Thermal Pressures in Neutral Clouds inside the Local Bubble, as Determined from C I Fine-Structure Excitations

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Abstract
High-resolution spectra covering absorption features from interstellar C I were recorded for four early-type stars with spectrographs on HST, in a program to measure the fine-structure excitation of this atom within neutral clouds inside or near the edge of the Local Bubble, a volume of hot (T ~ 10^6 K) gas that emits soft x-rays and extends out to about 100 pc away from the Sun. The excited levels of C I are populated by collisions, and the ratio of excited atoms to those in the ground level give a measure of the local thermal pressure. Absorptions from the two lowest levels of C I were detected toward alpha Del and delta Cyg, while only marginal indications of excited C I were obtained for gamma Ori, and lambda Lup. Along with temperature limits derived by other means, the C I fine-structure populations indicate that for the clouds in front of gamma Ori, delta Cyg and alpha Del, 10^3 < p/k < 10^4 cm^{-3}K at about the +-1 sigma confidence level in each case. The results for lambda Lup are not as well constrained, but still consistent with the other three stars. The results indicate that the thermal pressures are below generally accepted estimates p/k > 10^4 cm^{-3}K for the Local Bubble, based on the strength of x-ray and EUV emission from the hot gas. This inequality of pressure for these neutral clouds and their surroundings duplicates a condition that exists for the local, partly-ionized cloud that surrounds the Sun. An appendix in the paper describes a direct method for determining and eliminating small spectral artifacts arising from variations of detector sensitivity with position.

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