Digging Deeper for RR Lyrae Stars with Low Modulation Amplitudes

TL;DR

This study used a specialized search method on Kepler and K2 data to identify low-amplitude Blazhko modulation in RR Lyrae stars, significantly increasing the known incidence rate and revealing many stars with very subtle modulation signals.

Contribution

The paper introduces a task-oriented code for detecting low-amplitude modulation in RR Lyrae stars and provides the first comprehensive survey of such stars in Kepler and K2 data, revealing a high occurrence rate.

Findings

Identified 7 new low-amplitude modulated RR Lyrae stars in Kepler data.

Extended the survey to K2 fields, finding 514 Blazhko stars among 1061.

Estimated an underlying Blazhko star occurrence rate of approximately 75%.

Abstract

With the goal of searching for very low modulation amplitudes among fundamental mode RR Lyrae stars and assess their incidence rate, we performed a survey of 36 stars observed by the Kepler satellite during the entire four-year period of its mission. The search was conducted by a task-oriented code, designed to find low-amplitude signals in the presence of high-amplitude components and instrumental systematics. We found 7 new modulated stars and negate one earlier claimed star, whereby increasing the number of known Blazhko stars from 17 to 24 and yielding an observed occurrence rate of 67% for the Kepler field. Six of the new stars have the lowest modulation amplitudes found so far, with ~250 ppm Fourier side-lobe amplitudes near the fundamental mode frequency. Because of the small sample size in the Kepler field, we extended the survey to 12 campaign fields observed by K2, the ``two-wheeled'' mission of Kepler. From the 1061 stars we found 514 Blazhko stars. After correcting for the short duration of the time spent on each field, and for the noise dependence of the detections, we arrived at an underlying occurrence rate of ~75% - likely a lower limit for the true rate of Blazhko stars in the K2 fields.