Growth of Massive Disk and Early Disk Fragmentation in the Primordial Star Formation

TL;DR

This paper presents a combined analytical and numerical study showing that primordial protostellar disks become massive and fragment early due to gravitational instability, with the disk's evolution driven by infall and mass distribution.

Contribution

The paper introduces a 1D non-steady model of primordial disks that explains early fragmentation, validated by 3D simulations, highlighting the importance of mass supply and disk mass growth.

Findings

Disks become gravitationally unstable with Q near 1.

Disks grow more massive than the protostar before fragmentation.

The 1D model accurately reproduces the disk evolution seen in 3D simulations.

Abstract

Recent high-resolution simulations demonstrate that disks around primordial protostars easily fragment in the accretion phase before the protostars accrete less than a solar mass. To understand why the gravitational instability generally causes the fragmentation so early, we develop a one-dimensional (1D) non-steady model of the circumstellar disk that takes the mass supply from an accretion envelope into account. We also compare the model results to a three-dimensional (3D) numerical simulation performed with a code employing the adaptive mesh refinement. Our model shows that the self-gravitating disk, through which the Toomre $Q$ parameter is nearly constant at $Q \sim 1$, gradually spreads as the disk is fed by the gas infalling from the envelope. We further find that the accretion rate onto the star is an order of magnitude smaller than the mass supply rate onto the disk. This discrepancy makes the disk more massive than the protostar in an early evolutionary stage. Most of the infalling gas is used to extend the outer part of the self-gravitating disk rather than transferred inward toward the star through the disk. We find that similar evolution also occurs in the 3D simulation, where the disk becomes three times more massive than the star before the first fragmentation occurs. Our 1D disk model well explains the evolution of the disk-to-star mass ratio observed in the simulation. We argue that the formation of such a massive disk leads to the early disk fragmentation.