2013ApJ...765L...3C

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 ( ≲ 1 M_☉) 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= 10¹⁰, 10¹², and 10¹⁴/cm3 for metallicities Z = 10^–4, 10^–5, and 10^–6 Z_☉, 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∼ 10^–5.5 Z_☉ for the initial grain radius r_MgSiO3 ≲ 0.01 µm and Z_crit∼ 10^–4.5 Z_☉for r_MgSiO3_, 0_ ≳ 0.1 µm . The formation of a recently discovered low-mass star with extremely low metallicity ( ≤ 4.5x10^–5 Z_☉) could have been triggered by grain growth.