Testing viscous disc theory using the balance between stellar accretion and external photoevaporation of protoplanetary discs

TL;DR

This paper introduces a novel method to study angular momentum transport in protoplanetary discs by comparing stellar accretion rates with external photoevaporative winds, providing new insights into disc evolution.

Contribution

The study demonstrates how external photoevaporation can be used to empirically constrain viscous spreading in the outer regions of protoplanetary discs, a previously challenging area.

Findings

External photoevaporation explains disc depletion in $\sigma$ Orionis.

Mass flux due to viscosity balances photoevaporative mass loss.

Comparison of wind and accretion rates constrains viscous models.

Abstract

The nature and rate of (viscous) angular momentum transport in protoplanetary discs (PPDs) has important consequences for the formation process of planetary systems. While accretion rates onto the central star yield constraints on such transport in the inner regions of a PPD, empirical constraints on viscous spreading in the outer regions remain challenging to obtain. Here we demonstrate a novel method to probe the angular momentum transport at the outer edge of the disc. This method applies to PPDs that have lost a significant fraction of their mass due to thermal winds driven by UV irradiation from a neighbouring OB star. We demonstrate that this external photoevaporation can explain the observed depletion of discs in the 3-5 Myr old $\sigma$ Orionis region, and use our model to make predictions motivating future empirical investigations of disc winds. For populations of intermediate-age PPDs, in viscous models we show that the mass flux outwards due to angular momentum redistribution is balanced by the mass-loss in the photoevaporative wind. A comparison between wind mass-loss and stellar accretion rates therefore offers an independent constraint on viscous models in the outer regions of PPDs.