2014ApJ...781..114C

We present Herschel/HIFI observations of the light hydride H₂ S 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 H₂³²S, H₂³⁴S, and H₂³³S, 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(H₂³²S) = 9.5±1.9x10¹⁷/cm2, gas kinetic temperature, T_kin= 120± ₁₀¹³ K, and constrain the H₂ volume density, n_H2 ≳ 9x10 ⁷/cm3, 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 <4.9x10 ^–3.