Stars as Laboratories for Fundamental Physics - MPP Theory Group

The Energy-Loss Argument 3
example is the axion (Chapter 14) which arises as the Nambu-Goldstone
boson of the Peccei-Quinn symmetry which explains the puzzling absence
of a neutron electric dipole moment, i.e. it explains CP conservation
in strong interactions. The production of axions in stars, like that
of neutrinos, is not impeded by threshold effects.
Clearly, the emission of novel weakly interacting particles or the
emission of neutrinos with novel properties would have a strong impact
on the evolution and properties of stars. Sato and Sato (1975) were
the first to use this “energy-loss argument” to derive bounds on the
coupling strength of a putative low-mass Higgs particle. Following this
lead, the argument has been applied to a great variety of particlephysics
hypotheses, and to a great variety of stars.
In the remainder of this chapter the impact of a novel energy-loss
mechanism on stars will be discussed in simple terms. The main message
will be that the emission of weakly interacting particles usually
leads to a modification of evolutionary time scales. By losing energy in
a new channel, the star effectively burns or cools faster and thus shines
for a shorter time. In the case of low-mass red giants, however, particle
emission leads to a delay of helium ignition and thus to an extension
of the red-giant phase. Either way, what needs to be observationally
established is the duration of those phases of stellar evolution which
are most sensitive to a novel energy-loss mechanism. In Chapter 2 such
evolutionary phases will be identified, and the observational evidence
for their duration will be discussed. In the end one will be able to
state simple criteria for the allowed rate of energy loss from plasmas at
certain temperatures and densities.
One may be tempted to think that one should consider the hottest
and densest possible stars because no doubt the emission of weakly interacting
particles is most efficient there. However, this emission competes
with standard neutrinos whose production is also more efficient
in hotter and denser objects. Because neutrinos are thermally emitted
in pairs their emission rates involve favorable phase-space factors which
lead to a temperature dependence which is steeper than that for the
emission of, say, axions. Thus, for a given axion coupling strength the
relative importance of axion emission is greater for lower temperatures.
Of course, the temperatures must not be so low that neither neutrino
nor axion emission is important at all relative to the photon luminosity.
Consequently, the best objects to use are those where neutrinos just
begin to have an observational impact on stellar observables. Examples
are low-mass red giants, horizontal-branch stars, white dwarfs, and
old neutron stars. They all have masses of around 1 M ⊙ (solar mass).