Stars as Laboratories for Fundamental Physics - MPP Theory Group

108 Chapter 3
Fig. 3.6. Energy-loss rate of a helium plasma at T = 10 8 K as a function
of density for pseudoscalars with the coupling α ′ = 10 −26 . The Compton
rate is given in Eq. (3.23), suppressed with the factor F deg of Eq. (3.18) at
high density. The nondegenerate (ND) and degenerate (D) bremsstrahlung
rates are from Eqs. (3.28) and (3.35), respectively. The solid line is the
interpolation formula of Raffelt and Weiss (1995).
that was used in a numerical study of axion emission from red giants
by Raffelt and Weiss (1995); details of how it was constructed can be
found there.
At T = 10 8 K the compound rate (solid line) coincidentally is almost
independent of density. The requirement ϵ ∼ < 10 erg g −1 s −1 then yields
a constraint α ′ ∼ < 10 −26 for any density whence the helium-burning lifetime
of HB stars as well as the core mass at helium ignition yield an
almost identical constraint. In detail one needs to apply the heliumignition
argument at an average core density of 2×10 5 g/cm 3 (the central
density is about 10 6 g/cm 3 ) and at the almost constant temperature
of T = 10 8 K. The degenerate emission rate is then approximately
α ′ 2×10 27 erg g −1 s −1 so that
α ′ ∼ < 0.5×10 −26 . (3.44)
This is the most restrictive available bound on the Yukawa coupling of
pseudoscalars to electrons.