Fourier analysis of non-Blazhko ab-type RR Lyrae stars observed with the Kepler space telescope

Nineteen of the ∼40 RR Lyrae stars in the Kepler field have been identified as candidate non-Blazhko (or unmodulated) stars. In this paper we present the results of Fourier decomposition of the time-series photometry of these stars acquired during the first 417 days of operation (Q0-Q5) of the Kepler telescope. Fourier parameters based on ∼18400 long-cadence observations per star (and ∼150000 short-cadence observations for FN Lyr and for AW Dra) are derived. None of the stars shows the recently discovered ‘period-doubling’ effect seen in Blazhko variables; however, KIC 7021124 has been found to pulsate simultaneously in the fundamental and second overtone modes with a period ratio P2/P0 ∼ 0.59305 and is similar to the double-mode star V350 Lyr. Period change rates are derived from O−C diagrams spanning, in some cases, over 100 years; these are compared with high-precision periods derived from the Kepler data alone. Extant Fourier correlations by Kovács, Jurcsik et al. (with minor transformations from the V to the Kp passband) have been used to derive underlying physical characteristics for all the stars. This procedure seems to be validated through comparisons of the Kepler variables with galactic and LMC RR Lyrae stars. The most metal-poor star in the sample is NR Lyr, with [Fe/H]= −2.3 dex; and the four most metal-rich stars have [Fe/H] ranging from −0.6 to +0.1 dex. Pulsational luminosities and masses are found to be systematically smaller than L and M values derived from stellar evolution models, and are favoured over the evolutionary values when periods are computed with the Warsaw linear hydrodynamics code. Finally, the Fourier parameters are compared with theoretical values derived using the Warsaw non-linear convective pulsation code.