Irradiated Disks May Settle into Staircases

TL;DR

This study demonstrates that irradiated protoplanetary disks can settle into a stable staircase shape with bright rings and dark gaps, challenging previous beliefs of smooth disk structures and impacting theories of disk evolution and planet formation.

Contribution

The paper introduces a new stable staircase configuration for irradiated disks, supported by simulations and thermal equilibrium analysis, contrasting prior studies claiming smooth disk shapes.

Findings

Disks settle into staircase shapes with bright rings and dark gaps.

Bright rings intercept most stellar illumination, shadows create dark gaps.

The staircase state is in good thermal equilibrium according to radiative transfer modeling.

Abstract

Much of a protoplanetary disk is thermally controlled by irradiation from the central star. Such a disk, long thought to have a smoothly flaring shape, is unstable to the so-called 'irradiation instability'. But what's the outcome of such an instability? In particular, is it possible that such a disk settles into a shape that is immune to the instability? We combine Athena++ with a simplified thermal treatment to show that passively heated disks settle into a 'staircase' shape. Here, the disk is punctuated by bright rings and dark gaps, with the bright rings intercepting the lion's share of stellar illumination, and the dark gaps hidden in their shadows. The optical surface of such a disk (height at which starlight is absorbed) resembles a staircase. Although our simulations do not have realistic radiative transfer, we use the RADMC3d code to show that this steady state is in good thermal equilibrium. It is possible that realistic disks reach such a state via ways not captured by our simulations. In contrast to our results here, two previous studies have claimed that irradiated disks stay smooth. We show here that they err on different issues. The staircase state, if confirmed by more sophisticated radiative hydrodynamic simulations, has a range of implications for disk evolution and planet formation.