Detection of warm SO2 gas in oxygen-rich AGB stars

We report the discovery of the upsilon(3) band of SO2 at 7.3 mu m in the IS O/SWS spectra of oxygen-rich AGB stars. The band is clearly detected in thr ee stars, UX Cyg, o 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 emis sion in o Get. Seven spectra of T Cep taken at different phases show that t he feature changes from emission to absorption on a time scale of twice the pulsation period. Using an LTE model, we find that the excitation temperat ure of SO2 is typically 600 K, and that in T Cep the molecule occupies a re gion with dimensions of several stellar radii. The total number of molecule s contained in this region is of order 10(47), which requires a local gas d ensity of at least 10(9) H-2 cm(-3), and possibly up to 10(11) H-2 cm(-3) d epending on the SO2 abundance. The variation with phase of the T Cep spectr a can be explained by photodissociation of SO2 molecules by soft UV photons .