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Astron. Astrophys. 330, L17-L20 (1998)

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Letter to the Editor

Discovery of CO2 emission in AGB stars with the 13 µm dust feature *

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

1 SRON-Groningen, P.O.Box 800, 9700 AV Groningen, The Netherlands
2 Max-Planck-Institut für extraterrestrische Physik, Postfach 1603, D-85740 Garching, Germany
3 Astronomical Institute `Anton Pannekoek', University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
4 Department of Astronomy, University of Tokyo, 2-11-16 Yayoi-cho, Bunkyo-ku, Tokyo 113, Japan

Received 16 September 1997 / Accepted 7 October 1997


We have detected strong emission lines at 13.48, 13.87, 14.97, 15.40 and 16.28 µm in the ISO/SWS spectra of O-rich AGB stars. These lines are only found in the spectra of Miras and semi-regular variables when they also show the 13 µm dust emission feature. The lines appear just resolved in the high-resolution (AOT06, resolution [FORMULA] 1500) SWS spectra that we recently obtained.

Here we report the identification of these emission lines as the Q-branches of ro-vibrational transitions in CO2 molecules. This identification is corroborated by our SWS Fabry-Perot observation of the 13.87 µm line in W Hya where individual Q-branch components of the [FORMULA] - [FORMULA] transition of CO2 have been detected. The 15.40 µm line is probably due to 13 CO2.

We speculate that the simultaneous occurence of the 13 µm dust feature and the CO2 emission lines indicates the existence of a warm ([FORMULA] 650-1250 K) gas layer close to the star where both the 13 µm dust and the CO2 emission lines are formed.

Key words: line: identification – stars: AGB and post-AGB – 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: K. Justtanont (e-mail: kay@sron.rug.nl)

© European Southern Observatory (ESO) 1998

Online publication: January 8, 1998