The evolution of the Mira variable R Hydrae
Albert A. Zijlstra, T. R. Bedding, J.A. Mattei, 2002, MNRAS, 334, 498
Abstract
The Mira variable R Hydrae is well known for its declining period,
which Wood & Zarro (1981) 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 includes
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 semi-amplitude. We show that the
period decreased linearly between 1770 and 1950; since 1950 the period
has stabilized at 385 days. The semi-amplitude is shown to closely
follow the period evolution. Analysis of the oldest data shows that
before 1770 the period was about 495 days. 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 shows 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
Bloecker 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 occuring around AD 1600,
or an 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 5000 yr 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 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 the order of 104yr. R Hya
shows that significant evolution can also occur on intermediate time scales of
order 102-103yr.
paper (MNRAS reprint, pdf, 1.9Mb)
tif file of Figure 4 Hevelius (1690) chart
of the region showing R Hya and U Hya. (12Mb)