RR Lyrae Variables in the Globular Cluster M5

Abstract

We present V-band CCD photometry of 65 RR Lyr variables from the globular cluster M5. We have estimated the basic physical parameters for 16 RRc stars and 26 RRab stars using a Fourier decomposition of the light curves of the variables. The mean values of mass, luminosity, effective temperature and relative helium abundance for the RRc stars are measured to be ${\cal M}=0.54{\cal M}_\odot$, log L = 1.69 T_{eff}=7353 K and Y=0.28, respectively. For the RRab variables the derived mean values of absolute magnitude, metallicity and effective temperature are: M_V=0.81, [Fe/H]=-1.23 and T_{eff}=6465 K. We find that the V amplitude of an RRab star for a given period is a function of metal abundance rather than Oosterhoff type. We find significant problems with the calibration of both the zero point and the scale of the luminosities measured with the Fourier technique. The apparent distance modulus derived from RRc stars is equal to $14.47\pm0.11$ and it is in good agreement with recent determinations. On the other hand distance modulus obtained from the sample of RRab stars (calibrated by the Baade-Wesselink observations of field RR Lyr variables) is significantly smaller and equal to $14.27\pm0.04$Comment: accepted for publication in Astronomy and Astrophysics, 15 pages with 6 figure