AAS 195th Meeting, January 2000
Session 58. Solar System
Oral, Thursday, January 13, 2000, 10:00-11:30am, Regency VI

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[58.03] Silicate Mineralogy of C/1995 O1 (Hale-Bopp) and its Implications to the Study of Pre-Main Sequence Stars and the Origins of Solar Systems

David Harker (NRC Research Associate - NASA/Ames Research Center)

The evolution of the gas chemistry and dust mineralogy in disks surrounding newly forming stars is fundamental to elucidating processes associated with the origins of solar systems. In particular, silicate mineralogy now can be deduced from the 10~\micron\ and 18~--~33~\micron \ resonances. As reservoirs of pristine solar nebula materials, comets are one of the best sources for investigating early solar nebula dust properties that currently would be observable in the optically thin dust in external PMS objects. Observation and analysis of the silicate features of solar system comets provide a direct probe of the chemical and physical conditions in these potentially planet-forming environments, the condensation of dust from the gas-disk, and the aggregation and accretion of these solids into planetesimals and comets.

Mid-infrared spectrophotometry of comet C/1995 O1 (Hale-Bopp) was obtained with the NASA/Ames HIFOGS at four pre- and post-perihelion epochs from 1996 October through 1997 June. Synthetic spectra calculated from Mie Scattering Theory and the most recent optical constants are fit to the observed 10~\micron\ spectral feature of Hale-Bopp. Our analysis suggests that the observed spectra can be modeled with the Hanner grain size distribution (Hanner 1983) peaked at ap = 0.2~\micron\ of fractal porous grains with porosity parameter D = 2.5. This model spectrum also fits photometry points in the 3 -- 5~\micron\ region. Comparison with the ISO SWS spectrum of Hale-Bopp obtained 1996 October suggests that the crystalline olivine grains must be at a temperature hotter than computed from Mie theory.

This work was supported by grants from the NASA Graduate Student Research Program NGT2-52218, the NSF (AST94--53354), NASA (GRSP NGT2--52218, NAG5--7906), and the Office of Research, University of Wyoming.

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