Rapid radiative clearing of protoplanetary discs

TL;DR

This paper revises the understanding of how photoevaporation, specifically thermal sweeping, clears protoplanetary discs, showing it is less efficient than previously thought and raising questions about the rapid removal of outer disc gas.

Contribution

It presents a new criterion for thermal sweeping based on pressure maxima, refining the conditions under which rapid disc clearing occurs.

Findings

Thermal sweeping occurs when the pressure maximum drops below the X-ray heated gas pressure.

New critical density estimates suggest less efficient disc clearing than earlier models.

The study questions the sufficiency of X-ray driven thermal sweeping to explain rapid outer disc dispersal.

Abstract

The lack of observed transition discs with inner gas holes of radii greater than ~50AU implies that protoplanetary discs dispersed from the inside out must remove gas from the outer regions rapidly. We investigate the role of photoevaporation in the final clearing of gas from low mass discs with inner holes. In particular, we study the so-called "thermal sweeping" mechanism which results in rapid clearing of the disc. Thermal sweeping was originally thought to arise when the radial and vertical pressure scale lengths at the X-ray heated inner edge of the disc match. We demonstrate that this criterion is not fundamental. Rather, thermal sweeping occurs when the pressure maximum at the inner edge of the dust heated disc falls below the maximum possible pressure of X-ray heated gas (which depends on the local X-ray flux). We derive new critical peak volume and surface density estimates for rapid radiative clearing which, in general, result in rapid dispersal happening less readily than in previous estimates. This less efficient clearing of discs by X-ray driven thermal sweeping leaves open the issue of what mechanism can clear gas from the outer disc sufficiently quickly to explain the non-detection of cold gas around weak line T Tauri stars.