Star Formation in Atomic Gas
  Observations of nearby galaxies have firmly established, over a broad range
of galactic environments and metallicities, that star formation occurs
exclusively in the molecular phase of the interstellar medium (ISM).
Theoretical models show that this association results from the correlation
between chemical phase, shielding, and temperature. Interstellar gas converts
from atomic to molecular only in regions that are well shielded from
interstellar ultraviolet (UV) photons, and since UV photons are also the
dominant source of interstellar heating, only in these shielded regions does
the gas become cold enough to be subject to Jeans instability. However, while
the equilibrium temperature and chemical state of interstellar gas are
well-correlated, the time scale required to reach chemical equilibrium is much
longer than that required to reach thermal equilibrium, and both timescales are
metallicity-dependent. Here I show that the difference in time scales implies
that, at metallicities below a few percent of the Solar value, well-shielded
gas will reach low temperatures and proceed to star formation before the bulk
of it is able to convert from atomic to molecular. As a result, at extremely
low metallicities, star formation will occur in a cold atomic phase of the ISM
rather than a molecular phase. I calculate the observable consequences of this
result for star formation in low metallicity galaxies, and I discuss how some
current numerical models for H2-regulated star-formation may need to be
modified.
