Water vapor as a probe of the origin of gas in debris disks

TL;DR

This paper explores how observing cold water vapor in debris disks can distinguish between leftover protoplanetary gas and collision-produced gas, emphasizing the need for future far-IR space observatories.

Contribution

It proposes that cold water vapor observations can differentiate gas origins in debris disks, a novel approach not previously utilized.

Findings

Different hypotheses predict different water vapor masses.

Cold water vapor results from photodesorption by UV radiation.

Current facilities cannot detect the relevant FIR emission lines.

Abstract

Debris disks embrace the formation and evolution histories of planetary systems. Recent detections of gas in these disks have received considerable attention, as its origin ties up ongoing disk evolution and the present composition of planet-forming materials. Observations of the CO gas alone, however, cannot reliably differentiate between two leading, competing hypotheses: (1) the observed gas is the leftover of protoplanetary disk gas, and (2) the gas is the outcome of collisions between icy bodies. We propose that such differentiation may become possible by observing cold water vapor. Order-of-magnitude analyses and comparison with existing observations are performed. We show that different hypotheses lead to different masses of water vapor. This occurs because, for both hypotheses, the presence of cold water vapor is attributed to photodesorption from dust particles by attenuated interstellar UV radiation. Cold water vapor cannot be observed by current astronomical facilities as most of its emission lines fall in the far-IR (FIR) range. This work highlights the need for a future FIR space observatory to reveal the origin of gas in debris disks and the evolution of planet-forming disks in general.