Observations of DNC and DCO${}^{+}$ toward the $\int$-shaped Filament and Starless Cores in the Orion Molecular Clouds

Abstract

Although the deuterium fraction is known to be a powerful evolutionary tracer, its variation within individual molecular cloud cores is still poorly understood. The northern $\int$-shaped filament and 20 individual starless cores in the Orion A and B clouds were mapped in the deuterated molecules of DNC and DCO${}^{+}$ with the Receiver 7BEE installed on the Nobeyama 45~m radio telescope. In a ~ 5' X 30' map of the northern $\int$-shaped filament in the Orion A cloud, the DNC emission is detected over the filament, whereas the DCO${}^{+}$ emission is localized toward OMC-3, the northernmost region of the filament. The difference in distribution between DNC and DCO${}^{+}$ can be attributed to that between N- and C-bearing molecules as previously suggested by Tatematsu et al. High DNC/HN${}^{13}$C column density ratios were observed in OMC-2 and OMC-3, and low ratios in OMC-1. It seems that OMC-2 and OMC-3 still contain molecular gas close to the onset of star formation. In 3' X 3' maps of the individual starless cores in Orion, the column density ratios of DNC/HN${}^{13}$C and DCO${}^{+}$/H${}^{13}$CO${}^{+}$ are found to be rather constant locally within each core, although the core-to-core variation is not small. Similar timescales of deuterization, depletion, and dynamical evolution might explain the locally constant ratio.

33 pages, 14 figures, 3 tables