Star formation from large to small scales

Abstract

The origin of the empirical laws of star formation, i.e., the Schmidt law and the Kennicutt critical column density relation, are discussed in terms of various models, including gravitational instabilities, turbulence compression and sequential triggering. All three have a sensitivity to the critical column density if a cool phase of atomic gas can exist in equilibrium. Observations of star formation on large scales in galaxies favor the instability models, but intermediate scales, from 1 pc to 1 kpc, bring out the features of turbulence compression. Both of these processes differ in morphology from those involved with the formation of individual clusters, which are often triggered sequentially by older stars. We suggest that the three models combine in various ways to initiate star formation. Each gives the appearance that it dominates on a certain scale. The final event that makes clusters in the Solar neighborhood is usually sequential triggering. This operates in a medium that is pre-conditioned by turbulence and sensitive to self-gravity, so the resulting young stars display features characteristic of the other two models.