Isotopic abundance variations in interstellar HCN

A study of the relative abundances of the rare isotopic species H12C15N and H13C14N has been made in six dense molecule clouds through observations of the 3.5 mm wavelength rotational transitions. The data indicate that in all sources the (HC15N)/(H13CN) integrated line-intensity ratio is smaller than that implied by terrestrial isotopic abundances. The excitation of HCN is discussed with special attention to the conditions which permit accurate abundance determination even when optical depths are large. We make a quantitative estimate of the line saturation and find this to be a small effect for the present measurements. There appears to be at least a factor of 6 between the average ratio for the sources in the galactic disk and that for two sources associated with the galactic center. This result is discussed in terms of increased nuclear processing in the galactic center region