An Extinction Threshold for Protostellar Cores in Ophiuchus

We have observed continuum emission at λ = 850 μm over ~4 deg 2 of the Ophiuchus star-forming cloud using the Submillimeter Common-User Bolometric Array on the James Clerk Maxwell Telescope, producing a submillimeter continuum map 20 times larger than previous Ophiuchus surveys. Our sensitivity is 40 mJy beam -1 , a factor of ~2 less sensitive than earlier maps. Using an automated identification algorithm, we detect 100 candidate objects. Only two new objects are detected outside the boundary of previous maps, despite the much wider area surveyed. We compare the submillimeter continuum map with a map of visual extinction across the Ophiuchus cloud derived using a combination of Two Micron All Sky Survey and R -band data. The total mass in submillimeter objects is ≈50 M ☉ compared with ≈2000 M ☉ in observed cloud mass estimated from the extinction. The submillimeter objects represent only 2.5% of the cloud mass. A clear association is seen between the locations of detected submillimeter objects and high visual extinction, with no objects detected at A V < 7 mag. Using the extinction map, we estimate pressures within the cloud from P / k ≈ 2 × 10 5 cm -3 K in the less-extincted regions to P / k ≈ 2 × 10 6 cm -3 K at the cloud center. Given our sensitivities, cold ( T d ≈ 15 K) clumps supported by thermal pressure, had they existed, should have been detected throughout the majority of the map. Such objects may not be present at low A V because they may form only where A V > 15, by some mechanism (e.g., loss of nonthermal support).