The validity of 21 cm spin temperature as a kinetic temperature
  indicator in atomic and molecular gas
  The gas kinetic temperature ($T_K$) of various interstellar environments is
often inferred from observations that can deduce level populations of atoms,
ions, or molecules using spectral line observations; H I 21 cm is perhaps the
most widely used with a long history. Usually the H I 21 cm line is assumed to
be in thermal equilibrium and the populations are given by the Boltzmann
distribution. A variety of processes, many involving Lyman alpha ($Ly\alpha$),
can affect the 21 cm line. Here we show how this is treated in the spectral
simulation code Cloudy, and present numerical simulations of environments where
this temperature indicator is used, with a detailed treatment of the physical
processes that determine level populations within $H^0$. We discuss situations
where this temperature indicator traces $T_K$, cases where they fail, as well
as the effects of $Ly\alpha$ pumping on the 21 cm spin temperature. We also
show that the $Ly\alpha$ excitation temperature rarely traces the gas kinetic
temperature.
