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Infrared emission of hot water in the atmosphere of Mira

The ISO/SWS spectrum of o Cet taken at its maximum exhibits an absorption-like feature between 3.5 and 4.0 μm. We present evidence that the feature is due to emission of H2O and SiO molecules, in a layer extended to about two stellar radii with an excitation temperature of 2000 K. These hot molecules are also observed in a spectrum of Z Cas near minimum, but this time in absorption. A simple plane-parallel model is used to fit the spectra of these two stars. The H 2O column densities and excitation temperatures in the layers are found to be similar in both stars. The difference of the H 2O band is thus primarily due to the layer size. The H 2O layers seem to be more extended at visual maximum, probably related to the stellar pulsation. The estimated lower limit to the local gas density in the layers of∼ 1011 cm−3 is in good agreement with theoretical predictions from dynamical model atmospheres.