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

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: salinas@strw.leidenuniv.nl
² Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH, Amsterdam, The Netherlands
³ Institute for Astronomy, University of Hawaii, 2680 Woodlawn Dr., Honolulu, HI 96826, USA
⁴ Department of Astronomy, Harvard University, Cambridge, MA 02138, USA

Received: 23 May 2017
Accepted: 19 July 2017

Abstract

Context. Deuterium fractionation has been used to study the thermal history of prestellar environments. Their formation pathways trace different regions of the disk and may shed light into the physical structure of the disk, including locations of important features for planetary formation.

Aims. We aim to constrain the radial extent of the main deuterated species; we are particularly interested in spatially characterizing the high and low temperature pathways for enhancing deuteration of these species.

Methods. 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 N₂D⁺ (J = 3 − 2) with synthesized beam sizes of 0.̋53 × 0.̋42, 0.̋53 × 0.̋42, and 0.̋50 × 0.̋39, respectively. We adopted a physical model of the disk from the literature and use the 3D radiative transfer code LIME to estimate an excitation temperature profile for our detected lines. We modeled the radial emission profiles of DCO⁺, DCN, and N₂D⁺, assuming their emission is optically thin, using a parametric model of their abundances and our excitation temperature estimates.

Results. 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 N₂D⁺ 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 1.6–2.6×10¹² cm^-2 (0.04–0.07), 2.9–5.2×10¹² cm^-2 (~0.02), and 1.6–2.5×10¹¹ cm^-2 (0.34–0.45) for DCO⁺, DCN, and N₂D⁺, respectively.

Conclusions. Our simple best-fit models show a correlation between the radial location of the first two rings in DCO⁺ and the DCN and N₂D⁺ 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. The derived D_f values agree with previous estimates of 0.05 for DCO⁺/HCO⁺ and 0.02 for DCN/HCN in HD 163296, and 0.3−0.5 for N₂D⁺/N₂H⁺ in AS 209, a T Tauri disk. The high N₂D⁺/N₂H⁺ confirms N₂D⁺ as a good candidate for tracing ionization in the cold outer disk.