Testing protoplanetary disc dispersal with radio emission

Abstract

We consider continuum free-free radio emission from the upper atmosphere of protoplanetary discs as a probe of the ionized luminosity impinging upon the disc. Making use of previously computed hydrodynamic models of disc photoevaporation within the framework of EUV and X-ray irradiation, we use radiative transfer post-processing techniques to predict the expected free-free emission from protoplanetary discs. In general, the free-free luminosity scales roughly linearly with ionizing luminosity in both EUV and X-ray driven scenarios, where the emission dominates over the dust tail of the disc and is partial optically thin at cm wavelengths. We perform a test observation of GM Aur at 14-18 Ghz and detect an excess of radio emission above the dust tail to a very high level of confidence. The observed flux density and spectral index are consistent with free-free emission from the ionized disc in either the EUV or X-ray driven scenario. Finally, we suggest a possible route to testing the EUV and X-ray driven dispersal model of protoplanetary discs, by combining observed free-free flux densities with measurements of mass-accretion rates. On the point of disc dispersal one would expect to find a M_dot^2 scaling with free-free flux in the case of EUV driven disc dispersal or a M_dot scaling in the case of X-ray driven disc dispersal.