The Luminosity of Population III Star Clusters

TL;DR

This paper models the luminosity evolution of Population III star clusters, revealing episodic bursts driven by disk fragmentation and accretion, which can significantly elevate cluster brightness in the early universe.

Contribution

It introduces a detailed 2+1D hydrodynamics simulation approach to study luminosity bursts in primordial star clusters, highlighting the impact of gravitational instability and fragmentation.

Findings

Cluster luminosity can be 5-10 times higher during burst events.

Episodic accretion bursts can reach luminosities over 10^8 solar luminosities.

Approximately 15% of star-forming time involves elevated luminosity levels.

Abstract

We analyze the time evolution of the luminosity of a cluster of Population III protostars formed in the early universe. We argue from the Jeans criterion that primordial gas can collapse to form a cluster of first stars that evolve relatively independently of one another (i.e., with negligible gravitational interaction). We model the collapse of individual protostellar clumps using 2+1D nonaxisymmetric numerical hydrodynamics simulations. Each collapse produces a protostar surrounded by a massive disk (i.e., $M_{\rm disk} / M_{*} \gtrsim 0.1$), whose evolution we follow for a further 30--40 kyr. Gravitational instabilities result in the fragmentation and the formation of gravitationally bound clumps within the disk. The accretion of these fragments by the host protostar produces accretion and luminosity bursts on the order of $10^6\,\LSun$. Within the cluster, we show that a simultaneity of such events across several protostellar cluster members can elevate the cluster luminosity to 5--10${\times}$ greater than expected, and that the cluster spends $\sim15\%$ of it's star-forming history at these levels. This enhanced luminosity effect is particularly enabled in clusters of modest size with $\simeq$ 10--20 members. In one such instance, we identify a confluence of burst events that raise the luminosity to nearly $1000{\times}$ greater than the cluster mean luminosity, resulting in $L > 10^8\,\LSun$. This phenomenon arises solely through the gravitational-instability--driven episodic fragmentation and accretion that characterizes this early stage of protostellar evolution.