Modes of Star Formation and Constraints on the Birth Aggregate of our Solar System

Abstract

This work suggests that star forming environments should be classified into finer divisions than the traditional isolated and clustered modes. Using the observed open cluster system, we estimate the fraction of star formation that takes place within cluster environments: Only about 10% of the stellar population originates from star forming regions destined to become open clusters. The smallest clusters included in the observational surveys (containing N ~ 100 – 300 members) roughly coincide with the smallest stellar systems that evolve as clusters in a dynamical sense. Specifically, 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 thus 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 < N * ~ 100. The groups evolve and disperse much more rapidly than do open clusters and thus represent a different type of astronomical entity. Furthermore, groups and clusters affect star formation in different ways. Groups have a low probability of containing massive stars and are thus unaffected by supernovae and intense ultraviolet radiation fields. In addition, interactions between the young stellar objects are relatively rare in groups because of their short lifetimes and small stellar membership. Finally, we can apply these considerations to the formation of our own solar system and thereby find tight constraints on the solar birth aggregate.KeywordsSolar SystemStar FormationMassive StarOpen ClusterStar Formation RateThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.