Infrared emission of hot water in the atmosphere of Mira

The ISD/SWS spectrum of o Cet taken at its maximum exhibits an absorption-l ike feature between 3.5 and 4.0 mu m We present evidence that the feature i s due to emission of H2O and SiO molecules, in a layer extended to about tw o stellar radii with an excitation temperature of 2000 K. These hot molecul es 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 the se two stars. The H2O column densities and excitation temperatures in the l ayers are found to be similar in both stars. The difference of the H2O band is thus primarily due to the layer size. The H2O layers seem to be more ex tended at visual maximum, probably related to the stellar pulsation. The es timated lower limit to the local gas density in the layers of similar to 10 (11) cm(-3) is in good agreement with theoretical predictions from dynamica l model atmospheres.