A submillimeter dust and gas study of the Orion B molecular cloud

Using SCUBA on the James Clerk Maxwell Telescope, we obtained a map of 850 mum continuum emission from the Orion B molecular cloud. The map is 20'x40' in extent and covers much of the northern half of the giant molecular clou d. A total of 67 discrete continuum sources, or clumps, have been identifie d, many of which are grouped in three regions, near NGC 2071IR, NGC 2068, a nd HH 24/25/26. Masses of the sources range from 0.2 to 12 M-.. About half of the area of our 850 mum map is covered by the current release of the 2MA SS infrared survey. Of 40 clumps covered by the 2MASS, 14 have associated i nfrared sources detected in J, H, and K. Maps of (CO)-C-13 J = 2-1 and (CO) -O-18 J = 2-1 line emission were obtained for two regions in order to find the gas column density. Formaldehyde spectra were obtained toward eight of the continuum clumps to determine the gas kinetic temperature. Three of the clumps with measured temperature are hot (T-kin greater than or equal to 8 0 K) while the other five are cold K) (T-kin less than or equal to 20 K). T he gas-to-dust ratios differ substantially between the two regions mapped i n CO. In the NGC 2068 region we find close to constant ratios of dust-to-ga s emission, except in one compact source. However, in the HH 24/25/26 regio n the dust-to-gas emission ratio varies substantially with some of the brig htest dust continuum sources almost absent in CO emission. One explanation is that CO molecules have frozen onto grains in the dense cores. Why this f reeze-out should happen in the HH 24/25/26 cores but not in the NGC 2068 co res remains unexplained. A (CO)-C-12 J = 3-2 map of the NGC 2068 region sho ws patches of high-velocity gas associated with five of the compact continu um sources. The presence of outflows provides strong evidence that the grou p of sources south of NGC 2068 is actively forming stars.