Additional modes and cycle-to-cycle variations of non-Blazhko RR Lyrae stars József M. Benkő1, Johanna Jurcsik1, Aliz Derekas2,1 & Margit Paparó1 1Konkoly Observatory, MTA CSFK, Konkoly Thege u. 15-17. H-1121 Budapest, Hungary 2ELTE Eötvös Loránd University, Gothard Astrophysical Observatory, Szombathely, Hungary Questions? Comments? Let me know! benko@konkoly.hu Cycle-to-cycle variations Acknowledgements This work was supported by the Hungarian National Research, Development and Innovation Office by the Grants NKFIH K-115709 and K-119517. AD was supported by the ÚNKP 17 4 New National Excellence Program of the Ministry of Human Capacities. AD would like to thank the City of Szombathely for support under Agreement No. 67.177 21/2016. Reality of cycle-to-cycle variation Appearing additional modes Data handling origin? The local instrumental trends was handled three different ways. We applied both (i) our moving average subtraction method described in Benkő et al. (2014) and (ii) a method in which we adjust each pulsation cycle to a common zero point. Moreover, (iii) if there were no evident instrumental trend in the flux curves, the data without further processing were also investigated. These two (or three) data sets show qualitatively similar C2C variations. Photometric problems? We choose high and low maxima pairs from the light curves and plotted the flux in the pixel maps either at the high maxima phase or the flux differences between the high and low maxima phases. In Fig. 2 can be seen such a plot pairs for NR Lyr. The figure shows that (1) the amplitude difference is connected to the star itself and there is no other source of flux, and (2) the position of the star is unchanged in the pixel mask. These detection make doubts any serious photometric defects as an origin of the C2C variation. Unknown instrumental effects? For testing unknown instrumental effects as an explanation of C2C variations similar observations from a different instrument is the best chance. Currently the only independent instrument which observed high precision time series for non-Blazhko RR Lyrae stars is the CoRoT satellite (Baglin et al. 2016). Three non-Blazhko stars were observed with the so-called oversampled mode (32 sec sampling) during the CoRoT runs. Two of them proved to be bright enough for our purpose: CM Ori (CoRoT 617282043, rCoRoT= 12.64 mag, Benkő et al. 2016) and the BT Ser (CoRoT 105173544, rCoRoT=12.99 mag). Investigating the CoRoT light curves as we did for Kepler stars' data we found C2C variation for the two brighter stars CM Ori and BT Ser. Fig. 3 shows their amplitude variation as panel c) of Fig. 1. Even though for these stars the scatter is evidently higher than NR Lyr the amplitude variations are obvious. The maximal difference between high and low amplitude maxima is about 0.005-0.006 mag. This value is the same as we estimated for Kepler stars. Figure 3. The amplitude difference of consecutive cycles in CoRoT non- Blazhko stars CM Ori and BT Ser. The purple plus signs mean the original light curve points while the green `x' symbols show the light curve points shifted one pulsation cycle. Figure 2. The pixel mask of NR Lyr during the SC (Q11.1) observation. The left panel shows the flux in `high' maxima signed by `C' in Fig. 1, while the right-hand-side panel contains the flux difference between the `high' (`C') and `low' (`B') maxima points. Figure 4. Sample spectra showing additional modes around the main pulsation frequency f0 and its first harmonic 2f0. In the left-hand-side and in the middle: The black curves show the LC spectra. Spectra computed from the SC data are signed red and blue curves (see also labels in panels). Providing comparable spectra either we divided the frequency scales with the actual f0 and also the flux scales are normalized to the signal to noise ratio of the LC spectra. In the right hand side: Typical time-frequency plots. The colour scales show the power values. The C2C behaviour of NR Lyr