The Distribution of Thermal Pressures in the Diffuse, Cold Neutral
  Medium of our Galaxy. II. An Expanded Survey of Interstellar C I
  Fine-Structure Excitations
  We analyzed absorption features arising from interstellar neutral carbon that
appeared in the UV spectra of 89 stars recorded in the highest resolution
echelle modes of STIS on HST so that we could determine the relative
populations of collisionally excited fine structure levels in the atom's
electronic ground state. From this information we derive the distribution of
thermal pressures in the diffuse, cold neutral medium. We find a lognormal
pressure distribution (weighted by mass) with a mean in log (p/k) equal to 3.58
and an rms dispersion of at least 0.175 dex that plausibly arises from
turbulence with a characteristic Mach number in the range 1 < M < 4. The
extreme tails in the distribution are above the lognormal function however.
Overall, pressures are well correlated with local starlight intensities and
extreme kinematics. Approximately 23% of the gas is at a pressure that is below
that allowed for a static cold neutral medium. Accompanying nearly all of the
gas is a small fraction (~0.05%) that has an extraordinarily large pressure,
log (p/k) > 5.5, and this condition is more prevalent at high velocities or for
regions with enhanced starlight densities. This survey suggests that the
dispersion of thermal pressures in the cold, neutral ISM is predominantly
governed by microscopic turbulence driven by star forming regions, with some
additional effects from macroscopic events (e.g., SN explosions), and these
measurements provide constraints for future studies of the broader impact of
turbulence on the ISM and star formation.
