2010ApJ...725L.135C

Water is an important species that has a large influence on the chemical and physical structure of protoplanetary disks. Unfortunately, its main isotope is difficult to observe from the ground because of Earth's atmosphere, with the exception of a few maser lines. A viable alternative is offered by its isotopologue HDO, for which several lines are observable (and observed) with ground telescopes. In addition, observations of the HDO lines permit us to estimate the HDO/H₂ O ratio, which can be a powerful diagnostic of the past history of the disk. In this Letter, we present a theoretical study of the HDO lines. We show that a few HDO lines are bright enough to be observable with ground-based telescopes. Among them, the 225 GHz line is the brightest because of a maser effect taking place in the inner dense and warm regions of the disk. The high-frequency lines are all in absorption, with absorption that can reach almost the totality of the continuum. We discuss how the line spectra arise from a delicate equilibrium between emission and absorption and are, therefore, highly dependent on the assumed physical and chemical structure. We also show that the HDO/H₂ O ratio can not be derived with a simple prescription but it requires multi-frequency observations coupled with careful modeling.