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Deuterium fractionation and CO depletion in Barnard 5

Igor Petrashkevich, Anna Punanova, Paola Caselli, Jaime E. Pineda, Olli Sipila, Anton I. Vasyunin

TL;DR

This paper addresses how deuterium fractionation and CO depletion trace the chemical evolution of cold dense gas in Barnard 5 by delivering spatially resolved maps of $R_D^{\rm N_2H^+}$, $R_D^{\rm HCO^+}$, and $f_d$ across a starless core and a neighboring protostellar core. Using IRAM 30 m data for N$_2$H$^+$, N$_2$D$^+$, H$^{13}$CO$^+$, DCO$^+$, and C$^{18}$O, complemented by NH$_3$ and ^{13}CO from literature, the authors derive column densities, deuterium fractions, and CO depletion, tying the spatial patterns to physical conditions. They find $R_D^{\rm N_2H^+}$ up to $0.52$ and $R_D^{\rm HCO^+}$ up to $0.094$, with higher deuteration in the starless core ($R_D^{\rm N_2H^+}=0.43\pm0.10$, $R_D^{\rm HCO^+}=0.09\pm0.02$) and a gradient toward the protostar ($R_D^{\rm N_2H^+}=0.15\pm0.03$, $R_D^{\rm HCO^+}=0.05\pm0.01$). The CO depletion factor ranges from $f_d\sim1.2$ to $5.0$, peaking in the starless core, consistent with CO freeze-out driving enhanced deuteration. These results support a scenario where CO depletion and protostellar heating jointly regulate deuterium chemistry and provide empirical benchmarks for chemical models of low-mass star-forming regions.

Abstract

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.

Deuterium fractionation and CO depletion in Barnard 5

TL;DR

This paper addresses how deuterium fractionation and CO depletion trace the chemical evolution of cold dense gas in Barnard 5 by delivering spatially resolved maps of , , and across a starless core and a neighboring protostellar core. Using IRAM 30 m data for NH, ND, HCO, DCO, and CO, complemented by NH and ^{13}CO from literature, the authors derive column densities, deuterium fractions, and CO depletion, tying the spatial patterns to physical conditions. They find up to and up to , with higher deuteration in the starless core (, ) and a gradient toward the protostar (, ). The CO depletion factor ranges from to , peaking in the starless core, consistent with CO freeze-out driving enhanced deuteration. These results support a scenario where CO depletion and protostellar heating jointly regulate deuterium chemistry and provide empirical benchmarks for chemical models of low-mass star-forming regions.

Abstract

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 NH(1--0), ND(1--0), HCO(1--0), and DCO(2--1), complemented by CO(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 and in the starless core, compared with and , respectively, in the protostellar core. The CO depletion factor also rises from to 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.

Paper Structure

This paper contains 21 sections, 5 equations, 15 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Observation Data
  3. IRAM 30 M Data
  4. Data from Literature
  5. Data Reduction and Analysis
  6. Spectral Cubes
  7. Spectral Line Analysis
  8. HFS Analysis
  9. NH3, C^18O, $^{13}$CO Data
  10. Results
  11. Fit Parameters
  12. Column Density and Abundance
  13. Abundances
  14. CO Depletion
  15. Deuterium Fraction
  16. ...and 6 more sections

Figures (15)

  • Figure 1: Molecular hydrogen column density ($N_{\rm tot}$(H$_2$), color scale) toward B5 Pezzuto2021. The first contour starts at 0.1$\times$10$^{22}$ cm$^{-2}$ with a contour step of 0.5$\times$10$^{22}$ cm$^{-2}$. The black rectangle shows the area of our maps.
  • Figure 2: Integrated intensities ($W$) of the N$_2$H$^+$(1--0), N$_2$D$^+$(1--0), H$^{13}$CO$^+$(1--0), DCO$^+$(2--1), and C$^{18}$O(2--1) toward B5. The black contour shows the column density of molecular hydrogen. The first contour starts at 0.8$\times$10$^{22}$ cm$^{-2}$ with a contour step of 0.5$\times$10$^{22}$ cm$^{-2}$Pezzuto2021. The star shows the position of the YSO Yu1999. The beam size is shown in the bottom left corner of each map.
  • Figure 3: Distribution of the velocity dispersion $\sigma$ of N2D^+(1--0), DCO^+(2--1), and C$^{18}$O(2--1). The step is 0.025 km s$^{-1}$.
  • Figure 4: The column density maps of N$_2$H$^+$, N$_2$D$^+$, DCO$^+$,HCO$^+$, and C$^{18}$O toward B5. The black contour shows the column density of molecular hydrogen (the first contour starts at 0.8$\times$10$^{22}$ cm$^{-2}$ with a contour step of 0.5$\times$10$^{22}$ cm$^{-2}$). The star shows the position of the YSO Yu1999. The beam size is shown in the bottom left corner of each map.
  • Figure 5: $^{13}$CO depletion ($f_d$), based on the ^13CO(1--0) data Ridge2006 toward the entire Perseus molecular cloud and L1688 region, obtained with different CO canonical abundances. The canonical CO abundances were taken from Lacy1994, Wannier1980, Frerking1982, Tafalla2004. The black line shows $f_d$ = 1 at which all CO is in the gas phase.
  • ...and 10 more figures