The chemical evolution of self-gravitating primordial disks

TL;DR

This paper develops a one-zone model to study the chemical evolution of primordial self-gravitating disks, highlighting how viscous heating influences their thermal state and stability, impacting the formation of massive early universe objects.

Contribution

It introduces a simplified framework to analyze how viscous heating affects the chemical and thermal evolution of primordial disks, emphasizing its role in stabilizing the gas against collapse.

Findings

Viscous heating causes collisional dissociation of molecular gas in primordial disks.

Atomic gas remains atomic during evolution if initially atomic, reducing the impact of viscous heating.

Viscous heating stabilizes disks, aiding the formation of supermassive black hole progenitors.

Abstract

Numerical simulations show the formation of self-gravitating primordial disks during the assembly of the first structures in the Universe, in particular during the formation of Pop.~III and supermassive stars. Their subsequent evolution is expected to be crucial to determine the mass scale of the first cosmological objects, which depends on the temperature of the gas and the dominant cooling mechanism. Here, we derive a one-zone framework to explore the chemical evolution of such disks and show that viscous heating leads to the collisional dissociation of an initially molecular gas. The effect is relevant on scales of 10 AU (1000 AU) for a central mass of 10 M_sun (10^4 M_sun) at an accretion rate of 10^{-1} M_sun yr^{-1}, and provides a substantial heat input to stabilize the disk. If the gas is initially atomic, it remains atomic during the further evolution, and the effect of viscous heating is less significant. The additional thermal support is particularly relevant for the formation of very massive objects, such as the progenitors of the first supermassive black holes. The stabilizing impact of viscous heating thus alleviates the need for a strong radiation background as a means of keeping the gas atomic.