The distribution of thermal pressures in the interstellar medium from a survey of CI fine-structure excitation

We used the Space Telescope Imaging Spectrograph (STIS) with its smallest e ntrance aperture (0 " .03 wide slit) and highest resolution echelle grating s (E140H and E230H) to record the interstellar absorption features for 10 d ifferent multiplets of neutral carbon at a resolving power of lambda/Delta lambda = 200,000 in the UV spectra of 21 early-type stars. Our objective wa s to measure the amount of C I in each of its three fine-structure levels o f the ground electronic state, so that we could determine the thermal press ures in the absorbing gas and how much they vary in different regions. Our observations are principally along directions out to several kiloparsecs in the Galactic plane near longitudes 1 = 120 degrees and 300 degrees, with t he more distant stars penetrating nearby portions of the Perseus and Sagitt arius-Carina arms of the Galaxy. We devised a special analysis technique to decipher the overlapping absorption features in the different multiplets, each with different arrangements of the closely spaced transitions. In orde r to derive internally consistent results for all multiplets, we found that we had to modify the relative transition f-values in a way that made gener ally weak transitions stronger than amounts indicated in the current litera ture. We compared our measured relative populations of the excited fine-str ucture levels to those expected from equilibria calculated with collisional rate constants for various densities, temperatures, and compositions. The median thermal pressure for our entire sample was p/k=2240 cm(-3) K, or sli ghtly higher if the representative temperatures of the material are much ab ove or below a most favorable temperature of 40 K for the excitation of the first excited level at a given pressure. For gas that is moving outside th e range of radial velocities permitted by differential Galactic rotation be tween us and the targets, about 15% of the C I indicates a thermal pressure p/k>5000 cm(-3) K. For gas within the allowed velocities, this fraction is only 1.5%. This contrast reveals a relationship between pressure enhanceme nts and the kinematics of the gas. Regardless of velocity, we usually can r egister the presence of a very small proportion of the gas that seems to be at p/k greater than or similar to 10(5) cm(-3) K. We interpret these ubiqu itous wisps of high-pressure material to arise either from small-scale dens ity enhancements created by converging flows in a turbulent medium or from warm turbulent boundary layers on the surfaces of dense clouds moving throu gh an intercloud medium. For turbulent compression, our C I excitations ind icate that the barytropic index gamma (eff) greater than or similar to 0.90 must apply if the unperturbed gas starts out with representative densities and temperatures n=10 cm(-3) and T=100 K. This value for gamma (eff) is la rger than that expected for interstellar material that remains in thermal e quilibrium after it is compressed from the same initial n and T. However, i f regions of enhanced pressure reach a size smaller than similar to0.01 pc, the dynamical time starts to become shorter than the cooling time, and gam ma (eff) should start to approach the adiabatic value c(p)/c(v) = 5/3. Some of the excited C I may arise from the target stars' H II regions or by the effects of optical pumping from the submillimeter line radiation emitted b y them. We argue that these contributions are small, and our comparisons of the velocities of strongly excited C I to those of excited Si ii seem to s upport this outlook. For six stars in the survey, absorption features from interstellar excited O 1 could be detected at velocities slightly shifted from the persistent fe atures of telluric origin. These O I* and O I** features were especially st rong in the spectra of HD 93843 and HD 210839, the same stars that show exc eptionally large C I excitations. In appendices of this paper, we present e vidence that (1) the wavelength resolving power of STIS in the E140H mode i s indeed about 200,000, and (2) the telluric O I* and O I** features exhibi t some evidence for macroscopic motions, since their broadenings are in exc ess of that expected for thermal Doppler broadening at an exospheric temper ature T=1000 K.