An excess due to small grains around the nearby K0V star HD 69830: Asteroid or cometary debris?

Spitzer photometry and spectroscopy of the star HD 69830 reveal an excess of emission relative to the stellar photosphere between 8 and 35 mu m dominated by strong features attributable to crystalline silicates with an emitting surface area more than 1000 times that of our zodiacal cloud. The spectrum closely resembles that of the comet C/1995 O1 (Hale-Bopp). Since no excess is detected at 70 mu m, the emitting material must be quite warm, be confined within a few AU of the star, and originate in grains with low, long-wavelength emissivity, i.e., grains much smaller than 70 mu m/2 pi similar to 10 mu m. The strong mineralogical features are evidence for even smaller, possibly submicronsized grains. This small grain size is in direct contrast to the 10-100 mu m grains that dominate the relatively featureless spectra of our zodiacal dust cloud and most other main-sequence stars with excesses. The upper limit at 70 mu m also implies that any Kuiper Belt analog must be either very cold or less massive than similar to 5 times our own Kuiper Belt. With collisional and Poynting-Robertson drag times of less than 1000 yr for small grains, the emitting material must either (1) be created through continual grinding down of material in a dense asteroid belt, or ( 2) originate in cometary debris arising from either a single supercomet or a very large number of individual comets arriving from a distant reservoir. In the case of a cometary origin for the emission, the mass requirements for continuous generation by many individual comets are unreasonable, and we favor the capture of a single super comet into a 0.5-1 AU orbit, where it can evolve a large number of small grains over a 2 Myr period.