Mapping Observations of DNC and HN13C in Dark Cloud Cores

We present results of mapping observations of the DNC, HN^13C, and H^13CO^+lines (J=1-0) toward 4 nearby dark cloud cores, TMC-1, L1512, L1544, and L63,along with observations of the DNC and HN^13C lines (J=2-1) toward selectedpositions. By use of statistical equilibrium calculations based on the LVGmodel, the H_2 densities are derived to be (1.4-5.5)*10^5 cm^-3, and the[DNC]/[HN^13C] ratios are derived to be 1.25-5.44 with a typical uncertainty bya factor of 2. The observed [DNC]/[HNC] ratios range from 0.02 to 0.09,assuming the [^12C]/[^13C] ratio of 60. Distributions of DNC and HN^13C aregenerally similar to each other, whereas the distribution of H^13CO^+ is moreextended than those of DNC and HN^13C, indicating that they reside in an innerpart of the cores than HCO^+. The [DNC]/[HN^13C] ratio is rather constantwithin each core, although a small systematic gradients are observed in TMC-1and L63. Particularly, no such systematic gradient is found in L1512 and L1544,where a significant effect of depletion of molecules is reported toward thecentral part of the cores. This suggests that the [DNC]/[HNC] ratio would notbe very sensitive to depletion factor, unlike the [DCO^+]/[HCO^+] ratio. On theother hand, the core to core variation of the [DNC]/[HNC] ratio, which range anorder of magnitude, is more remarkable than the variation within each core.These results are interpreted qualitatively by a combination of three competingtime-dependent processes; gas-phase deuterium fractionation, depletion ofmolecules onto grain surface, and dynamical evolution of a core.Comment: 22 pages, 8 EPS figures, aasLaTex 5.0, accepted to The Astrophysical Journa