Weak, extended water vapor emission in the Horsehead nebula

We have analyzed archival Herschel observations of water vapor emission toward the Horsehead photon dominated region (PDR), along with supporting ground-based and airborne observations of CO isotopologues and fine structure lines of ionized and atomic carbon to determine the distribution and abundance of water vapor in this low-UV illumination PDR. Water emission in the Horsehead nebula is very weak and, surprisingly, extends outward beyond other PDR tracers such as CO isotopologues or [C i] 609 µm, and as far out as [C ii] 158 µm. We model the observations using a newly developed a PDR wrapper that takes into account the geometry of this region. PDR modeling provides strong constraints on the thermal pressure, but not on the UV illumination. Maximum model line intensities agree to within 40% with the observations. Spatial profiles are also well reproduced, except for CO isotopologues, where the increase on the illuminated side of the PDR is steeper than observed. Water vapor abundance in the model reaches 3.6 ×10−7 at AV∼3 mag. However, the ground state o-H2O 557 GHz line is systematically overestimated by the models. This line has a very high optical depth and the emergent line intensity is sensitive to radiative transfer effects such as line scattering by low-density halos and the assumed microturbulent line width. A more accurate model of the water surface chemistry is required.