An RR Lyrae family portrait: 33 stars observed in Pisces with K2-E2

TL;DR

This study analyzes 33 RR Lyrae stars observed with Kepler during a short test, revealing new pulsation modes, characterizing their properties, and identifying additional double-mode stars, demonstrating K2's potential for stellar research.

Contribution

First space-based detection of double-mode RR Lyrae stars and low-amplitude modes, with preliminary modeling and a new catalog of double-mode stars from Catalina Sky Survey data.

Findings

Detection of low-amplitude additional modes in all RR Lyrae subtypes.

Identification of the first space-observed modulated first-overtone RR Lyrae.

Discovery of 130 new double-mode RR Lyrae stars from sky survey data.

Abstract

A detailed analysis is presented of 33 RR Lyrae stars in Pisces observed with the Kepler space telescope over the 8.9-day long K2 Two-Wheel Concept Engineering Test. The sample includes not only fundamental-mode and first overtone (RRab and RRc) stars but the first two double-mode (RRd) stars that Kepler detected and the only modulated first-overtone star ever observed from space so far. The precision of the extracted K2 light curves made it possible to detect low-amplitude additional modes in all subtypes. All RRd and non-modulated RRc stars show the additional mode at P_X/P_1~0.61 that was detected in previous space-based photometric measurements. A periodicity longer than the fundamental mode was tentatively identified in one RRab star that might belong to a gravity mode. We determined the photometric [Fe/H] values for all fundamental-mode stars and provide the preliminary results of our efforts to fit the double-mode stars with non-linear hydrodynamic pulsation models. The results from this short test run indicate that the K2 mission will be, and has started to be, an ideal tool to expand our knowledge about RR Lyrae stars. As a by-product of the target search and analysis, we identified 165 bona-fide double-mode RR Lyrae stars from the Catalina Sky Survey observations throughout the sky, 130 of which are new discoveries.