arXiv subjects

Jean Zorec

Publications and source records attributed to Jean Zorec.

18 recordsLinked to original sources

arXiv2011

Massive Be and Oe stars at low metallicity and long gamma ray bursts

According to recent theoretical studies, the progenitors of Long Gamma Ray Bursts should be very fast rotating stars, massive enough but not so for collapsing into a black hole. In addition, recent observations seem to show that stars of about 20 solar masses could be at the origin of LGRBs. At low metallicity B-type stars rotate faster than at higher metallicity. We found with the ESO-WFI an occurrence of Be/Oe stars, that are quasi critical rotators, 3 to 5 times larger in the SMC than in the Galaxy. According to our results, and using observational clues on the SMC WR stars, as well as the theoretical predictions of the characteristics must have the LGRB progenitors, we have identified the low metallicity massive Be/Oe stars as potential LGRB progenitors. To support this identification, the expected rates and the numbers of LGRB were then calculated and compared to the observed ones: 3 to 6 LGRBs were found in the local universe in 11 years while 8 were actually observed.

astro-ph.SR↗

arXiv2010

X-shooter, NACO, and AMBER observations of the LBV Pistol Star \footnote{Based on ESO runs 85.D-0182A, 085.D-0625AC}

We present multi-instruments and multi-wavelengths observations of the famous LBV star Pistol Star. These observations are part of a larger program about early O stars at different metallicities. The Pistol star has been claimed as the most massive star known, with 250 solar masses. We present the preliminary results based on X-Shooter spectra, as well as the observations performed with the VLTI-AMBER and the VLT-NACO adaptive optics. The X-shooter spectrograph allows to obtain simultaneously a spectrum from the UV to the K-band with a resolving power of $\sim$15000. The preliminary results obtained indicate that Pistol Star has similar properties of Eta Car, including shells of matter, but also the binarity. Other objects of the program, here briefly presented, were selected for their particular nature: early O stars with mass discrepancies between stellar evolution models and observations, discrepancies with the wind momentum luminosity relation.

astro-ph.SR↗

arXiv2010

Evolution of massive Be and Oe stars at low metallicity towards the Long Gamma Ray bursts\footnote{Based on ESO runs 069.D-0275(A), 072.D-0245(A) and (C).}

Several studies have shown recently that at low metallicity B-type stars rotate faster than in environments of high metallicity. This is a typical case in the SMC. As a consequence, it is expected that a larger number of fast rotators is found in the SMC than in the Galaxy, in particular a higher fraction of Be/Oe stars. Using the ESO-WFI in its slitless mode, the data from the SMC open clusters were examined and an occurrence of Be stars 3 to 5 times larger than in the Galaxy was found. The evolution of the angular rotational velocity at different metallicities seems to be the main key to understand the specific behavior and evolution of these stars. According to the results from this WFI study, the observational clues obtained from the SMC WR stars and massive stars, and the theoretical predictions of the characteristics must have the long gamma-ray burst progenitors, we have identified the low metallicity massive Be and Oe stars as potential LGRB progenitors. To this end, the ZAMS rotational velocities of the SMC Be/Oe stars were determined and compared to models. The expected rates and the numbers of LGRB were then calculated and compared to the observed ones. Thus, a high probability was found that low metallicity Be/Oe stars can be LGRB progenitors. In this document, we describe the different steps followed in these studies: determination of the number of Be/Oe stars at different metallicities, identification of the clues that lead to suppose the low metallicity Be/Oe stars as LGRB progenitors, comparison of models with observations.

astro-ph.SR↗

arXiv2010

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.

astro-ph.SR↗

arXiv2010

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.

astro-ph.SR↗

arXiv2010

High-angular resolution observations of the Pistol Star

First results of near-IR adaptive optics (AO)-assisted imaging, interferometry, and spectroscopy of this Luminous Blue Variable (LBV) are presented. They suggest that the Pistol Star is at least double. If the association is physical, it would reinforce questions concerning the importance of multiplicity for the formation and evolution of extremely massive stars.

astro-ph.SR↗

arXiv2010

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 ($Ω/Ω_{\rm c},M/M_{\odot}$) parameters of these stars with the area predicted theoretically for progenitors with metallicity $Z=0.002$, 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 $<Ω/Ω_{\rm c}>=0.95$ 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 $R^{\rm pred}_{\rm LGRB}\sim10^{-7}$ to $\sim10^{-6}$ 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)$\times10^{-7}$ 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 $R_{\rm LGRB}$ rate.

astro-ph.SR↗

arXiv2009

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.

astro-ph.SR↗

arXiv2008

The Gaia satellite: a tool for Emission Line Stars and Hot Stars

The Gaia satellite will be launched at the end of 2011. It will observe at least 1 billion stars, and among them several million emission line stars and hot stars. Gaia will provide parallaxes for each star and spectra for stars till V magnitude equal to 17. After a general description of Gaia, we present the codes and methods, which are currently developed by our team. They will provide automatically the astrophysical parameters and spectral classification for the hot and emission line stars in the Milky Way and other close Local Group galaxies such as the Magellanic Clouds.

astro-ph↗

arXiv2008

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.

astro-ph↗

arXiv2007

On the behaviour of B and Be stars at low metallicity

We present new results obtained with the VLT GIRAFFE for a large sample of B and Be stars belonging to the Magellanic Clouds, i.e. at low metallicity. First, we show the effects of the metallicity of the environment on their rotation (linear, angular, and at the ZAMS). Second, we present the analysis of the effects of metallicity and evolution on the appearance of Be stars. We also new present results about the proportions of Be stars to B stars. Third, by cross-correlation with large photometric surveys such as MACHO and OGLE, we report on the detection for the first time of short-term multi-periodicity in 9 Be stars in the Small Magellanic Cloud, which can be interpreted in terms of pulsations.

astro-ph↗

arXiv2006

Effects of metallicity, star-formation conditions, and evolution in B and Be stars. II: Small Magellanic Cloud, field of NGC 330

We search for effects of metallicity on B and Be stars in the Small and Large Magellanic Clouds (SMC and LMC) and in the Milky Way (MW). We extend our previous analysis of B and Be stars populations in the LMC to the SMC. The rotational velocities of massive stars and the evolutionary status of Be stars are examined with respect to their environments. Spectroscopic observations of hot stars belonging to the young cluster SMC-NGC 330 and its surrounding region have been obtained with the VLT-GIRAFFE facilities in MEDUSA mode. We determine fundamental parameters for B and Be stars with the GIRFIT code, taking into account the effect of fast rotation, and the age of observed clusters. We compare the mean vsini obtained by spectral type- and mass-selection for field and cluster B and Be stars in the SMC with the one in the LMC and MW. We find that (i) B and Be stars rotate faster in the SMC than in the LMC, and in the LMC than in the MW; (ii) at a given metallicity, Be stars begin their main sequence life with a higher initial rotational velocity than B stars. Consequently, only a fraction of B stars that reach the ZAMS with a sufficiently high initial rotational velocity can become Be stars; (iii) the distributions of initial rotational velocities at the ZAMS for Be stars in the SMC, LMC and MW are mass- and metallicity-dependent; (iv) the angular velocities of B and Be stars are higher in the SMC than in the LMC and MW; (v) in the SMC and LMC, massive Be stars appear in the second part of the main sequence, contrary to massive Be stars in the MW.

astro-ph↗

arXiv2006

Effects of metallicity, star formation conditions and evolution of B & Be stars

To study the effects of metallicity and evolution on the appearance of the Be phenomenon in the B stars population, we observed several fields in the Large and Small Magellanic Clouds (LMC and SMC, respectively) which have different metallicities. Thanks to the FLAMES-GIRAFFE multi-fibres spectrograph on the VLT-UT2, we obtained spectra of 520 stars in the LMC-NGC2004 and SMC-NGC330 regions. We used 2 settings at medium resolution: R=8600 for the red setting which contains H$α$ and R=6400 for the blue setting which contains H$γ$, H$δ$, \ion{He}{I} 4026, 4388, 4471 Å. The latter setting was used to obtain fundamental parameters of the stars by fitting the observed spectrum with theoretical spectra. We used TLUSTY (Hubeny & Lanz 1995) to compute a grid of model atmospheres with abundance adopted from Korn et al. (2002) for the LMC and from Jasniewicz & Thévenin (1994) for the SMC. Thanks to the GIRFIT code (Frémat et al. 2005a), we obtained the fundamental parameters \teff, \logg, \vsini~and radial velocity (RV) for each star of the samples.We took into account the effects of fast rotation (stellar flattening and gravitational darkening) for Be stars to correct their apparent fundamental parameters. Then we compared the rotational velocities between fields and clusters in the SMC and in the LMC respectively, between the LMC and the SMC, and between the MC and the Milky Way (MW). The results show an increase in Vsini with decreasing metallicity in B and Be stars populations. The evolutionary status and ages of Be stars were also investigated.

astro-ph↗

arXiv2006

Effects of metallicity, star formation conditions and evolution in B and Be stars. I: Large Magellanic Cloud, field of NGC 2004

Spectroscopic observations of hot stars belonging to the young cluster LMC-NGC 2004 and its surrounding region were carried out with the VLT-GIRAFFE facilities in MEDUSA mode. We determine fundamental parameters (Teff, logg, vsini, and radial velocity), for all B and Be stars in the sample thanks to a code developed in our group. The effect of fast rotation (stellar flattening and gravitational darkening) are taken into account in this study. We also determine the age of observed clusters. We compare the mean vsini obtained for field and cluster B and Be stars in the Large Magellanic Cloud (LMC) with the ones in the Milky Way (MW). We find, in particular, that Be stars rotate faster in the LMC than in the MW, in the field as well as in clusters. We discuss the relations between vsini, metallicity, star formation conditions and stellar evolution by comparing the LMC with the MW. We conclude that Be stars begin their Main Sequence life with an initial rotational velocity greater than the one of B stars. It is probable that only part of the B stars, with a sufficient initial rotational velocity, can become Be stars. This result may explain the differences in the proportion of Be stars in clusters with similar ages.

astro-ph↗

arXiv2005

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.

astro-ph↗

arXiv2005

B and Be stars in the Magellanic Clouds: rotation, evolution and variability

Thanks to observations at the ESO VLT with the GIRAFFE multifibers spectrograph, we have obtained spectra of 177 and 346 B-type stars in the Large and Small Magellanic Clouds respectively. We have discovered 25 and 90 new Be stars among the 47 and 131 Be stars observed in the LMC and SMC respectively. We have determined the fundamental parameters of these stars and examined the effect of the metallicity, star formation conditions and evolution on the behaviour of the rotational velocities. We discovered that only the future B-type stars with a strong rotational velocity in ZAMS will become a Be star. We have also discovered short term variability in several SMC Be stars thanks to a cross-correlation with the MACHO database.

astro-ph↗

arXiv2005

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$ alpha$, H$ gamma$ and H$ delta$ 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 ($ simeq$ 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.

astro-ph↗

arXiv2005

A study of the B and Be star population in the field of the LMC open cluster NGC2004 with VLT-FLAMES

Observations of hot stars belonging to the young cluster LMC-NGC2004 and its surrounding region have been obtained with the VLT-GIRAFFE facilities in MEDUSA mode. 25 Be stars were discovered; the proportion of Be stars compared to B-type stars is found to be of the same order in the LMC and in the Galaxy fields. 23 hot stars were discovered as spectroscopic binaries (SB1 and SB2), 5 of these are found to be eclipsing systems from the MACHO database, with periods of a few days. About 75% of the spectra in our sample are polluted by hydrogen (Halpha and Hgamma), [SII] and [NII] nebular lines. These lines are typical of HII regions. They could be associated with patchy nebulosities with a bi-modal distribution in radial velocity, with higher values (+335 kms^{-1}) preferentially seen inside the southern part of the known bubble LMC4 observed in HI at 21 cm.

astro-ph↗