THE SENSITIVITY OF GAS-PHASE CHEMICAL MODELS OF INTERSTELLAR CLOUDS TO C AND O ELEMENTAL ABUNDANCES AND TO A NEW FORMATION MECHANISM FOR AMMONIA

The e†ects of variations in the gas-phase carbon-to-oxygen elemental abundance ratio (0.42 C/O 1.2) and the absolute gas-phase carbon and oxygen elemental abundances on calculated molecu-lar concentrations have been studied for three gas-phase chemical models of dense interstellar clouds. Both the C and O elemental abundances were varied from their “ “ low metal ÏÏ values, in which C/ O \ 0.42. The results were compared with observations of the dark interstellar clouds TMC-1 and L134N, the latter being chosen because TMC-1, with its singularly rich component of large hydrocar-bons and cyanopolyynes, may not represent dense cores universally. In general, variations in the gas-phase C and O elemental abundances have a large and time-dependent e†ect on calculated molecular concentrations for all three models. For the “ “ new standard ÏÏ model, which does not contain many rapid neutral-neutral reactions, excellent “ “ early-time ÏÏ agreement with TMC-1 occurs for a variety of C/O ratios obtained by depleting the low metal O abundance, but the time of best agreement tends to increase with increasing C/O ratio. At these early times, B80 % of the calculated abundances are within an order of magnitude of the observed values. Agreement at this level also occurs at steady state if the C and O abundances are Ðrst depleted by a factor of 5 and then O is additionally depleted so that C