Left: radial distributions of the fractional abundances of selected species at t = 5 × 10⁵ yr in a protostellar core resembling IRAS 16293 according to the model of Crimier et al. (2010). 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 selected species, and the meta/ortho ratio of . The thin solid lines show the thermal spin-state ratios of the plotted species. Right: density and temperature distributions of the IRAS 16293 core model. The blue and red horizontal lines mark the critical densities of the first excited rotational transitions of oH₂D⁺ and pD₂H⁺, respectively.