The Blazko Project: Joint Efforts in Solving a Century-old Problem

Almost a century after its discovery, the phenomenon of amplitude and/or phase modulation, observed in a large percentage of the RR Lyrae stars, still lacks widely acceptable theoretical understanding. 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 this paper the Blazhko Project is presented, a larger international collaboration focused on understanding the Blazhko effect. The aim of the project is to combine spectroscopic and photometric data from a sample of well-selected Blazhko and non-Blazhko stars, in order to reveal crucial information on the physical mechanism responsible for the modulation. The possible key to the century-old Blazhko puzzle lies in the detailed study of the line profile variations (LPV) over the pulsation and Blazhko cycle. We briefly discuss the methodology of the project. 1. RR Lyrae Stars and the Blazhko Effect RR Lyrae stars have been studied for over a century now, and play an important role in astrophysics. These pulsating variables have periods of 0.2–1.1 day, and show brightness variations of the order of a magnitude. Until not so long ago, they were considered to be prototypes of radially pulsating stars. The most intriguing subclass of RR Lyrae stars consists of stars showing the Blazhko effect, the phenomenon of amplitude or phase modulation. These stars have light curves that are modulated on time scales of typically tens to hundreds of days (Fig. 1). Blazhko (1907) was the first to report this phenomenon in RW Dra. The estimated incidence rate of Blazhko variables among the galactic RRab stars (fundamental mode pulsators) is about 20–30% (Szeidl 1988; Moskalik & Poretti 2002). For the RRc Blazhko stars (first overtone pulsators) this rate is less than 5%. In the LMC the incidence rate for RRab stars is only half as large, which is probably a metallicity effect. 2. Past and Present Studies of the Blazhko Effect The Blazhko effect has been the frequent subject of photographic and photometric studies (e.g., Szeidl & Kollath 2000). Traditionally, the phenomenon was studied by means of O − C analysis. Therefore, the observations were in general strongly biased towards the ascending branch and maximum phase of the primary light curve, a sampling pernicious for the Fourier analysis recently used