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

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

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

Transferencia de materia en Sistemas Binarios Cerrados

An insight of the distribution of the distribution of the transferred material in a close binary system is obtained around the receiving mass star.Asociación Argentina de Astronomí

Transferencia de materia en Sistemas Binarios Cerrados

An insight of the distribution of the distribution of the transferred material in a close binary system is obtained around the receiving mass star.Asociación Argentina de Astronomí

Transferencia de materia en Sistemas Binarios Cerrados

An insight of the distribution of the distribution of the transferred material in a close binary system is obtained around the receiving mass star.Asociación Argentina de Astronomí

Massive Oe/Be stars at low metallicity: candidate progenitors of long GRBs?

At low metallicity B-type stars rotate faster than at higher metallicity, typically in the SMC. As a consequence, a larger number of fast rotators is expected in the SMC than in the Galaxy, in particular more Be/Oe stars. With the ESO-WFI in its slitless mode, we examined the SMC open clusters and found an occurence of Be stars 3 to 5 times larger than in the Galaxy. The evolution of the angular rotational velocity seems to be the main key on the understanding of the specific behaviour and 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 progenitor

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.Comment: sf2a 2007 PNP

ACHERNAR CAN BE A DIFFERENTIAL ROTATOR

We take advantage of interferometric measurements of Achernar to inquire on its internal rotational law. The reinterpretation of interferome-tric data and the use of fundamental parameters corrected for gravitational darkening effects and models of 2D-models of internal stellar structures, lead us to the conclusion that the star could not be a rigid, near critical, rotator but a differential rotator with the core rotating $\sim3$ times faster than the surface