HCO⁺ column densities for different initial conditions in chemical network W (top row) and the corresponding radial emission profiles for the J = 2−1 transition of HCO⁺ (middle row) and H¹³CO⁺ (bottom row). Left-hand column: results for a cosmic ray ionisation rate of 10⁻¹⁷, 10⁻¹⁸, and 10⁻¹⁹s⁻¹. Middle column: corresponding models for an initial CO abundance of 10⁻⁴, 10⁻⁵, and 10⁻⁶. Right-hand column: corresponding models for an initial abundance of gas-phase water of 3 × 10⁻⁵, 3 × 10⁻⁶, and 3.8 × 10⁻⁷. The fiducial model is indicated with the light blue line in each panel and uses an initial abundance of 3.8 × 10⁻⁷ for gas-phase water, 10⁻⁴ for gas-phase CO, and 10⁻¹⁷s⁻¹ for the cosmic ray ionisation rate. The water snowline is indicated with a dashed black line and the position of the star is indicated by the symbol of a black star. The radial emission profiles are convolved with a 0′′.05 beam.