Abundances of HCN and HNC in Dark Cloud Cores

We have determined the abundances of HCN and HNC toward 19 nearby dark cloud cores by observations of optically thin H13CN (J = 1-0) and HN13C (J = 1-0) lines. The column density of HCN is found to be correlated with 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 HCNH+ with electrons in dark cloud cores. Furthermore, the [HNC]/[HCN] ratio does not show any significant differences between star-forming cores and starless cores. The HCN and HNC abundances are compared with those for the OMC-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 HNC 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 high kinetic temperature regions.