Formation of OB Associations in Galaxies
Abstract
We consider the formation of OB associations from two perspectives: (a) the fractional gas consumption in star formation,epsilon, per dynamical time scale tdyn in a galaxy, and (b) the origin of the so-called Kennicutt-Schmidt law that the rate of star formation per unit area is proportional to a power, alpha, of the surface density in HI and H2 gas when certain thresholds are crossed. The empirical findings that epsilon is approximately 0.01 and alpha is approximately 1.4 or 1.5 have simple explanations if the rate of star formation is magnetically regulated. An empirical test of the ideas resides in an analysis of why giant OB associations are ``strung out like pearls along the arms" of spiral galaxies.
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