Constraints on photoevaporation models from (lack of) radio emission in
  the Corona Australis protoplanetary disks
  Photoevaporation due to high-energy stellar photons is thought to be one of
the main drivers of protoplanetary disk dispersal. The fully or partially
ionized disk surface is expected to produce free-free continuum emission at
centimeter (cm) wavelengths that can be routinely detected with interferometers
such as the upgraded Very Large Array (VLA). We use deep (rms noise down to 8
$\mu$Jy beam$^{-1}$ in the field of view center) 3.5 cm maps of the nearby (130
pc) Corona Australis (CrA) star formation (SF) region to constrain disk
photoevaporation models. We find that the radio emission from disk sources in
CrA is surprisingly faint. Only 3 out of 10 sources within the field of view
are detected, with flux densities of order $10^2$ $\mu$Jy. However, a
significant fraction of their emission is non-thermal. Typical upper limits for
non-detections are $3\sigma\sim 60~\mu$Jy beam$^{-1}$. Assuming analytic
expressions for the free-free emission from extreme-UV (EUV) irradiation, we
derive stringent upper limits to the ionizing photon luminosity impinging on
the disk surface $\Phi_\mathrm{EUV}<1-4\times10^{41}$ s$^{-1}$. These limits
constrain $\Phi_\mathrm{EUV}$ to the low end of the values needed by EUV-driven
photoevaporation models to clear protoplanetary disks in the observed few Myr
timescale. Therefore, at least in CrA, EUV-driven photoevaporation is not
likely to be the main agent of disk dispersal. We also compare the observed
X-ray luminosities $L_X$ of disk sources with models in which photoevaporation
is driven by such photons. Although predictions are less specific than for the
EUV case, most of the observed fluxes (upper limits) are roughly consistent
with the (scaled) predictions. Deeper observations, as well as predictions
spanning a wider parameter space, are needed to properly test X-ray driven
photoevaporation.
