Excitation of molecular hydrogen by cosmic-ray protons
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| Main Authors: | , , , , , , |
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| Format: | Preprint |
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2025
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| _version_ | 1866911294291968000 |
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| author | Padovani, Marco Galli, Daniele Plowman, Corey T. Scarlett, Liam H. Zammit, Mark C. Bray, Igor Fursa, Dmitry V. |
| author_facet | Padovani, Marco Galli, Daniele Plowman, Corey T. Scarlett, Liam H. Zammit, Mark C. Bray, Igor Fursa, Dmitry V. |
| contents | 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. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2512_00474 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Excitation of molecular hydrogen by cosmic-ray protons Padovani, Marco Galli, Daniele Plowman, Corey T. Scarlett, Liam H. Zammit, Mark C. Bray, Igor Fursa, Dmitry V. High Energy Astrophysical Phenomena Astrophysics of Galaxies Solar and Stellar Astrophysics 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. |
| title | Excitation of molecular hydrogen by cosmic-ray protons |
| topic | High Energy Astrophysical Phenomena Astrophysics of Galaxies Solar and Stellar Astrophysics |
| url | https://arxiv.org/abs/2512.00474 |