Roche tomography of cataclysmic variables - VI. Differential rotation of AE Aqr - Not tidally locked!

TL;DR

This study uses Roche tomography to analyze the secondary star in AE Aqr, revealing differential rotation and starspots, challenging the assumption of tidal locking in cataclysmic variables, and discussing implications for stellar dynamo theory and accretion.

Contribution

First measurement of differential rotation in AE Aqr's secondary star, showing it is not tidally locked, with detailed starspot mapping and system parameter determination.

Findings

AE Aqr's secondary exhibits differential rotation with a lap time of ~265 days.

Starspots are present at high latitudes, covering 15-17% of the northern hemisphere.

The star is not fully tidally locked, with a co-rotation latitude around 40°.

Abstract

We present Roche tomograms of the K4V secondary star in the cataclysmic variable AE Aqr, reconstructed from two datasets taken 9 days apart, and measure the differential rotation of the stellar surface. The tomograms show many large, cool starspots, including a large high-latitude spot and a prominent appendage down the trailing hemisphere. We find two distinct bands of spots around 22$^{\circ}$ and 43$^{\circ}$ latitude, and estimate a spot coverage of 15.4-17% on the northern hemisphere. Assuming a solar-like differential rotation law, the differential rotation of AE Aqr was measured using two different techniques. The first method yields an equator-pole lap time of 269 d and the second yields a lap time of 262 d. This shows the star is not fully tidally locked, as was previously assumed for CVs, but has a co-rotation latitude of $\sim 40^{\circ}$. We discuss the implications that these observations have on stellar dynamo theory, as well as the impact that spot traversal across the first Lagrangian point may have on accretion rates in CVs as well as some of their other observed properties. The entropy landscape technique was applied to determine the system parameters of AE Aqr. For the two independent datasets we find M$_1$ = 1.20 and 1.17 M$_{\odot}$, M$_2$ = 0.81 and 0.78 M$_{\odot}$, and orbital inclinations of $50^{\circ}$ to $51^{\circ}$ at optimal systemic velocities of {\gamma} = -64.7 and -62.9 kms$^{-1}$.