Probing the mass-loss history of the unusual Mira variable R Hydrae through its infrared CO wind

Context. The unusual Mira variable R Hya is well known for its declining period between ad 1770 and 1950, which is possibly attributed to a recent thermal pulse. Aims. The goal of this study is to probe the circumstellar envelope (CSE) around R Hya and to check for a correlation between the derived density structure and the declining period. Methods. We investigate the CSE around R Hya by performing an in-depth analysis of (1.) the photospheric light scattered by three vibration-rotation transitions in the fundamental band of CO at 4.6 mu m; and (2.) the pure rotational CO J = 1-0 through 6-5 emission lines excited in the CSE. The vibrational-rotational lines trace the inner CSE within 3.5 '', whereas the pure rotational CO lines are sensitive probes of the cooler gas further out in the CSE. Results. The combined analysis bear evidence of a change in mass-loss rate some 220 yr ago (at similar to 150 R(*) or similar to 1.9 arcsec from the star). While the mass-loss rate before ad 1770 is estimated to be similar to 2 x 10(-7) M(circle dot)/yr, the present day mass-loss rate is a factor of similar to 20 lower. The derived mass-loss history nicely agrees with the mass-loss rate estimates by Zijlstra et al. (2002) on the basis of the period decline. Moreover, the recent detection of an AGB-ISM bow shock around R Hya at 100 arcsec to the west by Wareing et al. (2006) shows that the detached shell seen in the 60 mu m IRAS images can be explained by a slowing-down of the stellar wind by surrounding matter and that no extra mass-loss modulation around 1-2 arcmin needs to be invoked. Conclusions. Our results give empirical evidence to the thermal-pulse model, which is capable of explaining both the period evolution and the mass-loss history of R Hya.