FORMATION OF PAHS, POLYYNES, AND OTHER MACROMOLECULES IN THE PHOTOSPHERE OF CARBON STARS

Polycycle Aromatic Hydrocarbons (PAH) have been suggested as a link in one of the possible routes from molecules to grains in carbon-rich st ellar atmospheres. Carbon grains, except SiC, may form by conversion o f C2H2 into PAHs. The most obvious site of this PAH production is the outer atmosphere of carbon stars. We present the first calculations of equilibrium partial pressures of PAHs and other complex carbon-bearin g molecules in stellar photosphere models for carbon si-ars, We have i ncluded 38 atomic species and 338 molecules in chemical equilibrium. O ur computed models have the following parameters: T-eff=(2800K, 2600K, 2400K), Z=Z(.) log(g) epsilon [-1,0.5], C/O epsilon [1.1,7.0]. The re sults of the hydrostatic model calculations show that or atmospheric r egions with T>1000K the partial pressures of PAHs and other large mole cules are negligible small. This result is independent of the choice o f fundamental stellar parameters. For shallower depths than our hydros tatic photosphere models, however, a systematic chemical equilibrium e xploration of the T-P-gas plane, T epsilon[750K, 1000K], logP(gas) eps ilon[-5,3] (P-gas in dyn/cm(2)), discloses PAHs as the species contain ing the largest fraction of carbon atoms not bound in CO for a range o f temperatures around 850K, More carbon is contained in PAHs than in C O at these temperatures for C/O greater than or similar to 5.0. Chemic al equilibrium considered in a dynamical atmospheric structure confirm s these results. The higher column density of PAHs is sufficient. for the dynamic model to have an effect on the structure of the model (lev itation) and on the emergent spectrum. However, some studies (Frenklac h & Feigelson 1989) on kinetic PAH formation require a residence time at favourable T and P-gas which is longer than the rime scales of real istic wind models. In the framework of our hydrostatic models we confi rm that the conditions for PAH formation (T, P-gas or residence time) are not met, and we can point al the lower boundary condition needed f or the wind models. Therefore either the observed dust grains form via other routes, or more complex stellar environments are required.