Other
Scientific paper
Jul 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008hst..prop11736m&link_type=abstract
HST Proposal ID #11736. Cycle 17
Other
Scientific paper
Optical observations of interstellar Na I absorption have recently discovered that a cold {20 K} diffuse cloud stretching over 10 degrees across the constellation Leo is nearby {d < 45 pc} and thus, deep inside the Local Bubble of hot, tenuous gas surrounding the Sun out to distances of 100 pc. Assuming thermal pressure equilibrium under these conditions leads to an extremely thin, sheetlike geometry for the cold Leo cloud. Such a cold cloud could also be the overpressured interface between colliding flows of warm gas in the Local Bubble. The keys to evaluating the gas pressure and other physical characteristics of the cold Leo cloud lie in the ultraviolet with its rich diversity of interstellar diagnostic transitions. Due in part to a lack of bright UV background sources, there has been no previous UV absorption line study of this cloud. We have identified 3 stars exhibiting Na I absorption from the cold Leo cloud that are suitable for such a study with STIS onboard HST. The main goals of our proposed observations are to: {1} determine the cloud's gas pressure through measurements of its C I fine-structure excitation, {2} assess the dust contribution to the cloud heating by measuring the depletion-sensitive Cr/Zn gas-phase abundance ratio, and {3} evaluate the cloud cooling rate and electron density through measurements of its C II fine-structure excitation.;
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