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arXiv · 2512.00474

Excitation of molecular hydrogen by cosmic-ray protons

Marco Padovani·Daniele Galli·Corey T. Plowman·Liam H. Scarlett·Mark C. Zammit·Igor Bray·Dmitry V. Fursa
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Abstract

Low-energy cosmic rays ($E\lesssim 1$ GeV) are responsible for the ionisation and heating of molecular clouds. While the role of supra-thermal electrons produced in the ionisation process in inducing excitation of the ambient gas (mostly molecular hydrogen) has been studied in detail, the role of primary cosmic-ray nuclei (protons and heavier nuclei) has been generally neglected. Here, we introduce, for the first time, cross sections for proton impact on H$_2$, calculated using the semi-classical implementation of the molecular convergent close-coupling method. Our findings show that proton-induced H$_2$ excitation is comparable in magnitude to that caused by electrons. We discuss the possible implications on the estimate of the cosmic-ray ionisation rate from observations in the near-infrared domain and on the cosmic-ray-induced H$_2$ ultraviolet luminescence. We also derive a new approximated analytical parameterisation of the spectrum of secondary electrons that can be easily incorporated in numerical codes.

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Marco Padovani, Daniele Galli, Corey T. Plowman, Liam H. Scarlett, Mark C. Zammit, Igor Bray, Dmitry V. Fursa. 2025-11-29. Excitation of molecular hydrogen by cosmic-ray protons. https://arxiv.org/abs/2512.00474

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