Ultraviolet H₂ luminescence in molecular clouds induced by cosmic rays

¹ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
e-mail: marco.padovani@inaf.it
² Department of Physics and Astronomy, Curtin University, Perth, Western Australia 6102, Australia
³ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
⁴ Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA

Received: 5 October 2023
Accepted: 1 December 2023

Abstract

Context. Galactic cosmic rays (CRs) play a crucial role in ionisation, dissociation, and excitation processes within dense cloud regions where UV radiation is absorbed by dust grains and gas species. CRs regulate the abundance of ions and radicals, leading to the formation of more and more complex molecular species, and determine the charge distribution on dust grains. A quantitative analysis of these effects is essential for understanding the dynamical and chemical evolution of star-forming regions.

Aims. The CR-induced photon flux has a significant impact on the evolution of the dense molecular medium in its gas and dust components. This study evaluates the flux of UV photons generated by CRs to calculate the photon-induced dissociation and ionisation rates of a vast number of atomic and molecular species, as well as the integrated UV photon flux.

Methods. To achieve these goals, we took advantage of recent developments in the determination of the spectra of secondary electrons, in the calculation of state-resolved excitation cross sections of H₂ by electron impact, and of photodissociation and photoionisation cross sections.

Results. We calculated the H₂ level population of each rovibrational level of the X, B, C, B′, D, B″, D′, and a states. We then computed the UV photon spectrum of H₂ in its line and continuum components between 72 and 700 nm, with unprecedented accuracy, as a function of the CR spectrum incident on a molecular cloud, the H₂ column density, the isomeric H₂ composition, and the dust properties. The resulting photodissociation and photoionisation rates are, on average, lower than previous determinations by a factor of about 2, with deviations of up to a factor of 5 for the photodissociation of species such as AlH, C₂H₂, C₂H₃, C₃H₃, LiH, N₂, NaCl, NaH, O₂⁺, S₂, SiH, l-C₄, and l-C₅H. A special focus is given to the photoionisation rates of H₂, HF, and N₂, as well as to the photodissociation of H₂, which we find to be orders of magnitude higher than previous estimates. We give parameterisations for both the photorates and the integrated UV photon flux as a function of the CR ionisation rate, which implicitly depends on the H₂ column density, as well as the dust properties.