A multitransition HCO+ study in NGC 2264G:: Anomalous emission of the J=1→0 line

We present multitransition observations of the HCO+ molecule toward the very young star-forming region associated with the NGC 22646 molecular outflow. Anomalous emission is observed in the lowest rotational transition; the J = 4 --> 3 and J = 3 --> 2 transitions clearly trace the dense core encompassing the exciting source of the molecular outflow, whereas the HCO+ J = 1 --> 0 is barely detected at a much lower intensity and has a much broader line shape. Analysis of the data strongly suggests that the HCO+ J = 1 --> 0 emission arising from the core is being absorbed efficiently by a cold low-density envelope around the core or a foreground cloud. This result seems exceptional, yet the J = 1 --> 0 HCO+ and HCN emission from other dense cores (especially those in giant molecular clouds) may be affected. In these cases, the rare isotopes of these molecules and higher rotational transitions of the main isotopes should be used to study these regions. Two quiescent clumps, JMG 99 G1 and G2, are detected in the blue lobe of the NGC 22646 molecular outflow, close to shock-excited near-IR H-2 knots. These clumps belong to the class of radiatively excited clumps, i.e., the radiation from the shock, evaporates the dust mantles and initiates a photochemical process, enhancing the emission of the HCO+.