Left: radial distributions of the fractional abundances of the spin states of H₂D⁺ and D₂H⁺ at t = 1 × 10⁶ yr in the innermost 10 000 AU of the L1544 model. Solid lines correspond to the species-to-species model (method 1), while dashed lines correspond to the ground-state-to-species model. Middle: radial distributions of the o/p ratios of H₂D⁺ and D₂H⁺. Linestyles are the same as in the left panel. Right: density and temperature distributions of the L1544 core model. The blue and red horizontal lines mark the critical densities of the first excited rotational transitions of oH₂D⁺ and pD₂H⁺, respectively.