2002ApJ...576L..77G

We present Infrared Space Observatory (ISO) observations of ¹⁶OH and ¹⁸OH toward Sgr B2 with a spectral resolution of ∼35 km.s^–1. The OH J=5/2⟶3/2 and J=3/2⟶1/2 rotational lines of the ²Π_1/2 ladder are seen in emission, while the cross-ladder transitions (from the ²Π_3/2 J=3/2 to the J=1/2, 3/2, and 5/2 levels of the ²Π_1/2 ladder) and the ²Π_3/2 J=5/2⟵3/2 and J=7/2⟵5/2 lines are detected in absorption. The ¹⁸OH ²Π_3/2 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 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 10⁴ cm^–3 and ≃300 K in temperature. The OH abundance is high, (2-5)x10^–6. We argue that a widespread photon-dominated region explains the enhancement of OH abundance.