HCO⁺ 1–0 brightness temperature computed with RADEX as a function of kinetic temperature (x-axis) and H₂ density (y-axis), assuming different column densities. In each panel, the solid line corresponds to the level of emission measured in the Mopra spectrum around 14 km s⁻¹ and the dashed line to the upper limit (3σ) over the velocity range from − 110 km s⁻¹ to − 10 km s⁻¹. The solid line is displayed only for the column densities that fall in the range determined from the ALMA absorption map at this velocity and the white star marks the median column density obtained from the ALMA absorption map at this velocity.