One-armed spirals in locally isothermal, radially structured self-gravitating discs

TL;DR

This paper introduces a new instability mechanism in astrophysical discs with radial temperature gradients, leading to the growth of one-armed spiral patterns without requiring star movement or disc self-gravity.

Contribution

It demonstrates that imposed radial temperature gradients can destabilize non-axisymmetric waves, causing one-armed spirals in both self-gravitating and non-self-gravitating discs.

Findings

Radial temperature gradients induce instability in non-axisymmetric waves.

Numerical simulations confirm exponential growth of one-armed spirals.

Mechanism applies to both self-gravitating and non-self-gravitating discs.

Abstract

We describe a new mechanism that leads to the destabilisation of non-axisymmetric waves in astrophysical discs with an imposed radial temperature gradient. This might apply, for example, to the outer parts of protoplanetary discs. We use linear density wave theory to show that non-axisymmetric perturbations generally do not conserve their angular momentum in the presence of a forced temperature gradient. This implies an exchange of angular momentum between linear perturbations and the background disc. In particular, when the disturbance is a low-frequency trailing wave and the disc temperature decreases outwards, this interaction is unstable and leads to the growth of the wave. We demonstrate this phenomenon through numerical hydrodynamic simulations of locally isothermal discs in 2D using the FARGO code and in 3D with the ZEUS-MP and PLUTO codes. We consider radially structured discs with a self-gravitating region which remains stable in the absence of a temperature gradient. However, when a temperature gradient is imposed we observe exponential growth of a one-armed spiral mode (azimuthal wavenumber $m=1$) with co-rotation radius outside the bulk of the spiral arm, resulting in a nearly-stationary one-armed spiral pattern. The development of this one-armed spiral does not require the movement of the central star, as found in previous studies. Because destabilisation by a forced temperature gradient does not explicitly require disc self-gravity, we suggest this mechanism may also affect low-frequency one-armed oscillations in non-self-gravitating discs.