Frequency analysis of the first-overtone RR Lyrae stars based on Extended Aperture Photometry from K2 data

Abstract

Context. Additional low-amplitude signals have been observed in many RR Lyrae stars separate from pulsations in radial modes. The most common of these are short-period signals forming a period ratio of around 0.60–0.65 with the first overtone and long-period signals forming a period ratio of around 0.68. The RR Lyrae stars may also exhibit quasi-periodic modulation in their light curves, the so-called Blazhko effect. Aims. We used the extensive sample of the first-overtone RR Lyrae stars observed by the Kepler telescope during the K2 mission to search for and characterize additional low-amplitude signals. The K2 data provides space-based photometry for a statistically significant sample. Hence, this data is excellent for studying the pulsation properties of RR Lyrae stars in detail. Methods. We used K2 space-based photometry for RR Lyrae candidates from Campaigns 0–19. We selected RR Lyrae stars pulsating in the first overtone and performed a frequency analysis for each star to characterize their frequency contents. Results. We classified 452 stars as first-overtone RR Lyrae. From that sample, we selected 281 RR0.61 stars, 67 RR0.68 stars, and 68 Blazhko stars. We found particularly interesting stars that show all of the above phenomena simultaneously. We detected signals in RR0.61 stars that form period ratios lower than those observed for the majority of stars of this type. These signals likely form a new sequence in the Petersen diagram, around a period ratio of 0.60. In 32 stars, we detected additional signals that form a period ratio close to that expected in RRd stars, but the classification of these stars as RRd is uncertain. We also report a discovery of additional signals in eight stars that form a new group in the Petersen diagram around the period ratio of 0.465–0.490. The nature of this periodicity remains unknown.

Similar additional frequency patterns on fundamental- and overtone-mode RR Lyrae stars showing f68 frequencies

“…The first frequency that did not fit into the previously known frequency structure of RR Lyrae stars was detected by Gruberbauer et al (2007) from the MOST data of the doublemode (RRd) star AQ Leo. Later, similar frequencies were found in other RRd stars, and in many RRc stars that pulsate purely in the overtone mode (Olech & Moskalik 2009;Chadid 2012;Szabó et al 2014;Moskalik et al 2015). After analysing the large number of RR Lyrae stars of the OGLE survey, it became clear that these frequencies are located in three horizontal roughly parallel sequences on the Petersen diagram, where the frequency ratio f 1 / f x of each sequence is 0.613, 0.622, and 0.631, respectively (Netzel et al 2015;Netzel & Smolec 2019); from now on, these frequencies are referred to as f 61 , f 62 , and f 63 .…”

“…The frequencies of the modes themselves are 0.5 f 61 and 0.5 f 63 . This picture was confirmed by further theoretical and observational studies (Netzel & Kolenberg 2021;Netzel & Smolec 2022;Molnár et al 2022;Benkő et al 2023;Netzel et al 2023).…”

“…The Fourier spectra of TESS RRc stars are taken from the work of Benkő et al (2023). For the K2 RRc stars in Table A.2, following Netzel et al (2023), we downloaded the Automated Extended Aperture Photometry data (Bódi et al 2022) than the spectra were calculated using the Period04 program (Lenz & Breger 2005). The lower five panels of Fig.…”

“…To do this, only those non-Blazhko RRc stars are considered; these do not show the f 61 / f 63 frequencies either. The CoRoT and the original Kepler samples do not contain such 'clear' cases; however, the works on TESS and K2 RRc stars identified a significant number of such stars (Benkő et al 2023;Netzel et al 2023). From the TESS L6, page 2 of 6 sample, 23 stars were selected that contain both the f 68 frequency and at least one linear combination of f 68 and f 1 .…”

“…Some parameters of the sample are shown in Table A.1. We identified nine of the K2 RRc stars in the work of Netzel et al (2023) that met our criteria (listed in Table A.2).…”