What causes the ionization rates observed in molecular clouds? The role of cosmic ray protons and electrons
Date Issued
2018
Author(s)
Abstract
Cosmic rays are usually assumed to be the main ionization agent for the
interior of molecular clouds where UV and X-ray photons cannot penetrate. Here
we test this hypothesis by limiting ourselves to the case of diffuse clouds and
assuming that the average cosmic ray spectrum inside the Galaxy is equal to the
one at the position of the Sun as measured by Voyager 1 and AMS-02. To
calculate the cosmic ray spectrum inside the clouds, we solve the
one-dimensional transport equation taking into account advection, diffusion and
energy losses. While outside the cloud particles diffuse, in its interior they
are assumed to gyrate along magnetic field lines because ion-neutral friction
is effective in damping all the magnetic turbulence. We show that ionization
losses effectively reduce the CR flux in the cloud interior for energies below
$\approx 100$ MeV, especially for electrons, in such a way that the ionization
rate decreases by roughly 2 order of magnitude with respect to the case where
losses are neglected. As a consequence, the predicted ionization rate is more
than 10 times smaller than the one inferred from the detection of molecular
lines. We discuss the implication of our finding in terms of spatial
fluctuation of the Galactic cosmic ray spectra and possible additional sources
of low energy cosmic rays.
Volume
480
Issue
4
Start page
5167
Issn Identifier
0035-8711
Ads BibCode
2018MNRAS.480.5167P
Rights
open.access
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