Evolution of Horizontal-Branch Stars in Globular Clusters: The Interesting Case of V79 in M3

摘要

New observations of variable stars in the globular cluster M3 reveal that the RR Lyrae variable V79 is a double-mode (RRd) variable with the first-overtone mode dominating. In all previous studies, V79 was found to be a fundamental-mode (RRab) pulsator with an irregular light curve. This is the first observed mode switch for an RR Lyrae variable, and it is direct observational evidence for blue ward evolution of horizontal-branch stars in the Oosterhoff type I cluster M3. It also demonstrates that there is a connection between the Blazhko effect and pulsational mode mixing in RR Lyrae variables. These new observations also show that the strength of the overtone oscillations in the RRd star V68 in M3 may have increased in the last 70 yr, thus indicating blueward evolution for V68 as well