Extended Aperture Photometry of K2 RR Lyrae stars

TL;DR

This paper presents an improved photometric method for RR Lyrae stars observed by Kepler K2, providing high-quality light curves, classifications, and insights into the Blazhko effect, including the first potential Blazhko-modulated anomalous Cepheid.

Contribution

The authors introduce the Extended Aperture Photometry method, enhancing flux conservation and light curve quality for K2 RR Lyrae stars, and analyze the Blazhko effect and anomalous Cepheid candidates.

Findings

Blazhko effect detected in 44.7% of RRab stars.

Identified four anomalous Cepheid candidates, including a potential Blazhko-modulated one.

Amplitude and phase modulation behaviors vary among Blazhko stars.

Abstract

The \textit{Kepler} space telescope observed thousands of RR Lyrae stars in the K2 mission. In this paper we present our photometric solutions using extended apertures in order to conserve the flux of the stars to the highest possible extent. With this method we are able to avoid most of the problems that RR Lyrae light curves produced by other pipelines suffer from. For post-processing we apply the K2SC pipeline to our light curves. We provide the EAP (Extended Aperture Photometry) of 432 RR Lyrae stars observed in campaigns 3, 4, 5, and 6. We also provide subclass classifications based on Fourier parameters. We investigated in particular the presence of the Blazhko effect in the stars, and found it to be 44.7\% among the RRab stars, in agreement with results from independent samples. We found that the amplitude and phase modulation in the Blazhko stars may behave rather differently, at least over the length of a K2 Campaign. We also identified four anomalous Cepheid candidates in the sample one of which is potentially the first Blazhko-modulated member of its class.