Deuterium fractionation and CO depletion in Barnard 5

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 (B5) 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 N$_2$H$^+$(1–0), N$_2$D$^+$(1–0), H$^{13}$CO$^+$(1–0), and DCO$^+$(2–1), complemented by C$^{18}$O(2–1) data, we derive column density, deuterium fraction, and CO depletion maps. We find that the deuterium fraction in both mentioned nitrogen- and carbon-bearing species increases from the protostellar to the starless core, reaching $R_D^{\rm N_2H^+}=0.43\pm0.10$ and $R_D^{\rm HCO^+}=0.09\pm0.02$ in the starless core, compared with $0.15\pm0.03$ and $0.05\pm0.01$, respectively, in the protostellar core. The CO depletion factor also rises from $4.1\pm0.1$ to $5.0\pm0.1$ across the same transition. While the embedded YSO 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.

💡 Research Summary

This paper presents the first spatially resolved study of deuterium fractionation and CO depletion in the Barnard 5 (B5) region of the Perseus molecular cloud, simultaneously covering a starless core and the Class 0/I protostellar core IRAS 03445+3242. Using the IRAM 30 m telescope, the authors observed the transitions N₂H⁺(1–0), N₂D⁺(1–0), H¹³CO⁺(1–0), DCO⁺(2–1), and C¹⁸O(2–1). Complementary data from Herschel (H₂ column density and dust temperature), GBT NH₃ maps, and the COMPLETE 13CO(1–0) survey were incorporated to provide total gas column densities and a reference CO abundance.

All maps were convolved to a common angular resolution of 33.6″ and regridded to 12″ pixels. Spectral fitting employed hyperfine structure (HFS) models via the pyspeckit package, with excitation temperature (T_ex) and optical depth (τ) derived for each line. To reduce uncertainties, a Markov Chain Monte Carlo (MCMC) exploration of the τ–T_ex parameter space was performed, allowing the authors to retain only pixels with relative uncertainties below 33 %.

Column densities were calculated from the fitted τ and T_ex, acknowledging that N₂H⁺ and N₂D⁺ column densities are particularly sensitive to T_ex (≈20–27 % change per 1 K). The deuterium fractionation ratios were defined as R_D(N₂H⁺)=N(N₂D⁺)/N(N₂H⁺) and R_D(HCO⁺)=N(DCO⁺)/N(H¹³CO⁺). Results show a clear gradient from the protostellar core to the starless core:

• Starless core: R_D(N₂H⁺)=0.43 ± 0.10, R_D(HCO⁺)=0.09 ± 0.02, CO depletion factor f_d=5.0 ± 0.1.
• Protostellar core: R_D(N₂H⁺)=0.15 ± 0.03, R_D(HCO⁺)=0.05 ± 0.01, f_d=4.1 ± 0.1.

CO depletion was derived by comparing the observed C¹⁸O column density with the expected CO abundance (based on the canonical CO/H₂ ratio of 10⁻⁴, calibrated using the 13CO survey). The higher f_d in the starless core indicates more severe CO freeze‑out onto dust grains.

Interpretation focuses on two coupled processes. In cold, dense gas where CO is frozen out, the main ion H₃⁺ is less efficiently destroyed by CO, allowing the reaction H₃⁺ + HD → H₂D⁺ + H₂ to dominate and boost the abundance of deuterated species. Consequently, N₂H⁺, which traces densities ≳10⁵ cm⁻³, exhibits a high deuterium fraction. In the protostellar core, heating by the embedded YSO raises temperatures to ≈15 K, causing CO to sublimate back into the gas phase. The increased CO abundance both destroys H₂D⁺ and shifts the H₃⁺/H₂D⁺ equilibrium back toward H₃⁺, reducing deuterium fractionation. HCO⁺, tracing lower densities (10⁴–10⁵ cm⁻³), is less affected by the protostellar heating, which explains the modest change in R_D(HCO⁺).

The paper resolves a previously reported anomaly in B5—high deuterium fraction with low CO depletion—by demonstrating that the region contains both a highly depleted starless core and a less depleted protostellar envelope, whose signals were blended in earlier single‑point studies. The spatially resolved maps reveal a layered chemical structure: high‑density, highly depleted interiors with strong deuteration, surrounded by a lower‑density envelope where CO remains partially in the gas phase and deuteration is milder.

The authors conclude that accurate modeling of early star formation chemistry must incorporate the interplay of temperature, density, and CO freeze‑out/desorption on sub‑core scales. They suggest that future high‑resolution (≤5″) interferometric observations, combined with time‑dependent chemical models, will further clarify how deuterium fractionation evolves during the transition from prestellar to protostellar phases.