DCO${}^{+}$, DCN and N${}_2$D${}^{+}$ reveal three different deuteration regimes in the disk around the Herbig Ae star HD163296

Abstract

The formation pathways of deuterated species trace different regions of protoplanetary disks and may shed light into their physical structure. We aim to constrain the radial extent of main deuterated species; we are particularly interested in spatially characterizing the high and low temperature pathways for enhancing deuteration of these species. We observed the disk surrounding the Herbig Ae star HD 163296 using ALMA in Band 6 and obtained resolved spectral imaging data of DCO${}^{+}$ ($J$=3-2), DCN ($J$=3-2) and N${}_2$D${}^{+}$ ($J$=3-2). We model the radial emission profiles of DCO${}^{+}$, DCN and N${}_2$D${}^{+}$, assuming their emission is optically thin, using a parametric model of their abundances and radial excitation temperature estimates. DCO${}^{+}$ can be described by a three-region model, with constant-abundance rings centered at 70 AU, 150 AU and 260 AU. The DCN radial profile peaks at about ~60 AU and N${}_2$D${}^{+}$ is seen in a ring at ~160 AU. Simple models of both molecules using constant abundances reproduce the data. Assuming reasonable average excitation temperatures for the whole disk, their disk-averaged column densities (and deuterium fractionation ratios) are 1.6-2.6$\times 10^{12}$ cm${}^{-2}$ (0.04-0.07), 2.9-5.2$\times 10^{12}$ cm${}^{-2}$ ($\sim$0.02) and 1.6-2.5 $\times 10^{11}$ cm${}^{-2}$ (0.34-0.45) for DCO${}^{+}$, DCN and N${}_2$D${}^{+}$, respectively. Our simple best-fit models show a correlation between the radial location of the first two rings in DCO${}^{+}$ and the DCN and N${}_2$D${}^{+}$ abundance distributions that can be interpreted as the high and low temperature deuteration pathways regimes. The origin of the third DCO${}^{+}$ ring at 260 AU is unknown but may be due to a local decrease of ultraviolet opacity allowing the photodesorption of CO or due to thermal desorption of CO as a consequence of radial drift and settlement of dust grains.

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