Isolated and Clustered Star Formation: Observations and Theoretical Models

Abstract

The formation of stars occurs within the densest regions of molecular clouds. The detection of infrared point sources embedded in these dense cores shows unequivocally this fundamental property. However, star formation is not simply the result of clouds breaking apart into these dense substructures. The onset of collapse is rather a highly localized occurrence within large complexes, and the character of collapse dictates the structure of the nascent protostar. It is also true that individual collapses can occur over extensive regions of a complex which give birth to large collection of stars in multiple systems, groups, associations and clusters, in order of increasing hierarchy. The observational evidence is that most clouds are not in a state of collapse: support against gravity comes from a balance of forces that persists over long periods of time. The main support is provided partially from thermal pressure, but also from turbulent motions and the interstellar magnetic field, especially on the largest scales. Typically, the star formation efficiency, defined as the ratio of the mass in stars and the total mass (gas+stars), is of order of few percent in giant molecular clouds. In localized dense clumps, the conversion of gas into stars can be very efficient, reaching values greater than 20–30%, and gravitationally bound clusters of stars can form. The dual character of star formation that gives rise to either isolated single and binary stars or to groups and clusters has come out clearly from a large body of observations of the most spectacular nearby star forming regions.