Formation and Early Evolution of Intermediate Mass Stars

Abstract

In this paper I discuss our current understanding of the formation of intermediate mass stars and the evolution of their circumstellar material. The results of the search for clusters around Herbig Ae/Be stars are discussed in the context of the possible relationship of the formation of massive stars via coalescence process. Current surveys are not able to offer a definite answer on whether there is a physical relationship between the most massive intermediate mass stars and the presence of clusters. The present day stellar densities in these clusters is too low to allow for frequent stellar collisions, but do not exclude the possibility of coalescence at earlier cluster ages. I review the evidence for and properties of circumstellar disks around Herbig Ae stars, as well as recent observational evidence for dust evolution within these disks. Evidence is presented for the presence of evolved dust in some of these disks, with grain growth to sizes as large as a few centimeters. The status of the search for the more elusive disks around Herbig Be stars is also discussed. Current observations show that disks around Herbig Be stars progenitors may have been common and massive in the protostellar phase, but optically visible Herbig Be stars, albeit very young, are never found to be surrounded by conspicuous dusty disks, suggesting dispersion timescales as short as 1 Myr.