A provisional RR Lyrae distance scale

Variable Stars in the Unusual, Metal-rich, Globular Cluster NGC 6441

“…The long periods of the RRab stars and the relatively large proportion of c-type pulsators are more typical of Oosterho † II than Oosterho † I systems, although we again note that is unusually large even for Ooster-SP ab T ho † II. In contrast, the mean luminosity of the NGC 6441 RRc stars, as found by applying the Simon & Clement (1993a, 1993b) method (see°5.1), is consistent with values found in Oosterho † I clusters such as M3. This is in conÑict with our assertion that the RRLs are unusually bright (see°5 .3).…”

“…The RRc with the best light curve was V70. Leaving out V71 and V77, for which the Simon & Clement (1993a, 1993b equations yield especially low masses, we Ðnd that the mean values for the RRc star mass, and log (L /L _ ), T eff , are 0.47 1.65, 7408 K, and 0.79, respectively. If we M V M _ , were to include V71 and V77, the mean values would change to 0.44 1.65, 7403 K, and 0.79, which are not M _ , signiÐcant changes.…”

“…D m Clement & Rowe (2000) also used the masses and luminosities of the RRc stars to illustrate the di †erences between the Oosterho † groups. Their Table 4 derived from Simon & Clement (1993a, 1993b, and the Fourier parameters listed in Table 6, we have calculated the masses, luminosities, temperatures, and absolute magnitudes for a small number of RRc stars in NGC 6441 (see Table 13). Following the criteria for acceptable stars in their paper, we excluded all long-period RRc stars (see°5.2), since these stars were shown to have abnormally low derived masses, and included only the RRc stars with an error in below a certain limit.…”

“…While this may lead to scatter in the derived RRc parameters, it cannot account for the systematically low values for the masses. One possible reason for the low mass values could be that a problem with the zero point in the Simon & Clement (1993a, 1993b calibration for the RRc masses exists. In any case, it appears that even in this Oosterho † classiÐcation based on derived physical parameters for the RRc stars, NGC 6441 has difficulty being classiÐed as either an Oosterho † type I or Oosterho † type II cluster.…”

“…(Further points on this classiÐcation scheme are discussed in°5.4,°5.5, and°6.) Another possible explanation for the low RRc masses is that the Fourier relations derived by Simon & Clement (1993a, 1993b may not be applicable to the RRc in NGC 6441, especially if some noncanonical e †ect is responsible for the blue extension of the HB. Given the existence of long-period RRc stars in NGC 6441 (°5.2) and the unusually low masses obtained for the RRc stars with periods near 0.3 days, one might wonder if some of the shorter period RRLs might be second-overtone pulsators.…”