THE STELLAR INITIAL MASS FUNCTION IN PRIMORDIAL GALAXIES

摘要

In the context of star formation through fragmentation of an extremely metal deficient protogalactic cloud, the gravitational collapse of filamentary gas clouds is explored with one-dimensional numerical hydrodynamics coupled with nonequilibrium chemistry of H_2 and HD. It is found that the cloud evolution is governed mainly by the initial central density (n_(c,0)) and H_2 abundance (x_(H_2,0)). In particular, the evolution of low-density filaments (n_(c,0) approx< 10~5 cm~9-3)) bifurcates at a threshold H_2 abundance of x(H_2,cr) approx= 3 x 10~(-3), beyond which HD cooling overwhelms H_2 cooling. The contraction of a filament with n_(c,0) approx< 10~5 cm~(-3) and x_(H_2,0) approx> x(H_2,cr) is strongly decelerated when the central density (n_c) reaches a critical density of HD at which LTE level populations are achieved, and therefore the filament is expected to fragment at ～10~7 cm~(-3). The fragment mass is lowered to be ≈ 10 sola mass. In contrast, the contraction of a filament with n_(c,0) approx< 10~5 cm~(-3) and x(H_2,0) approx< x(H_2,cr) is regulated by H_2 cooling. In this case, the filament tends to fragment at lower density as ～ 10~4 cm~(-3) owing to the low critical density of H_2, and the fragment mass is as high as ≈10~2 sola mass. For a high-density filament with n_(c,0) approx> 10~5 cm~(-3), the temperature stays at a relatively high value because both H_2 and HD cooling saturate, and the cloud evolution is governed by H_2 cooling. The contraction of a high-density filament is accelerated by effective three-body H_2 formation when the density reaches 10~8-10~9 cm~(-3). Fragmentation is not expected to take place until the cloud becomes opaque in H_2 lines at n_(c,0) ～ 10~(12)-10~(13) cm~(-3), so that the fragment mass is reduced to 1-2 sola mass. As a result, the stellar initial mass function could be bimodal and deficient in sub-solar mass stars, where the high-mass peak is around 10 or 10~2 sola mass, dependent on n_(c,0) and x_(H_2,0). If the protogalactic clouds are ionized by UV radiation or strong shocks, the H_2 abundance could exceed x_(H_2,cr) approx= 3 x 10~(-3) by reactions of H + e → H~- + hv and H + H~- → H_2 + e. The high-mass peak would then be 0(10) sola mass.