Opened 5 years ago

#252 new enhancement

attenuation of cosmic rays vs column density

Reported by: Gary J. Ferland Owned by: nobody
Priority: good to do Milestone:
Component: atomic/molecular data base Version: trunk
Keywords: cosmic ray attenuation Cc:


this paper gives attenuation as simple fit.

A&A 501, 619-631 (2009)
DOI: 10.1051/0004-6361/200911794
Cosmic-ray ionization of molecular clouds
M. Padovani1, 2, D. Galli2, and A.E. Glassgold3

1  Dipartimento di Astronomia e Scienza dello Spazio, Università di Firenze, Largo E. Fermi 2, 50125 Firenze, Italy
2  INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
3  University of California at Berkeley, Berkeley, CA, 94720, USA

Received 4 February 2009 / Accepted 24 April 2009

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 H2+ 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 

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.


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