Weak, extended water vapor emission in the Horsehead nebula

TL;DR

This study investigates the weak water vapor emission in the Horsehead nebula's PDR, revealing its extended distribution beyond other tracers and highlighting the need for improved models of water chemistry and radiative transfer effects.

Contribution

It provides detailed modeling of water vapor distribution in the Horsehead nebula, emphasizing the importance of geometry and radiative transfer in interpreting water emission.

Findings

Water vapor emission extends beyond other PDR tracers.

Models overestimate the ground state water line by at least a factor of 7.

Water abundance reaches 3.6 x 10^-7 at A_V ~3 mag.

Abstract

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 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 $^{12}$CO or [CI] 609 $\mu$m, reaching as far out as [CII] 158 $\mu$m. We model the observations using a newly developed PDR wrapper that takes into account the geometry of this region. 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 %typically agree to within ~40\% with the observations. and spatial profiles are 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 $3.6 \times 10^{-7}$ at $A_V \sim 3$ mag. However, the ground state $o$-H$_2$O 557 GHz line is systematically overestimated by the models by at least a factor of 7 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.