Large amplitude change in spot-induced rotational modulation of the Kepler Ap star KIC 2569073

TL;DR

This study reports a Kepler-observed Ap star with the largest known amplitude variation in rotational modulation, showing a significant increase in amplitude over two decades without change in period.

Contribution

It identifies a rare Ap star with unprecedented amplitude variation in Kepler data, highlighting long-term changes in stellar surface spots.

Findings

Largest amplitude variation observed in Kepler Ap star

Amplitude increased significantly over two decades

No change in rotational period during Kepler observations

Abstract

An investigation of the 200 $\times$ 200 pixel 'superstamp' images of the centres of the open clusters NGC 6791 and NGC 6819 allows for the identification and study of many variable stars that were not included in the Kepler target list. KIC 2569073 (V=14.22), is a particularly interesting variable Ap star that we discovered in the NGC 6791 superstamp. With a rotational period of 14.67 days and 0.034-mag variability, it has one of the largest peak-to-peak variations of any known Ap star. Colour photometry reveals an anti-phase correlation between the $B$ band, and the $V$, $R$ and $I$ bands. This Ap star is a rotational variable, also known as an $\alpha^2$ CVn, star, and is one of only a handful of Ap stars observed by Kepler. While no change in spot period or amplitude is observed within the 4-year Kepler timeseries, the amplitude shows a large increase compared to ground-based photometry obtained two decades ago.