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Paper: Fluctuations in ISM Thermal Pressures Measured from C I Observations
Volume: 365, SINS — Small Ionized and Neutral Structures in the Diffuse Interstellar Medium
Page: 51
Authors: Jenkins, E.B.; Tripp, T.M.
Abstract: The ground electronic state of the neutral carbon atom is split into three fine-structure levels whose relative populations are determined by the balance between collisional excitations (and de-excitations) and radiative decay. High resolution absorption-line spectroscopy of stars in the ultraviolet reveal how much these populations vary with radial velocity and from one location to the next, which in turn indicate the magnitude of changes in thermal pressures and how they relate to the gas kinematics. We now report on our analysis of nearly all of the suitable high-resolution spectra in the HST archive, which represents a five-fold increase in coverage beyond that of our original survey of 21 stars published in 2001 (ApJS, 137, 297). We conclude that converging and diverging flows within a turbulent medium are responsible for the pressure changes, and that the barytropic index of the gas is higher than that based on predictions for thermal equilibrium. This near adiabatic behavior suggests that most of the pressure changes occur on scales as small as about 50AU, where the characteristic times for such changes are much shorter than the thermal equilibration times. The raw results for measured pressures are weighted by the C I densities, which in turn are influenced by ionization equilibria. We correct for this effect when we derive an H I mass-weighted pressure distribution. This distribution is centered at p/k = 2700 cm−3 K, and there is evidence that very small amounts of gas at pressures greater than p/k = 105 cm−3K may also be present.
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