The Stellar IMF at Very Low Metallicities

Abstract

The theory for the formation of the first population of stars (Pop. III) predicts an initial mass function (IMF) dominated by high-mass stars, in contrast to the present-day IMF, which tends to yield mostly stars with masses less than 1 M_⊙. The leading theory for the transition in the characteristic stellar mass predicts that the cause is the extra cooling provided by increasing metallicity. In particular, dust can overtake H₂ as the leading coolant at very high densities. The aim of this work is to determine the influence of dust cooling on the fragmentation of very low metallicity gas. To investigate this, we make use of high-resolution hydrodynamic simulations with sink particles to replace contracting protostars, and analyze the collapse and further fragmentation of star-forming clouds. We follow the thermodynamic response of the gas by solving the full thermal energy equation, and also track the behavior of the dust temperature and the chemical evolution of the gas. We model four clouds with different metallicities (10^{− 4}, 10^{− 5}, 10^{− 6}Z_⊙, and 0), and determine the properties of each cloud at the point at which it undergoes gravitational fragmentation. We find evidence for fragmentation in all four cases, and hence conclude that there is no critical metallicity below which fragmentation is impossible. Nevertheless, there is a clear change in the behavior of the clouds at $\mathrm{Z} = 1{0}^{-5}$ Z_⊙, caused by the fact that at this metallicity, fragmentation takes longer to occur than accretion, leading to a flat mass function at lower metallicities.