Gas-phase CO₂ around O-rich AGB stars

J. Cami ¹, T. de Jong ², A.G.G.M. Tielens ³, K. Justtanont ⁴, L.B.F.M. Waters ⁵, & I. Yamamura ⁶

¹ SRON-Groningen, PO Box 800, 9700 AV Groningen, The Netherlands

² Astronomical Institute ``Anton Pannekoek'', University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands

³ Kapteijn Institute, PO BOx 800, 9700 AV Groningen, The Netherlands ⁴ Stockholm Observatory, 13336 Saltsjöbaden, Sweden

⁵ Department of Astronomy, University of Tokyo, 2-11-16 Yayoi-cho Bunkyo-ku, Tokyo 113, Japan

In the ISO-SWS AOT 1 spectra of O-rich AGB stars obtained as part of the guaranteed time program to study the physical and chemical evolution of AGB stars and their circumstellar envelopes (PI T. de Jong) strong emission lines were detected between 12.5 and 16.5 . New observations at a higher resoltion allowed identifications of these lines as due to rovibratinoal Q-branch transitions of both and .

We constructed a model to generate sunthetic spectra that can be directly compared with the SWS full grating scans (R1500), assuming the bands are optically thin and formed under LTE conditions. Instead of aiming at the best overall match between model sectra and the data, we dtermined for each band independently the excitation temperature and the number of emitting molecules.

We cab fit the data very well with our model spectra and derive parameters for the different lines that are in agreementwith each other. The excitation temperature found is typically 70 K and the number of molecules is of the order of 10⁴⁶. The derived parameters for the fundamental band at 14.98  however deviates systematically from those found for the other lines, and is even seen in absorption in some objects.

We argue that the gas kinetic temperature must be higher than the excitation temperature and that the gas must be located close to the star. We assess the relevance of radiative pumping through the 4.28  line and the possibility of gas thermalized by the radiation field. We conclude that the physical conditions in the layer must be close to LTE conditinos, implying that locally the density must be enhanced by several orders of magnitude, probably cuased by shocks in the outflows of these stars.