Mass and Temperature of the TWA 7 Debris Disk

Abstract

We present photometric detections of dust emission at 850 and 450 m around the pre-main-sequence M1 dwarf TWA 7 using the SCUBA camera on the James Clerk Maxwell Telescope. These data confirm the presence of a cold dust disk around TWA 7, a member of the TW Hydrae Association (TWA). Based on the 850 m flux, we estimate the mass of the disk to be18 M lunar (0.2 M È) assuming a mass opacity of 1.7 cm 2 g À1 with a temperature of 45 K. This makes the TWA 7 disk (d ¼ 55 pc) an order of magnitude more massive than the disk reported around AU Microscopii (GL 803), the closest (9.9 pc) debris disk detected around an M dwarf. This is consistent with TWA 7 being slightly younger than AU Mic. We find that the mid-IR and submillimeter data require the disk to be comprised of dust at a range of temperatures. A model in which the dust is at a single radius from the star, with a range of temperatures according to grain size, is as effective at fitting the emission spectrum as a model in which the dust is of uniform size, but has a range of temperatures according to distance. We discuss this disk in the context of known disks in the TWA and around low-mass stars; a comparison of masses of disks in the TWA reveals no trend in mass or evolutionary state (gas-rich vs. debris) as a function of spectral type.