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

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~10⁶K) 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, 10³cm^-3K<p/k<10⁴cm^-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>10⁴cm^-3K 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. Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.