On the Nature of the Dust in the Debris Disk around HD 69830

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Abstract

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 K0V star HD69830 (Beichman et al. 2005). 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 comet 9P/Tempel 1 (Lisse et al. 2006) or C/Hale-Bopp 1995 O1 and comet-dominated YSO HD100546 (Lisse et al. 2007). Lacking in carbonaceous and ferrous materials but including small icy grains, the composition of the HD 69830 dust 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 ~1.0 AU from the central source, coincident with the 2:1 and 5:2 mean motion resonances of the outermost of 3 Neptune-sized planets detected by Lovis et al. (2006). 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 (c.f. the Karin and Veritas families 5-8 Myrs 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.

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