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 the Kepler space telescope, based on data obtained over nearly 2.5 yr. All four stars are found to be multiperiodic. The strongest secondary mode with frequency f 2 has an amplitude of a few mmag, 20−45 times lower than the main radial mode with frequency f 1 . The two oscillations have a period ratio of P 2 /P 1 = 0.612 − 0.632 that cannot be reproduced by any two radial modes. Thus, the secondary mode is nonradial. Modes yielding similar period ratios have also recently been discovered in other variables of the RRc and RRd types. These objects form a homogenous group and constitute a new class of multimode RR Lyrae pulsators, analogous to a similar class of multimode classical Cepheids in the Magellanic Clouds. Because a secondary mode with P 2 /P 1 ∼ 0.61 is found in almost every RRc and RRd star observed from space, this form of multiperiodicity must be common. In all four Kepler RRc stars studied, we find subharmonics of f 2 at ∼ 1/2f 2 and at ∼ 3/2f 2 . This is a signature of period doubling of the secondary oscillation, and is the first detection of period doubling in RRc stars. The amplitudes and phases of f 2 and its subharmonics are variable on a timescale of 10 − 200 d. The dominant radial mode also shows variations on the same timescale, but with much smaller amplitude. In three Kepler RRc stars we detect additional periodicities, with amplitudes below 1 mmag, that must correspond to nonradial g-modes. Such modes never before have been observed in RR Lyrae variables.

The K2 RR Lyrae Survey

“…Molnár et al in these proceedings). Besides the half-integer frequencies due to period-doubling [12], extra frequencies have been found around the expected location of the second radial overtone frequency [13], around P X /P 1 = 0.61 [14], where P 1 and P X are the periods of first radial overtone and a presumably nonradial mode which seems to be ubiquitous in RRc and RRd stars [15]. In addition, anomalous first overtone / fundamental mode (P 1 /P 0 ) period ratios were uncovered [16], [17], [18], [19].…”

Blazhko modulation in first-overtone RR Lyrae stars from the OGLE Collection

“…In the case of stars in which we observed a remnant signal at the position of the main pulsation frequency due to e.g. a period change, we removed the effect using a time-dependent prewhitening method ( [6]). Use of this method resulted in a sharp drop visible in Figure 4 at f / f 1O = 1.…”

Triennial Report (2012-2015): The Legacy Issue

“…The Kepler (planet-hunting) spacecraft delivered ultra-precise, continuous photometric time series data of the ∼50 RR Lyrae known in the Kepler field and one Cepheid. These data revealed an amazing variety of pulsational behaviours: previously unseen dynamical phenomena, such as period doubling , Szabo et al 2010, resonances , additional frequencies (Benkö et al 2010, Nemec et al 2011, Kolenberg et al 2011, Guggenberger et al 2012) often with interesting, recurring frequency ratios (Moskalik et al 2015), and the diversity of the Blazhko modulation (including multiple modulation periods) . These findings led to valuable new insights, including a new explanation for the Blazhko effect (Buchler & Kolláth 2011).…”

“…What is the nature of the mysterious, newly found modes with frequency ratios of ∼0.62 seen in RRc (and RRd) stars? (Moskalik et al 2015). The one Cepheid observed with Kepler shows period jitter (Derekas et al 2012) -is this a common phenomenon?…”