Nobeyama 45-m Mapping Observations toward Orion A. III. Multi-Line Observations toward an Outflow-shocked Region, OMC-2 FIR
2019
We present the results of mapping observations toward an
outflow-shocked region, OMC-2 FIR 4 using the Nobeyama 45-m telescope. We observed the area in $^{13}$CO ($J=1-0$), C$^{18}$O ($J=1-0$), N$_2$H$^+$ ($J=1-0$), CCS ($J_N=8_7-7_6$), HCO$^+$ ($J=1-0$), H$^{13}$CO$^+$ ($J=1-0$), HN$^{13}$C ($J=1-0$), H$^{13}$CN ($J=1-0$), DNC ($J=1-0$), N$_2$D$^+$ ($J=1-0$), and DC$_3$N ($J=9-8$). We detected a dense molecular clump that contains FIR 4/5. We also detected in $^{13}$CO
blueshiftedand redshifted components driven presumably by protostellar
outflowsin this region. The axes of the FIR 3 and VLA 13
outflows, projected on the plane of the sky, appear to point toward the FIR 4 clump, suggesting that the clump may be compressed by protostellar
outflowsfrom Class I sources, FIR 3 and VLA 13. Applying the hyperfine fit of N$_2$H$^+$ lines, we estimated the
excitation temperatureto be $\sim$ 20 K. The high
excitation temperatureis consistent with the fact that the clump contains
protostars. The CCS emission was detected in this region for the first time. Its abundance is estimated to be a few $\times 10^{-12}$, indicating that the region is chemically evolved at $\sim 10^5$ years, which is comparable to the typical lifetime of the Class I
protostars. This timescale is consistent with the scenario that star formation in FIR 4 is triggered by dynamical compression of the protostellar
outflows. The [HNC]/[HCN] ratio is evaluated to be $\sim 0.5$ in the dense clump and the
outflowlobes, whereas it is somewhat larger in the envelope of the dense clump. The small [HNC]/[HCN] ratio indicates that the HNC formation was prevented due to high temperatures. Such high temperatures seem to be consistent with the scenario that either protostellar radiation or
outflowcompression, or both, affected the thermal properties of this region.
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