Frequency analysis of Cepheids in the Large Magellanic Cloud. New types of classical Cepheid pulsators

摘要

We have performed a detailed systematic search for multiperiodicity in thePopulation I Cepheids of the Large Magellanic Cloud. In this process we haveidentified for the first time several new types of Cepheid pulsationalbehaviour. We have found two triple-mode Cepheids pulsating simultaneously inthe first three radial overtones. In 9% of the first overtone Cepheids we havedetected weak, but well resolved secondary periodicities. They appear eithervery close to the primary pulsation frequency or at a much higher frequencywith a characteristic period ratio of 0.60-0.64. In either case, the secondaryperiodicities must correspond to nonradial modes of oscillation. This resultpresents a major challenge to the theory of stellar pulsations, which predictsthat such modes should not be exited in Cepheid variables. Nonradial modes havealso been found in three of the fundamental/first overtone double-modeCepheids, but no such oscillations have been detected in single mode Cepheidspulsating in the fundamental mode. In 19% of double-mode Cepheids pulsating inthe first two radial overtones (FO/SO type) we have detected a Blazhko-typeperiodic modulation of amplitudes and phases. Both modes are modulated with acommon period, which is always longer than 700 days. Variations of the twoamplitudes are anticorrelated and maximum of one amplitude always coincideswith minimum of the other. We have compared observations of modulated FO/SOCepheids with predictions of theoretical models of the Blazhko effect, showingthat currently most popular models cannot account for properties of thesestars. We propose that Blazhko effect in FO/SO Cepheids can be explained by anonstationary resonant interaction of one of the radial modes with another,perhaps nonradial, mode of oscillations.