Infrared Radiative Pumping of Triatomic Molecules in AGB Stars

Eduardo González-Alfonso  & Jose Cernicharo 

Consejo Superior de Investigaciones Cientificas Instituto de Estructura de la Materia Dpto Fisica Molecular C/Serrano 121. 28006 Madrid. Spain

We present detailed radiative transfer models for HCN and CO₂ in C-rich and O-rich evolved stars respectively. For HCN and CO₂ we have included 12 and 15 vibrational levels respectively and for each vibrational level 100 rotational levels have been included. All significant ro-vibrational transitions between these levels have been treated. The model, similar to that presented by González-Alfonso and Cernicharo (1997) for SiO in O-rich stars, contains 10 shells going from 2 to 100 stellar radii. The abundance of HCN and CO₂ is maintained constant through the envelope. No collisional excitation between the vibrational levels is assumed.

The results indicate that radiative pumping is a very efficient mechanism to produce emission in the bending mode, , of both molecules. However, due to the symmetry of CO₂, the vibrational transitions producing the emission in the =1 level and its associated hot and combination bands is very different from those of HCN. All the levels around 15 m are predicted to be in emission for stars of low mass loss rate as observed by Justannot et al. (1997)

In the innermost regions, and assuming no collisional deexcitation, HCN vibrational levels are populated from the transitions (000)=>(001) followed by radiative decay to the (01¹0). This mechanism is more efficient than direct absorption from the ground to the bending levels. In the external layers, where dust has been formed, the number of photons at 3 um is strongly reduced due to the large dust opacity. In these layers, absorption from the ground to the =2⁰ level followed by radiative decay to the =1¹ level produce a strong emission at 14.3 m as observed in IRC+10216 by Cernicharo et al. (1998). This mechanism works through the whole envelope.

For CO₂, and due to its symmetry, the radiative path is much complex than for HCN and involves several combination bands between the two stretching modes and the bending one.

For both molecules the inclusion of dust has a very important effect on the spectral features in the emerging infrared spectrum of AGB stars. For very large mass loss rates the bending mode (and its associated hot and combination bands) will be in absorption while for moderate and low mass loss rate they will be in emission.