Formation of PAHs, polyynes, and other macro-molecules in the photosphere of carbon stars

C. Helling, U.G. Jørgenson, B. Plez and H.R. Johnson

Astronomy & Astrophysics, 315, 194-203 (1996)

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Abstract:

Polycyclic Aromatic Hydrocarbons (PAH) have been suggested as a link in one of the possible routes from molecules to grains in carbon-rich stellar atmospheres. Carbon grains, except SiC, may form by conversion of C_2H_2 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-bearing molecules in stellar photosphere models for carbon stars. We have included 38 atomic species and 338 molecules in chemical equilibrium. Our computed models have the following parameters: T_{eff}=(2800K, 2600K, 2400K), Z=Z_odot, log(g) epsilon [-1,0.5], C/O epsilon [1.1,7.0]. The results of the hydrostatic model calculations show that for atmospheric regions with T>1000K the partial pressures of PAHs and other large molecules are negligible small. This result is independent of the choice of fundamental stellar parameters.

For shallower depths than our hydrostatic photosphere models, however, a systematic chemical equilibrium exploration of the T-P_{gas} plane, T epsilon [750K,1000K], log P_{gas} epsilon[-5,3] (P_{gas} in dyn/cm^2), discloses PAHs as the species containing the largest fraction of carbon atoms not bound in CO for a range of temperatures around 850K. More carbon is contained in PAHs than in CO at these temperatures for C/O stackg 5.0.

Chemical equilibrium considered in a dynamical atmospheric structure confirms these results. The higher column density of PAHs is sufficient for the dynamic model to have an effect on the structure of the model (levitation) and on the emergent spectrum. However, some studies (Frenklach & Feigelson 1989) on kinetic PAH formation require a residence time at favourable T and P_{gas} which is longer than the time scales of realistic wind models. In the framework of our hydrostatic models we confirm that the conditions for PAH formation (T, P_{gas} or residence time) are not met, and we can point at the lower boundary condition needed for the wind models. Therefore either the observed dust grains form via other routes, or more complex stellar environments are required.