The Chemical Imprint of Silicate Dust on the Most Metal-Poor Stars
  We investigate the impact of dust-induced gas fragmentation on the formation
of the first low-mass, metal-poor stars (< 1Msun) in the early universe.
Previous work has shown the existence of a critical dust-to-gas ratio, below
which dust thermal cooling cannot cause gas fragmentation. Assuming the first
dust is silicon-based, we compute critical dust-to-gas ratios and associated
critical silicon abundances ([Si/H]crit). At the density and temperature
associated with protostellar disks, we find that a standard Milky Way grain
size distribution gives [Si/H]crit = -4.5 +/- 0.1, while smaller grain sizes
created in a supernova reverse shock give [Si/H]crit = -5.3 +/- 0.1. Other
environments are not dense enough to be influenced by dust cooling. We test the
silicate dust cooling theory by comparing to silicon abundances observed in the
most iron-poor stars ([Fe/H] < -4.0). Several stars have silicon abundances low
enough to rule out dust-induced gas fragmentation with a standard grain size
distribution. Moreover, two of these stars have such low silicon abundances
that even dust with a shocked grain size distribution cannot explain their
formation. Adding small amounts of carbon dust does not significantly change
these conclusions. Additionally, we find that these stars exhibit either high
carbon with low silicon abundances or the reverse. A silicate dust scenario
thus suggests that the earliest low-mass star formation in the most metal-poor
regime may have proceeded through two distinct cooling pathways: fine structure
line cooling and dust cooling. This naturally explains both the carbon-rich and
carbon-normal stars at extremely low [Fe/H].
