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_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.
