Far - Infrared OH fluorescent emission in Sagittarius B 2 1

We present Infrared Space Observatory (ISO) observations of OH and OH toward Sgr B2 with a spectral resolution of ∼35 km s. The OH J=5/2→3/2 and J=3/2→1/2 rotational lines of the 2Π1/2 ladder are seen in emission while the cross-ladder transitions (from the 2Π3/2 J=3/2 to the J=1/2, 3/2 and 5/2 levels of the 2Π1/2 ladder), and the 2Π3/2 J=5/2←3/2 and J=7/2←5/2 lines are detected in absorption. The OH 2Π3/2 J=5/2←3/2 Λ–doublet at ∼120 μm is also observed in absorption. All OH Λ–doublets are resolved (except the ∼98 μm) and show, in addition to the strong absorption at the velocity of Sgr B2, several velocity components associated to the gas surrounding Sgr B2 and to the foreground clouds along the line of sight. No asymmetries in the line intensities of each doublet have been observed. We have modeled the observations using a non-local radiative transfer code and found that the OH absorption/emission must arise in a shell around Sgr B2 not resolved by the ISO/LWS beam. The gas density is moderate, with upper limits of 10 cm and ≃300 K in temperature. The OH abundance is high, (2–5)×10. We argue that a widespread photon dominated region explains the enhancement of OH abundance. Subject headings: infrared: ISM: lines and bands — ISM: individual (Sgr B2) — ISM: molecules — line: identification — molecular data — radiative transfer Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with participation of ISAS and NASA. Visiting Scientist, Division of Physics, Mathematics and Astronomy, California Institute of Technology, MS 320-47, Pasadena, CA 91125, USA