Period Evolution in Mira Variables

Volume 31 number 1 (2003)

Albert A. Zijlstra: University
Timothy R. Bedding: University

Abstract

We investigate a number of Mira variables which show evolution in their periods. Three different types of period changes are found: continuous changes, sudden changes, and meandering periods. On the order of 1% of Miras show evidence for period changes, but unstable periods may be common among the longest period Miras. The case of R Hya is studied in more detail, using archived data from AAVSO, AFOEV, BAAVSS, RASNZ, and VSOLJ, and historical records: we find that its period evolved from almost 500 days around its discovery (AD 1662) to about 385 days since AD 1950. The period change was accompanied by a dramatic change in its mass-loss rate. Such changes in mass-loss rates, especially for the Miras with meandering periods, could be one of the causes of the rings seen around many descendents of Mira variables, the planetary nebulae.