Top: regions of non-zero HCO⁺ (blue) and DCO⁺ (red) abundance, overlaid with contours indicating the hydrogen density (solid gray lines) and temperature (dashed gray lines). The 19 and 21 K contours are shown with a thick dark gray line. The region containing HCO⁺ is bounded on the upper and lower sides by CO photodissociation and by freezeout at 19 K, respectively. The DCO⁺ region is bounded by the temperature range 19–21 K. Bottom: the column density of HCO⁺ and its isotopologues (colored lines), with the 19 K CO snowline indicated by the thick dark gray dashed line. The broad plateau of the DCO⁺ column density from ~110–150 AU explains the ring morphology seen in line emission. The 3σ line sensitivities (from Sect. 3.2) are shown with the horizontal gray lines.