Herschel observations of EXtra-Ordinary Sources: H2S AS A PROBE OF DENSE GAS AND POSSIBLY HIDDEN LUMINOSITY TOWARD THE ORION KL HOT CORE1

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 HEXOS GT key program. In total, we observe 52, 24, and 8 unblended or slightly blended features from H2S, H2S, and H2S, 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 LTE 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(H2S) = 9.5 ± 1.9 × 1017 cm−2, gas kinetic temperature, Tkin = 120±13 10 K, and constrain the H2 volume density, nH2 & 9 × 107 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. Subject headings: astrochemistry – ISM: abundances – ISM: individual objects (Orion KL) – ISM: molecules 1Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.