Disk fragmentation and the formation of population III stars

Abstract

Our understanding of population III star formation is still in its infancy. They are formed in dark matter minihalos of $\rm 10^5-10^6 M_{\odot}$ at $z=20-30$. Recent high resolution cosmological simulations show that a protostellar disk forms as a consequence of gravitational collapse and fragments into multiple clumps. However, it is not entirely clear if these clumps will be able to survive to form multiple stars as simulations are unable to follow the disk evolution for longer times. In this study, we employ a simple analytical model to derive the properties of marginally stable steady-state disks. Our results show that the stability of the disk depends on the critical value of the viscous parameter $α$. For $α_{crit} = 1$, the disk is stable for an accretion rate of $\rm \leq 10^{-3} M_{\odot}/yr$ and becomes unstable at radii about $\rm \geq 100 AU$ in the presence of an accretion rate of $\rm 10^{-2} M_{\odot}/yr$. For $0.06 < α_{crit} < 1$, the disk can be unstable for both accretion rates. The comparison of the migration and the Kelvin-Helmholtz time scales shows that clumps are expected to migrate inward before reaching the main sequence. Furthermore, in the presence of a massive central star the clumps within the central 1 AU will be tidally disrupted. We also find that UV feedback from the central star is unable to disrupt the disk, and that photo-evaporation becomes important only once the accretion rate has dropped to $\rm 2 \times 10^{-4} M_{\odot}/yr$. As a result, the central star may reach a mass of 100 $\rm M_{\odot}$ or even higher.