H I 21 cm emission as a tracer of gas during the evolution from protoplanetary to debris disks

We present radiative transfer models for the H I 21 cm emission from circumstellar disks and use them to convert observed upper limits on the H I 21 cm flux to limits on the total disk gas mass. We also present fresh upper limits for the H I 21 cm emission of the disks around HD 135344, LkCa 15, and HD 163296, obtained with the Giant Meterwave Radio Telescope. Our observations and models span a range of disk types, from young protoplanetary disks to old debris disks. The models self-consistently calculate the gas chemistry ( H/H-2 balance) and the thermal structure of UV irradiated disks. Atomic hydrogen production is dominated by UV irradiation in transition phase objects as well as debris disks, but for very young disks, H I production by stellar X-rays is important. This irradiation produces H I 21 cm at the surface of the disks. In massive protoplanetary disks, UV produced H I constitutes less than 0.5% of the total disk mass, while X-rays clearly dominate the chemistry and thus the H I production. In debris disks, hydrogen is mainly molecular, since the high dust-to-gas mass ratio leads to warmer disks. The 21 cm flux cannot be detected with currently available radio telescopes in such disks. The strongest 21 cm fluxes are predicted for transition phase disks at distances of 100 pc. The expected H I fluxes in such disks are close to current detection limits. A telescope with about similar to 10% the area of the SKA will be able to detect the H I 21 cm emission from such disks, while the full SKA will provide resolved images. Such observations will probe the kinematics of disks, as well as the effects of irradiation and evaporation at their surface layer.