2026ApJ...997..296P

Deuterium fractionation provides a key diagnostic of the physical and chemical evolution of prestellar and protostellar cores, where it is strongly linked to CO depletion in cold, dense gas. We present the first spatially resolved maps of deuterium fraction and CO depletion in the Barnard 5 region of the Perseus molecular cloud, covering both a starless core and the protostellar core hosting the Class 0/I source IRAS 03445+3242. Using IRAM 30 m observations of N₂H⁺(1-0), N₂D⁺(1-0), H¹³CO⁺(1-0), and DCO⁺(2-1), complemented by C¹⁸O(2-1) data, we derive column density, deuterium fraction, and CO depletion maps. We find that the deuterium fraction in the abovementioned nitrogen- and carbon-bearing species increases from the protostellar to the starless core, reaching ${R}_{{\rm{D}}}^{{{\rm{N}}}_{2}{{\rm{H}}}^{+}}=0.43\pm 0.10$ and ${R}_{{\rm{D}}}^{{\mathrm{HCO}}^{+}}=0.09\pm 0.02$ in the starless core, compared with 0.15 ± 0.03 and 0.05 ± 0.01, respectively, in the protostellar core. The CO depletion factor also rises from 4.1 ± 0.1 to 5.0 ± 0.1 across the same transition. While the embedded young stellar object reduces deuteration in the dense inner gas, the less dense envelope traced by HCO⁺ is only slightly affected at our resolution. Our analysis confirms that CO freeze-out and the presence of a protostar jointly regulate deuterium chemistry in star-forming regions.