AAS 201st Meeting, January, 2003
Session 112. The ISM: Clouds and Regions
Poster, Thursday, January 9, 2003, 9:20am-4:00pm, Exhibit Hall AB

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[112.12] The Local Interstellar Cloud: Abundances, Physical Properties, and Gas-to-Dust Mass Ratio

P. C. Frisch (University of Chicago), J. D. Slavin (Smithsonian Astrophysical Observatory)

The physical properties of the heliosphere and the astrospheres of extrasolar planetary systems are constrained by interstellar clouds surrounding these systems. As the Sun, stars, and interstellar clouds travel through space, variations in the galactic environment of each star modify the interplanetary environments, with pronounced differences for inner and outer planets and variations in cosmic ray modulation. In the case of the interstellar cloud surrounding the Sun, observations of interstellar matter (ISM) inside of the heliosphere provide information on the properties of diffuse clouds, which when combined with ionization models constrain the cloud physical properties in a unique fashion (Frisch 1994, Slavin and Frisch 2002). We present a model for interstellar gas within 3 parsecs of the Sun, and show that the gas-phase O, N, and S abundances in this warm (~6500 K) low density cloud (n(HI)~0.24 /cc, n(HII)~0.1 /cc) are about 70 per cent of solar abundances. However, C is present at slightly subsolar abundance, consistent with the destruction of small dust grains. For assumed solar abundances, a gas-to-dust mass ratio of ~190 results when the missing mass principle is used (Frisch et al. 1999). However, in situ measurements of interstellar dust in the solar system give a ratio of ~125+/-18. Alternatively, gas and dust may not be fully mixed over the parsec-sized clouds near the Sun. Both HI and HII need to be included when evaluating abundances of ions found in warm partially ionized diffuse clouds. The interplanetary environment of the heliosphere is dominated by ISM, and the same will be true of extrasolar planetary systems; this indicates the physical properties of the ISM must be understood on a small scale to understand the conditions of the environments of extrasolar planetary sytems. The authors are grateful to NASA for supporting this research.

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