Abstract
Low-energy cosmic rays (up to the GeV energy domain) play a crucial role in the physics and chemistry of the densest phase of the interstellar medium. Unlike interstellar ionising radiation, they can penetrate large column densities of gas, and reach molecular cloud cores. By maintaining there a small but not negligible gas ionisation fraction, they dictate the coupling between the plasma and the magnetic field, which in turn affects the dynamical evolution of clouds and impacts on the process of star and planet formation. The cosmic-ray ionisation of molecular hydrogen in interstellar clouds also drives the rich interstellar chemistry revealed by observations of spectral lines in a broad region of the electromagnetic spectrum, spanning from the submillimetre to the visual band. Some recent developments in various branches of astrophysics provide us with an unprecedented view on low-energy cosmic rays. Accurate measurements and constraints on the intensity of such particles are now available both for the very local interstellar medium and for distant interstellar clouds. The interpretation of these recent data is currently debated, and the emerging picture calls for a reassessment of the scenario invoked to describe the origin and/or the transport of low-energy cosmic rays in the Galaxy.
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Image adapted from Tatischeff and Gabici (2018)
Image reproduced with permission from Gabici et al. (2019), copyright by World Scientific Publishing
Image adapted from Vos and Potgieter 2015
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Notes
Besides LiBeB, CR spallation also explains the existence of other rare isotopes, such as, e.g., tritium, \(^{14}\)C, \(^{26}\)Al, etc., called cosmogenic nuclides (Gosse and Phillips 2001).
Anomalous CRs are neutral atoms in the local ISM that, due to the motion of the Sun, enter the heliosphere, become partially (mainly singly) ionised due to charge exchange, solar radiation, or electron impact, and are eventually accelerated in the solar wind up to energies of \(\lesssim 100\) MeV/nucleon (Reames 1999; Potgieter 2013).
The rigidity of a fully ionised nucleus of momentum p is \(R = p c/Z e\), where e is the elementary charge. If pc is expressed in eV and the particle charge in natural units (\(e = 1\)) the rigidity has units of Volts. Particles of equal rigidity have the same gyration radius around a magnetic field of strength B, \(r_g = R/B\), and therefore follow the same trajectory.
Nuclei characterised by the same energy per nucleon move at the same speed.
At smaller particle energies, the effect of Solar modulation becomes significant.
At the International Cosmic-Ray Conference that took place (virtually) in Berlin in July 2021, the AMS-02 collaboration presented preliminary results on the measurements of the isotopic ratio \(^{10}\mathrm{Be}/^9\mathrm{Be}\) up to energies of \(\sim\) 10 GeV/nucleon.
Which provides a better representation of data with respect to the singly broken power law spectra often adopted in the literature. However, this is just a convenient descriptive expression, being neither a physically motivated choice, nor a formal fit to data.
The main problem is that even the lowest (rotational) excited energy level is too far from the ground state to be significantly populated, given the cold environments where H\(_2\) is generally found (see Stahler and Palla 2004, for a detailed discussion).
KIDA: Kinetic Database for Astrochemistry: https://kida.astrochem-tools.org/.
For a detailed description of H\(_3^+\) spectroscopy notation, see Lindsay and McCall (2001).
Assumed typical values are: \(n_{{\mathrm{H}}} = 200\) cm\(^{-3}\), \(x_3 = 1.5 \times 10^{-4}\), \(f_{{\mathrm{H}}_2} = 0.67\), \(T = 70\) K.
PAHs are molecules made entirely of carbon and hydrogen arranged in cyclic (ring shaped) structures called aromatic rings (Tielens 2008).
Assumed typical values: \(T = 100\) K, \(x_e = 1.5 \times 10^{-4}\), \(n_{\mathrm{H}} = 35\) cm\(^{-3}\).
This claim is true unless an unknown and steep spectral component pops up in the sub-MeV domain. We will briefly discuss this possibility in Sect. 7.
See, e.g., Hollweg (1974) for a formal approach to MHD waves.
For a detailed derivation of the spatial diffusion coefficient from the spectrum of magnetic fluctuations based on a state-of-the-art theory of magnetic turbulence, see, for example, Fornieri et al. (2021) and the list of references therein.
Assumed values: \(n_n = 300\) cm\(^{-3}\), \(T = 50\) K, \(x_e \sim 10^{-4}\), H\(_2\) and C\(^+\) dominant neutral and ion species, respectively.
We neglect the small correction due to the Compton–Getting effect.
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Acknowledgements
I would like to thank the editors at A&A Rev, and in particular Luigina Feretti for her advices and infinite patience. I also thank Phan Vo Hong Minh, Sarah Recchia, and Thibault Vieu, who read and commented on the entire manuscript, Paola Caselli, with whom I had (almost 20 years ago) my first discussion on LECRs, and Cecilia Ceccarelli and Thierry Montmerle who gave me the opportunity to start working in this field. While writing this review, I enjoyed discussing LECRs and related topics with G. Bernardi, D. Breitschwerdt, J. Duprat, C. Evoli, D. Gaggero, D. Galli, R. Giuffrida, D. Grasso, A. Ivlev, D. Maurin, P. Mertsch, M. Miceli, M. Padovani, G. Peron, L. Podio, S. Ravikularaman, J. Raymond, G. Sabatini, F. Schulze, A. Strong, V. Tatischeff, R. Terrier, and F. van der Tak. Finally, I acknowledge support from Agence Nationale de la Recherche (grants ANR- 17-CE31-0014 and CRitiLISM).
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Gabici, S. Low-energy cosmic rays: regulators of the dense interstellar medium. Astron Astrophys Rev 30, 4 (2022). https://doi.org/10.1007/s00159-022-00141-2
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DOI: https://doi.org/10.1007/s00159-022-00141-2