George F. Mitchell (Saint Mary's University), Doug Johnstone (University of Toronto), Gerald Moriarty-Schieven (Joint Astronomy Centre/National Research Council of Canada), Michel Fich (University of Waterloo), & N. F. H. Tothill (Saint Mary's University)

Using SCUBA on the JCMT, we obtained a map of 850 um continuum emission from the Orion B molecular cloud. The map (shown above right) is 20'x40' in extent and covers much of the northern half of the GMC. Above left we show an image extracted from the Digitized Sky Survey with contours of 850 um superposed. We identify 67 discrete continuum sources. Many of the sources, or clumps, are grouped in three regions, near NGC 2071IR, NGC 2068, and HH 24-26 (LBS23). Masses of the sources range from 0.2 to 12 M_o. About half of the area of our 850 um 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 towards eight of the continuum clumps to determine the gas kinetic temperature. Three of the clumps with measured temperatures are hot (T_kin > 80K) while the other five are cold (T_kin > 20K). 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 24-26 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 24-26 cores but not in the NGC 2068 cores remains unexplained.

The paper has been submitted to The Astrophysical Journal. A gzipped preprint (1.9MB) is available here.