Enhancement of the Triple Alpha Process in Burning Stars

The triple alpha process is responsible for the conversion in stars of the alpha particles (or 4He nuclei) made in the Big Bang to the carbon we find around us and in us. This process underlies Carl Sagan’s famous statement that “we are made of star-stuff.”

How this occurs involves the famous Hoyle state, named after Sir Fred Hoyle who predicted that a state at an excitation energy of about 7.6 MeV in 12C was necessary if the triple-alpha process was to make the amount of 12C seen in the cosmos.  It was soon found experimentally, one of the triumphs of nuclear astrophysics. 

In the triple alpha process, two alpha particles combine to form the nucleus 8Be which then captures another alpha particle to form the Hoyle state in 12C.  This state can then decay to the ground state of 12C by emitting gamma rays and thereby making 12C, or decay back into three α’s. The production of 12C in nature is proportional to the probability of decay of the Hoyle state to the ground state of 12C.  This is the basis of what was investigated in a recent letter article [PRL 119, 112701 (2017)]


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For the conditions found in helium burning stars, the Hoyle state decays to the ground state by emitting two gamma rays, with a probability that can be measured in the laboratory.  However, since the reaction takes place in the presence of other particles, protons neutrons, or alpha particles, these particles can induce the decays.  This increases the probability of decay to the ground state and so the rate of the triple alpha process. It turns out that at densities sometimes found in supernovae, these enhancements can be large, a factor of 50 or more if the particles are neutrons.

The next step is to estimate the size of effects produced by the new enhanced rates for realistic astrophysical processes where large densities can occur, for example in supernovae or x-ray bursters.

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