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E. B. Jenkins (Princeton U. Obs.)
Spectra of early-type stars covering the absorption features from interstellar C~I were recorded by either GHRS or STIS on the Hubble Space Telescope, in a program to measure the fine-structure excitation of this atom in neutral clouds. Targets were chosen such that their lines of sight were dominated by material inside or near the edge of the Local Bubble, a volume of hot (T~106\,K) gas that emits soft x-rays and extends to about 100~pc away from the Sun. The excited fine-structure 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 (HD~196867, \ell=60.3, b=-15.3, d=74\,pc) and \delta~Cyg (HD~186882, \ell=78.7, b=+10.2, d=52\,pc), indicating thermal pressures p/k ranging somewhere between 500\,{\rm cm}-3\,K (if T=40\,K) and 5000\,{\rm cm}-3\,K (if T=250\,K). A less restrictive limit p/k<104\,{\rm cm}-3\,K can be placed on a lack of detection of excited C~I toward \gamma~Ori (HD~35468, \ell=196.9, b=-16.0, d=75\,pc). Repeating a condition found for a local, partly ionized cloud around the Sun, it is evident that the thermal pressures in other clouds within the Local Bubble are likewise below generally accepted estimates of p/k>104\,{\rm cm}-3\,K based on the strength of x-ray and EUV emission from the surrounding hot gas.
This research was supported through grant number GO-6415.01-95A from the Space Telescope Science Institute.