The Distribution of Thermal Pressures in the Interstellar Medium from a Survey of C i Fine‐Structure Excitation

We used the smallest entrance aperture (0.03 arc-sec wide slit) and highestresolution echelle gratings (E140H and E230H) of STIS on HST to record theinterstellar absorption features for 10 different multiplets of neutral carbonat a wavelength resolving power of 200,000 in the UV spectra of 21 early-typestars. We measured the amount of C I in each of its three fine-structurelevels, so that we could determine nT of the absorbing gas and how much itvaries in different regions. To obtain internally consistent results for allmultiplets, we had to modify the transition f-values so that generally weaktransitions were stronger than their published values. The fine-structurepopulations indicate that the median thermal pressure for our entire sample isp/k=2240 cm^-3 K, or slightly higher for temperatures that differ appreciablyfrom 40 K. About 15% of the gas moving at peculiar velocities shows p/k > 10^4cm^-3 K, and this fraction drops to 1.5% for quiescent gas. Often, there aresmall amounts of gas at p/k > 10^5 cm^-3 K. These ubiquitous wisps of highpressure material probably arise from small-scale density enhancements createdby converging flows in a turbulent medium. The C I excitations indicate abarytropic index gamma_eff > 0.90 when the gas is compressed, which is largerthan the value 0.72 expected for material in thermal equilibrium. Thisindicates that the regions have a dimension r < 0.01 pc which allows them tocompress very quickly and nearly adiabatically. Appendices of this paperpresent evidence that STIS can indeed achieve R = 200,000, and that telluricoxygen absorption lines are broadened by macroscopic motions.Comment: 97 pages, 27 figures (including 12 in color), to appear in the Dec 2001 issue of Apj Suppl., larger file with better quality figures is available at ftp://astro.princeton.edu/library/preprints/pop843.ps.g