Hi21 cm Emission as a Tracer of Gas During the Evolution from Protoplanetary to Debris Disks

We present models for the HI 21 cm emission from circumstellar disks and usethem to convert observed upper limits on the HI 21 cm flux to limits on thetotal disk gas mass. The upper limits we use come from earlier AustraliaTelescope Compact Array observations of the debris disk around beta Pictoris aswell as fresh Giant Meterwave Radio Telescope observations of HD135344, LkCa 15and HD163296. Our observations and models span a range of disk types, fromyoung proto-planetary disks to old debris disks. The models self-consistentlycalculate the gas chemistry (H/H2 balance) and the thermal structure of UVirradiated disks. Atomic hydrogen production is dominated by UV irradiation intransition phase objects as well as debris disks, but for very young disks, HIproduction by stellar X-rays (which we do not account for) is important. We usea simple radiative transfer approach to convert the model disk parameters intopredicted HI 21 cm line maps and spectral profiles. This allows a directcomparison of the observations to the model. We find that the HI traces thedisk surface layers, and that the Hi emission, if detected, could be used tostudy the e ects of irradiation and evaporation, in addition to the kinematicsof the disk. Our models cover massive protoplanetary disks, transition phasedisks and dusty debris disks. In massive protoplanetary disks, UV produced HIconstitutes less than 0.5% of the total disk mass, while X-rays clearlydominate the chemistry and thus the HI production. For the two such disks thatwe have observed, viz. those around LkCa 15 and HD163296, the predicted 21 cmflux is below the current detection limit. On the other hand, transition phasedisks at distances of 100 pc have predicted 21 cm fluxes that are close to thedetection limit.... (abstract abbreviated)Comment: 35 pages, 9 figures, accepted for publication in Ap