Modes of Multiple Star Formation

Abstract

This paper argues that star-forming environments should be classified into finer divisions than the traditional isolated and clustered modes. Using the observed set of Galactic open clusters and theoretical considerations regarding cluster formation, we estimate the fraction of star formation that takes place within clusters. We find that less than ~10% of the stellar population originates from star-forming regions destined to become open clusters, confirming earlier estimates. The smallest clusters included in the observational surveys (having at least N ~ 100 members) roughly coincide with the smallest stellar systems that are expected to evolve as clusters in a dynamical sense. We show that stellar systems with too few members N < N_* have dynamical relaxation times that are shorter than their formation times (~1-2 Myr), where the critical number of stars N_* ≈ 100. Our results suggest that star formation can be characterized by (at least) three principal modes: (I) isolated singles and binaries, (II) groups (N < N_*), and (III) clusters (N > N_*). Many—if not most—stars form through the intermediate mode in stellar groups with 10 < N < 100. Such groups evolve and disperse much more rapidly than do open clusters; groups also have a low probability of containing massive stars and are unaffected by supernovae and intense ultraviolet radiation fields. Because of their short lifetimes and small stellar membership, groups have relatively little effect on the star formation process (on average) compared to larger open clusters.