Early Stages of Protostellar Disk Evolution: A Link to the Initial Cloud Core

TL;DR

This study models the early evolution of protostellar disks, emphasizing the role of gravitational torques and infall rates in disk stability and angular momentum redistribution during the initial 2000 years after protostar formation.

Contribution

It introduces a self-similar model of early protostellar disks that links initial cloud core properties to disk stability and angular momentum profiles, extending understanding of early star formation stages.

Findings

Protostellar disks from unstable cores are more likely to be Toomre-unstable.

Young disks have specific angular momentum in the range 10^{19}–10^{20} cm^2 s^{-1}.

Rotation velocity profiles are shallower than Keplerian during early stages.

Abstract

We study the structure and evolution of the very early protostellar disk (``protodisk'') just after protostar formation, where disk self-gravity dominates and the stellar contribution is dynamically minor. The disk redistributes angular momentum outward through outflows and gravitational torques, thereby helping to resolve the angular momentum problem of star formation. We develop a self-similar model and carry out a parameter study that examines disk stability as a function of the key drivers of early evolution, notably the infall rate from the envelope and the strength of the gravitational torques. The mass infall rate onto the disk is estimated to be that from the collapse of a Bonnor-Ebert sphere. Our results indicate that protostellar disks that form from more unstable initial cores are more likely to be Toomre-unstable. We also find that the specific angular momentum of young protostellar disks lie in the range $10^{19}\text{--}10^{20}\,{\rm cm^2\,s^{-1}}$. We find distinct power-law profiles of physical quantities in the protodisk stage, including a rotation velocity profile that is shallower than the Keplerian profile that would be established at a later stage. As a rough validity window, our assumptions are most secure during the first $\lesssim 2\times 10^{3}$\,yr after protostar formation and may plausibly extend to $\sim(0.5\text{--}1)\times 10^{4}$\,yr under weak magnetic braking and strong infall.