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Dust grains in low-metallicity star-forming regions may be responsible forthe formation of the first low-mass stars. The minimal conditions to activatedust-induced fragmentation require the gas to be pre-enriched above a criticaldust-to-gas mass ratio Dcr=[2.6--6.3]x10^-9 with the spread reflecting thedependence on the grain properties. The recently discovered Galactic halo starSDSS J102915+172927 has a stellar mass of 0.8 Msun and a metallicity ofZ=4.5x10^-5 Zsun and represents an optimal candidate for the dust-inducedlow-mass star formation. Indeed, for the two most plausible Population IIIsupernova progenitors, with 20 Msun and 35 Msun, the critical dust-to-gas massratio can be overcome provided that at least 0.4 Msun of dust condenses in theejecta, allowing for moderate destruction by the reverse shock. Here we showthat even if dust formation in the first supernovae is less efficient or strongdust destruction does occur, grain growth during the collapse of the parent gascloud is sufficiently rapid to activate dust cooling and likely fragmentationinto low-mass and long-lived stars. Silicates and magnetite grains canexperience significant grain growth in the density range 10^9 /cc < nH<10^12/cc by accreting gas-phase species (SiO, SiO2, and Fe) until their gas-phaseabundance drops to zero, reaching condensation efficiencies =1. Thecorresponding increase in the dust-to-gas mass ratio allows dust-inducedcooling and fragmentation to be activated at 10^12 /cc < nH < 10^14 /cc, beforethe collapsing cloud becomes optically thick to continuum radiation. We showthat for all the initial conditions that apply to the parent cloud of SDSSJ102915+172927, dust-driven fragmentation is able to account for the formationof the star.

Dust grain growth and the formation of the extremely primitive star SDSS J102915+172927