A Multicolor and Fourier Study of RR Lyrae Variables in the Globular Cluster NGC 5272 (M3)

We have performed a detailed study of the pulsational and evolutionarycharacteristics of 133 RR Lyrae stars in the globular cluster NGC5272 (M3)using highly accurate BVI data taken on 5 separate epochs. M3 seems to containno less than ~32% of Blazhko stars, and the occurrence and characteristics ofthe Blazhko effect have been analyzed in detail. We have identified a goodnumber (~ 14%) of overluminous RR Lyrae stars that are likely in a moreadvanced evolutionary stage off the Zero Age Horizontal Branch (ZAHB). Physicalparameters (i.e. temperature, luminosity, mass) have been derived from (B--V)colors and accurate color-temperature calibration, and compared with HorizontalBranch evolutionary models and with the requirements of stellar pulsationtheory. Additional analysis by means of Fourier decomposition of the V lightcurves confirms, as expected, that no metallicity spread is present in M3.Evolution off the ZAHB does not affect [Fe/H] determinations, whereas Blazhkostars at low amplitude phase do affect [Fe/H] distributions as they appear moremetal-rich. Absolute magnitudes derived from Fourier coefficients might provideuseful average estimates for groups of stars, if applicable, but do not givereliable {\em individual} values. Intrinsic colors derived from Fouriercoefficients show significant discrepancies with the observed ones, hence theresulting temperatures and temperature-related parameters are unreliable.Comment: 86 pages, 19 figures, 13 tables, in press A