Stars as Laboratories for Fundamental Physics - MPP Theory Group

100 Chapter 3
Fig. 3.4. Pair annihilation processes for the production of neutrino pairs or
new bosons where a second amplitude with the vertices interchanged is not
shown.
importance of stellar energy-loss rates into new scalars relative to neutrino
pairs decreases with increasing temperature and density. The
impact of new bosons on stellar evolution relative to neutrinos is then
expected to be most pronounced for low-mass stars where other processes
such as photoproduction dominate. Consequently, the pair process
has not played any significant role at constraining the interactions
of new bosons.
3.4 Free-Bound and Bound-Free Transitions
Photons, new bosons, or neutrino pairs can be emitted in transitions
where a free electron is captured by an ion to form a bound state.
For the case of axions this effect was dubbed “axio-recombination”
(Dimopoulos et al. 1986). In the Sun, it contributes about 4% of the
total axion flux which is mostly from bremsstrahlung. The energy-loss
rate scales as T 3/2 , bremsstrahlung as T 5/2 , and Compton emission as
T 6 . Thus, axio-recombination is of importance in low-mass stars which
have low internal temperatures; for main-sequence stars with M ∼ <
0.2 M ⊙ it would be the dominant axion emission process. However,
given the limits on the coupling of pseudoscalars to electrons from other
arguments, no observable effects can be expected.
Of some practical interest is the inverse process where an axion unbinds
an atomic electron, the “axio-electric effect” (Dimopoulos, Starkman,
and Lynn 1986a,b). It serves to constrain the solar flux of axions
or other pseudoscalars which could produce keV electrons in a Ge spectrometer
designed to search for double-β decay (Avignone et al. 1987).
Unfortunately, the resulting bound of α ′ ∼ < 10 −21 is not very restrictive.
Pseudoscalars saturating this limit would be a major energy drain of
the Sun and thus not compatible with its observed properties.
For neutrinos, free-bound transitions were first discussed by Pinaev
(1963). Recently, Kohyama et al. (1993) studied the corresponding