Theoretical Mass Function for Secondaries Forming via Gravitational Instability in Circumstellar Disks

Abstract

This paper constructs a theoretical framework for calculating the distribution of masses for secondary bodies forming via gravitational instability in the outer regions of circumstellar disks. We show that several alternate ways to specify the mass scale of forming objects converge to the same result under the constraint that the parental disks are marginally stable with stability parameter $Q=1$. Next we show that the well-known constraint that the formation of secondary bodies requires rapid cooling is equivalent to that of opacity limited fragmentation. These results are then used to derive a mass function for secondary objects forming through disk instablity. The resulting distribution is relatively narrow, with log-normal-like shape, a characteristic mass scale of order $M_{\scriptstyle \rm P}\sim10M_{\scriptstyle \rm Jup}$ and an approximate range of $4-80M_{\scriptstyle \rm Jup}$. Current estimates for the occurrence rate suggest that these objects are outnumbered by both stars and planets formed via core accretion.