Nobeyama 45-m Mapping Observations toward Orion A. III. Multi-Line
  Observations toward an Outflow-shocked Region, OMC-2 FIR
  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 blueshifted and redshifted components driven
presumably by protostellar outflows in 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
outflows from Class I sources, FIR 3 and VLA 13. Applying the hyperfine fit of
N$_2$H$^+$ lines, we estimated the excitation temperature to be $\sim$ 20 K.
The high excitation temperature is 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 outflow lobes, 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 outflow compression, or both, affected the thermal
properties of this region.
