Disk mass form of the Toomre criterion
= Disk mass form of the Toomre criterion
{title2=$Q\sim(H/r)(M_*/M_D)$}
In a star-dominated <Keplerian disk> with $H\simeq c_s/\Omega$ and characteristic disk mass $M_D\sim\pi r^2\Sigma$, the <Toomre stability criterion> becomes $Q\sim(H/r)(M_*/M_D)$. Thus instability can occur at a disk-to-star mass ratio comparable to the small <disk aspect ratio>. The numerical coefficient depends on whether $M_D$ means a local annular estimate or integrated enclosed mass.