Towards the solution of the Blazhko enigma

RR Lyrae stars are one of the most important groups of variable stars. They can be used as standard candles to measure the distances within our Galaxy and beyond. Many of these old, population II objects are found in globular clusters and in the Galactic bulge. Thus, they are important tools to study the structure of our Galaxy, its formation and chemical evolution. Despite their importance we still don`t understand nearly half of these variables - those which show the mysterious Blazhko effect, amplitude and phase modulation on time scales from a few to hundreds of days. The goal of the project is to shed more light on this phenomenon through study of the period doubling effect discovered recently in three Blazhko RR Lyrae stars by the satellite mission Kepler. In principle, the pulsation of the RR Lyrae stars is very simple. These are radial pulsators most of which pulsate in a single pulsation mode, fundamental (RRab stars) or first overtone (RRc stars). However, nearly half of the stars, as recent results from ground-based campaigns and space missions indicate, show the Blazhko effect. We still lack a satisfactory explanation for the phenomenon. The recent discovery of the period doubling effect - alternating maxima and minima of the pulsational cycles in the light curves of three RRab Blazhko stars stimulated the research. It was soon explained that period doubling effect is caused by the resonant interaction between the fundamental mode and a higher order overtone. In the new model proposed to explain the Blazhko modulation, the same resonant mechanism is also responsible for the Blazhko modulation. Several questions arise, however. Since only three Blazhko RRab stars show the period doubling effect on top of modulations, are the two effects indeed directly connected? Is period doubling a cause or an effect of Blazhko modulation? If it is a cause, does the same mechanism work in Blazhko stars pulsating in the first overtone mode (RRc stars)? To answer these questions we plan to conduct a search for period doubling effect in the largest sample of RR Lyrae star available up to date - in the ground-based observations of the Optical Gravitational Lensing Experiment, containing more than 40000 RR Lyrae stars. Due to the high accuracy of the photometry and long observation base-line these data promise the best chance for success. Of primary importance is the search for the period doubling effect in non-modulated stars. The possible detection of such stars would put the direct connection between the period doubling effect and Blazhko modulation in question. The other important issue is the search for the period doubling effect in RRc stars, as well as a theoretical investigation of the possible resonances that might cause the period doubling behaviour in these stars. Two important by-products of the project are also expected: a precise characterisation of the Blazhko effect in the largest sample of of RR Lyrae stars so far and the discovery of new multiperiodic stars, in particular the members of a new class of double-periodic pulsators with a period ratio of the excited modes in a range of 0.60-0.64, which cannot be interpreted in terms of two radial modes.