Stars as Laboratories for Fundamental Physics - MPP Theory Group

60 Chapter 2
limit of µ < B ∼ 3×10 −12 µ B (Sect. 6.5.6), I interpret these results to mean
that a neutrino magnetic dipole moment leaves neutron-star cooling
unaffected—one less nonstandard effect to worry about.
The rough agreement between the reference cooling curves and the
data points in Fig. 2.15, notably for the old pulsars, suggests that “nonstandard”
effects cannot be much more efficient than standard neutrino
cooling unless all of the old isolated pulsar surface temperatures have
been incorrectly assigned—not a likely scenario. Therefore, it is clear
that these and future observations of cooling neutron stars will be pivotal
as laboratories to study novel phenomena such as the occurrence
of nonstandard phases of nuclear matter.
However, at the present time it is not clear if the emission of weakly
interacting particles such as axions could still have an interesting impact
on neutron star cooling. At the present time it appears easier to make
the reverse statement that such cooling effects likely are not important
in view of the restrictive limits on the interaction strength of axions or
nonstandard neutrinos set by other astrophysical objects. Therefore,
at present it appears that for the more narrowly defined purposes of
particle physics the role of old neutron stars as laboratories is less useful
than had been thought in the early works discussed above. It also
appears that novel weakly interacting particles usually would have a
more dramatic impact on the first few seconds of Kelvin-Helmholtz
cooling of a protoneutron star (Chapter 11) than they do on the cooling
of old pulsars.
2.4 Globular-Cluster Stars
2.4.1 Observables in the Color-Magnitude Diagram
Globular clusters are gravitationally bound associations of typically
10 6 stars (Fig. 2.1); the clusters themselves (about 150 in our galaxy)
form an approximately spherical galactic halo. The metallicity is in the
range Z = 10 −4 −10 −2 ; it is usually the same for all stars in one cluster.
The low metallicity is one indicator for their great age—like isolated
halo stars they belong to the Population II which formed early from
a relatively “uncontaminated” hydrogen-helium mixture left over from
the big bang of the universe. Stars found in the galactic disk belong
to the later Population I which continue to form even today from the
interstellar gas. Clusters of disk stars are usually less populous and less
tightly bound—the so-called “open clusters.”