The evolution of the Mira variable R Hydrae

The Mira variable R Hydrae is well known for its declining period, which Wood & Zarro attributed to a possible recent thermal pulse. Here we investigate the long-term period evolution, covering 340 years, going back to its discovery in AD 1662. The data include photometric monitoring by amateur and other astronomers over the last century, and recorded dates of maximum for earlier times. Wavelets are used to determine both the period and the semi-amplitude. We show that the period decreased linearly between 1770 and 1950; since 1950 the period has stabilized at 385d. The semi-amplitude is shown to follow the period evolution closely. Analysis of the oldest data shows that before 1770 the period was about 495d. We find no evidence for an increasing period during this time as found by Wood & Zarro. We discuss the mass-loss history of R Hya: the IRAS data show that the mass loss dropped dramatically around AD 1750. The evolution of the mass loss as function of period agrees with the mass-loss formalism from Vassiliadis & Wood; it is much larger than predicted by the Blöcker law. An outer detached IRAS shell suggests that R Hya has experienced mass-loss interruptions before. The period evolution can be explained by two models: a thermal pulse occurring around AD 1600, or a non-linear instability leading to an internal relaxation of the stellar structure. The elapsed time between the mass-loss decline giving rise to the outer detached shell and the recent event, of approximately 5000yr, suggests that only one of these events could be due to a thermal pulse. Further monitoring of R Hya is recommended, as both models make strong predictions for the future period evolution. We argue that R Hya-type events could provide part of the explanation for the rings seen around some asymptotic giant branch (AGB) and post-AGB stars. Changes in Mira properties were already known on a cycle-to-cycle basis, and on the thermal pulse time-scale of ~10⁴yr. R Hya shows that significant evolution can also occur on intermediate time-scales of the order of 10²-10³yr.