Kepler Pixel Project -- background RR Lyrae stars in the primary Kepler mission field of view

TL;DR

This study used Kepler pixel data to discover 26 new faint RR Lyrae stars in the background of primary targets, providing high-precision light curves that reveal their pulsation properties and expand the known population.

Contribution

It presents the first detailed analysis of faint RR Lyrae stars using Kepler's high-cadence, precise photometry, increasing the known sample by 50% and offering new insights into their properties.

Findings

Discovered 26 new RR Lyrae stars in Kepler background pixels.

Provided four-year continuous light curves for these faint stars.

Confirmed that faint RR Lyrae share properties with brighter counterparts.

Abstract

In this paper we describe a project we initiated to investigate individual pixels in the downloaded Kepler apertures in order to find objects in the background of the main targets with variable brightness. In the first paper of this series we discovered and investigated 547 short-period eclipsing binaries (Bienias et al. 2021). Here we present the independent discovery of 26 new RR Lyrae stars in the Kepler background pixels obtained during the primary mission, and provide continuous and precise photometry for these objects. Twenty-one of these stars were already noted by Gaia or the Pan-STARRS survey. This new population of dominantly faint and distant RR Lyrae stars increases by 50% and complements nicely the 52 already known main target RR Lyrae stars in the original Kepler field. Despite their faintness, the four-year quasi-uninterrupted light curves of these stars allow an unprecedented view of these faint halo objects. We present an analysis of the light curves of the new RR Lyrae sample, verify their classification using Fourier parameters, and discuss the properties of these newly found pulsating variable stars. Most notably, this is the first time that such faint RR Lyrae stars have been investigated with the help of a photometric data set with outstanding cadence and precision. Interestingly, these objects share the properties of their brighter siblings in terms of sub-class characteristics, additional mode content, and modulation occurrence rates.