Orion S: a test for Oxygen chemistry

Oxygen is the third most abundant element in the Universe, and such, it plays and important role in the physics and chemistry of the interstellar medium. O_2 and H_2O are among the primary reservoirs of oxygen in space (e.g. Bergin et al. 2000). Motivated by the very low O_2 and H_2O abundances (and upper limits) derived from the SWAS and Odin missions (e.g. Goldsmith et al. 2000, Pagani et al. 2003, Larsson et al. 2007), several key programs of the Herschel Space Observatory are dedicated to the chemistry of oxygen-bearing species. We present results from the HEXOS (Herschel/HIFI Observations of EXtraOrdinary Sources) and HOP (Herschel Oxygen Project) Herschel HIFI key programs. Our target is Orion S, a nearby high-mass star forming region. Orion S has a fairly large N(H_2) compared to other column density peaks in Orion but suffers much less potential line confusion. Three transitions of O_2 have been measured as part of the HOP key program: O_2 NJ = 3_3 → 1_2 at 487.2 GHz, O_2 NJ = 5_4 → 3_4 at 773.8 GHz and O_2 NJ = 7_6 → 5_6 at 1120.7 GHz. Several p-H_2O and o-H_2O transitions have been detected by the HEXOS key program, including o-H_2O 110-101 at 556.9 GHz, o-H_2O 532-441 at 620.7 GHz, o-H18_2O 110-101 at 547.7 GHz, p-H_2O 211-202 at 752.0 GHz, p-H_2O 422 - 331 at 916.2 GHz, p-H_2O 202 - 111 at 987.9 GHz and p-H_2O 111-000 at 1113.3 GHz. We report H_2O abundances and O_2 abundance upper limits and compare to chemical models. We discuss the possible explanations of the low O_2 and H_2O abundances