Gap formation and stability in non-isothermal protoplanetary discs

Abstract

Several observations of transition discs show lopsided dust-distributions. A potential explanation is the formation of a large-scale vortex acting as a dust-trap at the edge of a gap opened by a giant planet. Numerical models of gap-edge vortices have thus far employed locally isothermal discs, but the theory of this vortex-forming or `Rossby wave' instability was originally developed for adiabatic discs. We generalise the study of planetary gap stability to non-isothermal discs using customised numerical simulations of disc-planet systems where the planet opens an unstable gap. We include in the energy equation a simple cooling function with cooling timescale $t_c=\beta {\Omega }_k^{-1}$, where ${\Omega }_k$ is the Keplerian frequency, and examine the effect of $\beta$ on the stability of gap edges and vortex lifetimes. We find increasing $\beta$ lowers the growth rate of non-axisymmetric perturbations, and the dominant azimuthal wavenumber $m$ decreases. We find a quasi-steady state consisting of one large-scale, over-dense vortex circulating the outer gap edge, typically lasting $O(10^3)$ orbits. Vortex lifetimes were found to generally increase with cooling times up to an optimal value, beyond which vortex lifetimes decrease. This non-monotonic dependence is qualitatively consistent with recent studies using strictly isothermal discs that vary the disc aspect ratio.

12 pages, 13 figures, 1 table, accepted by MNRAS

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• On the vortex evolution in non-isothermal protoplanetary discs
• Self-Sustaining Vortices in Protoplanetary Disks: Setting the Stage for Planetary System Formation
• Dynamics of Dusty Vortices II: Stability of 2D dust laden vortices
• Planetary nurseries: vortices formed at smooth viscosity transition
• The Prospects for Hurricane-like Vortices in Protoplanetary Disks