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
Received 23 September 1998 / Accepted 3 November 1998
We report the discovery of the 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 , which requires a local gas density of at least H2 cm-3, and possibly up to 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 (firstname.lastname@example.org)
© European Southern Observatory (ESO) 1999
Online publication: November 26, 1998