Fractional abundances of o-H₂D⁺ and N₂D⁺ (left panel) and their abundance ratio (right panel) plotted as a function of the potential source evolutionary sequence (see text for details). The arrows pointing up and down indicate the lower and upper limits, respectively. The red, vertical dashed line separates the prestellar phase of evolution on the left from the protostellar phase on the right.