An Excess Due to Small Grains around the Nearby K0 V Star HD 69830: Asteroid or Cometary Debris?

Abstract

Spitzer photometry and spectroscopy of the star HD 69830 reveal an excess of emission relative to the stellar photosphere between 8 and 35 μm 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 μm, 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 μm/2π ~ 10 μm. The strong mineralogical features are evidence for even smaller, possibly submicron-sized grains. This small grain size is in direct contrast to the 10-100 μm 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 μm also implies that any Kuiper Belt analog must be either very cold or less massive than ~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.