Negative superhumps in cataclysmic variables driven by retrograde apsidal disk precession

TL;DR

This paper proposes that negative superhumps in cataclysmic variables are caused by retrograde apsidal precession of eccentric disks, driven by pressure effects and disk size, challenging the traditional tilt-based explanation.

Contribution

It introduces a new model attributing negative superhumps to apsidal precession, emphasizing pressure effects and disk dynamics over disk tilt.

Findings

Pressure effects can induce retrograde precession in cool disks.

Disk expansion during outbursts can cause coexistence of positive and negative superhumps.

The mechanism explains negative superhumps across various mass ratios and states.

Abstract

Negative superhumps are photometric modulations in cataclysmic variables with periods slightly shorter than the orbital period. They are usually attributed to retrograde nodal precession of a tilted accretion disk, although the origin and persistence of the tilt remains unexplained. We propose instead that negative superhumps arise from retrograde apsidal precession of an eccentric disk. Using linear eccentric disk theory, we show that the direction of apsidal precession is highly sensitive to disk size and temperature, and that pressure effects can drive retrograde precession even in cool disks. In low mass ratio systems where the 3:1 resonance is within the disk, disk expansion during outbursts may produce opposite precession directions in the inner and outer disk, allowing the temporary coexistence of positive and negative superhumps, and driving dissipation in an extended superoutburst. In higher mass ratio systems where the resonance location is outside of the disk, the resonance width can still extend into the outer parts of the disk, excite eccentricity, and drive apsidal precession. This mechanism explains the prevalence of negative superhumps across a wide range of mass ratios and accretion states, without requiring a long-lived disk tilt. It may also explain how positive superhumps can occur in high mass ratio systems if the disk density builds up in the outer parts of the disk.