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 μm continuum emission from the Orion B molecular cloud. The map is 20"² × 40"² in extent and covers much of the northern half of the giant molecular cloud. A total of 67 discrete continuum sources, or clumps, have been identified, many of which are grouped in three regions, near NGC 2071IR, NGC 2068, and HH 24/25/26. Masses of the sources range from 0.2 to 12 M⊙. About half of the area of our 850 μm map is covered by the current release of the 2MASS infrared survey. Of 40 clumps covered by the 2MASS, 14 have associated infrared sources detected in J, H, and K. Maps of 13CO J = 2-1 and C18O 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 (Tkin ≥ 80 K) while the other five are cold (Tkin ≤ 20 K). The gas-to-dust ratios differ substantially between the two regions mapped in CO. In the NGC 2068 region we find close to constant ratios of dust-to-gas emission, except in one compact source. However, in the HH 24/25/26 region the dust-to-gas emission ratio varies substantially with some of the brightest dust continuum sources almost absent in CO emission. One explanation is that CO molecules have frozen onto grains in the dense cores. Why this freeze-out should happen in the HH 24/25/26 cores but not in the NGC 2068 cores remains unexplained. A 12CO J = 3-2 map of the NGC 2068 region shows patches of high-velocity gas associated with five of the compact continuum sources. The presence of outflows provides strong evidence that the group of sources south of NGC 2068 is actively forming stars.