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

Context. Debris disks contain the formation and evolution histories of planetary systems. Recent detections of gas in these disks have received considerable attention, as the origin of the gas sheds light on ongoing disk evolution and the current composition of planet-forming materials. Aims. Observations of CO gas alone, however, cannot reliably differentiate between two leading and competing hypotheses: (1) that the observed gas is a leftover of protoplanetary disk gas, and (2) that the gas is the outcome of collisions between icy bodies. We propose that such a differentiation may become possible by observing cold water vapor. Methods. We performed order-of-magnitude analyses and compared these with existing observations. Results. 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. Conclusions. 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.