Water gas discs in exo-asteroid belts

Observations of tens of secondary CO gas discs associated with cold exo-Kuiper belts together with other arguments have led to the idea initially investigated in the Solar System that water ice could also sublimate in exo-asteroid belts, suggesting a new pathway for the delivery of water to terrestrial planets, including Earth. Our aim was to model such water vapour discs and to characterise their physical properties across a range of extrasolar systems with different host stars. We further investigated the implications for the accretion of this water by potential planets located in the inner regions of these systems. We adapted and extended previous gas production and evolution models to follow the outgassing, photodissociation, and viscous evolution of water vapour discs. We performed a suite of simulations exploring the parameter space, focusing on the stellar mass, the mass of the parent belt, and its orbital location. We also included an inner planet to estimate the mass of water accreted as a function of disc properties and system architecture. We find that systems hosting more massive stars (M_⋆ ≳ 1,M_⊙) produce water vapour very efficiently, sublimating nearly all of the ice initially present in the belt. In most cases, the bulk of the gas mass is generated early, when the stellar luminosity is highest. Long-term shielding of water vapour against photodissociation occurs only in the most massive belts. The amount of water accreted by inner planets can approach the initial ice mass of the belt, leading to planets with water inventories comparable to or exceeding those of Earth, potentially creating ocean planets. We find that water outgassing occurs soon after the protoplanetary disc dissipates in systems containing exo-asteroid belts. ALMA, JWST, and ELT are capable of detecting this water vapour for several tens of millions of years, even in relatively low-mass water discs containing down to a few 10^-9 M_⊕ at 100 pc. Hence, if such water gas discs are present, they should be detectable with the current facilities.