Nobeyama 45 m mapping observations toward Orion A. III. Multi-line observations toward an outflow-shocked region, Orion Molecular Cloud 2 FIR 4

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 13CO (J = 1–0), C18O (J = 1–0), N2H+ (J = 1–0), CCS (JN = 87–76), HCO+ (J = 1–0), H13CO+ (J = 1–0), HN13C (J = 1–0), H13CN (J = 1–0), DNC (J = 1–0), N2D+ (J = 1–0), and DC3N (J = 9–8). We detected a dense molecular clump that contains FIR 4/5. We also detected, in the 13CO line, 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 to the FIR 4 clump, suggesting that it may be compressed by protostellar outflows from Class I sources, FIR 3 and VLA 13. Applying a hyperfine fit of N2H+ lines, we estimated the excitation temperature to be ∼20 K. The high excitation temperature is consistent with the fact that the clump contains protostars. CCS emission was detected in this region for the first time. Its abundance is estimated to be a few × 10−12, indicating that the region is chemically evolved at ∼105 yr, which is comparable to the typical lifetime of 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 was evaluated to be ∼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 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.

Massive and low-mass protostars in massive “starless” cores

The infrared dark clouds (IRDCs) G11.11−0.12 and G28.34+0.06 are two of the best-studied IRDCs in our Galaxy. These two clouds host clumps at different stages of evolution, including a massive dense clump in both clouds that is dark even at 70 and 100 μm. Such seemingly quiescent massive dense clumps have been speculated to harbor cores that are precursors of high-mass stars and clusters. We observed these two “prestellar” regions at 1 mm with the Submillimeter Array (SMA) with the aim of characterizing the nature of such cores. We show that the clumps fragment into several low- to high-mass cores within the filamentary structure of the enveloping cloud. However, while the overall physical properties of the clump may indicate a starless phase, we find that both regions host multiple outflows. The most massive core though 70 μm dark in both clumps is clearly associated with compact outflows. Such low-luminosity, massive cores are potentially the earliest stage in the evolution of a massive protostar. We also identify several outflow features distributed in the large environment around the most massive core. We infer that these outflows are being powered by young, low-mass protostars whose core mass is below our detection limit. These findings suggest that low-mass protostars have already formed or are coevally formed at the earliest phase of high-mass star formation.

Molecular gas in high-mass filament WB673

We studied the distribution of dense gas in a filamentary molecular cloud containing several dense clumps. The center of the filament is given by the dense clump WB673. The clumps are high-mass and intermediate-mass starforming regions. We observed CS (2-1),