Letter to the Editor

Double mode RR Lyrae stars in Omega Centauri

Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warszawa, Poland e-mail: [olech;pam]@camk.edu.pl

Received: 28 November 2008
Accepted: 23 December 2008

Abstract

Aims. The aim of this work is to search for double mode pulsators among RR Lyr variables of globular cluster ω Cen.

Methods. We conducted a systematic frequency analysis of CASE photometry of ω Cen RR Lyr stars. We searched for periodicities using Fourier and ANOVA periodograms, combined with a consecutive prewhitening technique.

Results. We discovered six double mode pulsators, with the first overtone and a secondary mode of higher frequency simultaneously excited. These are the first double mode RR Lyr stars identified in ω Cen. In variable V10 the period ratio of the two modes is 0.80, which corresponds to pulsations in the first and second radial overtones. In V19 and V105 we found an unexpected period ratio of 0.61. Three other stars display period ratios of either ~ 0.80 or ~ 0.61, depending on the choice of aliases.

Conclusions. While the period ratio of ~0.80 is easy to interpret in terms of the two lowest radial overtones, the value of ~0.61 cannot be explained by any two radial modes. Thus, V19 and V105 are the first members of a new class of double mode RR Lyr pulsators.