GROWTH OF DUST GRAINS IN A LOW-METALLICITY GAS AND ITS EFFECT ON THE CLOUD FRAGMENTATION

In a low-metallicity gas, rapid cooling by dust thermal emission is considered to induce cloud fragmentation and play a vital role in the formation of low-mass stars (less than or similar to 1 M-circle dot) in metal-poor environments. We investigate how the growth of dust grains through accretion of heavy elements in the gas phase onto grain surfaces alters the thermal evolution and fragmentation properties of a collapsing gas cloud. We directly calculate grain growth and dust emission cooling in a self-consistent manner. We show that MgSiO3 grains grow sufficiently at gas densities n(H) = 10(10), 10(12), and 10(14) cm(-3) for metallicities Z = 10(-4), 10(-5), and 10(-6) Z(circle dot), respectively, where the cooling of the collapsing gas cloud is enhanced. The condition for efficient dust cooling is insensitive to the initial condensation factor of pre-existing grains within the realistic range of 0.001-0.1, but sensitive to metallicity. The critical metallicity is Z(crit) similar to 10(-5.5) Z(circle dot) for the initial grain radius r(MgSiO3,0) less than or similar to 0.01 mu m and Z(crit) similar to 10(-4.5) Z(circle dot) for r(MgSiO3,0) greater than or similar to 0.1 mu m. The formation of a recently discovered low-mass star with extremely low metallicity (<= 4.5 x 10(-5) Z(circle dot)) could have been triggered by grain growth.