Inelastic H + H₃⁺ collision rates and their impact on the determination of the excitation temperature of H₃⁺

¹ Unidad Asociada UAM-IFF-CSIC, Departamento de Química Física Aplicada, Facultad de Ciencias M-14, Universidad Autónoma de Madrid, 28049 Madrid, Spain
² Instituto de Física Fundamental (IFF-CSIC), C.S.I.C., Serrano 123, 28006 Madrid, Spain
³ Sorbonne Université, Observatoire de Paris, Université PSL, CNRS, LERMA, 92190 Meudon, France
⁴ Université Paris-Cité, Paris, France

^★ Corresponding author; octavio.roncero@csic.es

Received: 13 November 2024
Accepted: 6 December 2024

Abstract

Context. In diffuse interstellar clouds, the excitation temperature derived from the lowest levels of H₃⁺ is systematically lower than that derived from H₂. The differences may be attributed to the lack of state-specific formation and destruction rates of H₃⁺, which are needed to thermalize the two species.

Aims. In this work, we aim to investigate the possible influence of rotational excitation collisions of H₃⁺ with atomic hydrogen on its excitation temperature.

Methods. We used a time-independent close-coupling method to calculate the state-to-state rate coefficients, incorporating a very accurate and full-dimensional potential energy surface recently developed for H₄⁺. We take a symmetric top approach to describe a frozen H₃⁺ as an equilateral triangle.

Results. We derive rotational excitation collision rate coefficients of H₃⁺ with atomic hydrogen in a temperature range corresponding to diffuse interstellar conditions up to (J, K, ±) = (7, 6, +) and (J, K, ±) = (6, 4, +) for its ortho and para forms. This allows us to obtain a consistent set of collisional excitation rate coefficients and to improve on a previous study that included speculations regarding these contributions.

Conclusions. The new state-specific inelastic H₃⁺ + H rate coefficients yield differences of up to 20% in the excitation temperature, and their impact increases with decreasing molecular fraction. We also confirm the impact of chemical state-to-state destruction reactions on the excitation balance of H₃⁺, and that reactive H + H₃⁺ collisions are also needed to account for possible further ortho to para transitions.