SIMBAD references

We have observed the DNC (J=1-0) and HN¹³C (J=1-0) lines toward 29 nearby dark cloud cores to investigate a variation of deuterium fractionation. The [DNC]/[HN¹³C] ratio is found to be different from core to core; the lowest ratio (0.50) is observed toward L1521E and the highest ratio (7.3) toward L183(S). The [DNC]/[HNC] ratios range from 0.008 to 0.122 under the assumption that the [¹²C]/[¹³C] ratio is 60. We have found that the [DNC]/[HN¹³C] ratio in ``carbon-chain-producing regions'', where CCS and the other carbon-chain molecules are abundant, is significantly lower than those in the other cores. We have also made mapping observations of the DNC (J=1-0) and HN¹³C (J=1-0) lines toward TMC-1. The [DNC]/[HN¹³C] ratio in the northwest part of the TMC-1 ridge including the NH₃ peak is found to be larger than that of the southeast part around the cyanopolyyne peak. Thus, we have found that the observed cores can be divided into two groups according to the [DNC]/[HN¹³C] ratio and the [NH₃]/[CCS] ratio. This systematic variation of the deuterium fractionation would be interpreted in terms of the chemical evolution of dark cloud cores.