The Distribution of Interstellar Thermal Pressures in the Milky Way: New Data on the Fine-structure Excitation of C I from STIS Spectra in the HST Archive

Astronomy and Astrophysics – Astronomy

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Scientific paper

We have used the method of Jenkins & Tripp (2001: ApJS, 137, 297) to extract velocity profiles of the absorption by interstellar neutral carbon atoms in the 3 different fine-structure levels of the ground electronic state. The populations of these levels are governed by the balance between collisional excitations (and de-excitations) against radiative decay. By studying these populations, we can determine the different combinations of local temperatures and densities of the C I-bearing gas. We have analyzed virtually all of the stellar spectra in HST Archive that show clearly the multiplets of C I recorded with the E140H echelle grating of STIS. We have now expanded our coverage from the 21 stars in our original survey to a new total of 102 stars. This new study increases the coverage in Galactic longitudes well beyond that of the earlier observing program, which was mostly restricted to the intersections of the two HST continuous viewing zones and the plane of the Galaxy. We propose that substantial deviations of thermal pressures above and below a mean value are caused by interstellar turbulence, and we correlate these deviations with the kinematic properties of the gas. We derive an overall C I-weighted distribution function for the thermal pressures, and this distribution can be converted to one that applies to all of the neutral gas after we correct for changes in the ionization equilibrium between the neutral and ionized forms of carbon.This research was supported by grant HST-AR-09534.01 from the Space Telescope Science Institute, operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.

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