Far-Infrared OH Fluorescent Emission in Sagittarius B2

We present Infrared Space Observatory (ISO) observations of 16OH and 18OHtoward Sgr B2 with a spectral resolution of 35 km/s. The OH J=5/2-3/2 andJ=3/2-1/2 rotational lines of the 2Pi1/2 ladder are seen in emission while thecross-ladder transitions (from the 2Pi3/2 J=3/2 to the J=1/2, 3/2 and 5/2levels of the 2Pi1/2ladder), and the 2Pi3/2 J=5/2-3/2 and J=7/2-5/2 lines aredetected in absorption. The 18OH 2Pi3/2 J=5/2-3/2 Lambda-doublet at 120 mu isalso observed in absorption. All OH Lambda-doublets are resolved (except the 98mu) 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 theforeground clouds along the line of sight. No asymmetries in the line intensities of each doublet have been observed. Wehave modeled the observations using a non-local radiative transfer code andfound that the OH absorption/emission must arise in a shell around Sgr B2 notresolved by the ISO/LWS beam. The gas density is moderate, with upper limits of10^4 cm^{-3} and 300 K in temperature. The OH abundance is high,(2-5)10^{-6}.We argue that a widespread photon dominated region explains the enhancement ofOH abundance.Comment: 13 pages, 2 figures, accepted in ApJ Letter