ALBERT

Description: We present the results from a large $850\text{-}\mu$ m survey of the  Orionis cluster using the SCUBA-2 camera on the James Clerk Maxwell Telescope. The 0 $_{.}^{\circ}$ 5 diameter circular region we surveyed contains 297 young stellar objects with an age estimated at about 3 Myr. We detect nine of these objects, eight of which have infrared excesses from an inner disc. We also serendipitously detect three non-stellar sources at 〉5 that are likely background submillimetre galaxies. The nine detected stars have inferred disc masses ranging from 5 to about 17 M Jup , assuming similar dust properties as Taurus discs and an interstellar medium gas-to-dust ratio of 100. There is a net positive signal towards the positions of the individually undetected infrared excess sources indicating a mean disc mass of 0.5 M Jup . Stacking the emission towards those stars without infrared excesses constrains their mean disc mass to less than 0.3 M Jup , 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. Submillimetre Array observations reveal CO emission towards four 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.

Description: We present deep 450 μm and 850 μm observations of a large, uniformly covered 394 arcmin 2 area in the Cosmic Evolution Survey (COSMOS) field obtained with the S cuba-2 instrument on the James Clerk Maxwell Telescope (JCMT). We achieve root-mean-square noise values of 450  = 4.13 mJy and 850  = 0.80 mJy. The differential and cumulative number counts are presented and compared to similar previous works. Individual point sources are identified at 〉3.6 significance, a threshold corresponding to a 3–5 per cent sample contamination rate. We identify 78 sources at 450 μm and 99 at 850 μm, with flux densities S 450  = 13–37 mJy and S 850  = 2–16 mJy. Only 62–76 per cent of 450 μm sources are 850 μm detected and 61–81 per cent of 850 μm sources are 450 μm detected. The positional uncertainties at 450 μm are small (1–2.5 arcsec) and therefore allow a precise identification of multiwavelength counterparts without reliance on detection at 24 μm or radio wavelengths; we find that only 44 per cent of 450 μm sources and 60 per cent of 850 μm sources have 24 μm or radio counterparts. 450 μm selected galaxies peak at 〈 z 〉 = 1.95 ± 0.19 and 850 μm selected galaxies peak at 〈 z 〉 = 2.16 ± 0.11. The two samples occupy similar parameter space in redshift and luminosity, while their median SED peak wavelengths differ by ~20–50 μm (translating to T dust  = 8–12 K, where 450 μm selected galaxies are warmer). The similarities of the 450 μm and 850 μm populations, yet lack of direct overlap between them, suggests that submillimetre surveys conducted at any single far-infrared wavelength will be significantly incomplete (30 per cent) at censusing infrared-luminous star formation at high z .