Long‐Term Changes in Mira Stars. I. Period Fluctuations in Mira Stars

Abstract

It has been known since the pioneering work of Eddington & Plakidis that the period changes in Mira stars are dominated by the effects of random cycle‐to‐cycle fluctuations in period, at least in the limited sample of stars that were studied. We have extended their work, using a portion of the American Association of Variable Star Observers (AAVSO) international database of variable star observations: times and magnitudes of maximum and minimum magnitude of 391 bright Mira stars over 75 yr (Kowalsky et al.). The Eddington & Plakidis model fits the data in almost every case, at least over a time interval of several tens of cycles. There is a weak correlation between the fractional period fluctuation, per cycle, and the period (or color) of the star, but not with the amplitude or chemical type (M, S, or C).