Abundance ratios calculated when a 10% branching ratio is considered for the reaction N₂H ⁺  + e ⁻  → NH + H. The hashed bands show the observed ratios with their 1σ uncertainties. Left panel: varying O/P, with the initial abundances of Flower & Pineau des Forêts (2003) ([O] = 1.24( − 4) and [C] = 8.27( − 5)). Right panel: varying [O] while keeping [C] constant, at constant O/P = 10^-3. The low-metal abundances of Wakelam & Herbst (2008) are [O] = 1.76( − 4) and [C] = 7.30( − 5), or C:O = 0.41.