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

Abstract

We present results from a James Clerk Maxwell Telescope/SCUBA 850μm search for cold dust around eight nearby young stars belongingto the β Pic (t~12 Myr) and the Local Association (t~50 Myr) movinggroups. Unlike most past submillimeter studies, our sample was chosensolely on the basis of stellar age. Our observations achieve aboutan order of magnitude greater sensitivity in dust mass compared toprevious work in this age range. We detected two of the three M dwarfsin our sample at 850 μm, GJ 182 and GJ 803 (M*~0.5 Msolar), withinferred dust masses of only ~0.01-0.03 M⊕. GJ 182 may also possessa 25 μm excess, which is indicative of warm dust in the inner fewAU of its disk. For GJ 803 (AU Mic; HD 197481), submillimeter mappingfinds that the 850 μm emission is unresolved. A nondetection of theCO 3-2 line indicates the system is gas-poor, and the spectral energydistribution suggests the presence of a large inner disk hole (~17AU=1.7"in radius for blackbody grains). These are possible indications thatplanets at large separations can form around M dwarfs within ~10Myr. In a companion paper, we confirm the existence of a dust diskaround GJ 803 using optical coronagraphic imaging. Given its youthfulness,proximity, and detectability, the GJ 803 disk will be a valuablesystem for studying disk, and perhaps planet, formation in greatdetail. Overall, submillimeter measurements of debris disks pointto a drop in dust mass by a factor of ~103 within the first ~10 Myr,with the subsequent decline in the masses of submillimeter-detecteddisks consistent with t-0.5-t-1.