Stars as Laboratories for Fundamental Physics - MPP Theory Group

Chapter 1
The Energy-Loss Argument
Weakly interacting, low-mass particles such as neutrinos or axions contribute
to the energy loss or energy transfer in stars. The impact of
an anomalous energy-loss mechanism is discussed qualitatively and in
terms of homology relations between standard and perturbed stellar
models. The example of massive pseudoscalar particles is used to illustrate
the impact of a new energy-loss and a new radiative-transfer
mechanism on the Sun.
1.1 Introduction
More than half a century ago, Gamow and Schoenberg (1940, 1941)
ushered in the advent of particle astrophysics when they speculated that
neutrinos may play an important role in stellar evolution, particularly
in the collapse of evolved stars. Such a hypothesis was quite bold for
the time because neutrinos, which had been proposed by Pauli in 1930,
were not directly detected until 1954. That their existence was far from
being an established belief when Gamow and Schoenberg wrote their
papers is illustrated by Bethe’s (1939) complete silence about them in
his seminal paper on the solar nuclear fusion chains.
Even after the existence of neutrinos had been established they
seemed to interact only by β reactions of the sort e − + (A, Z) →
(A, Z−1) + ν e or (A, Z−1) → (A, Z) + e − + ν e , the so-called URCA
reactions which Gamow and Schoenberg had in mind, or by fusion processes
like pp → de + ν e . The URCA reactions and related processes
become important only at very high temperatures or densities because
of their energy threshold. While the Sun emits two neutrinos for every
helium nucleus fused from hydrogen, the energy loss in neutrinos is only
a few percent of the total luminosity and thus plays a minor role.
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