Cosmic-ray ionisation in circumstellar discs

Context. Galactic cosmic rays (CRs) are a ubiquitous source of ionisation of the interstellar gas, competing with UV and X-ray photons as well as natural radioactivity in determining the fractional abundance of electrons, ions, and charged dust grains in molecular clouds and circumstellar discs. Aim...

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Bibliographic Details
Published in:Astronomy and astrophysics (Berlin) 2018-06, Vol.614, p.A111
Main Authors: Padovani, Marco, Ivlev, Alexei V., Galli, Daniele, Caselli, Paola
Format: Article
Language:English
Subjects:
Accretion disks
Approximation
atomic processes
Atoms & subatomic particles
Clouds
Cosmic dust
Cosmic rays
Density
Diffusion rate
Electron-positron pairs
Electrons
Fluxes
Galactic cosmic rays
Interstellar gas
Ionization
ISM: clouds
Mathematical analysis
Molecular chains
Molecular clouds
molecular processes
Natural radioactivity
Nuclei (nuclear physics)
Particle physics
Photons
Positrons
Propagation
Radioactivity
stars: protostars
Transport
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Summary:Context. Galactic cosmic rays (CRs) are a ubiquitous source of ionisation of the interstellar gas, competing with UV and X-ray photons as well as natural radioactivity in determining the fractional abundance of electrons, ions, and charged dust grains in molecular clouds and circumstellar discs. Aims. We model the propagation of various components of Galactic CRs versus the column density of the gas. Our study is focussed on the propagation at high densities, above a few g cm−2, especially relevant for the inner regions of collapsing clouds and circumstellar discs. Methods. The propagation of primary and secondary CR particles (protons and heavier nuclei, electrons, positrons, and photons) is computed in the continuous slowing down approximation, diffusion approximation, or catastrophic approximation by adopting a matching procedure for the various transport regimes. A choice of the proper regime depends on the nature of the dominant loss process modelled as continuous or catastrophic. Results. The CR ionisation rate is determined by CR protons and their secondary electrons below ≈130 g cm−2 and by electron-positron pairs created by photon decay above ≈600 g cm−2. We show that a proper description of the particle transport is essential to compute the ionisation rate in the latter case, since the electron and positron differential fluxes depend sensitively on the fluxes of both protons and photons. Conclusions. Our results show that the CR ionisation rate in high-density environments, such as the inner parts of collapsing molecular clouds or the mid-plane of circumstellar discs, is higher than previously assumed. It does not decline exponentially with increasing column density, but follows a more complex behaviour because of the interplay of the different processes governing the generation and propagation of secondary particles.
ISSN:0004-6361
1432-0746
DOI:10.1051/0004-6361/201732202