Thermal Pressures in Neutral Clouds inside the Local Bubble, as Determined from CiFine‐Structure Excitations

High-resolution spectra covering absorption features from interstellar C Iwere recorded for four early-type stars with spectrographs on HST, in a programto measure the fine-structure excitation of this atom within neutral cloudsinside or near the edge of the Local Bubble, a volume of hot (T ~ 10^6 K) gasthat emits soft x-rays and extends out to about 100 pc away from the Sun. Theexcited levels of C I are populated by collisions, and the ratio of excitedatoms to those in the ground level give a measure of the local thermalpressure. Absorptions from the two lowest levels of C I were detected towardalpha Del and delta Cyg, while only marginal indications of excited C I wereobtained for gamma Ori, and lambda Lup. Along with temperature limits derivedby other means, the C I fine-structure populations indicate that for the cloudsin front of gamma Ori, delta Cyg and alpha Del, 10^3 < p/k < 10^4 cm^{-3}K atabout the +-1 sigma confidence level in each case. The results for lambda Lupare not as well constrained, but still consistent with the other three stars.The results indicate that the thermal pressures are below generally acceptedestimates p/k > 10^4 cm^{-3}K for the Local Bubble, based on the strength ofx-ray and EUV emission from the hot gas. This inequality of pressure for theseneutral clouds and their surroundings duplicates a condition that exists forthe local, partly-ionized cloud that surrounds the Sun. An appendix in thepaper describes a direct method for determining and eliminating small spectralartifacts arising from variations of detector sensitivity with position.Comment: 33 pages, 7 figures, submitted to the Astrophysical Journa