Stars as Laboratories for Fundamental Physics - MPP Theory Group

110 Chapter 3
The requirement L < a ∼ L ⊙ yields a constraint α < a ∼ 1.7×10 −22 .
A much more restrictive limit arises from the white-dwarf luminosity
function as axion emission would accelerate white-dwarf cooling.
This argument was studied in detail in Sect. 2.2.4 as a generic case
for the use of the white-dwarf luminosity function; the resulting constraint
is given in Tab. 3.1. The cooling speed of white dwarfs was
also established from a measurement of the period decrease of the DA
variable (ZZ Ceti) star G117–B15A which thus yields a similar limit.
However, the period decrease of this star may be slightly faster than
can be attributed to standard cooling processes; it has been speculated
that axions with a coupling strength of about α a ≈ 0.5×10 −26 could be
responsible (Sect. 2.2.5). The limit Eq. (3.44) that was derived in the
previous section from the helium-ignition argument in globular clusters
is of a similar magnitude, but not restrictive enough to exclude this
hypothesis.
All of these constraints apply to low-mass bosons. The most restrictive
one is based on the helium ignition argument with T ≈ 10 8 K =
8.6 keV. Therefore, these constraints apply if m < ∼ 10 keV. However,
it would be incorrect to think that for larger masses there was no constraint.
There is one, but it is degraded because threshold effects limit
the particle production to the high-energy tails of the thermal distributions
of the plasma constituents. For massive pseudoscalars, this
question has been studied in Sect. 1.3.5 in the context of general solar
particle constraints. For the more restrictive globular-cluster limits,
such a detailed investigation does not exist in the literature.
3.6.2 Energy Loss by Scalar and Vector Bosons
The couplings of low-mass scalar or vector particles ϕ are easy to constrain
by the same methods. Because the energy-loss rates have been
calculated only for nondegenerate conditions, only the arguments involving
the solar age and the helium-burning lifetime can be employed.
The latter yields a constraint on the ϕ-e coupling of
α ϕe ∼ < 1.4×10 −29 . (3.46)
It is based on the bremsstrahlung process e + 4 He → 4 He + e + ϕ as
discussed above in Sect. 3.5.5. For vector bosons which couple by a
current-current structure analogous to photons the same results apply
except for a factor of two in the emission rate which improves the limit
by a factor of 2.