Herschel Observations of EXtra-Ordinary Sources: H₂S as a Probe of Dense Gas and Possibly Hidden Luminosity Toward the Orion KL Hot Core

Abstract

We present Herschel/HIFI observations of the light hydride H₂S obtained from the full spectral scan of the Orion Kleinmann-Low nebula (Orion KL) taken as part of the Herschel Observations of EXtra-Ordinary Sources GT (guaranteed time) key program. In total, we observe 52, 24, and 8 unblended or slightly blended features from H₂ ³²S, H₂ ³⁴S, and H₂ ³³S, respectively. We only analyze emission from the so-called hot core, but emission from the plateau, extended ridge, and/or compact ridge are also detected. Rotation diagrams for ortho and para H₂S follow straight lines given the uncertainties and yield T _rot = 141 ± 12 K. This indicates H₂S is in local thermodynamic equilibrium and is well characterized by a single kinetic temperature or an intense far-IR radiation field is redistributing the population to produce the observed trend. We argue the latter scenario is more probable and find that the most highly excited states (E _up >~ 1000 K) are likely populated primarily by radiation pumping. We derive a column density, N _tot(H₂ ³²S) = 9.5 ± 1.9 × 10¹⁷ cm^-2, gas kinetic temperature, T _kin = 120+/- ^{13}_{10} K, and constrain the H₂ volume density, n_H_2 >~ 9 × 10 ⁷ cm^-3, for the H₂S emitting gas. These results point to an H₂S origin in markedly dense, heavily embedded gas, possibly in close proximity to a hidden self-luminous source (or sources), which are conceivably responsible for Orion KL's high luminosity. We also derive an H₂S ortho/para ratio of 1.7 ± 0.8 and set an upper limit for HDS/H₂S of <4.9 × 10 ^-3.

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