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

¹ Department of Physics and Astronomy, Institute of Astronomy, K.U. Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium e-mail: Leen.Decin@ster.kuleuven.ac.be
² Sterrenkundig Instituut Anton Pannekoek, University of Amsterdam, Kruislaan 4031098 Amsterdam, The Netherlands
³ Department of Astronomy and Space Physics, Uppsala University, Box 515, 5120 Uppsala, Sweden
⁴ Lund Observatory, Box 43, 22100 Lund, Sweden
⁵ National Optical Observatories, PO Box 26732, Tucson, ZA 85726, USA

Received: 21 December 2007
Accepted: 23 March 2008

Abstract

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 μm; 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 ~150  or ~1.9 arcsec from the star). While the mass-loss rate before ad 1770 is estimated to be ~ /yr, the present day mass-loss rate is a factor of ~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 μm 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.