Herschel Far-infrared Spectral-mapping of Orion BN/KL Outflows: Spatial Distribution of Excited CO, H₂O, OH, O, and C⁺ in Shocked Gas

Abstract

We present ~2' × 2' spectral-maps of Orion Becklin-Neugebauer/Kleinmann-Low (BN/KL) outflows taken with Herschel at ~12'' resolution. For the first time in the far-IR domain, we spatially resolve the emission associated with the bright H₂ shocked regions "Peak 1" and "Peak 2" from that of the hot core and ambient cloud. We analyze the ~54-310 μm spectra taken with the PACS and SPIRE spectrometers. More than 100 lines are detected, most of them rotationally excited lines of ¹²CO (up to J = 48-47), H₂O, OH, ¹³CO, and HCN. Peaks 1/2 are characterized by a very high L(CO)/L _FIR ≈ 5 × 10^-3 ratio and a plethora of far-IR H₂O emission lines. The high-J CO and OH lines are a factor of ≈2 brighter toward Peak 1 whereas several excited H₂O lines are lsim50% brighter toward Peak 2. Most of the CO column density arises from T _k ~ 200-500 K gas that we associate with low-velocity shocks that fail to sputter grain ice mantles and show a maximum gas-phase H₂O/CO lsim 10^-2 abundance ratio. In addition, the very excited CO (J > 35) and H₂O lines reveal a hotter gas component (T _k ~ 2500 K) from faster (v _S > 25 km s^-1) shocks that are able to sputter the frozen-out H₂O and lead to high H₂O/CO gsim 1 abundance ratios. The H₂O and OH luminosities cannot be reproduced by shock models that assume high (undepleted) abundances of atomic oxygen in the preshock gas and/or neglect the presence of UV radiation in the postshock gas. Although massive outflows are a common feature in other massive star-forming cores, Orion BN/KL seems more peculiar because of its higher molecular luminosities and strong outflows caused by a recent explosive event.

Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.