Formation of OB Associations in Galaxies

TL;DR

This paper explores the formation of OB associations in galaxies through the lens of gas consumption efficiency and the Kennicutt-Schmidt law, proposing magnetic regulation as a key factor and analyzing their distribution along spiral arms.

Contribution

It offers a novel explanation for star formation rates and the spatial distribution of OB associations based on magnetic regulation and empirical laws.

Findings

Gas consumption efficiency epsilon is about 0.01.

Star formation rate follows a power law with surface density, alpha ~ 1.4-1.5.

OB associations are distributed along spiral arms like pearls.

Abstract

We consider the formation of OB associations from two perspectives: (a) the fractional gas consumption in star formation,epsilon, per dynamical time scale t_dyn 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 H_2 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.