Modulated RR Lyrae Stars

What causes some stars to have regular oscillations, while other stars have oscillations which have strengths changing in time? This question was posed for the pulsating RR Lyrae stars exactly a century ago, and despite multiple attempts to explain the so-called Blazhko effect, it remains a mystery. The most plausible hypotheses have included magnetic cycles, resonance effects, and beating between simultaneously excited pulsation modes with similar periods. In recent years, the experimental precision in photometric and spectroscopic techniques has increased remarkably. For example, we can now detect small waves running around the surface of the star. To do this, we will analyze the photometric and spectroscopic variations of the stars during their oscillations. We will also use specialized techniques to detect magnetic fields on the stellar surface of modulated and non-modulated RR lyrae stars. Since these are thought to be responsible in some models, their detection or non-detection is crucial. As a next step, we will apply new statistical analyses that have been made possibly only recently due to computing advances. These will allow us to disentangle and identify the wave patterns running around the surface of the star, and get clues about the origin of the modulation. Our findings will add important constraints to the physical models of RR Lyrae stars. The answer to this stubborn and century-old problem may clarify similar phenomena also observed in other classes of pulsating stars.

What causes some stars to have regular oscillations, while other stars have oscillations which have strengths changing in time? This question was posed for the pulsating RR Lyrae stars exactly a century ago, and despite multiple attempts to explain the so-called Blazhko effect, it has remained a mystery for over a century. The most plausible hypotheses have included magnetic cycles, resonance effects, and beating between simultaneously excited pulsation modes with similar periods.In recent years, the experimental precision in photometric and spectroscopic techniques has increased remarkably, especially with the availability of the ultra-precise data of the two satellite missions CoRoT and Kepler. It is now possible to detect incredibly small waves running around the surface of the star. We have used the data provided by these satellites to analyze a selected sample of RR Lyrae stars in great detail, which led to the discovery of several phenomena that had never been found before in this type of stars and which were described by us for the first time. Examples are the presence of additional pulsation frequencies, the signs of chaos in the pulsation behavior as well as strong irregularities in the modulation.In addition to the above, we have also used specialized techniques to detect magnetic fields on the stellar surface of modulated and non-modulated RR Lyrae stars. Since these are a requirement in one of the most promising theoretical models, their detection or non-detection was crucial. We demonstrated the absence of a strong dipole field not only for the brightest prototype of the class of stars, but also for a larger sample, which allows us to exclude unfavorable geometric effects as a cause for the non-detection, and to finally rule out the magnetic model as the mechanism behind the Blazhko effect. We also analyzed the chemical composition of the prototype RR Lyrae, which is a great challenge because of the strong pulsation. We first determined the parameters during a carefully selected "quiescent" phase that is least disturbed by the pulsation, and then we used these results as the basis for further analyses. We could subsequently determine the variation of the fundamental physical parameters during the course of a pulsation cycle - a task that had never before been so consistently completed with such a high accuracy.Our findings have added important constraints to the physical models of RR Lyrae stars, and at the end of the project we are significantly closer to an understanding of the modulation in RR Lyrae stars. The answer to this stubborn and century-old problem may clarify similar phenomena also observed in other classes of pulsating stars.