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Aims. We study whether or not rotational excitation can make a large difference to chemical models of the abundances of the H₃⁺ isotopologs, including spin states, in physical conditions corresponding to starless cores and protostellar envelopes.
Methods. We developed a new rate coefficient set for the chemistry of the H₃⁺ isotopologs, allowing for rotational excitation, using previously published state-to-state rate coefficients. These new so-called species-to-species rate coefficients are compared with previously-used ground-state-to-species rate coefficients by calculating chemical evolution in variable physical conditions using a pseudo-time-dependent chemical code.
Results. We find that the new species-to-species model produces different results to the ground state-to-species model at high density and toward increasing temperatures (T>10K). The most prominent difference is that the species-to-species model predicts a lower H₃⁺ deuteration degree at high density owing to an increase of the rate coefficients of endothermic reactions that tend to decrease deuteration. For example at 20 K, the ground-state-to-species model overestimates the abundance of H₂D⁺ by a factor of about two, while the abundance of D₃⁺ can differ by up to an order of magnitude between the models. The spin-state abundance ratios of the various H₃⁺ isotopologs are also affected, and the new model better reproduces recent observations of the abundances of ortho and para H₂D⁺ and D₂H⁺. The main caveat is that the applicability regime of the new rate coefficients depends on the critical densities of the various rotational transitions which vary with the abundances of the species and the temperature in dense clouds.
Conclusions. The difference in the abundances of the H₃⁺ isotopologs predicted by the species-to-species and ground state-to-species models is negligible at 10K corresponding to physical conditions in starless cores, but inclusion of the excited states is very important in studies of deuteration at higher temperatures, for example in protostellar envelopes. The species-to-species rate coefficients provide a more realistic approach to the chemistry of the H₃⁺ isotopologs than the ground-state-to-species rate coefficients do, and so the former should be adopted in chemical models describing the chemistry of the H₃⁺ + H₂ reacting system.