Se p 20 04 A multi - color and Fourier study of RR Lyrae variables in the globular cluster

We have performed a detailed study of the pulsational and evolutionary characteristics of 133 RR Lyrae stars in the globular cluster NGC5272 (M3) using highly accurate BVI data taken on 5 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, 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 – 2 – coefficients show significant discrepancies with the observed ones, hence the resulting temperatures and temperature-related parameters are unreliable.