Measurements of the Thermal Pressures of Neutral Gas Clouds Inside the Local Bubble

Details Measurements of the Thermal Pressures of Neutral Gas Clouds Inside the Local Bubble Measurements of the Thermal Pressures of Neutral Gas Clouds Inside the Local Bubble

Dec 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001aas...199.1105j&link_type=abstract

American Astronomical Society, 199th AAS Meeting, #11.05; Bulletin of the American Astronomical Society, Vol. 33, p.1324

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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 α Del (HD 196867, l=60.3, b=-15.3, d=74 pc) and δ Cyg (HD 186882, l=78.7, b=+10.2, d=52 pc), indicating thermal pressures p/k ranging somewhere between 500 cm-3 K (if T=40 K) and 5000 cm-3 K (if T=250 K). A less restrictive limit p/k<104 cm-3 K can be placed on a lack of detection of excited C I toward γ Ori (HD 35468, l=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 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.

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