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 (≲ 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 cm^-3 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 and Z_⊙crit ∼ 10^-4.5 Z _⊙ for . The formation of a recently discovered low-mass star with extremely low metallicity (≤4.5 × 10^-5 Z _⊙) could have been triggered by grain growth.