Low EUV Luminosities Impinging on Protoplanetary Disks

TL;DR

This study constrains the EUV radiation reaching protoplanetary disks using radio data, finding it too low to solely drive disk dispersal via photoevaporation, implying other high-energy photons play a significant role.

Contribution

First to use millimetric and centimetric radio data to set upper limits on EUV luminosity impinging on protoplanetary disks around young stars.

Findings

EUV luminosity upper limits are below 10^42 photons/s for most sources.

EUV-driven photoevaporation alone cannot explain observed disk dispersal timescales.

[NeII] emission likely traces neutral winds ionized by X-ray photons, not EUV.

Abstract

The amount of high-energy stellar radiation reaching the surface of protoplanetary disks is essential to determine their chemistry and physical evolution. Here, we use millimetric and centimetric radio data to constrain the EUV luminosity impinging on 14 disks around young (~2-10Myr) sun-like stars. For each object we identify the long-wavelength emission in excess to the dust thermal emission, attribute that to free-free disk emission, and thereby compute an upper limit to the EUV reaching the disk. We find upper limits lower than 10$^{42}$ photons/s for all sources without jets and lower than $5 \times 10^{40}$ photons/s for the three older sources in our sample. These latter values are low for EUV-driven photoevaporation alone to clear out protoplanetary material in the timescale inferred by observations. In addition, our EUV upper limits are too low to reproduce the [NeII] 12.81 micron luminosities from three disks with slow [NeII]-detected winds. This indicates that the [NeII] line in these sources primarily traces a mostly neutral wind where Ne is ionized by 1 keV X-ray photons, implying higher photoevaporative mass loss rates than those predicted by EUV-driven models alone. In summary, our results suggest that high-energy stellar photons other than EUV may dominate the dispersal of protoplanetary disks around sun-like stars.