Warm H2O and OH in the disk around the Herbig star HD 163296

We present observations of far-infrared (50-200 mu m) OH and H2O 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 [OI] emission, a number of OH doublets and a few weak highly excited lines of H2O are detected. The presence of warm H2O 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 H2O 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 N-mol of 10(14)-10(15) cm(-2) and T-ex similar to 300-500 K. For both species, we find an emitting region of r similar to 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 H2O 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 H2O. Given the similar column density and emitting region, OH and H2O 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 H2O compared to OH suggests a vertical stratification of the molecular gas with OH located higher and H2O deeper in the disk, consistent with thermo-chemical models.