A Submillimeter Dust and Gas Study of the Orion B Molecular Cloud

Abstract

Using SCUBA on the James Clerk Maxwell Telescope, we obtained a map of 850 km continuum emission from the Orion B molecular cloud. The map is 20@ ] 40@ 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 identiÐed, 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 12About half of the area of our 850 km map is M _ . 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 \ 2È1 and C18O J \ 2È1 line emission were obtained for two regions in order to Ðnd 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 K) while the other Ðve are cold K). (T kin º 80 (T kin ¹ 20 The gas-to-dust ratios di †er substantially between the two regions mapped in CO. In the NGC 2068 region we Ðnd 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 \ 3È2 map of the NGC 2068 region shows patches of high-velocity gas associated with Ðve of the compact continuum sources. The presence of outÑows provides strong evidence that the group of sources south of NGC 2068 is actively forming stars.

Magnetic Fields in Star‐forming Molecular Clouds. IV. Polarimetry of the Filamentary NGC 2068 Cloud in Orion B

“…The comparison between the Palomar Observatory Sky Survey optical data and the dust emission has been made by Mitchell et al (2001). We produce a similar image in Figure 1, which shows that while the western dust emission lines up well with the optically dark dust lane, the eastern dust emission has no corresponding dark lane.…”

“…Chopping removes the effects of slow sky variability; however, fast variations remain in the data, which require sky subtraction using array bolometers devoid of significant flux. We used between one and three bolometers to determine the sky variability, using the existing scan maps of total intensity at 850 lm (Mitchell et al 2001) to help select empty bolometers. The methods of sky subtraction are discussed in detail in Jenness, Lightfoot, & Holland (1998).…”

“…Using the 850 lm map of Mitchell et al (2001), a radial profile can be built up by taking several slices across the filament. We have attempted to confine the cuts to the regions between cores along NGC 2068.…”

“…Within it, star formation is concentrated into five distinct regions: NGC 2071, NGC 2068, NGC 2024, and NGC 2023, as determined from unbiased surveys for young stellar objects (Lada, Bally, & Stark 1991a) and dense CS gas (Lada et al 1991b). Large-scale 850 lm dust emission from these regions has been mapped by Mitchell et al (2001) and Motte et al (2001). Maps of the polarized emission from NGC 2071, LBS 23, and NGC 2024 have already appeared in the literature (see Matthews, Fiege, & Moriarty-Schieven 2002, hereafter Paper III).…”

“…1), which is seen optically and contains an infrared cluster (Lada et al 1991a). The overall structure of the dust emission is that of a '' clumpy filament '' in which 18 distinct compact sources are identified (Mitchell et al 2001). Most of the submillimeter sources fall outside the boundary of the cluster identified in CS by Lada et al (1991b).…”