Stars as Laboratories for Fundamental Physics - MPP Theory Group

Miscellaneous Exotica 551
Most recently, a similar study was completed by Guenther et al.
(1995) who focussed on the predicted g-mode spectrum. At the present
time there is no generally accepted observation of solar g-modes (recall
that they are evanescent in the convection zone). If one were to
take the claimed observations by Hill and Gu (1990) seriously, a bound
|β| ∼ < 0.05 would obtain. Therefore, if an unambiguous identification
of g-modes would emerge from a number of forthcoming observational
projects, the Sun may yet provide one of the most restrictive limits on
the constancy of Newton’s constant.
15.2.4 White Dwarfs
A large impact of a time-varying gravitational constant can be expected
on the oldest stars which “integrate” G N (t) into the more distant past
than does the evolution of the Sun. One well understood case are
white dwarfs, the faintest of which likely formed shortly after the birth
of the galactic disk. Therefore, the age of the galactic disk implied by
the fast drop of the white-dwarf luminosity function at the faint end
(Sect. 2.2.1) depends on the G N evolution in the past.
In an early study Vila (1976) concluded on the basis of the observations
then available that ĠN/G N as large as 75×10 −12 yr −1 was not excluded.
García-Berro et al. (1995) constructed detailed luminosity functions
under the assumption of a decreasing G N . For an assumed age of
the galactic disk of 7 Gyr, which probably is a lower plausible limit, the
best fit for the faintest data point requires ĠN/G N = −10×10 −12 yr −1
while the curves for 0 and −30×10 −12 yr −1 lie somewhat outside of the
1σ error bar of this all-important data point. Still, the white-dwarf
luminosity function does not seem to yield significant limits relative to
the celestial-mechanics ones.
15.2.5 Globular Clusters
The oldest stellar objects in the galaxy are globular-cluster stars which
are thus expected to yield the most restrictive stellar-evolution limits
on ĠN/G N . A color-magnitude diagram for an intermediate-aged galactic
cluster was constructed by Roeder (1967) while detailed studies of
globular clusters were performed by Prather (1976) and Degl’Innocenti
et al. (1995). Roeder (1967) and Prather (1976) used a time variation
for a specific Brans-Dicke cosmology where G N decreases approximately
as in Eq. (15.1) with β ≈ 0.03 while Degl’Innocenti et al. (1995) considered
more generic cases of G N (t).