The Distribution of Thermal Pressures in the Interstellar Medium from a Survey of CiFine‐Structure Excitation

Edward B. Jenkins, Todd M. Tripp
2001 Astrophysical Journal Supplement Series  
We used the smallest entrance aperture (0.03 arc-sec wide slit) and highest resolution echelle gratings (E140H and E230H) of STIS on HST to record the interstellar absorption features for 10 different multiplets of neutral carbon at a wavelength resolving power of 200,000 in the UV spectra of 21 early-type stars. We measured the amount of C I in each of its three fine-structure levels, so that we could determine nT of the absorbing gas and how much it varies in different regions. To obtain
more » ... ons. To obtain internally consistent results for all multiplets, we had to modify the transition f-values so that generally weak transitions were stronger than their published values. The fine-structure populations indicate that the median thermal pressure for our entire sample is p/k=2240 cm^-3 K, or slightly higher for temperatures that differ appreciably from 40 K. About 15% of the gas moving at peculiar velocities shows p/k > 10^4 cm^-3 K, and this fraction drops to 1.5% for quiescent gas. Often, there are small amounts of gas at p/k > 10^5 cm^-3 K. These ubiquitous wisps of high pressure material probably arise from small-scale density enhancements created by converging flows in a turbulent medium. The C I excitations indicate a barytropic index gamma_eff > 0.90 when the gas is compressed, which is larger than the value 0.72 expected for material in thermal equilibrium. This indicates that the regions have a dimension r < 0.01 pc which allows them to compress very quickly and nearly adiabatically. Appendices of this paper present evidence that STIS can indeed achieve R = 200,000, and that telluric oxygen absorption lines are broadened by macroscopic motions.
doi:10.1086/323326 fatcat:vqr5rg6yljdcdjrq2gkwbjl72u