Stars as Laboratories for Fundamental Physics - MPP Theory Group

566 Chapter 15
Fig. 15.3. Summary on limits on the electric charge e x and mass m x of
generic millicharged particles which may be sequential neutrinos or novel
particles. (Adapted from Davidson, Bailey, and Campbell 1991.) In order
to avoid overclosing the universe, additional model-dependent parameter
regions are excluded. The big-bang nucleosynthesis (BBN) excluded region
is larger in some models.
undue shortening of the lifetime of horizontal-branch stars, one needs
to require (Sect. 6.5.6)
e x ∼ < 2×10 −14 e. (15.11)
It applies for m x ∼ < ω P /2 with ω P the plasma frequency. It is larger for
red giants before helium ignition than for HB stars because of the larger
average density of 2×10 5 g cm −3 which corresponds to ω P ≈ 8.6 keV.
The white-dwarf luminosity function yields about the same limit.
If e x exceeds around 10 −8 e the mean free path of the millicharged
particles will be less than the physical size of a white dwarf or redgiant
core. For larger e x the particles will be trapped and contribute
to the transfer of energy. Their impact on stellar evolution will become
negligible when e x is so large that other forms of energy transfer (photon
radiation, convection) are more important. Because the new particles
act essentially as radiation their mean free path must be less than that
of photons, or very crudely, their charge must be of order an electron
charge. (The main opacity source is probably Coulomb scattering on
charged particles.) Therefore, if their mass is below a few keV, even the
properties of the Sun would imply that there is not an allowed range of
large e x on the trapping side of the red-giant argument, except perhaps
for e x so large that it is excluded by experimental arguments.