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

I. Large Magellanic Cloud, field of NGC 2004

¹ GEPI, UMR 8111 du CNRS, Observatoire de Paris-Meudon, 92195 Meudon Cedex, France e-mail: christophe.martayan@obspm.fr
² Royal Observatory of Belgium, 3 avenue circulaire, 1180 Brussels, Belgium
³ Institut d'Astrophysique de Paris (IAP), 98bis boulevard Arago, 75014 Paris, France
⁴ Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200B, 3001 Belgium

Received: 19 July 2005
Accepted: 15 January 2006

Abstract

Aims.To statistically study the effects of the metallicity, star-formation conditions, and evolution on the behaviour of massive stars and, more particularly, of B and Be stars, we observed large samples of stars in the Magellanic Clouds for the first time. In this article we present the first part of this study.

Methods.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 determined the fundamental parameters (T_eff, , , 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 determined the age of observed clusters. We then compared the mean  obtained for field and cluster B and Be stars in the Large Magellanic Cloud (LMC) with the ones in the Milky Way (MW).

Results.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 , metallicity, star-formation conditions, and stellar evolution by comparing the LMC with the MW. We conclude that Be stars began their main sequence life with an initial rotational velocity higher than the one for B stars. It is probable that only part of the B stars, those 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.