Chi-square maps from comparison between the observed and simulated oH₂D⁺ integrated intensity maps described in Sect. 6. The y-axis in the plots is the logarithm of the cosmic-ray ionisation rate, ${\zeta }_{{\text{H}}_2}$, and the x-axis is logarithm of time. The χ² values are also presented on a logarithmic scale: a darker shade means a better fit. The plot on the left (a) represents simulations with the grain radius a = 0.1 µm and low elemental abundances of O, C, and N, assumed in several previous models of H-MM1, and that in the middle (b) simulations with some 40%–60% higher elemental O, C, and N abundances. The chi-square map on the right (c), is from a simulation with the grain radius a = 0.3 µm and low O, C, and N abundances (see Sect. 5).