61,368 research outputs found
Particle acceleration by shocks in supernova remnants
Particle acceleration occurs on a range of scales from AU in the heliosphere
to Mpc in clusters of galaxies and to energies ranging from MeV to EeV. A
number of acceleration processes have been proposed, but diffusive shock
acceleration (DSA) is widely invoked as the predominant mechanism. DSA operates
on all these scales and probably to the highest energies. DSA is simple, robust
and predicts a universal spectrum. However there are still many unknowns
regarding particle acceleration. This paper focuses on the particular question
of whether supernova remnants (SNR) can produce the Galactic CR spectrum up to
the knee at a few PeV. The answer depends in large part on the detailed physics
of diffusive shock acceleration.Comment: Invited talk at the 33rd International Cosmic Ray Conference, Rio de
Janeiro, Brazil, 2-9 July 2013. Submitted for publication in a special issue
of the Brazilian Journal of Physic
Calibration of the CH and CN Variations Among Main Sequence Stars in M71 and in M13
An analysis of the CN and CH band strengths measured in a large sample of M71
and M13 main sequence stars by Cohen (1999a,b) is undertaken using synthetic
spectra to quantify the underlying C and N abundances. In the case of M71 it is
found that the observed CN and CH band strengths are best matched by the
{\it{identical}} C/N/O abundances which fit the bright giants, implying: 1)
little if any mixing is taking place during red giant branch ascent in M71, and
2) a substantial component of the C and N abundance inhomogeneities is in place
before the main sequence turn-off. The unlikelihood of mixing while on the main
sequence requires an explanation for the abundance variations which lies
outside the present stars (primordial inhomogeneities or intra-cluster self
enrichment). For M13 it is shown that the 3883\AA CN bands are too weak to be
measured in the spectra for any reasonable set of expected compositions. A
similar situation exists for CH as well. However, two of the more luminous
program stars do appear to have C abundances considerably greater than those
found among the bright giants thereby suggesting deep mixing has taken place on
the M13 red giant branch.Comment: 14 pages, 4 figures, accepted for publication by A
A filamentation instability for streaming cosmic-rays
We demonstrate that cosmic rays form filamentary structures in the precursors
of supernova remnant shocks due to their self-generated magnetic fields. The
cosmic-ray filamentation results in the growth of a long wavelength
instability, and naturally couples the rapid non-linear amplification on small
scales to larger length scales. Hybrid magnetohydrodynamics--particle
simulations are performed to confirm the effect. The resulting large scale
magnetic field may facilitate the scattering of high energy cosmic rays as
required to accelerate protons beyond the knee in the cosmic-ray spectrum at
supernova remnant shocks. Filamentation far upstream of the shock may also
assist in the escape of cosmic rays from the accelerator.Comment: Accepted for publication in MNRA
Magnetic field amplification by cosmic rays in supernova remnants
Magnetic field amplification is needed to accelerate cosmic cays to PeV
energies in supernova remants. Escaping cosmic rays trigger a return current in
the plasma that drives a non-resonant hybrid instability. We run simulations in
which we represent the escaping cosmic rays with the plasma return current,
keeping the maximum cosmic ray energy fixed, and evaluate its effects on the
upstream medium. In addition to magnetic field amplification, density
perturbations arise that, when passing through the shock, further increase
amplification levels downstream. As the growth rate of the instability is most
rapid for the smaller scales, the resolution is a limiting factor in the
amplification that can be reached with these simulations.Comment: 4 pages, 2 figures, to appear in the proceedings of the conference
"370 years of Astronomy in Utrecht", eds. G. Pugliese, A. de Koter and M.
Wijbur
Cosmic ray acceleration in young supernova remnants
We investigate the appearance of magnetic field amplification resulting from
a cosmic ray escape current in the context of supernova remnant shock waves.
The current is inversely proportional to the maximum energy of cosmic rays, and
is a strong function of the shock velocity. Depending on the evolution of the
shock wave, which is drastically different for different circumstellar
environments, the maximum energy of cosmic rays as required to generate enough
current to trigger the non-resonant hybrid instability that confines the cosmic
rays follows a different evolution and reaches different values. We find that
the best candidates to accelerate cosmic rays to ~few PeV energies are young
remnants in a dense environment, such as a red supergiant wind, as may be
applicable to Cassiopeia A. We also find that for a typical background magnetic
field strength of 5 microG the instability is quenched in about 1000 years,
making SN1006 just at the border of candidates for cosmic ray acceleration to
high energies.Comment: 14 pages, 8 figures. Accepted for publication in MNRA
Confining the high-energy cosmic rays
Diffusive shock acceleration is the prime candidate for efficient
acceleration of cosmic rays. Galactic cosmic rays are believed to originate
predominantly from this process in supernova remnant shock waves. Confinement
of the cosmic rays in the shock region is key in making the mechanism
effective. It has been known that on small scales (smaller than the typical
gyroradius) high-amplitude non-resonant instabilities arise due to cosmic ray
streaming ahead of the shock. For the efficiency of scattering of the highest
energy cosmic rays it is of interest to determine the type of instabilities
that act on longer length scales, i.e. larger than the cosmic ray gyroradius.
We will present the results of our analysis of an instability that acts in this
regime and will discuss its driving mechanism and typical growth times.Comment: 4 pages, 1 figure. Proceedings for the conference on Cosmic Rays and
the Interstellar Medium (CRISM) in June 2011, Montpellier, France. To appear
in MSA
From cosmic ray source to the Galactic pool
The Galactic cosmic ray spectrum is a remarkably straight power law. Our
current understanding is that the dominant sources that accelerate cosmic rays
up to the knee ( eV) or perhaps even the ankle ( eV), are young Galactic supernova remnants. In theory, however, there
are various reasons why the spectrum may be different for different sources,
and may not even be a power law if nonlinear shock acceleration applies during
the most efficient stages of acceleration. We show how the spectrum at the
accelerator translates to the spectrum that make up the escaping cosmic rays
that replenish the Galactic pool of cosmic rays. We assume that cosmic ray
confinement, and thus escape, is linked to the level of magnetic field
amplification, and that the magnetic field is amplified by streaming cosmic
rays according to the non-resonant hybrid or resonant instability. When a fixed
fraction of the energy is transferred to cosmic rays, it turns out that a
source spectrum that is flatter than will result in a escape
spectrum, whereas a steeper source spectrum will result in an escape spectrum
with equal steepening. This alleviates some of the concern that may arise from
expected flat or concave cosmic ray spectra associated with nonlinear shock
modification.Comment: 5 pages, 1 figure. Accepted for publication in MNRA
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