1999A&A...348L..55Y

The ISO/SWS spectrum of omicron Cet taken at its maximum exhibits an absorption-like feature between 3.5 and 4.0µm. 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 2000K. 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 H₂O column densities and excitation temperatures in the layers 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 extended at visual maximum, probably related to the stellar pulsation. The estimated lower limit to the local gas density in the layers of ∼10¹¹cm^–3 is in good agreement with theoretical predictions from dynamical model atmospheres.