  We present observations of far-infrared (50-200 micron) 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 an LTE slab model including line
opacity. The water 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 a column density of 10^14 - 2x10^15 cm^-2 and T_ex ~ 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
from an 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 due to strong photodissociation of
water. Given the similarity in column density and emitting region, OH and H2O
emission seems to arise from an upper layer of the disk atmosphere of HD
163296, probing 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 water deeper in the disk, consistent with thermo-chemical
models.
