Variations of the HCO+, HCN, HNC, N2H+, and NH3 deuterium fractionation in high-mass star-forming regions

Abstract

ABSTRACT We use spectra and maps of the J = 1 − 0 and J = 2 − 1 DCO+, DCN, DNC, $\rm N_2D^+$ lines, and 111−101 ortho- and para-NH2D lines, obtained with the Institut de Radioastronomie Millimétrique (IRAM)-30 m 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 3CCH lines, also observed with the IRAM-30 m 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 H13CN and HN13C lines and their main isotopologues. Applying a non-local thermodynamic equilibrium 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 NH2D. The D/H ratios decrease with increasing temperature in the range of 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 20–25 K and at a density of ∼105 cm−3. We also estimate relative abundances and find ∼10−11–10−9 for DCO+ and DNC, ∼10−11–10−10 for $\rm N_2D^+$, and ∼10−10–10−8 for NH2D. The relative abundances of these species decrease with increasing temperature. However, the DCN/H2 ratio is almost constant (∼10−10). The observational results agree with the predictions of chemical models (although in some cases there are significant differences).