Stars as Laboratories for Fundamental Physics - MPP Theory Group

The Energy-Loss Argument 5
trinos, to be harmless in stars. Of course, depending on whether the
particles are bosons or fermions, and depending on the details of their
interaction structure, this statement must be refined. Still, the impact
of novel particles on stellar structure and evolution is maximized when
their mean free path is of order the geometric dimension of the system.
The energy-transfer argument is equally powerful as the energy-loss argument.
The only reason why it has not been elaborated much in the
literature is because there is usually little motivation for considering
“strongly” interacting novel particles.
When it comes to the evolution of a supernova (SN) core after collapse
even neutrinos are trapped. Such a newborn neutron star is so
hot (T of order 30 MeV) and dense (ρ exceeding nuclear density of
3×10 14 g cm −3 ) that neutrinos take several seconds to diffuse to the
surface. Particles like axions can then compete in spite of the extreme
conditions because they freely stream out if their coupling is weak
enough. The energy-loss argument can be applied because the neutrino
cooling time scale has been established by the SN 1987A neutrino
observations. While the energy-loss argument in this case is fundamentally
no different from, say, white-dwarf cooling, the detailed reasoning
is closely intertwined with the issue of neutrino physics in supernovae,
and with the details of the SN 1987A neutrino observations. Therefore,
it is taken up only in Chapter 13. The groundwork concerning
the interactions of neutrinos and axions with nucleons, however, is laid
in Chapter 4 within the series of chapters devoted to various modes of
particle interactions with the constituents of stellar plasmas.
1.2 Equations of Stellar Structure
1.2.1 Hydrostatic Equilibrium
To understand the impact of a novel energy-loss mechanism on the
evolution of stars one must understand the basic physical principles that
govern stellar structure. While a number of simplifying assumptions
need to be made, the theory of stellar structure and evolution has been
extremely successful at modelling stars with a vast range of properties.
For a more detailed account than is possible here the reader is referred
to the textbook literature, e.g. Kippenhahn and Weigert (1990).
One usually assumes spherical symmetry and thus excludes the effects
of rotation, magnetic fields, tidal effects from a binary companion,
and large-scale convective currents. While any of those effects can be
important in special cases, none of them appears to have a noticeable