Chemical variation in molecular cloud cores in the Orion A cloud. II.

Abstract

Abstract We have mapped six molecular cloud cores in the Orion A giant molecular cloud (GMC), whose kinetic temperatures range from 10 to 30 K, in CCS and N2H+ with the Nobeyama 45 m radio telescope to study their chemical characteristics. We identified 31 intensity peaks in the CCS and N2H+ emission in these molecular cloud cores. We found, for cores with temperatures lower than ∼ 25 K, that the column density ratio of N(N2H+)/N(CCS) is low toward starless core regions while it is high toward star-forming core regions, in cases where we detected both the CCS and N2H+ emission. This is very similar to the tendency found in dark clouds (kinetic temperature ∼ 10 K). The criterion found in the Orion A GMC is N(N2H+)/N(CCS) ∼ 2–3. In some cases, both CCS and N2H+ emission is detected toward protostars. A secondary late-stage CCS peak in the chemical evolution caused by CO depletion may be a possible explanation for this. We found that the chemical variation of CCS and N2H+ can also be used as a tracer of evolution in warm (10–25 K) GMC cores. On the other hand, some protostars do not accompany N2H+ intensity peaks but are associated with dust continuum emitting regions, suggesting that the N2H+ abundance might be decreased due to CO evaporation in warmer star-forming sites.