Stars as Laboratories for Fundamental Physics - MPP Theory Group

88 Chapter 2
nonstandard energy-loss mechanism in stars are the white-dwarf luminosity
function, the helium-burning lifetime of horizontal-branch stars,
and the nondelay of helium ignition in low-mass red giants as observed
by the brightness of the tip of the red-giant branch in globular clusters.
It was possible to condense the latter two arguments into two
exceedingly simple criteria, namely that an anomalous energy-loss rate
in the cores of HB stars as well as in red-giant cores before helium
ignition must not exceed about 10 erg g −1 s −1 . The emission rate is to
be calculated at the pertinent plasma conditions, i.e. at an approximate
temperature of 10 8 K = 8.6 keV, an electron concentration of
Y e = 0.5, and an average density of about 0.6×10 4 g cm −3 (HB stars)
or 2×10 5 g cm −3 (red giants). The former case corresponds to roughly
nondegenerate conditions, the latter case to degenerate ones so that it
depends on the density dependence of the emission rates which of these
cases will yield a more restrictive limit.
A red-giant core is essentially a 0.5 M ⊙ helium white dwarf. The
observed “real” white dwarfs have typical masses of about 0.6 M ⊙ ;
they are thought to consist mostly of carbon and oxygen. Both the
helium-ignition argument and the white-dwarf luminosity function allow
one to constrain a novel energy-loss mechanism roughly on the level
of standard neutrino emission. Therefore, it is no surprise that bounds
derived from both arguments tend to be very similar.
There may be other objects or phenomena in the universe that measure
novel particle-physics hypotheses even more sensitively than the
cases discussed here. They still need to make their way into the particle
astrophysics literature.