Secular Evolution in Mira Variable Pulsations

Stellar evolution theory predicts that asymptotic giant branch stars undergoa series of short thermal pulses that significantly change their luminosity andmass on timescales of hundreds to thousands of years. Secular changes in thesestars resulting from thermal pulses can be detected as measurable changes inperiod if the star is undergoing Mira pulsations. The American Association ofVariable Star Observers (AAVSO) International Database currently containsvisual data for over 1500 Mira variables. Light curves for these stars spannearly a century in some cases, making it possible to study the secularevolution of the pulsation behavior on these timescales. In this paper, wepresent the results of our study of period change in 547 Mira variables usingdata from the AAVSO. We find non-zero rates of period change, dlnP/dt, at the2-sigma significance level in 57 of the 547 stars, at the 3-sigma level in 21stars, and at the level of 6-sigma or greater in eight of the 547. The lattereight stars have been previously noted in the literature, and our derived ratesof period changes largely agree with published values. The largest and moststatistically significant dlnP/dt are consistent with the rates of periodchange expected during thermal pulses on the AGB. A number of other starsexhibit non-monotonic period changes on decades-long timescales, the cause ofwhich is not yet known.Comment: 37 pages, with 9 figures and 1 table. The complete electronic version of Table 1 is available from the authors upon request. Accepted for publication in The Astronomical Journa