Stars as Laboratories for Fundamental Physics - MPP Theory Group

Preface
xix
cuts of higher-order graphs. These methods have been applied only
in a few cases of direct interest to particle astrophysics where the results
seem to agree with those from kinetic theory in the limits which
have been relevant in practice. A systematic formulation of astrophysically
relevant particle dispersion, emission, and absorption processes
in the framework of FTD field theory is a project for another author.
Tanguy Altherr had begun to take up this challenge with his collaborators
in a series of papers. However, his premature death in a tragic
climbing accident on 14 July 1994 has put an abrupt end to this line
of research.
My presentation of dispersion, emission, and absorption processes
is based entirely on the old-fashioned tools of kinetic theory where, say,
an axion emission rate is given by an integral of a squared matrix element
over the thermally occupied phase space of the reaction partners.
Usually this approach is not problematic, but it does require some tinkering
when it comes to the problem of electromagnetic screening or
other collective effects. I would not be surprised if subtle but important
collective effects had been overlooked in some cases.
A simple kinetic approach is not adequate in the hot nuclear medium
characteristic for a young SN core. However, in practice quantities like
the neutrino opacities and axion emissivities have been calculated as
if the medium constituents were freely propagating particles. At least
for the dominant axial-vector current interactions this approach is not
consistent as one needs to assume that the spin-fluctuation rate is small
compared with typical thermal energies while a naive calculation yields
a result much larger than T . Realistically, it probably saturates at
O(T ), independently of details of the assumed interaction potential.
This conjecture appears to be supported by our recent “calibration” of
SN opacities from the SN 1987A neutrino signal. However, a calculation
of either neutrino opacities or axion emissivities on the basis of first
principles is not available at the present time because FTD effects as
well as nuclear-physics complications dominate the problem.
⋄
It has been challenging to hammer the multifarious and intertwined
aspects of my topic into the linear shape required by the nature of
a book. Chapters 1−6 are mainly devoted to the stellar energy-loss
argument. It is introduced in Chapter 1 where its general aspects are
developed on the basis of the stellar structure equations. Chapter 2
establishes the observational limits on those stellar evolutionary time