Star formation in metal-poor gas clouds
  Observations of molecular clouds in metal-poor environments typically find
that they have much higher star formation rates than one would expect based on
their observed CO luminosities and the molecular gas masses that are inferred
from them. This finding can be understood if one assumes that the conversion
factor between CO luminosity and H2 mass is much larger in these low
metallicity systems than in nearby molecular clouds. However, it is unclear
whether this is the only factor at work, or whether the star formation rate of
the clouds is directly sensitive to the metallicity of the gas.
  To investigate this, we have performed numerical simulations of the coupled
dynamical, chemical and thermal evolution of model clouds with metallicities
ranging from 0.01 Z_solar to Z_solar. We find that the star formation rate in
our model clouds has little sensitivity to the metallicity. Reducing the
metallicity of the gas by two orders of magnitude delays the onset of star
formation in the clouds by no more than a cloud free-fall time and reduces the
time-averaged star formation rate by at most a factor of two. On the other
hand, the chemical state of the clouds is highly sensitive to the metallicity,
and at the lowest metallicities, the clouds are completely dominated by atomic
gas. Our results confirm that the CO-to-H2 conversion factor in these systems
depends strongly on the metallicity, but also show that the precise value is
highly time-dependent, as the integrated CO luminosity of the most metal-poor
clouds is dominated by emission from short-lived gravitationally collapsing
regions. Finally, we find evidence that the star formation rate per unit H2
mass increases with decreasing metallicity, owing to the much smaller H2
fractions present in our low metallicity clouds.
