A New Steady State of Irradiated Disks: the Staircase

Much of a protoplanetary disk is thermally controlled by the central irradiation. Such a disk, thought to have a flaring shape, is likely 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 settle 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-like steady state. The steady state is punctuated by bright rings and dark gaps, with the bright rings intercepting the lion’s share of stellar illumination, and the dark gaps lying in shadows. The optical surface of such a disk (height at which starlight is absorbed) resembles a staircase. We use the RADMC3d code to show that the steady state we find is also in good thermal equilibrium. The novel staircase state, if confirmed by more sophisticated radiative hydrodynamic simulations, has a range of implications for disk evolution and planet formation.