Physics
Scientific paper
Jul 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996hst..prop.6415j&link_type=abstract
HST Proposal ID #6415
Physics
Hst Proposal Id #6415 Interstellar Medium
Scientific paper
The thermal pressure nkT of the interstellar medium is a fundamental quantity whose average value and fluctuations tell us much about key physical processes. Theorists place heavy reliance on good measurements of this pressure to construct global theories or explain specific phenomena. The observations proposed here seek to measure this pressure inside H I clouds in a very local part of the Milky Way, with the promise of expanding into a more distant survey in later HST observing cycles. Our method relies on measuring absorption features produced by C I and Si II in different levels of fine-structure excitation that are populated by collisions with neutral atoms and electrons. Observations of the soft x-ray background indicate that our solar system is located near the middle of a bubble of hot gas {T=10^6K} that has a diameter somewhere between 100 and 200 pc. By measuring the pressure of the ambient medium just outside the boundary of this region, we can help to differentiate between two models for the Local Bubble: {1} it is bounded by an expanding shock produced by an explosion some 10^5 yr ago, or {2} it is a pressure-confined region that is about 10^7 yr old.
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