On the nature of the dust in the debris disk around HD 69830

We have used the infrared mineralogical model derived from the Spitzer IRS observations of the Deep Impact experiment to study the nature of the dust in the debris found around the K0 V star HD 69830. Using a robust approach to determine the bulk average mineralogical composition of the dust, we show it to be substantially different from that found for comets 9P/Tempel 1 and C/Hale-Bopp 1995 O1 or for the comet-dominated YSO HD 100546. Lacking in carbonaceous and ferrous materials but including small icy grains, the composition of the HD 69830 dust most closely resembles that of a disrupted P- or D- type asteroid. The amount of mass responsible for the observed emission is the equivalent of a 30 km radius, 2500 kg m(-3) sphere, while the radiative temperature of the dust implies that the bulk of the observed material is at similar to 1.0 AU from the central source, coincident with the 2:1 and 5:2 mean motion resonances of the outermost of three Neptune-sized planets detected by Lovis and coworkers. In our solar system, P- and D-type asteroids are both large and numerous in the outer main belt and near Jupiter ( e. g., the Hildas and Trojans) and have undergone major disruptive events to produce debris disk-like structures ( cf. the Karin and Veritas families 5-8 Myr ago). The short-lived nature of the small and icy dust implies that the disruption occurred within the last year, or that replenishment due to ongoing collisional fragmentation is occurring.