Warm dioxides around O-rich AGB stars

J. Cami ¹, I. Yamamura ², T. de Jong ^3,4, K. Justtanont ⁵, A.G.G.M. Tielens ⁶, T. Onaka ², & L.B.F.M. Waters ³

¹ SRON-Groningen, PO Box 800, 9700 AV Groningen, The Netherlands

² Department of Astronomy, University of Tokyo, 2-11-16 Yayoi-cho Bunkyo-ku, Tokyo 113, Japan

³ Astronomical Institute ``Anton Pannekoek'', University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands

⁴ SRON-Utrecht, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands

⁵ Stockholm Observatory, 13336 Saltsjöbaden, Sweden

⁶ Kapteijn Institute, PO BOx 800, 9700 AV Groningen, The Netherlands

A significant part of the O-rich AGB stars that were observed with the ISO-SWS as part of the guaranteed time program to study the physical and chemical evolution of AGB stars and their circumstellar envelopes (P.I. T. de Jong) shows prominent emissino bands between 12.5 and 16.5 , originating from the ro-vibrational Q-branch transitions of and . The fundamental band at 14.98  is sometimes seen in absorption.

In three O-rich stars the SO₂ band at 7.3 is clearly detected. The feature is seen as a deep absorption in UX Cyg, while it is in emission in o Cet. Seven spectra of T Cep taken at different optical phases show that the feature changed from emission to absorption in a time scale of one year.

A model assuming that the CO₂ lines are optically thin and formed under LTE conditions is able to reproduce the SWS full grating scans very well in most cases with excitation temperatures of typically 700 K and a total number of emitting CO₂ molecules; only the fundamental band cannot be explained by this model as it shows optical effects. We argue that the gas kinetic temperature must be higher than the excitation temperature and the the CO₂ gas must thus be close to LTE conditions, implying that locally the density must be enhanced by several orders of magnitude.

An LTE model including optical depth effects can reproduce the SO₂ bands very well in all circumstances and yields an excitation temperature of typically 600 K and a total number of emitting SO₂ molecules of the order of 10⁴⁷, indicating similar conditins as for the CO₂ bands. The emitting region extends over several stellar radii, and the spectral variations observed in the T Cep spectra can be explained by dissociation of the molecules in the outer part of the layer. In this contribution we discuss the nature of the emitting region for both molecules, comment on the formation andc destruction of these molecules and assess the importance of these results for the process of dust formation.