Rotational Line Emission from Water in Protoplanetary Disks

Circumstellar disks provide the material reservoir for the growth of young stars and for planet formation. We combine a high-level radiative transfer program with a thermal-chemical model of a typical T Tauri star disk to investigate the diagnostic potential of the far-infrared lines of water for probing disk structure. We discuss the observability of pure rotational H2O lines with the Herschel Space Observatory, specifically the residual gas where water is mainly frozen out. We find that measuring both the line profile of the ground 110 − 101 ortho-H2O transition and the ratio of this line to the 312 − 303 and 221 − 212 line can provide information on the gas phase water between 5-100 AU, but not on the snow line which is expected to occur at smaller radii. Subject headings: accretion, accretion disks – infrared: stars – planetary systems: protoplanetary disks – stars: formation: pre-main sequence – X-rays: stars – radiative transfer