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

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

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

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 ($\Omega/\Omega_{\rm c},M/M_{\odot}$) 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 $=0.95$ 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

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.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 ($\tau$) normalized to the respective inferred time that each rotating star can spend in the main sequence phase ($\tau\_{\rm MS}$) 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 ($\tau/\tau\_{\rm MS},M/M\_{\odot}$) diagram indicates that in massive stars (M \ga 12M\_{\odot}) the Be phenomenon is present at smaller $\tau/\tau\_{\rm MS}$ age ratios than for less massive stars (M \la 12M\_{\odot}). This distribution can be due to: $i$) higher mass-loss rates in massive objets, which can act to reduce the surface fast rotation; $ii$) 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