3. The chemistry
We consider 68 chemical species and 752 reactions and include all possible chemical routes at work in a dense gas, that is, termolecular and bimolecular neutral-neutral reactions. No photo-processes (dissociation or ionisation) are considered as the UV stellar radiation field is low for the effective temperature quoted in Table 1. Also, we do not consider radiative processes in the post-shock gas on the ground of the theoretical models of post-shock structures by Fox & Wood (1985). Indeed, for a molecular, cool pre-shock gas, they find that the ionization level in the thermal cooling region is low and we consider the collisional dissociation of molecular hydrogen to be the dominant coolant in the immediate post-shock region (WC98, Cherchneff et al. 1998).
The reaction rates are taken from the RATE95 UMIST database (Millar et al. 1997), Baulch et al. (1992), Cherchneff et al. (1992), Mick et al. (1994) and the NIST database (Mallard et al. 1994). Details about assumptions involved in the chemistry used in the model are given by WC98.
We assume thermal equilibrium (TE) for the stellar photosphere and derive molecular abundances for the effective temperature, gas number density and C/O ratio given in Table 1 for IK Tau. We then "shock" the photosphere and investigate the chemistry in the immediate cooling layer and the hydrodynamical cooling part ( excursion) of the post-shock region. The model (the immediate post-shock region followed by excursion at one position in the envelope) is run over two pulsation cycles to check for periodicity. The output abundances for one model are then used as the input to that for the shock at the next distance and are rescaled according to the local gas number density.
© European Southern Observatory (ESO) 1999
Online publication: December 4, 1998