Can the growth of dust grains in low-metallicity star-forming clouds affect the formation of metal-poor low-mass stars?

Abstract

The discovery of a low-mass star with such a low metallicity as ≤4.5 × 10-5 Z ⊙ reveals the critical role of dust in the formation of extremely metal-poor stars. In this Letter, we explore the effect of the growth of dust grains through accretion of gaseous refractory elements in very low metallicity pre-stellar cores on cloud fragmentation induced by 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 ≃ 1010-10 14cm-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 μm) 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 to fragment into sub-solar mass clumps. We suggest that as long as the critical metal abundance is satisfied, grain growth could play an important role in the formation of low-mass stars with metallicity as low as 10-5 Z ⊙. © 2012 The American Astronomical Society. All rights reserved.