Species-to-species rate coefficients for the ${\mathrm{H}}_3^{+}+{\mathrm{H}}_2$ reacting system

Abstract

Aims. We study whether rotational excitation makes a difference to the abundances of the ${\mathrm{H}}_3^{+}$ isotopologs, including spin states, in physical conditions corresponding to starless cores and protostellar envelopes. Methods. We developed a new rate coefficient set for the ${\mathrm{H}}_3^{+}$ 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 ${\mathrm{H}}_3^{+}$ 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 ${\mathrm{H}}_2{\mathrm{D}}^{+}$ by a factor of about two while the abundance of ${\mathrm{D}}_3^{+}$ 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 ${\mathrm{H}}_2{\mathrm{D}}^{+}$ and ${\mathrm{D}}_2{\mathrm{H}}^{+}$. 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 ${\mathrm{H}}_3^{+}$ 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 ${\mathrm{H}}_3^{+}$ 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 ${\mathrm{H}}_3^{+}+{\mathrm{H}}_2$ reacting system.

arXiv commentaccepted to A&A; 22 pages, 8 figures, 2 appendices; arXiv abstract heavily redacted and somewhat modified

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• Chemical analysis of prestellar cores in Ophiuchus yields short timescales and rapid collapse