The long-term behavior of the population II Cepheid V1 in the globular cluster Messier 12

Abstract

Observations made over an interval of 73 yr have been used to study the period changes of the W Virginis star in M12. They show that the period has undergone a series of abrupt changes, both increasing and decreasing, instead of a smooth change as predicted by evolutionary theory. This result is compared with period changes of W Virginis and RV Tauri stars in other galactic globular clusters, and it is found that the long-term behavior of the periods of most of these stars is similar to that of the variable in M12. However, two stars, V42 in M5 and V1 in ω Centauri, may be in their final blueward evolutionary phase.