Modelling Observed Stellar Cycles

Investigations of the pulsating RR Lyrae stars have contributed to almost every branch of modern astronomy. However, these stars are far from being fully understood. Exactly a century ago, it was discovered that not all RR Lyrae stars pulsate as regularly as was thought. In fact, their curves are modulated on longer time scales of typically tens to hundreds of days. A hundred years after its discovery, the cause of the so-called Blazhko effect remains a mystery. In preparation for this project, a large data set of selected modulated and non-modulated RR Lyrae stars has been gathered from 16 different observatories over several years in an international network focused on the Blazhko problem (the Blazhko Project, http://ww.univie.ac.atops/blazhko, gathering more than 40 scientists from 18 countries). The results from our own studies and from many others in the field reveal that the existing theoretical models are insufficient to explain the variety of phenomena observed in Blazhko stars. A next logical step is to use the observations to improve the models. In the models of the modulation, the geometry of the movements of the stellar layers plays a crucial role. It is thought that these movements are not spherically symmetric. In two dimensions, this can be compared to the patterns of vibration on a beaten drum. We want to use the existing data to unravel these patterns. Therefore, appropriate models and techniques will be developed. Complementary to the modelling endeavours, both photometric and spectroscopic data will continue to be gathered through the network set up over the past years. These data will be used to challenge the outcome of the modelling. Besides getting to the bottom of a stubborn, century-old problem, identifying the mechanisms behind the Blazhko effect may clarify similar phenomena observed in other classes of pulsating stars.