Short-Lived Gravitational Instability in Isolated Irradiated Discs

TL;DR

This study uses advanced 3D simulations to show that stellar irradiation prevents long-term gravitational instability regulation in protoplanetary discs, leading to transient spiral structures and potential fragmentation in more massive discs.

Contribution

It introduces coupled hydrodynamics and radiative transfer simulations revealing the dominant role of stellar irradiation in disc thermal regulation and instability behavior.

Findings

Temperature remains approximately constant due to stellar irradiation.

Spiral arms in irradiated discs are short-lived and hard to detect.

Higher mass discs are prone to fragmentation despite irradiation effects.

Abstract

Irradiation from the central star controls the temperature structure in protoplanetary discs. Yet simulations of gravitational instability typically use models of stellar irradiation with varying complexity, or ignore it altogether, assuming heat generated by spiral shocks is balanced by cooling, leading to a self-regulated state. In this paper, we perform simulations of irradiated, gravitationally unstable protoplanetary discs using 3D hydrodynamics coupled with live Monte-Carlo radiative transfer. We find that the resulting temperature profile is approximately constant in time, since the thermal effects of the star dominate. Hence, the disc cannot regulate gravitational instabilities by adjusting the temperatures in the disc. In a 0.1 Solar mass disc, the disc instead adjusts by angular momentum transport induced by the spiral arms, leading to steadily decreasing surface density, and hence quenching of the instability. Thus, strong spiral arms caused by self-gravity would not persist for longer than ten thousand years in the absence of fresh infall, although weak spiral structures remain present over longer timescales. Using synthetic images at 1.3mm, we find that spirals formed in irradiated discs are challenging to detect. In higher mass discs, we find that fragmentation is likely because the dominant stellar irradiation overwhelms the stabilising influence of PdV work and shock heating in the spiral arms.