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Period Evolution in Mira Variables

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Albert A. Zijlstra

University of Manchester, Institute of Science and Technology, Department of Physics, Manchester M60 1QD, United Kingdom

Timothy R. Bedding

University of Sydney, School of Physics, Sydney 2006, Australia

Presented at the 91st Spring Meeting of the AAVSO, July 2, 2002

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.

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