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
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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 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.
