The foggy disks surrounding herbig Ae stars: A theoretical study of the H₂O line spectra

Water is a key species in many astrophysical environments, but it is particularly important in protoplanetary disks. So far, observations of water in these objects have been scarce, but the situation should soon change thanks to the Herschel satellite. We report here a theoretical study of the water line spectrum of a protoplanetary disk surrounding Ae stars. We show that several lines will be observable with the HIFI instrument onboard the Herschel Space Observatory. We predict that some maser lines also could be observable with ground telescopes and we discuss how the predictions depend not only on the adopted physical and chemical model but also on the set of collisional coefficients used and on the H₂ ortho-to-para ratio through its effect on collisional excitation. This makes the water line observations a powerful, but dangerous - if misused - diagnostic tool.