We present Herschel/HIFI observations of the light hydride H$_{ 2}$S 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 H$_{ 2}$$^{ 32}$S, H$_{ 2}$$^{ 34}$S, and H$_{ 2}$$^{ 33}$S, respectively. We only analyze emission from the so called hot core, but emission from the plateau, extended ridge, and/or compact ridge are also detected. Rotation diagrams for ortho and para H$_{ 2}$S follow straight lines given the uncertainties and yield T$_{ \rm rot}$=141$\pm$12 K. This indicates H$_{ 2}$S is in LTE and is well characterized by a single kinetic temperature or an intense far-IR radiation field is redistributing the population to produce the observed trend. We argue the latter scenario is more probable and find that the most highly excited states (E$_{ \rm up}$>1000 K) are likely populated primarily by radiation pumping. We derive an H$_{ 2}$$^{ 32}$S column density, N$_{ \rm tot}$(H$_{ 2}$$^{ 32}$S)=9.5$\pm$1.9$\times$10$^{ 17}$ cm$^{ -2}$, gas kinetic temperature, T$_{ \rm kin}$=120$\pm^{ 13}_{ 10}$ K, and constrain the H$_{ 2}$ volume density, n$_{ H2}$>9$\times$10$^{ 7}$ cm$^{ -3}$, for the H$_{ 2}$S emitting gas. These results point to an H$_{ 2}$S 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$_{ 2}$S ortho/para ratio of 1.7$\pm$0.8 and set an upper limit for HDS/H2S of <4.9$\times$10$^{ -3}$.
Categories: astro-ph.GA