Browsing by Author "Kovács, G"

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    The Macho project Large Magellanic Cloud variable star inventory.: XI.: Frequency analysis of the fundamental-mode RR Lyrae stars
    (2003) Alcock, C; Alves, DR; Becker, A; Bennett, D; Cook, KH; Drake, A; Freeman, K; Geha, M; Griest, K; Kovács, G; Lehner, M; Marshall, S; Minniti, D; Nelson, C; Peterson, B; Popowski, P; Pratt, M; Quinn, P; Rodgers, A; Stubbs, C; Sutherland, W; Vandehei, T; Welch, DL
    We have frequency-analyzed 6391 variables classified earlier as fundamental-mode RR Lyrae (RR0) stars in the MACHO database on the Large Magellanic Cloud (LMC). The overwhelming majority (i.e., 96%) of these variables have been proved to be indeed RR0 stars, whereas the remaining ones have fallen into one of the following categories: single- and double-mode Cepheids, binaries, first-overtone and double-mode RR Lyrae stars, and nonclassified variables. Special attention has been paid to the properties of the amplitude- and phase-modulated RR0 stars ( the Blazhko stars). We found altogether 731 Blazhko variables showing either a doublet or an equidistant triplet pattern at the main pulsation component in their frequency spectra. This sample overwhelmingly exceeds the number of Blazhko stars known in all other systems combined. The incidence rate of the Blazhko variables among the RR0 stars in the LMC is 11.9%, which is 3 times higher than their rate among the first-overtone RR Lyrae stars. No difference is found in the average brightness between the single- mode and Blazhko variables. However, the latter ones show a somewhat lower degree of skewness in their average light curves and a concomitant lower total amplitude in their modulation-free light curves. From the frequency spectra we found that variables with larger modulation amplitudes at the higher frequency side of the main pulsation component are 3 times more common than the ones showing the opposite amplitude pattern. A search for a modulation component with the Blazhko period in the average brightness of the individual variables showed the existence of such a modulation with an overall amplitude of approximate to 0.006 mag. On the other hand, a similar search for quadruple modulation patterns around the main pulsation component has failed to clearly detect such components at the approximate to 0.004 mag level. This means that the amplitudes of the quadruple components ( if they exist) should be, on average, at least 10 times smaller than those of the triplet components. This finding and the existence of Blazhko variables with highly asymmetric modulation amplitudes not only question the validity of the magnetic oblique rotator model but also put stringent constraints on models based on mode-coupling theories.
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    The MACHO project Large Magellanic Cloud variable-star inventory.: IX.: Frequency analysis of the first-overtone RR Lyrae stars and the indication for nonradial pulsations
    (2000) Alcock, C; Allsman, R; Alves, DR; Axelrod, T; Becker, A; Bennett, D; Clement, C; Cook, KH; Drake, A; Freeman, K; Geha, M; Griest, K; Kovács, G; Kurtz, DW; Lehner, M; Marshall, S; Minniti, D; Nelson, C; Peterson, B; Popowski, P; Pratt, M; Quinn, P; Rodgers, A; Rowe, J; Stubbs, C; Sutherland, W; Tomaney, A; Vandehei, T; Welch, DL
    More than 1300 variables classified provisionally as first-overtone RR Lyrae pulsators in the MACHO variable-star database of the Large Magellanic Cloud (LMC) have been subjected to standard frequency analysis. Based on the remnant power in the prewhitened spectra, we found 70% of the total population to be monoperiodic. The remaining 30% (411 stars) are classified as one of nine types according to their frequency spectra. Several types of RR Lyrae pulsational behavior are clearly identified here for the first time. Together with the earlier discovered double-mode (fundamental and first-overtone) variables, this study increased the number of known double-mode stars in the LMC to 181. During the total 6.5 yr time span of the data, 10% of the stars showed strong period changes. The size, and in general also the patterns of the period changes, exclude a simple evolutionary explanation. We also discovered two additional types of multifrequency pulsators with low occurrence rates of 2% for each. In the first type, there remains one closely spaced component after prewhitening by the main pulsation frequency. In the second type, the number of remnant components is two; they are also closely spaced, and are symmetric in their frequency spacing relative to the central component. This latter type of variables are associated with their relatives among the fundamental pulsators, known as Blazhko variables. Their high frequency (approximate to 20%) among the fundamental-mode variables versus the low occurrence rate of their first-overtone counterparts makes it more difficult to explain the Blazhko phenomenon by any theory depending mainly on the role of aspect angle or magnetic field. None of the current theoretical models are able to explain the observed close frequency components without invoking nonradial pulsation components in these stars.