A slim disc approach to external photoevaporation of discs

TL;DR

This paper introduces a slim disc model for external photoevaporation of protoplanetary discs, providing a smooth transition between the disc and outflow, and predicts observable signatures in dust properties.

Contribution

It develops a self-consistent slim disc framework that captures the gradual transition to photoevaporative outflows, improving upon previous abrupt transition models.

Findings

Transition occurs over ~4-5 scale heights

UV penetration sets the transition radius

Dust surface density changes rapidly in the transition region

Abstract

The photoevaporation of protoplanetary discs by nearby massive stars present in their birth cluster plays a vital role in their evolution. Previous modelling assumes that the disc behaves like a classical Keplerian accretion disc out to a radius where the photoevaporative outflow is launched. There is then an abrupt change in the angular velocity profile, and the outflow is modelled by forcing the fluid parcels to conserve their specific angular momenta. Instead, we model externally photoevaporating discs using the slim disc formalism. The slim disc approach self consistently includes the advection of radial and angular momentum as well as angular momentum redistribution by internal viscous torques. Our resulting models produce a smooth transition from a rotationally supported Keplerian disc to a photoevaporative driven outflow, where this transition typically occurs over ~4-5 scale heights. The penetration of UV photons predominately sets the radius of the transition and the viscosity's strength plays a minor role. By studying the entrainment of dust particles in the outflow, we find a rapid change in the dust size and surface density distribution in the transition region due to the steep gas density gradients present. This rapid change in the dust properties leaves a potentially observable signature in the continuum spectral index of the disc at mm wavelengths. Using the slim disc formalism in future evolutionary calculations will reveal how both the gas and dust evolves in their outer regions and the observable imprints of the external photoevaporation process.