Herschel Observations of EXtra-Ordinary Sources: H_2S 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 H_2S 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 H_2^(32)S, H_2^(34)S, and H_2^(33)S, 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 H_2S follow straight lines given the uncertainties and yield T_(rot) = 141 ± 12 K. This indicates H_2S 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)(H_2^(32)S) = 9.5 ± 1.9 × 10^(17) cm^(–2), gas kinetic temperature, T_(kin) = 120±^(13)_(10) K, and constrain the H_2 volume density, n_H_2 ≳ 9 × 10^7 cm^(–3), for the H_2S emitting gas. These results point to an H_2S 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 H_2S ortho/para ratio of 1.7 ± 0.8 and set an upper limit for HDS/H_2S of <4.9 × 10^(–3).

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