Pushing the limits, episode 2: K2 observations of extragalactic RR Lyrae stars in the dwarf galaxy Leo IV

TL;DR

This study reports the first detection of extragalactic RR Lyrae stars in Leo IV using K2 data, revealing modulation periods and demonstrating effective photometry extraction techniques for faint, blended sources.

Contribution

First measurement of Blazhko periods in extragalactic RR Lyrae stars and validation of image subtraction for faint source photometry in space data.

Findings

Detected variability in all three RR Lyrae stars in Leo IV.

Measured Blazhko period and cycle variations for the first extragalactic star.

Validated image subtraction for faint, blended source photometry.

Abstract

We present the first observations of extragalactic pulsating stars in the K2 ecliptic survey of the Kepler space telescope. Variability of all three RR Lyrae stars in the dwarf spheroidal galaxy Leo IV were successfully detected, at a brightness of Kp~21.5 mag, from data collected during Campaign 1. We identified one modulated star and another likely Blazhko candidate with periods of 29.8+-0.9 d and more than 80 d, respectively. EPIC 210282473 represents the first star beyond the Magellanic Clouds for which the Blazhko period and cycle-to-cycle variations in the modulation were unambiguously measured.The photometric [Fe/H] indices of the stars agree with earlier results that Leo IV is a very metal-poor galaxy. Two out of three stars blend with brighter background galaxies in the K2 frames. We demonstrate that image subtraction can be reliably used to extract photometry from faint confused sources that will be crucial not only for the K2 mission but for future space photometric missions as well.