From Pre-Stellar Cores to Protostars: The Initial Conditions of Star Formation

Abstract

The last decade has witnessed significant advances in our observational understanding of the earliest stages of low-mass star formation. The advent of sensitive receivers on large radio telescopes such as the JCMT and IRAM 30m MRT has led to the identification of young protostars at the beginning of the main accretion phase (`Class 0' objects), and has made it possible to probe, for the first time, the inner density structure of pre-collapse cores. Class 0 objects are characterized by strong, centrally-condensed dust continuum emission at submillimeter wavelengths, very little emission shortward of 10 microns, and powerful jet-like outflows. Direct evidence for gravitational infall has been observed toward several of them. They are interpreted as accreting protostars which have not yet accumulated the majority of their final stellar mass. In contrast to protostars, pre-stellar cores have flat inner density profiles, suggesting the initial conditions for fast protostellar collapse depart sometimes significantly from a singular isothermal sphere. In the case of non-singular initial conditions, the beginning of protostellar evolution is expected to feature a brief phase of vigorous accretion/ejection which may coincide with Class 0 objects. In addition, submillimeter continuum imaging surveys of regions of multiple star formation such as Ophiuchus and Serpens suggest a picture according to which each star in an embedded cluster is built from a finite reservoir of mass and the associated IMF is primarily determined at the pre-stellar stage of evolution.