Abundances of HCN and HNC in dark cloud cores

We have determined the abundances of HCN and HNC toward 19 nearby dark clou d cores by observations of optically thin (HCN)-C-13 (J = 1-0) and (HNC)-C- 13 (J = 1-0) lines. The column density of HCN is found to be correlated wit h that of HNC. The abundance ratio of [HNC]/[HCN] is determined to be 0.54- 4.5 in the observed dark cloud cores. These results are consistent with the idea that HCN and HNC are produced mainly by a recombination reaction of H CNH+ with electrons in dark cloud cores. Furthermore, the [HNC]/[HCN] ratio does not show any significant differences between star-forming cores and s tarless cores. The HCN and HNC abundances are compared with those fbr the O MC-1 cores previously reported. Although the abundances of HCN in the OMC-1 cores are comparable to those in the dark cloud cores, the abundances of H NC in OMC-1 are 1-2 orders of magnitude less than those in dark cloud cores . It is suggested that HNC is destroyed by neutral-neutral reactions in hig h kinetic temperature regions.