Period Changes of Mira Variables, RR Lyrae Stars, and Type II Cepheids

Abstract

Mira variables, RR Lyrae variables, and type II Cepheids all represent evolved states of low-mass stars, and long term observations have revealed that changes in pulsation period occur for each of these classes of variable. Most Mira variables show small or no period changes, but a few show large period changes that can plausibly be associated with thermal pulses on the asymptotic red giant branch. Individual RR Lyrae stars show period changes that do not accord with the predictions of stellar evolution theory. This may be especially true for RR Lyrae stars that exhibit the Blazhko effect. However, the average period changes of all of the RR Lyrae variables within a globular cluster prove a better but still imperfect match for the predictions of evolutionary theory. The observed period changes of short period type II Cepheids (BL Her stars) as well as those of long period type II Cepheids (W Vir stars) are in broad agreement with the rates of period changes expected from their evolutionary motions through the instability strip.

A search for period changes of eight short-period Type II Cepheids

“…Several period change studies of BLH stars in globular clusters (Osborn 1969;Wehlau & Bohlender 1982;Jurcsik et al 2001;Osborn et al 2019) and in the field (Christianson 1983;Diethelm 1996;Provencal 1986) have found that the majority of BLH stars do indeed show period increases while a minority, within observational uncertainties and limited data, show no period change. However, the changes are broadly consistent with theoretically predicted period changes (Neilson et al 2016;Smith 2013).…”

Near-ultraviolet detections of four dwarf nova candidates in the globular cluster 47 Tucanae

“…Typical RR Lyrae 'period changers' have β < 0.1 d Myr −1 (see e.g. Jurcsik et al 2001;Smith 2013;Arellano Ferro et al 2016, which is much smaller than the period change we determine for HV 810. However, a small number of RR Lyrae have been observed with very large period change rates of 1 < β < 10 d Myr −1 (Jurcsik et al 2001).…”