Cold water emission cannot be used to infer depletion of bulk elemental oxygen [O/H] in disks

TL;DR

This study shows that cold water emission in protoplanetary disks does not necessarily indicate oxygen depletion, as previous models suggested, but can be explained with standard ISM-like oxygen abundance and typical disk parameters.

Contribution

The paper demonstrates that cold water emission can be modeled without assuming oxygen depletion, challenging prior interpretations based on Herschel data.

Findings

Cold water vapor formation is mainly due to photodesorption at the molecular layer interface.

Water emission is largely independent of disk mass and gas/dust ratio.

Water line emission is optically thick and more sensitive to temperature than abundance.

Abstract

We re-examine the constraints provided by Herschel Space Observatory data regarding cold water emission from protoplanetary disks. Previous disk models that were used to interpret observed water emission concluded that oxygen (O/H) is depleted by at least 2 orders of magnitude if a standard, interstellar gas/dust mass ratio is assumed in the disk. In this work, we use model results from a recent disk parameter survey and show that most of the \textit{Herschel} constraints obtained for cold water (i.e. for transitions with an upper energy level $E_\mathrm{up}<200$ K, where the bulk of the disk water lies) can be explained with disk models adopting ISM-like oxygen elemental abundance (i.e. O/H=$3.2\times10^{-4}$) and the canonical gas/dust mass ratio of 100. We show that cold water vapor is mainly formed by photodesorption of water ice at the interface between the molecular layer and the midplane, and that its emission is relatively independent of the main disk properties like the disk gas mass and gas/dust mass ratio. We find that the abundance of water vapor in the outer disk is set by photoprocesses and depends on the (constant) vertical column density of water ice needed to attenuate the FUV photon flux, resulting in roughly constant emission for the parameters (gas mass, dust mass, disk radius) varied in our survey. Importantly, water line emission is found to be optically thick and hence sensitive to temperature more than abundance, possibly driving previous inferences of large scale oxygen depletion.