Double mode RR Lyrae stars in Omega Centauri

Abstract

\n Aims. The aim of this work is to search for double mode pulsators among\n RR Lyr variables of globular cluster ω Cen.\n Methods. We conducted a systematic frequency analysis of CASE photometry of\nω Cen RR Lyr stars. We searched for periodicities using\nFourier and ANOVA periodograms, combined with a consecutive prewhitening\n technique.\n Results. We discovered six double mode pulsators, with the first overtone\nand a secondary mode of higher frequency simultaneously excited.\nThese are the first double mode RR Lyr stars identified in\nω Cen. In variable V10 the period ratio of the two modes is 0.80, which corresponds to pulsations in the first and second\n radial overtones. In V19 and V105 we found an unexpected period ratio\n of 0.61. Three other stars display period ratios of either ~\n 0.80 or ~ 0.61, depending on the choice of aliases.\n Conclusions. While the period ratio of ~0.80 is easy to interpret in\n terms of the two lowest radial overtones, the value of ~0.61\n cannot be explained by any two radial modes. Thus, V19 and V105 are\n the first members of a new class of double mode RR Lyr pulsators.\n