Gap opening beyond dead zones by photoevaporation

TL;DR

This paper introduces a new model explaining large gaps in protoplanetary discs through layered accretion and photoevaporation, accounting for observed features of transition discs.

Contribution

It presents a novel hypothesis combining layered accretion and photoevaporative winds to explain gap formation beyond dead zones in protoplanetary discs.

Findings

Gap opens outside dead zones when photoevaporative mass loss exceeds accretion.

Dead zones persist after gap formation, allowing continued mass accretion.

Model reproduces observed large gaps and high accretion rates in transition discs.

Abstract

We propose a new hypothesis for the origin of protoplanetary discs with large inner holes (or gaps), so-called transition discs. Our gas disc model takes into account layered accretion, in which poorly-ionized low-viscosity dead zones are sandwiched by high-viscosity surface layers, and photoevaporative winds induced by X-rays from the central stars. We find that a gap opens at a radius outside a dead zone, if the mass loss rate due to photoevaporative winds exceeds the mass accretion rate in the dead zone region. Since the dead zone survives even after the gap opens, mass accretion onto the central star continues for a long time. This model can reproduce large gap sizes and high mass accretion rates seen in observed transition discs.