Variations of the HCO${}^{+}$, HCN, HNC, N${}_2$H${}^{+}$ and NH${}_3$ deuterium fractionation in high-mass star-forming regions

Abstract

We use spectra and maps of the $J=1-0$ and $J=2-1$ DCO${}^{+}$, DCN, DNC, ${\mathrm{N}}_2{\mathrm{D}}^{+}$ lines and $1_{11}-1_{01}$ ortho- and para-NH${}_2$D lines, obtained with the IRAM-30m telescope, as well as observations of their hydrogenated isotopologues to study deuteration processes in five high-mass star-forming regions. The temperature was estimated from CH${}_3$CCH lines, also observed with the IRAM-30m telescope, and from NH${}_3$ lines, observed with the 100-m radio telescope in Effelsberg, as well as using the integrated intensity ratios of the $J=1-0$ H${}^{13}$CN and HN${}^{13}$C lines and their main isotopologues. Applying a non-LTE radiative transfer model with RADEX, the gas density and the molecular column densities were estimated. D/H ratios are $0.001-0.05$ for DCO${}^{+}$, $0.001-0.02$ for DCN, $0.001-0.05$ for DNC and $0.02-0.4$ for NH${}_2$D. The D/H ratios decrease with increasing temperature in the range of $20-40\mathrm{K}$ and slightly vary at densities $n({\mathrm{H}}_2)\sim 10^4-10^6\mathrm{c}{\mathrm{m}}^{-3}$. The deuterium fraction of ${\mathrm{N}}_2{\mathrm{H}}^{+}$ is $0.008-0.1$ at temperatures in the range of $20-25\mathrm{K}$ and at a density of $\sim 10^5\mathrm{c}{\mathrm{m}}^{-3}$. We also estimate relative abundances and find $\sim 10^{-11}-10^{-9}$ for DCO${}^{+}$ and DNC, $\sim 10^{-11}-10^{-10}$ for ${\mathrm{N}}_2{\mathrm{D}}^{+}$ and $\sim 10^{-10}-10^{-8}$ for NH${}_2$D. The relative abundances of these species decrease with increasing temperature. However, the DCN/H${}_2$ ratio is almost constant ($\sim 10^{-10}$). The observational results agree with the predictions of chemical models (although in some cases there are significant differences).

13 pages + appendix (25 pages total), 11 figures. Accepted for publication in MNRAS

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