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Register a new userUSNO-B1.0 1171-0309158: An RR Lyrae Star that Switched from a Double- to Single-mode Pulsation
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We report the discovery of a new case of an RR Lyrae star that experienced a switching of its pulsation mode. We detected USNO-B1.0 1171-0309158 as a double-mode RR Lyrae star from observations of the Catalina surveys (CSS) that showed additional scattering on the light curve. Our analysis of the time-series of CSS data showed gradual increase in scattering and in the amplitude of fundamental pulsation mode. Our CCD observations carried out in 2015 reveal that this object is now a fundamental-mode RRab star, with no sign of the first-overtone pulsation.
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RR Lyrae stars play an important role as distance indicators and stellar population tracers. In this context the construction of accurate pulsation models is crucial to understand the observed properties and to constrain the intrinsic stellar parameters of these pulsators. The physical mechanism driving pulsation in RR Lyrae stars has been known since the middle of the 20th century and many efforts have been performed during the last few decades in the construction of more and more refined pulsation models. In particular, nonlinear pulsation models including a nonlocal time-dependent treatment of convection, such as the ones originally developed in Los Alamos in the seventies, allow us to reproduce all the relevant observables of radial pulsation and to establish accurate relations and methods to constrain the intrinsic stellar properties and the distance of these variables. The most recent results on RR Lyrae pulsation obtained through these kinds of models will be presented and a few still debated problems will be discussed.
Classical double-mode pulsators (RR Lyrae stars and delta Cepheids) are important for their simultaneous pulsation in low-order radial modes. This enables us to put stringent constraints on their physical parameters. We use 30 bright galactic double-mode RR~Lyrae (RRd) stars to estimate their luminosities and compare them with those derived from the parallaxes of the recent data release (EDR3) of the Gaia survey. We employ pulsation and evolutionary models, together with observationally determined effective temperatures to derive the basic stellar parameters. Excluding 6 outlying stars (e.g., with blending issues) the RRd and Gaia luminosities correlate well. With the adopted temperature zero point from one of the works based on the infrared flux method, we find it necessary to increase the Gaia parallaxes by 0.02 mas to bring the RRd and Gaia luminosities into agreement. This value is consonant with those derived from studies on binary stars in the context of Gaia. We examine also the resulting period-luminosity-metallicity (PLZ) relation in the 2MASS K band as follows from the RRd parameters. This leads to the verification of two independently derived other PLZs. No significant zero point differences are found. Furthermore, the predicted K absolute magnitudes agree within sigma=0.005-0.01mag.
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 $0.28<P_{rm D}<0.41$,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 $R_{mathrm{21}}$ and $R_{mathrm{31}}$ 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 $varphi_{mathrm{21}}$ and $varphi_{mathrm{31}}$, 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.
We present the frequency analysis of the multicolour time series photometry of the field RRd variable BS Comae. The large number of data points in each of the BV(RI)_c bands and the ~0.01 magnitude accuracy of the individual measurements allow us a high precision analysis of the properties of the combination frequencies due to nonlinear coupling. Through the combination of the frequency spectra in different colors we show that except for the components corresponding to the linear combinations of the two pulsation modes, there are no other components present above the millimagnitude amplitude level.
We have discovered a new, rare triple-mode RR Lyr star, EPIC 201585823, in the Kepler K2 mission Campaign 1 data. This star pulsates primarily in the fundamental and first-overtone radial modes, and, in addition, a third nonradial mode. The ratio of the period of the nonradial mode to that of the first-overtone radial mode, 0.616285, is remarkably similar to that seen in 11 other triple-mode RR Lyr stars, and in 260 RRc stars observed in the Galactic Bulge. This systematic character promises new constraints on RR Lyr star models. We detected subharmonics of the nonradial mode frequency, which are a signature of period doubling of this oscillation; we note that this phenomenon is ubiquitous in RRc and RRd stars observed from space, and from ground with sufficient precision. The nonradial mode and subharmonic frequencies are not constant in frequency or in amplitude. The amplitude spectrum of EPIC 201585823 is dominated by many combination frequencies among the three interacting pulsation mode frequencies. Inspection of the phase relationships of the combination frequencies in a phasor plot explains the `upward shape of the light curve. We also found that raw data with custom masks encompassing all pixels with significant signal for the star, but without correction for pointing changes, is best for frequency analysis of this star, and, by implication, other RR Lyr stars observed by the K2 mission. We compare several pipeline reductions of the K2 mission data for this star.
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