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

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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.
Funding Information
  • Ministerio de Ciencia e Innovación (PID2021-122549NB-C21 and PID2021- 122549NB-C22,)
  • Programme National « Physique et Chimie du Milieu Interstellaire » (PCMI) of the CNRS/INSU with INC/INP co-funded by CEA and CNES.

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