky Way. Finally, we also present
results on a statistical study of variability of Be stars with OGLE.Comment: sf2a 200
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