Growth of dust grains in a low-metallicity gas and its effect on the cloud fragmentation

TL;DR

This study examines how dust grain growth in low-metallicity gas clouds influences thermal evolution and fragmentation, potentially enabling low-mass star formation in metal-poor environments.

Contribution

It provides a self-consistent calculation of dust grain growth and cooling, identifying critical metallicities for efficient dust cooling and star formation.

Findings

MgSiO3 grains grow significantly at specific densities and metallicities.

Critical metallicity for dust cooling depends on initial grain size.

Grain growth may trigger formation of extremely low-metallicity stars.

Abstract

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_sun) 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 calculate directly grain growth and dust emission cooling in a self-consistent manner. We show that MgSiO3 grains grow sufficiently at gas densities nH = 10^{10}, 10^{12}, and 10^{14} /cc for metallicities Z = 10^{-4}, 10^{-5}, and 10^{-6} Zsun, 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 Zcrit ~ 10^{-5.5} Zsun for the initial grain radius r_{MgSiO3,0} <~ 0.01 um and Zcrit ~ 10^{-4.5} Zsun for r_{MgSiO3,0} >~ 0.1 um. The formation of a recently discovered low-mass star with extremely low metallicity (<= 4.5x10^{-5} Zsun) could have been triggered by grain growth.