A Submillimeter Search of Nearby Young Stars for Cold Dust: Discovery of Debris Disks around Two Low-Mass Stars

We present results from a James Clerk Maxwell Telescope/SCUBA 850 μm search for cold dust around eight nearby young stars belonging to the β Pic (t ≈ 12 Myr) and the Local Association (t ≈ 50 Myr) moving groups. Unlike most past submillimeter studies, our sample was chosen solely on the basis of stellar age. Our observations achieve about an order of magnitude greater sensitivity in dust mass compared to previous work in this age range. We detected two of the three M dwarfs in our sample at 850 μm, GJ 182 and GJ 803 (M* ≈ 0.5 M☉), with inferred dust masses of only ≈0.01-0.03 M⊕. GJ 182 may also possess a 25 μm excess, which is indicative of warm dust in the inner few AU of its disk. For GJ 803 (AU Mic; HD 197481), submillimeter mapping finds that the 850 μm emission is unresolved. A nondetection of the CO 3-2 line indicates the system is gas-poor, and the spectral energy distribution suggests the presence of a large inner disk hole (≈17 AU = 1.″7 in radius for blackbody grains). These are possible indications that planets at large separations can form around M dwarfs within ~10 Myr. In a companion paper, we confirm the existence of a dust disk around GJ 803 using optical coronagraphic imaging. Given its youthfulness, proximity, and detectability, the GJ 803 disk will be a valuable system for studying disk, and perhaps planet, formation in great detail. Overall, submillimeter measurements of debris disks point to a drop in dust mass by a factor of ~103 within the first ~10 Myr, with the subsequent decline in the masses of submillimeter-detected disks consistent with t-0.5-t-1.