Blazhko-type fundamental-mode RR Lyrae stars in the globular cluster M3

Abstract

ABSTRACT Blazhko stars from the extended BVIC time series of RRab stars in the globular cluster M3 are analysed. This is the largest sample of Blazhko stars with full details of their Blazhko properties in a homogeneous stellar system. Blazhko periods and light-curve solutions are determined/estimated for 83 fundamental-mode variables. The lack of phase modulation in Oosterhoff type II stars and the diminishing relative strength of the modulation in long-period Oosterhoff type I stars are regular tendencies found between the pulsation and modulation properties of Blazko stars. Similarly to previous results, no modulation of the longest-period stars is detected. The onset of modulation in a previously regular RRab star and the similar distribution of modulated and non-modulated stars imply that the modulation is a temporal property of RR Lyrae (RRL) stars, which may occur at any time in any RRab star except the coolest ones. Comparing the modulation periods in M3 and in other samples of Blazhko stars, the mean log(Pmod) value is found to depend on the metallicity of the system. The separation of temperature- and radius-change induced variations supports our previous finding that the photometric radius variation does not show any modulation. The pulsation-averaged mean brightness and temperature of Blazhko stars are found to be larger in the large-amplitude phase of the modulation than in the small-amplitude phase. The larger the amplitude of the modulation, the larger the changes of the mean parameters detected.