An RR Lyrae family portrait: 33 stars observed in Pisces with K2-E2
L. Moln\'ar, R. Szab\'o, P. A. Moskalik, J. M. Nemec, E. Guggenberger,, R. Smolec, R. Poleski, E. Plachy, K. Kolenberg, Z. Koll\'ath

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…
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