DCO+, DCN, and N2D+ reveal three different deuteration regimes in the disk around the Herbig Ae star HD 163296

Context. Deuterium fractionation has been used to study the thermalhistory of prestellar environments. Their formation pathways tracedifferent regions of the disk and may shed light into the physicalstructure of the disk, including locations of important features forplanetary formation.
Aims: We aim to constrain the radial extentof the main deuterated species; we are particularly interested inspatially characterizing the high and low temperature pathways forenhancing deuteration of these species.
Methods: We observed thedisk surrounding the Herbig Ae star HD 163296 using ALMA in Band 6 andobtained resolved spectral imaging data of DCO+ (J = 3 - 2),DCN (J = 3 - 2) and N2D+ (J = 3 - 2) withsynthesized beam sizes of 0.̋53 × 0.̋42, 0.̋53× 0.̋42, and 0.̋50 × 0.̋39, respectively. Weadopted a physical model of the disk from the literature and use the 3Dradiative transfer code LIME to estimate an excitation temperatureprofile for our detected lines. We modeled the radial emission profilesof DCO+, DCN, and N2D+, assuming theiremission is optically thin, using a parametric model of their abundancesand our excitation temperature estimates.
Results:DCO+ can be described by a three-region model withconstant-abundance rings centered at 70 AU, 150 AU, and 260 AU. The DCNradial profile peaks at about 60 AU and N2D+ isseen in a ring at 160 AU. Simple models of both molecules using constantabundances reproduce the data. Assuming reasonable average excitationtemperatures for the whole disk, their disk-averaged column densities(and deuterium fractionation ratios) are 1.6-2.6×1012cm-2 (0.04-0.07), 2.9-5.2×1012cm-2 ( 0.02), and 1.6-2.5×1011cm-2 (0.34-0.45) for DCO+, DCN, andN2D+, respectively. Conclusions: Our simplebest-fit models show a correlation between the radial location of thefirst two rings in DCO+ and the DCN andN2D+ abundance distributions that can beinterpreted as the high and low temperature deuteration pathwaysregimes. The origin of the third DCO+ ring at 260 AU isunknown but may be due to a local decrease of ultraviolet opacityallowing the photodesorption of CO or due to thermal desorption of CO asa consequence of radial drift and settlement of dust grains. The derivedDf values agree with previous estimates of 0.05 forDCO+/HCO+ and 0.02 for DCN/HCN in HD 163296, and0.3-0.5 for N2D+/N2H+ in AS209, a T Tauri disk. The highN2D+/N2H+ confirmsN2D+ as a good candidate for tracing ionization inthe cold outer disk.The reduced images (FITS files) are only available at the CDS viaanonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/606/A125