The stellar IMF at very low metallicities

Abstract

The theory for the formation of the first population of stars (Pop III) predicts a 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. 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 model four clouds with different metallicities (10−4, 10−5, 10−6Z⊙, 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 Z = 105Z⊙, caused by the fact that at this metallicity, fragmentation takes longer to occur than accretion, leading to a flat mass function at lower metallicities.