Species-to-species rate coefficients for the $\rm H_3^+ + H_2$ reacting system

Abstract: Aims. We study whether rotational excitation makes a difference to the abundances of the $\rm H_3^+$ isotopologs, including spin states, in physical conditions corresponding to starless cores and protostellar envelopes. Methods. We developed a new rate coefficient set for the $\rm H_3^+$ isotopologs, allowing for rotational excitation, using the state-to-state rate coefficients from Hugo et al. These new so-called species-to-species rate coefficients are compared with previously-used ground state-to-species rate coefficients. Results. The species-to-species and ground state-to-species model results differ at high density and toward increasing temperatures ($T > 10$ K). The species-to-species model predicts a lower $\rm H_3^+$ deuteration degree at high density owing to an increase of the rate coefficients of endothermic reactions that decrease deuteration. At 20 K the ground state-to-species model overestimates the abundance of $\rm H_2D^+$ by a factor of about two while the abundance of $\rm D_3^+$ can differ by an order of magnitude between the models. Spin-state abundance ratios are also affected, and the new model better reproduces recent observations of ortho and para $\rm H_2D^+$ and $\rm D_2H^+$. The applicability regime of the new rate coefficients depends on the critical densities of the various rotational transitions. Conclusions. The difference in the abundances of the $\rm H_3^+$ isotopologs predicted by the two models is negligible at 10 K but excited states are 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 $\rm H_3^+$ 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 $\rm H_3^+ + H_2$ reacting system.