Kepler photometry of RRc stars: peculiar double-mode pulsations and period doubling

Abstract

We present the analysis of four first overtone RR Lyrae stars observed with\nthe Kepler space telescope, based on data obtained over nearly 2.5yr. All four\nstars are found to be multiperiodic. The strongest secondary mode with\nfrequency f_2 has an amplitude of a few mmag, 20 - 45 times lower than the main\nradial mode with frequency f_1. The two oscillations have a period ratio of\nP_2/P_1 = 0.612 - 0.632 that cannot be reproduced by any two radial modes.\nThus, the secondary mode is nonradial. Modes yielding similar period ratios\nhave also recently been discovered in other variables of the RRc and RRd types.\nThese objects form a homogenous group and constitute a new class of multimode\nRR Lyrae pulsators, analogous to a similar class of multimode classical\nCepheids in the Magellanic Clouds. Because a secondary mode with P_2/P_1 ~ 0.61\nis found in almost every RRc and RRd star observed from space, this form of\nmultiperiodicity must be common. In all four Kepler RRc stars studied, we find\nsubharmonics of f_2 at ~1/2 f_2 and at ~3/2 f_2. This is a signature of period\ndoubling of the secondary oscillation, and is the first detection of period\ndoubling in RRc stars. The amplitudes and phases of f_2 and its subharmonics\nare variable on a timescale of 10 - 200d. The dominant radial mode also shows\nvariations on the same timescale, but with much smaller amplitude. In three\nKepler RRc stars we detect additional periodicities, with amplitudes below\n1mmag, that must correspond to nonradial g-modes. Such modes never before have\nbeen observed in RR Lyrae variables.\n