We present results from a JCMT/SCUBA 850 µm search for cold dust around eight nearby young stars belonging to the \beta Pic ( t \approx 12 Myr ) and the Local Association ( t \approx 50 Myr ) moving groups .
Unlike most past sub-mm 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 ( \mbox { $M _ { \star } $ } \approx 0.5 ~ { } \mbox { $M _ { \sun } $ } ) , with inferred dust masses of only \approx 0.01–0.03 M _ { \oplus } .
GJ 182 may also possess a 25 µm excess , indicative of warm dust in the inner few AU of its disk .
For GJ 803 ( AU Mic ; HD 197481 ) , sub-mm mapping finds that the 850 µm emission is unresolved .
A non-detection 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 ( \approx 17 AU = 1.7″ in radius for blackbody grains ) .
These are possible indications that planets at large separations can form around M dwarfs within \sim 10 Myr .
In a companion paper ( Kalas , Liu & Matthews 2004 ) , 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 , sub-mm measurements of debris disks point to a drop in dust mass by a factor of \sim 10 ^ { 3 } within the first \sim 10 Myr , with the subsequent decline in the masses of sub-mm detected disks consistent with t ^ { -0.5 } to t ^ { -1 } .