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

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Abstract
High-resolution spectra covering the absorption features from interstellar C I were recorded for four early-type stars with spectrographs on the Hubble Space Telescope, 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 ~ 106 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 α Del and δ Cyg, while only marginal indications of excited C I were obtained for γ Ori and λ Lup. Along with temperature limits derived by other means, the C I fine-structure populations indicate that for the clouds in front of γ Ori, δ Cyg, and α Del, 103 cm-3 K < p/k < 104 cm-3 K at about the ±1 σ confidence level in each case. The results for λ 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 > 104 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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