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 HD69830 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 1,000 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\\sim 10 $ $\\mu$m. The strong mineralogical features are evidence for even smaller, possibly sub-micron sized 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 either be very cold or be less massive than $\\sim$ 5 times our own Kuiper Belt. With collisional and Poynting-Robertson drag times of about a thousand years for small grains, the emitting material must either: a) be created through continual grinding down of material in a dense asteroid belt; or b) originate in cometary debris arising from either a single ``super-comet'' 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.