K2 Observations of Galactic RRd Stars

TL;DR

This study analyzes high-precision Kepler-K2 photometry of 77 RRd stars, revealing common non-radial pulsation modes and identifying subclasses with unique period ratios, advancing understanding of RR Lyrae star pulsations.

Contribution

It provides the first comprehensive analysis of non-radial modes in a large sample of RRd stars observed by Kepler-K2, identifying their prevalence and characteristics.

Findings

Non-radial mode detected in most RRd stars.

Subharmonics of the non-radial frequency are commonly observed.

Non-radial pulsation properties are similar in RRd and RRc stars.

Abstract

We have analysed high-precision photometry of 77 double-mode RR Lyrae (RRd) stars observed by NASA's Kepler-K2 mission during Campaigns 1-18. Among those stars, we have identified several low-period ratio variables, belonging either to a subclass of "anomalous" RRd stars (Soszy\'nski et al., 2016) or to a separate shorter-period subclass recently identified by Prudil et al. (2017). A non-radial mode with the period ratio of $P_{\rm x}/P_{\rm 1} \sim 0.615$ has been detected in most RRd stars of our sample. In majority of these variables, at least one subharmonic of the non-radial frequency is also present. Our findings indicate that excitation of the non-radial mode is a common property of the RRd stars. The same non-radial pulsation is also commonly detected in the RRc stars. We show that properties of this puzzling mode are essentially the same in both groups of RR Lyrae-type pulsators.