THE LUMINOSITY SCALE OF RR LYRAE STARS WITH THE BAADE-WESSELINK METHOD .3. THE ABSOLUTE MAGNITUDES OF 4 RR LYRAE STARS IN THE GLOBULAR-CLUSTER M4

The surface brightness technique is applied to four RR Lyrae stars in the globular cluster M4 to determine their absolute magnitudes. Our study indicates that an accuracy comparable to the field star <Mv> determination (∼0.2 mag) can be achieved for cluster RR Lyraes. The mean of the <Mv> values for the four stars agrees with our field star <Mv> calibration appropriate to the cluster's metallicity ([Fe/H] = -1.3). Because there are no systematic effects in the <Mv> comparison, this agreement implies that there is no fundamental difference between field and cluster RR Lyraes in their physical characteristics. We find that there is a few tenths of a magnitude intrinsic dispersion in the derived values of <Mv>, which is expected due to the effect of evolution off the zero-age horizontal branch. In addition, the four cluster RR Lyraes obey the <MK>-log P relation found for the field RR Lyrae Stars.