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 evolutionary characteristics of 133 RR Lyrae stars in the globular cluster NGC 5272 (M3) using highly accurate BVI data taken on five separate epochs. M3 seems to contain no less than ∼32% of Blazhko stars, and the occurrence and characteristics of the Blazhko effect have been analyzed in detail. We have identified a good number (∼14%) of overluminous RR Lyrae stars that are likely in a more advanced evolutionary stage off the zero-age horizontal branch (ZAHB). Physical parameters (i.e., temperature, luminosity, and mass) have been derived from B - V colors and accurate color-temperature calibration, and compared with horizontal-branch evolutionary models and with the requirements of stellar pulsation theory. Additional analysis by means of Fourier decomposition of the V light curves confirms, as expected, that no metallicity spread is present in M3. Evolution off the ZAHB does not affect [Fe/H] determinations, whereas Blazhko stars at low amplitude phase do affect [Fe/H] distributions, as they appear more metal-rich. Absolute magnitudes derived from Fourier coefficients might provide useful average estimates for groups of stars, if applicable, but do not give reliable individual values. Intrinsic colors derived from Fourier coefficients show significant discrepancies with the observed ones, and hence the resulting temperatures and temperature-related parameters are unreliable.