Stars as Laboratories for Fundamental Physics - MPP Theory Group

352 Chapter 10
of details that have to be minded it is sometimes difficult to compare
exactly the solar models of different workers. However, wherever a
detailed comparison has been performed, different results for such output
quantities as predicted neutrino fluxes usually can be understood
in terms of different choices regarding some of the input physics (e.g.
Bahcall and Pinsonneault 1992; Turck-Chièze and Lopes 1993). Thus,
there is very little ambiguity in solar modelling for a common choice of
input physics.
Nonstandard solar models are ones where standard input parameters
have been chosen outside the range of recognized uncertainties (for
example the opacities), or which include entirely new physical effects
such as strong magnetic fields, fast rotation in the deep interior, nonstandard
nuclear reaction rates (involving free quarks, for example),
energy transfer by a nonstandard mechanism (e.g. by trapped massive
weakly interacting particles), and others. Some such possibilities can
be excluded by the helioseismologically determined sound-speed profile
of the Sun. I will not discuss nonstandard solar models any further—
for an overview see Bahcall (1989). Naturally, it is always possible that
a new “nonstandard” effect is recognized which could be important for
the solar structure and neutrino fluxes.
One standard physical effect that has made its way into standard
solar models only recently is the gravitational settling of helium and
metals. This effect leads to a stronger concentration of helium in the
central region than is caused by nuclear burning alone so that the
present-day Sun is “more evolved.” Consequently, the central temperature
is slightly higher, leading to an increased flux of boron neutrinos.
The settling of metals leads to an increased opacity, causing
a further increase of the temperature and of the neutrino fluxes by
a similar amount. Solar models with gravitational settling of helium
significantly improve the already good agreement with the helioseismologically
inferred sound-speed profile (Christensen-Dalsgaard, Proffitt,
and Thompson 1993) while the settling of metals does not seem to have
a strong additional impact on the p-mode frequencies (Proffitt 1994).
Bahcall and Pinsonneault (1992) were the first to include helium settling
in a standard solar model; in their 1995 paper they included metal
settling as well. They find that the compound effect is to change the 8 B
flux by +36%, the 7 Be flux by +14%, and the pp flux by −1.7%. Similar
changes (+31%, +13%, and −1.7%) were found by Proffitt (1994)
while Kovetz and Shaviv (1994), who included only helium settling,
obtained smaller effects relative to the corresponding case of Bahcall
and Pinsonneault (1992).