The Blazhko Effect

Abstract

We review the recent progress in our understanding of the Blazhko effect in RR Lyrae stars.1 Introduction RR Lyrae stars are low-mass, typically metal poor, Population II stars. In the HR diagram, they are located within the so-called classical instability strip. Their apparent large-amplitude brightness changes are due to oscillation in the radial modes: radial fundamental mode (RRab stars) or radial first overtone mode (RRc stars). Simultaneous pulsation in these two radial modes is also possible (RRd stars). As excellent distance indicators, RR Lyrae stars are invaluable in studies of the Galactic structure and evolution.RR Lyrae stars are considered a textbook examples of simple, single-periodic radially pulsating stars. In fact, they are much more complex and display phenomena that are not well understood. Recent analyses of top-quality ground-based and space-based photometry revealed that additional low-amplitude variability, attributed to nonradial modes, may be common, at least in RRc stars -see e.g. Szabó et al. (2014); Moskalik et al. (2015); Netzel et al. (2015); for recent review, see Moskalik (2014).Another puzzling effect, the Blazhko modulation, was discovered more than hundred years ago (Blažko, 1907). It manifests as a quasi-periodic modulation of pulsation amplitude and/or phase. Its cause remains a mystery. These proceedings are focused on recent progress in our understanding of the Blazhko phenomenon. Blazhko effect: new discoveries from the ground-based and space-based observationsBlazhko effect is one of the most stubborn problems of stellar astrophysics. Many efforts are devoted to study the phenomenon, both on observational side and on theoretical side. Until recently, the observations were limited to the ground-based only. RR Lyr, eponym of the class and a Blazhko variable, was a target of multi-site campaign (Kolenberg et al., 2006). Many Blazhko variables were discovered in the data gathered by the photometric sky surveys such as OGLE (e.g., Mizerski, 2003;Soszyński et al., 2011a) or MACHO (e.g. Alcock et al., 2003). Excellent observations, allowing detailed study of the light curve changes during the Blazhko cycle, were collected by the Konkoly Blazhko Survey (e.g., Jurcsik et al., 2008;Sódor et al., 2011). These observations also revealed that the Blazhko phenomenon might be common, at least in RRab stars, as it was detected in nearly 50 % of RRab stars for which high-quality photometry was gathered (Jurcsik et al., 2009).

The SERMON project: 48 new field Blazhko stars and an investigation of modulation-period distribution

“…There are many more interesting features of the BL effect. Extensive reviews of the BL effect are provided in recent overviews by Szabó (2014), Kovács (2015), and Smolec (2016).…”