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

Abstract

Nineteen of the ~40 RR Lyr stars in the Kepler field have been identified as\ncandidate non-Blazhko (or unmodulated) stars. In this paper we present the\nresults of Fourier decomposition of the time-series photometry of these stars\nacquired during the first 417 days of operation (Q0-Q5) of the Kepler\ntelescope. Fourier parameters based on ~18400 long-cadence observations per\nstar (and ~150000 short-cadence observations for FN Lyr and for AW Dra) are\nderived. None of the stars shows the recently discovered `period-doubling'\neffect seen in Blazhko variables; however, KIC 7021124 has been found to\npulsate simultaneously in the fundamental and second overtone modes with a\nperiod ratio P2/P0 ~ 0.59305 and is similar to the double-mode star V350 Lyr.\nPeriod change rates are derived from O-C diagrams spanning, in some cases, over\n100 years; these are compared with high-precision periods derived from the\nKepler data alone. Extant Fourier correlations by Kovacs, Jurcsik et al. (with\nminor transformations from the V to the Kp passband) have been used to derive\nunderlying physical characteristics for all the stars. This procedure seems to\nbe validated through comparisons of the Kepler variables with galactic and LMC\nRR Lyr stars. The most metal-poor star in the sample is NR Lyr, with\n[Fe/H]=-2.3 dex; and the four most metal-rich stars have [Fe/H] ranging from\n-0.6 to +0.1 dex. Pulsational luminosities and masses are found to be\nsystematically smaller than L and mass values derived from stellar evolution\nmodels, and are favoured over the evolutionary values when periods are computed\nwith the Warsaw linear hydrodynamics code. Finally, the Fourier parameters are\ncompared with theoretical values derived using the Warsaw non-linear convective\npulsation code.\n