W Hya: Molecular inventory by ISO-SWS

Abstract

Infrared spectroscopy is a powerful tool to probe the inventory of solid state and molecular species in circumstellar ejecta. Here we analyse the infrared spectrum of the Asymptotic Giant Branch star W Hya, obtained by the Short and Long Wavelength Spectrometers on board of the Infrared Satellite Observatory. These spectra show evidence for the presence of amorphous silicates, aluminum oxide, and magnesium-iron oxide grains. We have modelled the spectral energy distribution using laboratory-measured optical properties of these compounds and derive a total dust mass-loss rate of yr-1. We find no satisfactory fit to the 13 μm dust emission feature and the identification of its carrier is still an open issue. We have also modelled the molecular absorption bands due to H2O, OH, CO, CO2, SiO, and SO2 and estimated the excitation temperatures for different bands which range from 300 to 3000 K. It is clear that different molecules giving rise to these absorption bands originate from different gas layers. We present and analyse high-resolution Fabry-Perot spectra of the three CO2 bands in the 15 μm region. In these data, the bands are resolved into individual Q-lines in emission, which allows the direct determination of the excitation temperature and column density of the emitting gas. This reveals the presence of a warm (450 K) extended layer of CO2, somewhere between the photosphere and the dust formation zone. The gas in this layer is cooler than the 1000 K CO2 gas responsible for the low-resolution absorption bands at 4.25 and 15 μm. The rotational and vibrational excitation temperatures derived from the individual Q-branch lines of CO2 are different (~450 K and 150 K, respectively) so that the CO2 level population is not in LTE.