2026A&A...706A...7L

We 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 isotopologs 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 ¹²CO or [C I] 609 μm, reaching as far out as [C II] 158 μm. We modeled the observations using a newly developed PDR wrapper that takes into account the geometry of this region. The PDR modeling of the molecular and atomic lines studied here provides strong constraints on the thermal pressure, but not on the UV illumination. Maximum model line intensities and spatial profiles are well reproduced, except for CO isotopologs, 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 A_V ∼ 3 mag. However, the ground state o-H₂O 557 GHz line is systematically overestimated by the models by at least a factor of seven for any values of the model parameters. This line has a very high optical depth and the emergent line intensity is sensitive to radiative transfer effects such as line scattering by water molecules in a low-density halo surrounding the dense PDR and the assumed microturbulent line width. A more accurate model of the water surface chemistry is required.