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Tesi etd-11182018-174625


Thesis type
Tesi di laurea magistrale
Author
FRANCESCHI, RICCARDO
URN
etd-11182018-174625
Title
A semi-empirical approach to low energy cosmic ray propagation in the diffuse interstellar medium
Struttura
FISICA
Corso di studi
FISICA
Commissione
relatore Prof. Shore, Steven Neil
Parole chiave
  • interstellar medium
  • turbulence
  • cosmic ray
  • ionization
Data inizio appello
10/12/2018;
Consultabilità
secretata d'ufficio
Riassunto analitico
This thesis presents an approach to modeling how low energy cosmic rays (CR)
propagate by scattering and energy losses within diffuse interstellar clouds. The inter-
stellar medium (ISM), containing a mixture of dust and gas that is not bound into stars,
consists of an atomic and molecular state for the gas. It permeates the space between
stars in all galaxies and is the birthplace of stars and the relic matter released during
their lifetimes. The CRs have a variety of sources distributed throughout the Galaxy.
Their spatial diffusion is governed by a highly turbulent magnetic field coupled to the
interstellar gas that forces a random walk of magnetic field lines and induces diusive
particle propagation. In this study, I work in the strong turbulent regime, with no
constant mean ambient and statistically isotropic fluctuations.
The diusion of charged particles depends on particle's energy. Given the Larmor
radius RL of the charged particle and the outer lengthscale of the turbulence, lc, we
can identify three diffusion regimes: the high energy limit R_L=l_c > 1, and intermediate
region where R_L=l_c = 1, and the low energy range R_L=l_c < 1. In the high-energy limit,
particles experience only small kicks from their initial trajectory due to small angle scat-
tering on magnetic field irregularities. In the low energy limit, particles gyrate around
the local field produced by large-scale fluctuations, while occasionally experiencing per-
turbations (mainly by scattering on magnetic perturbation at scales comparable to the
gyroradius).
In this work I model the propagation of low energy CRs (less than a few hundreds
MeV), given their fundamental role in the evolution of the ISM. For instance, they are
able to significantly heat the medium, and drive its chemistry. I treat only the protons,
which do not suffer synchrotron losses and are responsible for the bulk contribution to
the ionization in optically thick media.
To our knowledge, the possible effects of reenergization of particles due to Fermi-like
processes has not been explored in the context of cloud interiors. I examine whether
particle reenergization can change the CR ionization rate, due to its strong dependence
on the particle energy.
Magnetic turbulence in these environments is not well constrained, though turbulent
motions can be described by a power-law energy distribution. Since it is beyond the
scope of this thesis to develop a self-consistent theory of MHD interstellar turbulence, I
base my model on an empirical description of the turbulent field observed in low density
(translucent and diffuse) molecular clouds without internal star formation or driving.
Rare large fluctuations of the turbulent velocity and density distribution dominate the
energy boosts. We follow the trajectory of each particle rather than the diffusion of the
distribution function. To avoid a formal treatment of the turbulence that is not needed
in this context, we take the novel approach of using an observationally determined set
of probability distribution functions (PDFs) for the turbulence instead of imposing a
particular power spectrum or statistical independence of the fluctuations.
The turbulence is driven by a large scale shearing ambient medium surrounding and
incorporating the cloud. Since there are no internal drivers, the flow satisfies, at least schematically, the requirements for a classical cascade of the sort first described by Kol-
mogorov (1941). We use the derived velocity structure functions and probability dis-
tribution functions derived from spectral line (12C16O and 13C16O) observations taken
from the translucent molecular clouds MBM 3 and MBM 40 to characterize the random
walk of the magnetic field lines, assuming MHD turbulence.
This is the essential ingredient in simulating the CR propagation inside the clouds.
CR in the energy range relevant to our problem have a gyroradius of the order of 105 km
in the typical magnetic field present in the diffuse ISM, a few microgauss. Turbulent motions
do not fall below a lengthscale of about 0.1 pc, therefore protons cannot be scattered
by magnetic fluctuations, and will simply stream across the local magnetic field lines of
force.
I designed the requisite algorithms and Monte Carlo code for the proton random walk
with energy losses from ionization of background atoms and molecules. In their travel,
protons are continuously loosing energy to the medium via ionization losses, but also
exchange energy with the turbulent field thorough Fermi-like processes. We find out
that the reenegization does not significantly affect the propagation in an homogeneous
medium. However reenergization is enhanced by some other effect, e.g. the presence of
inhomogeneities in the medium. In denser regions, more typical of dark clouds or even
cold cores (those in which star formation has not begun despite their densities being
suffcient for gravitational instability) some particles will be trapped inside those regions,
undergoing a large number of scattering events before escaping and I find indication of
reenergization and modified spectral energy distributions. This effect appears to be
too small to give an appreciable difference in the ionization rate of the diffuse medium.
Finally we discuss possible further implementation of the code, and what we expect to
see from a more extended treatment of the problem.
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