The role of detailed gas and dust opacities in shaping the evolution of the inner disc edge subject to episodic accretion

Context . The transition in turbulence in the inner regions of protoplanetary discs and the closely connected dust sublimation front lead to a periodic instability, manifesting as episodic accretion outbursts. For the corresponding interplay between heating and cooling, the opacity of the material needs to be treated carefully. Aims . We investigated the effects of different dust and gas opacity descriptions on the structure and evolution of the inner regions of protoplanetary discs. The effect on the episodic instability of the inner disc edge is of central interest here. Methods . Two-dimensional (2D) axisymmetric radiation hydrodynamic models were employed to simulate the evolution of the inner disc over the course of several thousand years. Our simulations greatly expand on previously published models by implementing detailed descriptions of the gas and dust opacities in terms of their mean and frequency-dependent values. This allowed us to also consider binned frequency-dependent irradiation from the central star. Results . The adaptive opacity description affects the structure of the inner disc rim to a great extent, and the contribution of the gas opacities is the most significant effect. The resulting effects include the shift in position of the dust sublimation front and the dead zone inner edge (DZIE), a significantly altered temperature in the dust-free region, and the manifestation of the equilibrium temperature degeneracy as a sharp temperature transition. The episodic instability due to the activation of the magneto-rotational instability in the dead zone still occurs, but at lower inner disc densities. While the gas opacities set the initial conditions for the instability by determining the location of the DZIE, the evolution of the outburst itself is mainly governed by the dust opacities. The analysis of criteria for non-axisymmetric instabilities reveals a possible breaking of the density peaks that is produced during the burst phase. However, due to the periodicity of the instability cycle, the DZIE itself may remain stable throughout the quiescent phases according to linear criteria applied to our axisymmetric models. Conclusions . Although the thermal structure of the inner disc is crucially affected by different opacity descriptions, especially by the contribution of gas, the mechanism of the periodic instability of the DZIE remains active and is only marginally affected by the gas opacities. The observational consequences of the severely altered temperatures can be significant and require further investigation.