Are RRab stars fully radial?

Abstract

Abstract. Thanks to the space missions CoRoT and Kepler new oscillation frequencies have been discovered in the Fourier spectra of Blazhko RR Lyrae stars. The period doubling (PD) yields half-integer frequencies between the fundamental mode and its harmonics. In many cases the first and/or second radial overtone frequencies also appear temporally. Some stars show extra frequencies that were identified as potential non-radial modes. We show here that all these frequencies can be explained by pure radial pulsation as linear combinations of the frequencies of radial fundamental and overtone modes. We homogeneously re-analyzed light curves of all the CoRoT and Kepler Blazhko RRab stars in which non-radial mode(s) were reported. We used the CoRoT 150-dayslong data co-added to get 8-min sampling and Kepler 3-years-long long-cadence (30-min sampling) data covering Q1-Q12. The CoRoT white fluxes were cleaned and de-trended. In the case of Kepler targets, we used the raw pixel frames applying our own proper tailor-made apertures for each star and quarter separately (Benkő et al., in preparation). The data were pre-whitened with the main pulsation frequencies and their harmonics, the modulation frequencies and as many modulation side peaks as possible. The resulting Fourier spectra and the frequency solutions from the literature are compared.V1127 Aql (CoRoT 100689962). Our identification of f 0 and f m are the same as in Chadid et al. (2010), but if we assume f = f 2 − f 0 , then f = 2(f 2 − f 0 ), where f 2 = 4.825397 d −1 is the frequency of the radial second overtone with the period ratio of 0.582. This identification eliminates the non-radial mode with its period ratio of 0.696. Poretti et al. (2010) have already noticed that V1127 Aql shows half-integer frequencies. If we accept the PD paradigm , the frequency f m1 can also be interpreted as a linear combination: 1.5f 0 − f . G12 found the following independent frequencies of CoRoT 105288363: f 0 , f m , f s (secondary Blazhko frequency), f 1 , f 2 , first and second radial overtone modes with the period ratios 0.745 and 0.590, respectively, and two non-radial modes, f N = 2.442 d

The Blazhko Effect

“…The effect is observed only at some phases of the modulation cycle; its nature is intermittent. In the frequency spectrum, the additional signals are not centered at (2n + 1)/2ν F , but significant offset is sometimes present, as already reported by Szabó et al (2010) or Benkő et al (2014) and also analysed by Bryant (2015a). For these reasons, Kovács (2015) suggests to call the effect 'amplitude alternation' rather than period doubling.…”

“…In several stars additional periodicities with P x /P F around 0.7 were detected and initially identified as corresponding to first overtone or to nonradial modes. Benkő & Szabó (2014) showed, that in the majority of cases, these frequencies can be interpreted as linear combinations of radial mode frequencies (F and 2O) and radial mode and halfinteger frequencies. This is the case for e.g., V360 Lyr and V354 Lyr (1O detection claimed in Benkő et al, 2010), and V1127 Aql (nonradial mode detection claimed in Chadid et al, 2010;Szabó et al, 2014), in which additional signals in the frequency spectrum can be interpreted as linear combination 2(ν 2O −ν F ) (Benkő & Szabó, 2014;Benkő et al, 2014).…”

“…In-depth study was done by Szabó et al (2010). Detailed analysis of the CoRoT and Kepler data (Benkő et al, 2014;Szabó et al, 2014) revealed the phenomenon in 14 stars i.e. in ∼58 % of Blazhko stars observed from space.…”

“…We stress that amplitude of the additional signals (including 2O) is always very low and sometimes complex structures are present in the frequency spectrum (close double, or multiple peaks, see fig. 6 in Benkő et al, 2014). Two stars deserve more attention.…”

“…4 and 6 in Szabó et al (2014) and fig. 4 in Benkő et al (2014). Modulation periods range from a few days to more than thousand of days.…”