Variations of the HCO$^{+}$, HCN, HNC, N$_2$H$^+$ and NH$_{3}$ deuterium fractionation in high-mass star-forming regions

TL;DR

This study investigates deuterium fractionation in high-mass star-forming regions using spectral observations and radiative transfer modeling, revealing temperature-dependent variations in D/H ratios and relative abundances of key molecules.

Contribution

It provides new observational data and analysis of deuteration processes in high-mass star-forming regions, comparing results with chemical models to enhance understanding of molecular evolution.

Findings

D/H ratios decrease with increasing temperature from 20 to 40 K.

Deuterium fraction of N2H+ is 0.008-0.1 at 20-25 K.

Relative abundances decrease with temperature, but DCN/H2 ratio remains constant.

Abstract

We use spectra and maps of the $J=1-0$ and $J=2-1$ DCO$^{+}$, DCN, DNC, $\rm N_2D^+$ 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 $\rm 20-40 \,K$ and slightly vary at densities $n(\rm H_2) \sim 10^4-10^6\, cm^{-3}$. The deuterium fraction of $\rm N_2H^{+}$ is $0.008-0.1$ at temperatures in the range of $\rm 20-25\, K$ and at a density of $\sim 10^5\, \rm cm^{-3}$. We also estimate relative abundances and find $ \sim 10^{-11}-10^{-9}$ for DCO$^{+}$ and DNC, $ \sim 10^{-11}-10^{-10}$ for $\rm N_2D^+$ 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).