H i 21 cm Emission as a Tracer of Gas During the Evolution from Protoplanetary to Debris Disks

Abstract

We present models for the HI 21 cm emission from circumstellar disks and use them to convert observed upper limits on the HI 21 cm flux to limits on the total disk gas mass. The upper limits we use come from earlier Australia Telescope Compact Array observations of the debris disk around beta Pictoris as well as fresh Giant Meterwave Radio Telescope observations of HD135344, LkCa 15 and HD163296. Our observations and models span a range of disk types, from young proto-planetary disks to old debris disks. The models self-consistently calculate the gas chemistry (H/H2 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, HI production by stellar X-rays (which we do not account for) is important. We use a simple radiative transfer approach to convert the model disk parameters into predicted HI 21 cm line maps and spectral profiles. This allows a direct comparison of the observations to the model. We find that the HI traces the disk surface layers, and that the Hi emission, if detected, could be used to study the e ects of irradiation and evaporation, in addition to the kinematics of the disk. Our models cover massive protoplanetary disks, transition phase disks and dusty debris disks. In massive protoplanetary disks, UV produced HI constitutes less than 0.5% of the total disk mass, while X-rays clearly dominate the chemistry and thus the HI production. For the two such disks that we have observed, viz. those around LkCa 15 and HD163296, the predicted 21 cm flux is below the current detection limit. On the other hand, transition phase disks at distances of 100 pc have predicted 21 cm fluxes that are close to the detection limit.... (abstract abbreviated)