Wave-like warp propagation in circumbinary discs II. Application to KH 15D

TL;DR

This paper models the warp propagation in circumbinary discs around KH 15D, showing that such discs can precess as a rigid body with small warps, and explores how viscosity and a third body influence their dynamics.

Contribution

It provides an analytical and numerical study of warp propagation in narrow circumbinary discs, applying it specifically to KH 15D, and suggests the presence of a third body could explain observed features.

Findings

Discs extend out to 6-10 AU, wider than previously thought.

Discs precess as a rigid body with small warps.

Viscosity causes tilt decay over ~3000 years.

Abstract

KH 15D is a protostellar binary system that shows a peculiar light curve. In order to model it, a narrow circumbinary precessing disc has been invoked, but a proper dynamical model has never been developed. In this paper, we analytically address the issue of whether such a disc can rigidly precess around KH 15D, and we relate the precessional period to the main parameters of the system. Then, we simulate the disc's dynamics by using a 1D model developed in a companion paper, such that the warp propagates into the disc as a bending wave, which is expected to be the case for protostellar discs. The validity of such an approach has been confirmed by comparing its results with full 3D SPH simulations on extended discs. In the present case, we use this 1D code to model the propagation of the warp in a narrow disc. If the inner truncation radius of the disc is set by the binary tidal torques at {\sim} 1 AU, we find that the disc should extend out to 6-10 AU (depending on the models), and is therefore wider than previously suggested. Our simulations show that such a disc does reach an almost steady state, and then precesses as a rigid body. The disc displays a very small warp, with a tilt inclination that increases with radius in order to keep the disc in equilibrium against the binary torque. However, for such wider discs, the presence of viscosity leads to a secular decay of the tilt on a timescale of {\approx} 3000 ({\alpha}/0.05)^(-1) years, where {\alpha} is the disc viscosity parameter. The presence of a third body (such as a planet), orbiting at roughly 10 AU might simultaneously explain the outer truncation of the disc and the maintenance of the tilt for a prolonged time.