Can the Growth of Dust Grains in Low-Metallicity Star-Forming Clouds Affect the Formation of Metal-Poor Low-Mass Stars?

TL;DR

This study investigates how dust grain growth in extremely low-metallicity star-forming clouds influences cloud fragmentation and the formation of low-mass, metal-poor stars, highlighting the significance of dust processes in early star formation.

Contribution

It demonstrates that dust grain growth via accretion can induce cloud fragmentation at very low metallicities, expanding understanding of low-mass star formation in metal-poor environments.

Findings

Fe and Si grains grow efficiently at high densities in low-metallicity clouds.

Critical metal abundances for grain growth-induced fragmentation are estimated to be 10^{-9}-10^{-8}.

Grain growth can enable low-mass star formation even with initially low dust-to-gas ratios.

Abstract

The discovery of a low-mass star with such low metallicity as < 4.5x10^{-5} Z_sun reveals the critical role of dust in the formation of extremely metal-poor stars. In this paper we explore the effect of the growth of dust grains through accretion of gaseous refractory elements in very low-metallicity pre-stellar cores on the cloud fragmentation induced by the dust emission cooling. Employing a simple model of grain growth in a gravitationally collapsing gas, we show that Fe and Si grains can grow efficiently at hydrogen densities of ~10^{10}-10^{14} cm^{-3} in the clouds with metal abundances of -5 <~ [Fe, Si/H] <~ -3. The critical metal number abundances, above which the grain growth could induce the fragmentation of the gas clouds, are estimated to be A_{crit} ~ 10^{-9}-10^{-8}, unless the initial grain radius is too large (>~ 1 um) or the sticking probability is too small (<~ 0.01). We find that even if the initial dust-to-gas mass ratio is well below the minimum value required for the dust-induced fragmentation, the grain growth increases the dust mass high enough to cause the gas fragmentation into sub-solar mass clumps. We suggest that as long as the critical metal abundance is satisfied, the grain growth could play an important role in the formation of low-mass stars with metallicity as low as 10^{-5} Z_sun.