1999A&A...341L...9Y

We report the discovery of the ν₃ band of SO₂ at 7.3µm in the ISO/SWS spectra of oxygen-rich AGB stars. The band is clearly detected in three stars, UX Cyg, ο Cet and T Cep, and marginally detected in at least four other stars. The band is seen in absorption in UX Cyg, while it is in emission in ο Cet. Seven spectra of T Cep taken at different phases show that the feature changes from emission to absorption on a time scale of twice the pulsation period. Using an LTE model, we find that the excitation temperature of SO₂ is typically 600K, and that in T Cep the molecule occupies a region with dimensions of several stellar radii. The total number of molecules contained in this region is of order 10⁴⁷, which requires a local gas density of at least 10⁹H₂.cm^–3, and possibly up to 10¹¹H₂.cm^–3 depending on the SO₂ abundance. The variation with phase of the T Cep spectra can be explained by photodissociation of SO₂ molecules by soft UV photons.