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Scientific paper
Dec 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997hst..prop.7305m&link_type=abstract
HST Proposal ID #7305
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Hst Proposal Id #7305 Interstellar Matter
Scientific paper
We propose to use STIS to accurately measure and compare the interstellar Zn II, S II, Cr II, Mg II, Mn II, CO, Cl I, and C I absorption line profiles toward a sample of seven multiple star systems with projected linear separations ranging from 550 to 9700 AU. Recent optical observations of Na I at very high spectral resolution toward six of these systems and more than a dozen others have found line profile variations in every case that are indicative of small-scale structure in the diffuse ISM@. A detailed physical understanding of this structure requires the diagnostic power of the many interstellar species observable in the ultraviolet. The specific scientific objectives of our proposed STIS observations are to: {1} assess the small-scale variations in terms of relatively undepleted, dominant ions like Zn II and S II; {2} determine the dust content of the structure using depleted, dominant ions such as Cr II, Mg II, and Mn II; {3} investigate the molecular content of the clumpiness with CO and Cl I; and {4} evaluate the thermal gas pressures of the small-scale structure using the fine-structure excitation of C I@. The last of these goals is especially important in that the Na I variations imply the existence of a significant population of small, dense {n_H> 10^3 cm^-3} clumps that should be short-lived due to their overpressure with respect to the ambient diffuse ISM@.
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