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Context. Physical and chemical processes in protoplanetary disks affect the disk structure and the midplane environment within which planets form. The simple deuterated molecular cation DCO⁺ has been proposed to act as a tracer of the disk midplane conditions.
Aims. This work aims to understand which midplane conditions are probed by the DCO⁺ emission in the disk around the Herbig Ae star HD 169142. We explore the sensitivity of the DCO⁺ formation pathways to gas temperature and CO abundance.
Methods. The DCO⁺ J = 3-2 transition was observed with Atacama Large Millimeter/submillimeter Array at a spatial resolution of ∼0.3'' (35 AU at 117 pc). We modeled the DCO⁺ emission in HD 169142 with a physical disk structure adapted from the literature, and employed a simple deuterium chemical network to investigate the formation of DCO⁺ through the cold deuterium fractionation pathway via H₂D⁺. Parameterized models are used to modify the gas temperature and CO abundance structure of the disk midplane to test their effect on DCO⁺ production. Contributions from the warm deuterium fractionation pathway via CH₂D⁺ are approximated using a constant abundance in the intermediate disk layers.
Results. The DCO⁺ line is detected in the HD 169142 disk with a total integrated line flux of 730 ± 73 mJy km s^–1. The radial intensity profile reveals a warm, inner component of the DCO⁺ emission at radii <=30 AU and a broad, ring-like structure from ∼50-230 AU with a peak at 100 AU just beyond the edge of the millimeter grain distribution. Parameterized models show that alterations to the midplane gas temperature and CO abundance are both needed to recover the observed DCO⁺ radial intensity profile. The alterations are relative to the fiducial physical structure of the literature model constrained by dust and CO observations. The best-fit model contains a shadowed, cold midplane in the region z/r < 0.1 with an 8 K decrease in T_gas and a factor of five CO depletion just beyond the millimeter grains (r = 83 AU), and a 2 K decrease in T_gas for r > 120 AU. The warm deuterium fractionation pathway is implemented as a constant DCO⁺ abundance of 2.0 x 10^–12 between 30-70 K and contributes >85% to the DCO⁺ emission at r < 83 AU in the best-fit model.
Conclusions. The DCO⁺ emission probes a reservoir of cold material in the HD 169142 outer disk that is not probed by the millimeter continuum, the spectral energy distribution, nor the emission from the ¹² CO, ¹³ CO, or C¹⁸O J = 2-1 lines. The DCO⁺ emission is a sensitive probe of gas temperature and CO abundance near the disk midplane and provides information about the outer disk beyond the millimeter continuum distribution that is largely absent in abundant gaseous tracers such as CO isotopologues.