GAS-GRAIN CHEMISTRY AND THE HCN CO ABUNDANCE RATIO IN THE NUCLEUS OF NGC-1068/

Based on recent interferometric observations of HCN and CO 1-0 molecul ar emission for the Seyfert galaxy NGC 1068, an exceptionally large HC N/CO emission-line ratio has been revealed. Such a large ratio is attr ibuted to the presence of high density, n(H) greater than or similar t o 10(5) cm(-3), molecular clouds, and also to a large HCN/CO abundance ratio in the narrow line region. We have modelled the chemistry of th ese molecular clouds using two time-dependent models: 1) pure gas-phas e model, 2) the dust/gas model in which the full complement of gas-gra in 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 < delta(o) < 0.8) is assumed can one account for a high HCN/CO abun dance ratio. An underabundance of O is required in both the dust/gas a s well as in the pure gas-phase model at ''steady state'' evolution ti me. 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 th at not only the oxygen depletion and the number density of the molecul ar clouds are crucial parameters affecting the HCN/CO ratio but also t he chemical evolution time. Even with the optimization of gas-grain in teractions they are very important parameters for obtaining a fair com parison 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 similar or equal to 10(6) yr; i. e., they have not yet reached steady state no matter what the physical state and elemental abundances are.