"A New Determination of the Distribution of Interstellar Thermal Pressures"
The ground electronic state of neutral atomic carbon has three fine-structure levels. In the interstellar medium, the relative populations of the upper two levels are established by collisional excitations (and de-excitations) balanced against spontaneous radiative decay. We have measured at high velocity resolution the absorption features from C I in the UV spectra of 102 stars. The level populations of C I reveal that much of the diffuse, cold, neutral medium has pressures that are distributed in an approximately log-normal fashion, spread mostly over a range 1000 < p/k < 10^4 cm^-3 K, with a median pressure of 2750 cm^-3 K. Other contributions that we can detect are a moderate amount of gas at thermally unstable temperatures (T ~ 1000 K) and parcels of gas with pressures in the general range 10^5 < p/k < 10^6 cm^-3 K. The latter probably reside within or at the edges of old supernova remnants or stellar mass-loss bubbles. In addition to distinct velocity components that show high pressures, we find that components identified with the cold, neutral medium show slightly less than one part in a thousand of their atoms elevated to pressures of order or greater than 10^5 cm^-3 K.