On the Pop. II IMF: Many More Brown Dwarfs

Abstract

One of the challenging questions about star formation which one can ask is; how does the frequency distribution of stellar mass (the IMF) change with cosmic time? — and in particular, for the lowest stellar masses including brown dwarfs. Although a general theory of star formation does not yet exist, we can try to extrapolate current models of fragmentation to predict whether the fraction of very low-mass stars and brown dwarfs in Pop. II is enhanced or reduced relative to the Pop. I IMF. The main difference between Pop. II and Pop. I star formation seems to lie in the role of high-velocity cloud-cloud collisions for star formation, as well as in metallicity effects. Thus, fragmentation of galactic halo clouds (Pop. II star formation) may be quite different from fragmentation of galactic disk clouds (Pop. I star formation), in the sense that Pop. II cloud fragmentation starts at higher column density (10²³ cm ^-2) and ends with lower mass objects (i. e. brown dwarfs). It may only be under Pop. II conditions that the minimum Jeans-mass (≈ 0. 01M_⊙) can be reached.