The effect of dust cooling on low-metallicity star-forming clouds

TL;DR

This study uses high-resolution hydrodynamic simulations to investigate how dust cooling influences the fragmentation of low-metallicity star-forming clouds, suggesting it can lead to low-mass star formation even at very low metallicities.

Contribution

It provides the first detailed simulation-based evidence that dust cooling can induce low-mass star formation at extremely low metallicities, impacting the initial mass function.

Findings

Dust cooling enables low-mass star formation at metallicities as low as 10^{-5} Z_Solar.

The characteristic fragment mass increases as metallicity decreases.

No abrupt transition in the initial mass function was observed within the studied metallicity range.

Abstract

The theory for the formation of the first population of stars (Pop III) predicts a IMF composed predominantly of high-mass stars, in contrast to the present-day IMF, which tends to yield stars with masses less than 1 M_Solar. The leading theory for the transition in the characteristic stellar mass predicts that the cause is the extra cooling provided by increasing metallicity and in particular the cooling provided at high densities by dust. The aim of this work is to test whether dust cooling can lead to fragmentation and be responsible for this transition. To investigate this, we make use of high-resolution hydrodynamic simulations. We follow the thermodynamic evolution of the gas by solving the full thermal energy equation, and also track the evolution of the dust temperature and the chemical evolution of the gas. We model clouds with different metallicities, and determine the properties of the cloud at the point at which it undergoes gravitational fragmentation. We follow the further collapse to scales of an AU when we replace very dense, gravitationally bound, and collapsing regions by a simple and nongaseous object, a sink particle. Our results suggest that for metallicities as small as 10^{-5}Z_Solar, dust cooling produces low-mass fragments and hence can potentially enable the formation of low mass stars. We conclude that dust cooling affects the fragmentation of low-metallicity gas clouds and plays an important role in shaping the stellar IMF even at these very low metallicities. We find that the characteristic fragment mass increases with decreasing metallicity, but find no evidence for a sudden transition in the behaviour of the IMF within the range of metallicites examined in our present study.