2 FREQUENCY ANALYSIS OF THE RR LYRAE STARS IN THE LMC

More than 1300 variables classified provisionally as first overtone RR Lyrae pulsators in the macho variable star database of the Large Magellanic Cloud (LMC) have been subjected to standard frequency analysis. Based on the remnant power in the prewhitened spectra, we found 70% of the total population to be monoperiodic. The remaining 30% (411 stars) are classified as one of 9 types according to their frequency spectra. Several types of RR Lyrae pulsational behavior are clearly identified here for the first time. Together with the earlier discovered double-mode (fundamental & first overtone) variables this study increased the number of the known double-mode stars in the LMC to 181. During the total 6.5 yr time span of the data, 10% of the stars show strong period changes. The size, and in general also the patterns of the period changes exclude simple evolutionary explanation. We also discovered two additional types of multifrequency pulsators with low occurrence rates of 2% for each. In the first type there remains one closely spaced component after prewhitening by the main pulsation frequency. In the second type the number of remnant components is two, they are also closely spaced, and, in addition, they are symmetric in their frequency spacing relative to the central component. This latter type of variables is associated with their relatives among the fundamental pulsators, known as Blazhko variables. Their high frequency (≈ 20%) among the fundamental mode variables versus the low occurrence rate of their first overtone counterparts makes it more difficult to explain Blazhko phenomenon by any theory depending mainly on the role of aspect angle or magnetic field. None of the current theoretical models are able to explain the observed close frequency components without invoking nonradial pulsation components in these stars. Subject headings: globular clusters: general — stars: horizontal-branch — stars: oscillations — stars: variables: other (RR Lyrae) Institute for Geophysics and Planetary Physics, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA alcock, kcook, mgeha, stuart, cnelson, popowski1@llnl.gov Mount Stromlo and Siding Spring Observatory, Australian National University, Weston Creek, ACT 0200, Australia ajd, kcf, peterson, tsa@mso.anu. edu.au, robyn@mindful.anu. edu.au Space Telescope Science Institute, Baltimore, MD 21218, USA alves@stsci.edu Department of Astronomy, University of Washington, Seattle, WA 98195, USA austin, becker, stubbs@astro.washington. edu Department of Physics, University of California, San Diego, CA 92093, USA kgriest, tvandehei@ucsd.edu Department Astronomia, Universidad Catolica, Chile dante@astro.puc. cl Department of Physics, University of California, Berkeley, CA 94720, USA European Southern Observatory, Garching, D-85748, Germany pjq@eso.org Department of Physics, Oxford University, OX1 3RH, England w.sutherland@physics.ox. ac.uk Department of Physics and Astronomy, McMaster University, Hamilton, L82 4M1, Canada welch@physics.mcmaster. ca Department of Physics, University of Sheffield, S3 7RH, England Department of Physics, University of Notre Dame, South Bend, IN 46556, USA bennett@nd.edu Konkoly Observatory, P.O. Box 67, H-1525 Budapest, Hungary kovacs@konkoly.hu Department of Astronomy, University of Cape Town, Rondebosch 7701, South Africa dkurtz@ma.saao. ac.za Department of Astronomy, University of Toronto, Toronto, M5S 3H8, Canada cclement, rowe@astro.utoronto. ca