Meandering periods and asymmetries in light curves of Miras: Observational evidence for low mass-loss rates

Abstract

Context. Some Miras (long-period variables in late evolutionary stages) have meandering pulsation periods and light-curve asymmetries, the causes of which are still unclear. Aims. We aim to better understand the origin of meandering periods and light-curve asymmetries by investigating a sample of Miras in the solar neighbourhood. We characterised this group of stars and related their variability characteristics to other stellar parameters. Methods. We analysed observations from several databases to obtain light curves with maximum time span and temporal coverage for a sample of 548 Miras. We determined their pulsation-period evolution over a time span of many decades, searched for changes in the periods, and determined the amplitude of the period change. We also analysed the Fourier spectra with respect to possible secondary frequency maxima. The sample was divided into two groups with respect to the presence of light-curve asymmetries (‘bumps’). Infrared colours and indicators of the third dredge-up were collected to study the mass loss and deep mixing properties of the stars of our sample. Results. Our analysis reveals one new star, T Lyn, with a continuously changing period. The group of Miras with meandering period changes is exclusively made up of M-type stars. The Fourier spectra of the meandering-period Miras have no prominent additional peaks, suggesting that additional pulsation modes are not the cause of the meandering periods. We confirm that light-curve bumps are more common among S and C Miras and show, for the first time, that Miras with such bumps have lower mass-loss rates than those with regular, symmetric light curves. Also, Miras with meandering period changes have relatively little mass loss. Conclusions. We conclude that Miras with strongly changing periods (including meandering periods) or asymmetries in their light curves have relatively low dust mass-loss rates. Meandering period changes and light-curve asymmetries could be connected to He-shell flashes and third dredge-up episodes.