Cosmic-ray ionization of molecular clouds

By M. Padovani, D. Galli, A. E. G. D. D. A. E. S. Spazio, U. Firenze, Italy., Inaf - Arcetri, Firenze, U. O. C. Berkeley, Usa. 2009

278 citations recorded by the source corpus.

Context. Low-energy cosmic rays are a fundamental source of ionization for molecular clouds, influencing their chemical, thermal, and dynamical evolution. Aims. The purpose of this work is to explore the possibility that a low-energy component of cosmic rays, not directly measurable from the Earth, can account for the discrepancy between the ionization rate measured in diffuse and dense interstellar clouds. Methods. We collected the most recent experimental and theoretical data on the cross sections for the production of H + and He + by electron and proton impact and discuss the available constraints on the cosmic-ray fluxes in the local interstellar medium. Starting from different extrapolations at low energies of the demodulated cosmic-ray proton and electron spectra, we computed the propagated spectra in molecular clouds in the continuous slowing-down approximation taking all the relevant energy loss processes into account. Results. The theoretical value of the cosmic-ray ionization rate as a function of the column density of traversed matter agrees with the observational data only if the flux of either cosmic-ray electrons or of protons increases at low energies. The most successful models are characterized by a significant (or even dominant) contribution of the electron component to the ionization rate, in agreement with previous suggestions. However, the large spread of cosmic-ray ionization rates inferred from chemical models of molecular cloud cores remains to be explained. Conclusions. Available data combined with simple propagation models support the existence of a low-energy component (below ∼100 MeV) of cosmic-ray electrons or protons responsible for the ionization of molecular cloud cores and dense protostellar envelopes.

Subfields: Astronomy and Astrophysics, Atomic and Molecular Physics, and Optics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/3e7cad219256215a9ef3c6ed1985cf5ab75a19f5

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Citations in the corpus

Physical Processes in the Interstellar Medium (2014, background): “This difference between diffuse and dense clouds may indicate that the cosmic rays that dominate the heating and ionization of the local ISM have energies of only a few MeV, allowing them to penetrate low column density, diffuse atomic or molecular gas, but not high column density clouds (Padovani e”
INVESTIGATING THE COSMIC-RAY IONIZATION RATE IN THE GALACTIC DIFFUSE INTERSTELLAR MEDIUM THROUGH OBSERVATIONS OF H+3 (2011, background): “A parameter with which ζ2 has been predicted to vary is the hydrogen column density, NH (Padovani et al. 2009).”
Photodissociation and photoionisation of atoms and molecules of astrophysical interest (2017, background): “A reduction of the ionisation rate likely occurs in dark clouds and protoplanetary disks because of the shielding effects of the surrounding material (Padovani et al. 2009) or stellar winds and magnetic fields (Cleeves et al. 2013, 2015).”
NON-IDEAL MHD EFFECTS AND MAGNETIC BRAKING CATASTROPHE IN PROTOSTELLAR DISK FORMATION (2011, background): “Padovani et al. (2009) compiled from the literature the values of ζ inferred for clouds of a wide range of column densities, from diffuse clouds to massive protostellar envelopes (see their Fig.”
Observational Signatures of Particle Acceleration in Supernova Remnants (2012, methodology): “Padovani et al. (2009) calculated the gas ionization rates in molecular clouds assuming a CR spectrum extrapolated to low energies from observed CR proton and electron spectra.”
HERSCHEL SURVEY OF GALACTIC OH+, H2O+, AND H3O+: PROBING THE MOLECULAR HYDROGEN FRACTION AND COSMIC-RAY IONIZATION RATE (2014, background): “…for ionization of H and H2 by cosmic rays increases with decreasing energy, meaning the flux of low-energy particles (E ≤ 100 MeV) is most important in controlling ζH, and such particles will quickly be removed from the cosmic-ray spectrum due to these energy losses (e.g., Padovani et al. 2009).”
EXCLUSION OF COSMIC RAYS IN PROTOPLANETARY DISKS: STELLAR AND MAGNETIC EFFECTS (2013, methodology): “5) Padovani et al. (2009) then numerically compute and fit ζCR(NH2) using the function provided in Equation (5).”