The origin of Galactic cosmic rays

Title: On the origin and composition of Galactic cosmic rays
Author: N. Prantzos
Author’s Institution: Institute d’Astrohpysique de Paris

If you have ever reduced astronomical data, then you have most likely come across a cosmic ray or two. These cosmic rays are actually charged particles that leave thin bright spots or streaks as they pass through detectors. While cosmic rays can be a nuisance in reducing astronomical data, they can also be tools that provide unique probes of the Galactic environment. This is due to the fact that cosmic rays can be directly sampled, rather than observed via electromagnetic radiation (like most particles). As a result, there are a number of cosmic ray research initiatives.

While some cosmic rays originate from outside the Galaxy or from the sun during solar flares, the cosmic rays this paper is directed at are Galactic Cosmic Rays (GCRs). These cosmic rays originate from an unknown location within the Galaxy, and are trapped by the Galactic magnetic field. These atomic nuclei have been stripped of all their electrons, and accelerated to near the speed of light.

The primary question at hand is what astrophysics site and physical process accelerates these particles to such high speeds? Several sites have been suggested including supernovae remnants, the interstellar medium, winds of massive stars, and the interiors of superbubbles. We can’t directly observe where GCRs come from, because their paths are altered by magnetic fields. By the time they reach Earth, they may have traveled around the Galaxy several times. Instead, we have to turn to indirect means. The best way to go about this is to determine the elemental compositions of GCRs. Most cosmic rays (about 90%) are hydrogen, or protons, about 9% are helium nulcei, and only about 1% of cosmic rays are made up of heavier nuclei.

A schematic representation of the model setup. The supernovae explodes outward in to the stars previous stellar winds and then the interstellar medium (ISM). MEJ shows the ejecta from the supernovae explosion and the remnant mass (MREM) is shown in blue. This particular diagram represents the schematic for a Wolf-Rayet star which has had two stages of mass loss. The first, represented by MENV, is the hydrogen rich envelope. The second, represented by MPROC, is the nuclearly processed layers.

The author of this paper, N. Prantzos, investigates the idea that GCRs are accelerated as supernovae blast waves pass through the pre-supernovae winds of massive stars and then the interstellar medium. The idea is that the high velocity ejecta from the supernovae explosion catches up with the circumstellar shell of mass from the stars previous mass loss owing to stellar winds and then reaches the interstellar medium. For a schematic representation of this, see the figure to the right. This is referred to as the forward shock mechanism. The particles gain energy as the are scattered across the supernovae shock front, so the point when this occurs in the expansion of the remnant tells us what the expected isotopic ratios of the observed GCRs should be.

The author investigates stars with masses ranging from 10-120 M with a variety of mass loss rates since stellar winds rates are fairly uncertain. He also uses multiple stellar models since yields (i.e. the amount of different elements the star produces) are very important. They use these yields and winds to determine the size and composition of the circumstellar shell surrounding the star prior to the supernovae explosion. When the explosion begins, the velocity of the shock wave is assumed to be constant. Once the amount of material swept up is approximately equal to the amount of material in the explosion, it reaches the Sedov-Taylor phase. At this point, the blast wave expands adiabatically, losing velocity rather than energy (because the internal energy is still much larger than the rate at which the system is losing energy to radiation). The Sedov-Taylor phase is the most energetically favorable time for the forward shock to accelerate particles to GCR energies, so the author assumes these accelerations begin during this phase in his models.

In order to constrain the models, the author primarily uses the isotopic ration of 22Ne and 20Ne. This particular ratio was chosen because it strays significantly from the isotopic ration of 22Ne and 20Ne in our own solar system. By matching this relative composition the author is not only able to show that the observed GCR abundances can be produced in supernovae remnants, he is also able to place constraints on stellar evolution models and the initial velocities of supernovae shock waves. The author further investigates whether GCRs can be accelerated in superbubbles, and finds that they cannot be a primary source of GCRs.

The final word: The results of this paper support previous suspicions that the bulk of Galactic cosmic rays are produced in supernovae remnants, and shows that the forward shock mechanism can sufficiently explain the observed isotope ratios.

About Kim Phifer

I am currently a first year graduate student at UCLA. I work with Andrea Ghez to study the dynamics of the old stars in the Galactic Center. Last year, I earned a M.Phil (Master of Philosophy) in astronomy at the University of Cambridge. While there, I studied the (theoretical) progenitors of electron capture supernovae with Chris Tout. I completed my undergraduate degree at Butler University where I studied the dynamics of galactic nuclei with Brian Murphy.

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