HERSCHEL OBSERVATIONS OF EXTRA-ORDINARY SOURCES: H2S AS A PROBE OF DENSE GAS AND POSSIBLY HIDDEN LUMINOSITY TOWARD THE ORION KL HOT CORE

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 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 Trot = 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 (Eup ≳ 1000 K) are likely populated primarily by radiation pumping. We derive a column density, Ntot(H232S) = 9.5 ± 1.9 × 1017 cm−2, gas kinetic temperature, Tkin = 120 K, and constrain the H2 volume density, ≳ 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.