A. J. Weinberger (Carnegie, DTM), I. Song (Gemini Observatory), B. Zuckerman, E. E. Becklin (UCLA)
The debris disk around the ~300 Myr old star HIP 8920 (SAO 75016) is extremely dusty at ~1 AU from the star (Zuckerman et al. 2004, BAAS, 205, 1554). We present mid-infrared spectra (8--13 microns) of its dust taken with the Long Wavelength Spectrograph at Keck Observatory and the MICHELLE spectrograph at Gemini Observatory. The dust exhibits a very strong silicate feature with broad peaks at 10 and 11 microns. We attempt to fit the spectra, assuming the disk is optically thin, with the sum of emission from pyroxenes and olivines of various sizes. The shape of the 8--13 micron excess indicates a mixture of amorphous and crystalline grains of sizes 0.1 -- 2.5 micron and at temperatures of ~300 K. While the interstellar medium, and therefore presumably proto-planetary disks, are dominated by small grains, this star is too old for the dust to be primordial. The very high line-to-continuum ratio, produced by the abundant small grains, is more similar to Solar System comet spectra than to debris disks or the total Zodiacal dust (Reach et al. 2003, Icarus, 164, 384; Jura et al. 2004, ApJS, 154, 453). We compare our spectra to that of comets and T Tauri stars to try to understand the origin of this debris.
This work is supported in part by the NASA Astrobiology Institute
Bulletin of the American Astronomical Society, 37 #2
© 2005. The American Astronomical Soceity.