The statistical properties of stars and their dependence on metallicity: the effects of opacity

TL;DR

This study uses radiation hydrodynamical simulations to investigate how metallicity and opacity influence star formation, finding that stellar properties are largely unaffected by opacity variations, with some effects on protostellar mergers.

Contribution

First comprehensive simulation-based analysis of metallicity effects on star formation, showing stellar properties are resilient to opacity changes.

Findings

Stellar properties show little dependence on opacity.

Protostellar mergers increase with lower opacities.

Simulated stellar populations align with observed properties.

Abstract

We report the statistical properties of stars and brown dwarfs obtained from four radiation hydrodynamical simulations of star cluster formation that resolve masses down to the opacity limit for fragmentation. The calculations are identical except for their dust and gas opacities. Assuming dust opacity is proportional to metallicity, the calculations span a range of metallicities from 1/100 to 3 times solar, although we emphasise that changing the metallicity has other thermodynamic effects that the calculations do not capture (e.g. on the thermal coupling between gas and dust). All four calculations produce stellar populations whose statistical properties are difficult to distinguish from observed stellar systems, and we find no significant dependence of stellar properties on opacity. The mass functions and properties of multiple stellar systems are consistent with each other. However, we find protostellar mergers are more common with lower opacities. Combining the results from the three calculations with the highest opacities, we obtain a stellar population consisting of more than 500 stars and brown dwarfs. Many of the statistical properties of this population are in good agreement with those observed in our Galaxy, implying that gravity, hydrodynamics, and radiative feedback may be the primary ingredients for determining the statistical properties of low-mass stars. However, we do find indications that the calculations may be slightly too dissipative. Although further calculations will be required to understand all of the effects of metallicity on stellar properties, we conclude that stellar properties are surprisingly resilient to variations of the dust and gas opacities.