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Variable Stars in M13. IV. The RR~Lyrae Variable Stars

Kopacki G., Osborn W., Smith H., Layden A., Pritzl B., Kuehn C.

Acta Astronomica

2024

Vol. 74, No. 3

157--200

New CCD photometry combined with published and unpublished archival observations have been used to study ten RR~Lyr-type variable stars in the globular cluster M13. The periods have been updated and U, B, V, RC, C-band light curves and their characteristics derived. Fourier analysis of the CCD observations showed that variables V31 and V36 are multiperiodic RRc stars with additional frequencies grouped close to the frequency of the overtone mode, but not forming equidistant triplets. For V36, we detect an additional weak component for which the period ratio with the overtone indicates variability of the RR0.61 kind. This makes M13 the third globular cluster, besides M3 and NGC 6362, where such RRc stars are known. We confirm V34 to be a biperiodic variable with very close frequencies. Searches for period changes using the (O-C) method for observations spanning up to 120~yr show that the cluster's only RRab star (V8) has undergone a very large period decrease, but rather than decreasing at a steady rate it most likely experienced an abrupt period decrease around 1970. Two of the nine RRc variables show steady period increases, one a period decrease, and five no significant period change. Rapid irregular light-curve phase changes observed for four of the RRc stars can be explained by their multiperiodicity. Expected evolutionary rates of period change have been determined for the RR Lyr stars using the PISA evolutionary tracks for horizontal branch models. The observed rates mostly do not agree with the theoretical ones, both in sign and in absolute value. The results for V25, however, indicate that this RRc star is much more evolved than other RR Lyr stars in M13 with its observed period change rate of 0.5 d/Myr being consistent with that predicted by theory. A comparison of the derived physical properties from different approaches suggests problems with the theory. Of most concern is the inconsistency found for the RR~Lyr stars between the predictions from HB evolutionary tracks and the well-determined observational parameters of these stars.