2012A&A...545A...3M

Observations of molecular spectral lines provide information on the gas kinematics and chemistry of star-forming regions. We attempt to achieve a better understanding of the gas distribution and velocity field around the deeply embedded Class 0 protostar SMM 3 in the Orion B9 star-forming region. Using the APEX 12-m telescope, we mapped the line emission from the J=2-1 rotational transition of two CO isotopologues, ¹³CO and C¹⁸O, over a 4'x4' region around Orion B9/SMM 3. Both the ¹³CO and C¹⁸O lines exhibit two well-separated velocity components at about 1.3 and 8.7km/s. The emission of both CO isotopologues is more widely distributed than the submillimetre dust continuum emission as probed by LABOCA. The LABOCA 870-µm peak position of SMM 3 is devoid of strong CO isotopologue emission, which is consistent with our earlier detection of strong CO depletion in the source. No signatures of a large-scale outflow were found towards SMM 3. The ¹³CO and C¹⁸O emission seen at ∼1.3km/s is concentrated into a single clump-like feature at the eastern part of the map. The peak H₂ column density towards a C¹⁸O maximum of the low-velocity component is estimated to be ∼10²²cm^–2. A velocity gradient was found across both the ¹³CO and C¹⁸O maps. Interestingly, SMM 3 lies on the border of this velocity gradient. The ¹³CO and C¹⁸O emission at ∼1.3km/s is likely to originate from the ``low-velocity part'' of Orion B. Our analysis suggests that it contains high density gas (∼10²² H₂ molecules per cm²), which conforms to our earlier detection of deuterated species at similarly low radial velocities. Higher-resolution observations would be needed to clarify the outflow activity of SMM 3. The sharp velocity gradient in the region might represent a shock front resulting from the feedback from the nearby expanding HII region NGC 2024. The formation of SMM 3, and possibly of the other members of Orion B9, might have been triggered by this feedback.