Late Type Stars and Water

The presence of water in the atmosphere of cool stars was already predicted by Russell in 1934. His calculations showed that water should be the most abundant molecule besides the H2 molecule and atomic hydrogen for cool stars at about 2800 K. The first detection of water was made in the spectrum of the well known star Mira. This is a pulsating variable star, an expansion and contraction produces large change in temperature and radius and therefore strong variations in its luminosity are observed. Kuiper (1963) observed Mira in the wavelength range from 1400 nm to 1900 nm. 

Searches for 1612-MHz OH and 22 335-MHz H2O maser emission from symbiotic Mira variables were reported by Seaquist, Ivison, and Hall, 1995 [303]. Because only two cases were found a mechanism for the deficiency was proposed. A powerful wind from the hot companion sweeps much of the volume of the dusty Mira wind away from the hot companion, thus exposing the Mira to UV emission. The UV emission then photodissociates molecules responsible for all known masers. 

Barlow et al., 1996 [17], investigated the rich far IR water vapor spectrum of the star W Hya (Fig. 7.9). This object is a cool (2700 K) M-dwarf at a distance of 130 pc. It has a high mass loss (about 10 M0yr ). Water vapor seems to be the major coolant and the H2O/H2 abundance is 8 x 10 for r 4.5 x 10 cm and 3 x 10 at large radii. Maercker et al., 2008 [210] found high amounts of H2O in the circumstellar envelope of that star and they speculate that evaporation of icy cometary or planetary bodies might be an effective ongoing mechanism in such systems. 

The importance of water vapor lines in the spectra of M stars was addressed by Jones et al., 2002 [174], 

2000 [342] detected water in emission in the spectrum of the M2 supergiant star /u, Cephei (M2 la) observed by the Short-Wavelength Spectrometer (SWL) on board ISO. The emission first appears in the 6 pm region and then in the 40 pm region (pure rotation lines) despite the rather strong dust emission. The intensity ratios of the emission features are far from those of the optically thin gaseous emission. From the observed emission features an optically thick water sphere of the inner radius about 2 R (corresponds to 1300 R ), T = 1500 K, and a colunm den- 

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