ar X iv : 0 80 4 . 01 11 v 1 [ as tr o - ph ] 1 A pr 2 00 8 N 2 H + and HC 3 N Observations of the Orion A Cloud

The “ ∫ -shaped filament” of the Orion A giant molecular cloud was mapped in N2H + J = 1→0, and its northern end, the OMC-2/3 region was observed also in HC3N J = 5→4 and CCS JN = 43 → 32 and 76 → 65. The results are compared with maps of other molecular lines and the dust continuum emission. The N2H + distribution is similar to the dust continuum distribution, except for the central part of the Orion Nebula. The distribution of HCO holds resemblance to that of dust continuum, but the N2H + distribution looks more similar to dust continuum distribution. The Nbearing molecules, N2H + and NH3 seem to be more intense in OMC-2, compared with the HCO and CS distribution. This suggests that OMC-2 has higher abundance of N-bearing molecules or higher filling factor of the quiescent gas. We identified 34 cloud cores from N2H + data. Their average physical parameters are Tex = 9.2±4.2 K, ∆v = 0.92±0.52 km s, R = 0.086±0.025 pc, and M = 46±32 M⊙. The masses of cores identified in both N2H + and HCO in the OMC-2/3 region are rather consistent. Over the Orion Nebula region, the N2H + linewidth is large (1.1−2.1 km s). In the OMC-2/3 region, it becomes moderate (0.5−1.3 km s), and it is smaller (0.3−1.1 km s) in the south of the Orion Nebula. On the other hand, the gas kinetic temperature of the quiescent cores observed in N2H + is rather constant (∼ 20 K) over the ∫ -shaped filament. The average N2H + excitation temperature in Orion cores is ∼ 1.6 times as high as that in Taurus cores (∼ 5.7 K). The N2H excitation temperature decreases toward the south, suggesting the core gas density or N2H + abundance decreases toward the south. We found one peculiar HCO core which is not prominent in either N2H , HC3N, or dust. This core overlaps with the lobe of the intense outflow from a nearby protostar. We detected no CCS emission in the OMC2/3 region. In general, N2H + and HC3N distribution is quite similar in the OMC-2/3 region, but we observed displacement between N2H + and HC3N over 2 ′ scale in OMC3, which has a chain of Class 0-I protostars (candidates). This displacement might be due to either chemical evolution or effect of protostellar outflows.