Quiescent formation of bound galactic clusters

A new theory for the origin of bound galactic clusters is presented. This theory assumes that new stars appear at random times in a cloud core, and that the stellar masses are distributed according to some initial mass function. The primary result is an explanation for the differences between bound galactic clusters and expanding OB associations: bound clusters are the occasional result of star formation in low mass cloud cores (M ≲$$10^3-10^4M_\odot$$) and OB associations are the usual result of star formation in high mass cloud cores ($$M\gt10^4 M_\odot$$). Star formation should proceed in a cloud until the luminosity of the embedded cluster exceeds a critical value proportional to the total cloud mass. Then the cloud core disrupts. If the efficiency for star formation at this time of disruption is small, as will be the case for high mass cloud cores, then the cluster will be unbound after the gas leaves. If the efficiency is large, then a bound cluster can remain. The masses, stellar densities and radii of galactic clusters can be derived using only the observed initial mass function and mass–luminosity law for stars, and the characteristic gas densities of star-forming regions. We predict that high mass stars will form in the most massive clouds, that these stars will not generally appear until long after the first low mass stars form, and that some bound clusters will have a large spread in stellar ages.