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

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 V sin i 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.

Constraining globular cluster formation through studies of young massive clusters – III. A lack of gas and dust in massive stellar clusters in the LMC and SMC

“…The fact that we do not detect the clusters at 70 µm (with the possible exception of NGC 2164) means that either the assumed number of evolved massive stars (and their mass-loss rates) are overestimated (as they are meant to be upper limits), or the clusters are efficient at removing any gas/dust released in the cluster. We note that some of the younger clusters (e.g., NGC 330) may have a significant number of Be stars within them that may also contribute gas and dust to the host cluster (e.g., Martayan et 2007), further emphasising the efficiency of the gas/dust removal process from young ( 300 Myr) clusters. Also, we note that the mass loss rates of RSGs and AGBs are expected to be highly time variable, with the high mass loss rate phase being short-lived (e.g., van Loon, Marshall, & Zijlstra 2005).…”

A slitless spectroscopic survey for Hαemission-line objects in SMC clusters

“…While extremely rapid rotation is a necessary condition for the Be phenomenon, it is not clear whether it is also sufficient or which other conditions need to be fulfilled. There are reports suggesting that the Be phenomenon also depends on metallicity (Maeder et al 1999) and Tables A.1, A.2, B.1, B.2, C.1-C9 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/509/A11 evolutionary phase (e.g., Fabregat & Torrejón 2000;Martayan et al 2007b). An obvious method for investigating the latter possibility is the study of open star clusters with a suitable range of ages.…”

Evolution and appearance of Be stars in SMC clusters

“…From our SMC data and results obtained by Zorec et al (2005) Martayan et al 2007), which holds the key to our understanding of the distribution seen in Figure 2: • In massive Milky Way Be stars, Ω Ωc is sufficient to develop Be-star symptoms at the beginning of the main sequence. Due to wind-driven mass loss, the angular momentum decreases until Ω Ωc drops to below the threshold of 0.7 and the initial Be-star appearance will be lost after ∼5 − 10 Myr.…”

“…In particular, rotational velocities were determined as well as the statistical, fractional angular breakup velocity for Be stars of different ages (see Martayan et al 2007). …”