W Hya: Molecular inventory by ISO-SWS ^*

¹ Stockholm Observatory, AlbaNova, Dept. of Astronomy, 106 91 Stockholm, Sweden
² SRON-Utrecht, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
³ University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
⁴ Kapteyn Institute, PO Box 800, 9700 AV Groningen, The Netherlands
⁵ SRON-Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
⁶ Max-Planck Institut fuer extraterrestrische Physik, Giessenbachstrasse, 85740 Garching, Germany

Corresponding author: K. Justtanont, kay@astro.su.se

Received: 28 February 2003
Accepted: 16 December 2003

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.