2003ApJ...594..859H

We present results of mapping observations of the DNC, HN¹³C, and H¹³CO⁺ lines (J=1-0) toward four nearby dark cloud cores, TMC-1, L1512, L1544, and L63, along with observations of the DNC and HN¹³C lines (J=2-1) toward selected positions. By use of statistical equilibrium calculations based on the large velocity gradient (LVG) model, the H₂densities are derived to be 1.4-5.5x10⁵ cm^–3, and the [DNC]/[HN¹³C] ratios are derived to be 1.25-5.44, with a typical uncertainty of a factor of 2. The observed [DNC]/[HNC] ratios range from 0.02 to 0.09, assuming a [¹²C]/[¹³C] ratio of 60. Distributions of DNC and HN¹³C are generally similar to each other, whereas the distribution of H¹³CO⁺ is more extended than those of DNC and HN¹³C, indicating that they reside in a more inward part of the cores than HCO⁺. The [DNC]/[HN¹³C] ratio is rather constant within each core, although small systematic gradients are observed in TMC-1 and L63. In particular, no such systematic gradient is found in L1512 and L1544, where a significant effect of depletion of molecules is reported toward the central part of the cores. This suggests that the [DNC]/[HNC] ratio would not be very sensitive to the depletion factor, unlike the [DCO⁺]/[HCO⁺] ratio. On the other hand, the core-to-core variation of the [DNC]/[HNC] ratio, which ranges over an order of magnitude, is more remarkable than the variation within each core. These results are interpreted qualitatively by a combination of three competing time-dependent processes: gas-phase deuterium fractionation, depletion of molecules onto grain surfaces, and dynamical evolution of a core.