Constraints on photoevaporation models from (lack of) radio emission in the Corona Australis protoplanetary disks^⋆

¹ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
e-mail: r.galvan@crya.unam.mx; rgalvan@eso.org
² Centro de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de México, 58090 Morelia, Mexico
³ Academia Sinica, Institute of Astronomy and Astrophysics, PO Box 23-141, Taipei 106, Taiwan
⁴ Department of Astrophysics, University of Vienna, Türkenschanzstraße 17, 1180 Vienna, Austria
⁵ Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA
⁶ Joint Institute for VLBI in Europe, Postbus 2, 7990 AA Dwingeloo, The Netherlands
⁷ INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 50125 Firenze, Italy
⁸ Excellence Cluster Universe, Boltzmannstr. 2, 85748 Garching, Germany

Received: 18 July 2014
Accepted: 24 September 2014

Abstract

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 μJy 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 three out of ten sources within the field of view are detected, with flux densities of about 10² μJy. However, a significant fraction of their emission is nonthermal. Typical upper limits for nondetections are 3σ ~ 60 μJy 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 that impinges on the disk surface Φ_EUV < 1−4 × 10⁴¹ s^-1. These limits constrain Φ_EUV to the low end of the values needed by EUV-driven photoevaporation models to clear protoplanetary disks within the observed few Myr timescale. Therefore, at least in CrA, EUV-driven photoevaporation is unlikely 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.