A SCUBA-2 850 micron survey of protoplanetary discs in the sigma Orionis
  cluster
  We present the results from a large 850 micron survey of the sigma Orionis
cluster using the SCUBA-2 camera on the James Clerk Maxwell Telescope. The
0.5-degree diameter circular region we surveyed contains 297 young stellar
objects with an age estimated at about 3Myr. We detect 9 of these objects, 8 of
which have infrared excesses from an inner disc. We also serendipitously detect
3 non-stellar sources at > 5sigma that are likely background submillimetre
galaxies. The 9 detected stars have inferred disc masses ranging from 5 to
about 17MJup, assuming similar dust properties as Taurus discs and an ISM
gas-to-dust ratio of 100. There is a net positive signal toward the positions
of the individually undetected infrared excess sources indicating a mean disc
mass of 0.5 MJup . Stacking the emission toward those stars without infrared
excesses constrains their mean disc mass to less than 0.3MJup, or an equivalent
Earth mass in dust. The submillimetre luminosity distribution is significantly
different from that in the younger Taurus region, indicating disc mass
evolution as star forming regions age and the infrared excess fraction
decreases. Submillimeter Array observations reveal CO emission toward 4 sources
demonstrating that some, but probably not much, molecular gas remains in these
relatively evolved discs. These observations provide new constraints on the
dust and gas mass of protoplanetary discs during the giant planet building
phase and provide a reference level for future studies of disc evolution.
