Far-infrared OH fluorescent emission in Sagittarius B2

We present Infrared Space Observatory (ISO) observations of (OH)-O-16 and (OH)-O-18 toward Sgr B2 with a spectral resolution of 35 km s(-1). The OH J = 5/2 --> 3/2 and 3/2 --> 1/2 rotational lines of the (2)Pi(1/2) ladder are seen in emission, while the cross-ladder transitions (from the (2)Pi(3/2) J = 3/2 to the J = 1/2, 3/2, and 5/2 levels of the ladder) and the and lines are detected in absorption. The (OH)-O-18 (2Pi3/2) J = 5/2 <-- 3/2 Lambda-doublet at similar to 120 mu m is also observed in absorption. All OH Lambda -doublets are resolved (except the similar to 98 mu m) and show, in addition to the strong absorption at the velocity of Sgr B2, several velocity components associated with the gas surrounding Sgr B2 and with the foreground clouds along the line of sight. No asymmetries in the line intensities of each doublet have been observed. We model the observations using a nonlocal radiative transfer code and find that the OH absorption/emission must arise in a shell around Sgr B2 not resolved by the ISO/Long-Wavelength Spectrometer beam. The gas density is moderate, with upper limits of 10(4) cm(-3) and similar or equal to 300 K in temperature. The OH abundance is high, (2-5) x 10(-6). We argue that a widespread photon-dominated region explains the enhancement of OH abundance.