Stars as Laboratories for Fundamental Physics - MPP Theory Group

Particles Interacting with Electrons and Baryons 101
stellar energy-loss rate in detail and found that it dominates the other
neutrino emission processes only in such regions of temperature and
density where the overall neutrino luminosity is very small. Therefore,
free-bound transitions do not seem to be of practical importance as a
stellar energy-loss mechanism.
3.5 Bremsstrahlung
3.5.1 Nondegenerate, Nonrelativistic Medium
The last emission process to be discussed is bremsstrahlung (Fig. 3.5)
where an electron emits a boson or a neutrino pair when scattering
off the Coulomb field of a nucleus. Conceptually, bremsstrahlung is
closely related to the Compton process (Fig. 3.1) because in both cases
the electron interacts with electromagnetic field fluctuations of the ambient
medium which have nonvanishing power for all wavenumbers and
frequencies. The Compton process corresponds to wavevectors which
satisfy the photon dispersion relation so that a real (on-shell) excitation
is absorbed. However, in a typical stellar plasma there is more
power in the “off-shell” electromagnetic field fluctuations associated
with the charged particles. 18 Moreover, degeneracy effects do not suppress
bremsstrahlung at high densities, in contrast with the Compton
process.
Fig. 3.5. Bremsstrahlung emission of bosons or neutrino pairs by an electron
which scatters from the Coulomb field of a nucleus of charge Ze. In each
case a second amplitude with the vertices interchanged is not shown.
For neutrino pairs bremsstrahlung dominates over other processes
only in the highly degenerate regime (Appendix C). For pseudoscalars,
18 In a typical stellar plasma there are many more charged particles than blackbody
photons. In the solar center, for example, the electron density is 6×10 25 cm −3
while for photons at a temperature of 1.3 keV it is 2ζ(3) T 3 /π 2 = 6×10 22 cm −3 .