Infrared emission of hot water in the atmosphere of Mira

Abstract

The ISO/SWS spectrum of o Cet taken at its maximum exhibits an absorption-like feature between 3.5 and 4.0 mu m. We present evidence that the feature is due to emission of H_2O 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 ~ 10(11) cm(-3) is in good agreement with theoretical predictions from dynamical model atmospheres. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) with the participation of ISAS and NASA. The SWS is a joint project of SRON and MPE.