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Astron. Astrophys. 341, L9-L12 (1999)

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Letter to the Editor

Detection of warm SO2 gas in oxygen-rich AGB stars *

I. Yamamura 1, T. de Jong 2,1, T. Onaka 3, J. Cami 4,1 and L.B.F.M. Waters 1,4

1 Astronomical Institute "Anton Pannekoek", University of Amsterdam, Kruislaan 403, 1098 SJ, Amsterdam, The Netherlands
2 SRON-Utrecht, Sorbonnelaan 2, 3584 CA, Utrecht, The Netherlands
3 Department of Astronomy, School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, 113-0033 Tokyo, Japan
4 SRON-Groningen, P.O. Box 800, 9700 AV, Groningen, The Netherlands

Received 23 September 1998 / Accepted 3 November 1998

Abstract

We report the discovery of the [FORMULA] band of SO2 at 7.3 µm in the ISO/SWS spectra of oxygen-rich AGB stars. The band is clearly detected in three 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 emission in o 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 600 K, 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 [FORMULA], which requires a local gas density of at least [FORMULA] H2 cm-3, and possibly up to [FORMULA] H2 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.

Key words: stars: AGB and post-AGB – stars: atmospheres – stars: circumstellar matter – stars: late-type – stars: mass-loss – infrared: stars

* Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) with the participation of ISAS and NASA. The SWS is a joint project of SRON and MPE.

Send offprint requests to: I. Yamamura (yamamura@astro.uva.nl)

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© European Southern Observatory (ESO) 1999

Online publication: November 26, 1998
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