2012A&A...544L...9F

We present observations of far-infrared (50-200µm) 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 10¹⁴-10¹⁵cm^–2, and the excitation temperature is 200-300K, implying warm gas with a density n>10⁵cm^–3. For OH, we find N_mol of 10¹⁴-10¹⁵cm^–2 and T_ex∼300-500K. For both species, we find an emitting region of r∼15-20AU 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.