Atlas of Tilted Accretion Disks & Source to Negative Superhumps

TL;DR

This paper uses simulations to create an atlas of tilted accretion discs in dwarf novae, revealing the conditions for negative superhumps and identifying their source as inner disc light modulated by stream overflow.

Contribution

It provides the first detailed simulation-based atlas of tilted accretion discs and clarifies the origin of negative superhumps in these systems.

Findings

Discs need to be tilted more than 3 degrees for negative superhumps to be significant.

A dense cooling ring forms when the disc is sufficiently tilted for the gas stream to hit the disc face.

The source of negative superhumps is inner disc light modulated by stream overflow.

Abstract

Using smoothed particle hydrodynamics, we numerically simulate steady state accretion discs for Cataclysmic Variable Dwarf Novae systems that have a secondary-to-primary mass ratio (0.35 \le q \le 0.55). After these accretion discs have come to quasi-equilibrium, we rotate each disc out of the orbital plane by (\delta = (1, 2, 3, 4, 5,) or (20)^{o}) to induce negative superhumps. For accretion discs tilted $5^{o}$, we generate light curves and associated Fourier transforms for an atlas on negative superhumps and retrograde precession. Our simulation results suggest that accretion discs need to be tilted more than three degrees for negative superhumps to be statistically significant. We also show that if the disc is tilted enough such that the gas stream strikes a disc face, then a dense cooling ring is generated near the radius of impact. In addition to the atlas, we study these artificially tilted accretion discs to find the source to negative superhumps. Our results suggest that the source is additional light from innermost disc annuli, and this additional light waxes and wanes with the amount of gas stream overflow received as the secondary orbits. The nodes, where the gas stream transitions from flowing over to under the disc rim (and vice versa), precess in the retrograde direction.