Application of time-dependent Fourier analysis to nonlinear pulsational stellar models

Monthly Notices of the Royal Astronomical Society

Kolenberg

et al. 2015

168

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.

Section: Time Domainmentioning

confidence: 99%

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

Moskalik

Monthly Notices of the Royal Astronomical Society

Kolenberg

et al. 2015

168

“…The merged OGLE-III and OGLE-IV data can be used to conduct time-dependent Fourier analysis (Kovács, Buchler & Davis 1987) for the fundamental mode. This is done on a season-to-season basis and results are presented in Fig.…”

Section: Notes On the Individual Starsmentioning

confidence: 99%

Peculiar double-periodic pulsation in RR Lyrae stars of the OGLE collection – I. Long-period stars with dominant radial fundamental mode

Mon. Not. R. Astron. Soc.

Prudil

Skarka

et al. 2016

We present the discovery of a new, peculiar form of double-periodic pulsation in RR Lyrae stars. In four, long-period (P > 0.6 d) stars observed by the Optical Gravitational Lensing Experiment, and classified as fundamental mode pulsators (RRab), we detect additional, low-amplitude variability, with period shorter than fundamental mode period. The period ratios fall in a range similar to double-mode fundamental and first overtone RR Lyrae stars (RRd), with the exception of one star, in which the period ratio is significantly lower and nearly exactly equals 0.7. Although period ratios are fairly different for the four stars, the light curve shapes corresponding to the dominant, fundamental mode are very similar. The peak-to-peak amplitudes and amplitude ratios (Fourier parameters R 21 and R 31 ) are among the highest observed in RRab stars of similar period, while Fourier phases (ϕ 21 and ϕ 31 ) are among the lowest observed in RRab stars. If the additional variability is interpreted as due to radial first overtone, then, the four stars are the most extreme RRd variables of the longest pulsation periods known. Indeed, the observed period ratios can be well modelled with high metallicity pulsation models. However, at such long pulsation periods, first overtone is typically damped. Five other candidates, with weak signature of additional variability, sharing the same characteristics, were also detected and are briefly discussed.

“…The numerous and high-quality data allowed us to use the time-dependent Fourier analysis (Kovács, Buchler & Davis 1987) using merged OGLE-II, OGLE-III and OGLE-IV data (see Fig. 7).…”

Section: Remarks On the Most Well-observed Starsmentioning

confidence: 99%

Peculiar double-periodic pulsation in RR Lyrae stars of the OGLE collection – II. Short-period stars with a dominant radial fundamental mode

Prudil

Mon. Not. R. Astron. Soc.

Skarka

et al. 2016

We report the discovery of a new group of double-periodic stars in the OGLE Galactic bulge photometry. In 38 stars identified as fundamental mode RR Lyrae and 4 classified as the first-overtone RR Lyrae, we detected additional shorter periodicity. Periods of the dominant variability in the newly discovered group are 0.28 < P D < 0.41 days. Period ratios (0.68−0.72) are smaller than the period ratios of the Galactic bulge RRd stars. The typical amplitude ratio (of the additional to the dominant periodicity) is 20 % for the stars identified as fundamental mode RR Lyrae and 50 % for stars classified as the first-overtone RR Lyrae. Ten stars from our sample exhibit equidistant peaks in the frequency spectrum, that suggest the Blazhko-type modulation of the main pulsation frequency and/or the additional periodicity. The Fourier coefficients R 21 and R 31 are one of the lowest among fundamental mode RR Lyrae stars, but among the highest for the first-overtone pulsators. For the phase Fourier coefficients ϕ 21 and ϕ 31 , our stars lie in between RRab and RRc stars. Discussed stars were compared with the radial, linear pulsation models. Their position in the Petersen diagram cannot be reproduced assuming that two radial modes are excited and their physical parameters are similar to that characteristic for RR Lyrae stars. The non-radial mode scenario also faces difficulties. We conclude that the dominant variability is most likely due to pulsation in the radial fundamental mode including stars classified as the first overtone mode pulsators. At this point, we cannot explain the nature of the additional periodicity. Even more, classification of the stars as RR Lyrae should be treated as tentative.