| Abstract
| - We present observations of far-infrared (50−200 μm) OH and H 2O emission of the disk around the Herbig Ae star HD 163296 obtained with Herschel/PACS in the context of the DIGIT key program. In addition to strong [O i] emission, a number of OH doublets and a few weak highly excited lines of H 2O are detected. The presence of warm H 2O in this Herbig disk is confirmed by a line stacking analysis, enabled by the full PACS spectral scan, and by lines seen in Spitzer data. The line fluxes are analyzed using a local-thermal-equilibrium slab model including line opacity. The H 2O column density is 10 14−10 15 cm -2, and the excitation temperature is 200−300 K, implying warm gas with a density n > 10 5 cm -3. For OH, we find Nmol of 10 14−10 15 cm -2 and Tex ~ 300−500 K. For both species, we find an emitting region of r ~ 15−20 AU from the star. We argue that the molecular emission arises from the protoplanetary disk rather than the outflow. This far-infrared detection of both H 2O and OH contrasts with near- and mid-infrared observations, which have generally found a lack of water in the inner disk around Herbig AeBe stars owing to the strong photodissociation of H 2O. Given the similar column density and emitting region, OH and H 2O emission seems to arise from an upper layer of the disk atmosphere of HD 163296, which probes a new reservoir of water. The slightly lower temperature of H 2O compared to OH suggests a vertical stratification of the molecular gas with OH located higher and H 2O deeper in the disk, consistent with thermo-chemical models.
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