Effects of metallicity, star formation conditions and evolution of B & Be stars.
Résumé
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$\alpha$ and R=6400 for the blue setting which contains H$\gamma$, H$\delta$, \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.