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.
Highlights
In nearby dark clouds, molecules such as CCS, HC3 N, NH3, and N2 H+, and the neutral carbon atom C0 are known to be good tracers of the chemical evolution (e.g., Hirahara et al.1992; Suzuki et al 1992; Benson, Caselli, & Myers 1998; Maezawa et al 1999; Lai & Crutcher2000; Hirota, Ito, & Yamamoto 2002; Hirota, Ohishi, & Yamamoto 2009)
The purpose of this study is to investigate whether our knowledge of the chemical evolution found in cold dark clouds is valid for cores in Orion A giant molecular clouds (GMCs)
It is found that the column density ratio of N(N2 H+ )/N(CCS) is low toward starless peaks while it is high toward star-forming peaks
Summary
Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588 of Astronomical Science, The Graduate University for Advanced Studies (Sokendai), 2-21-1 Osawa, Mitaka, Tokyo 181-8588. Astronomy and Space Science Institute, Daedeokdaero 776, Yuseong, Daejeon 305-348, South
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