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

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Abstract
Aims.We search for the effects of metallicity on B and Be stars in the Small and Large Magellanic Clouds (SMC and LMC) and in the Milky Way (MW), by extending our previous analysis of B and Be star 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.Methods.Spectroscopic observations of hot stars belonging to the young cluster SMC-NGC 330 and its surrounding region were obtained with the VLT-GIRAFFE facilities in MEDUSA mode. We determined fundamental parameters for B and Be stars with the GIRFIT code, taking the effect of fast rotation and the age of observed clusters into account. We compared the mean 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.Results.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 the 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, in contrast to massive Be stars in the MW.
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