1996A&A...307...52S

Based on recent interferometric observations of HCN and CO 1-0 molecular emission for the Seyfert galaxy NGC 1068, an exceptionally large HCN/CO emission-line ratio has been revealed. Such a large ratio is attributed to the presence of high density, n_H>10⁵cm^–3, molecular clouds, and also to a large HCN/CO abundance ratio in the narrow line region. We have modelled the chemistry of these molecular clouds using two time-dependent models: 1) pure gas-phase model, 2) the dust/gas model in which the full complement of gas-grain interactions are included along with the pure gas-phase chemistry. Our main results show that only at early or mid time of evolution and with the dust/gas chemical model in which moderate oxygen depletion (0.5<δ_O<0.8) is assumed can one account for a high HCN/CO abundance ratio. An underabundance of O is required in both the dust/gas as well as in the pure gas-phase model at "steady state" evolution time. However, the HCN/CO ratio for dust/gas models is in general higher than this ratio in the pure gas-phase model. Therefore, it appears that not only the oxygen depletion and the number density of the molecular clouds are crucial parameters affecting the HCN/CO ratio but also the chemical evolution time. Even with the optimization of gas-grain interactions they are very important parameters for obtaining a fair comparison with observations. In any case, we can assert that in order to achieve high HCN/CO ratios these molecular clouds must still be at an early state of chemical evolution, t≃10⁶yr; i.e., they have not yet reached steady state no matter what the physical state and elemental abundances are.