Integrated intensity 0th-moment maps. From left to right: HCO⁺, DCO⁺, H¹³CO⁺, and C¹⁸O, all J = 3–2. From the top to the bottom row: reference model (R), gas-poor gap model (A), gas-rich model (B), ratio (A)/(R), and ratio (B)/(R). The gap boundaries are marked by the two dotted lines. The optically thick HCO⁺ emission is not significantly affected by the presence of the gas-poor gap. In contrast, the C¹⁸O and particularly H¹³CO⁺ intensities decrease inside the gas-poor gap, while the DCO⁺ intensity increases inside this gap.