Stellar Cycles

Long-term cycles are observed in everyday phenomena like the weather and its long-term average: the climate. On Earth we have the well-known rhythm of the seasons, repeated every year as the Earth completes its orbit around the Sun. This annual cycle is as regular as the periods observed in pulsating stars. On Earth, however, there also exist longer cycles associated with climatic changes. The reasons for those are complex and not fully understood.
A similar long-term cyclic behavior is observed in several types of pulsating stars, where the regular pulsation is modulated by longer cycles. Almost a century after the discovery of this phenomenon, its physical origin still remains a mystery.
In this proposal we focus on the long-term cyclic behavior, which is frequently observed in one class of astrophysically important pulsating stars, the so-called RR Lyrae stars. The phenomenon is called Blazhko effect after one of its discoverers, and has been the frequent subject of numerous photometric studies. Recent attempts to theoretically explain the effect focus on two alternatives: the magnetic models and the resonances models, both involving the presence of nonradial pulsation components. In the course of the past years, large photometric data bases supplied by the microlensing surveys and the precise CCD observations of globular clusters, have yielded statistics on the phenomenology of the Blazhko effect and its occurence. These findings put important constraints upon the applicability of the models.
Spectroscopic analyses, made possible by recent developments in modern astronomical instrumentation, appear to be the most powerful approach to disentangle the pulsation changes. The new approach of studying the Blazhko effect through a detailed study of the variations in their spectral lines, can provide decisive information on its physical origin. We intend to carry out this research by studying simultaneous spectroscopic and photometric data of a well-selected sample of field RR Lyrae stars. The answer to this stubborn and century-old problem would definitely clarify similar phenomena also observed in other classes of pulsating stars.