Stars as Laboratories for Fundamental Physics - MPP Theory Group

54 Chapter 2
faster than predicted by these models (Kepler et al. 1991). It should
be noted, however, that part of the observed period change can be attributed
to a Doppler shift if G117–B15A is a physical binary with its
proper-motion companion G117–B15B.
Isern, Hernanz, and García-Berro (1992) speculated that an additional
cooling agent may be operating and notably that this star is
cooled by axion emission. For a fiducial WD model (M = 0.5 M ⊙ ,
internal temperature T = 1.8×10 7 K) the cooling time scale is found
to be T/ T ˙ = 1.0 Gyr while P/ P ˙ ≈ 1.4 Gyr. Therefore, Isern et al.
found that L x = 0.5−2.6 L γ was needed to account for the observed P ˙ ,
although other models needed little or no axion cooling. For their
fiducial model the required axion cooling yielded a coupling constant
α 26 = 0.2−0.8. This interpretation is speculative, of course, but apparently
not in conflict with any other constraints on the electron coupling
of pseudoscalars (Sect. 3.6.1).
The most conservative interpretation of the P ˙ measurement of the
star G117–B15A is that it agrees with theoretical calculations within
the observational and model uncertainties. Therefore, it provides independent
evidence that the WD cooling speed is known to within a
factor of O(1) so that any novel cooling agent is constrained to be less
efficient than O(L γ ).
2.3 Neutron Stars
2.3.1 Late-Time Cooling
Neutron stars are born when the degenerate iron core of an evolved
massive star becomes unstable and collapses to nuclear densities, an
implosion which is partly reflected at the core bounce and leads to a
type II supernova explosion (Sect. 2.1.8). These events and the first few
seconds of neutron star cooling are discussed more fully in Chapter 11.
For a few seconds the star emits most of its binding energy in the form
of MeV neutrinos which were observed from SN 1987A. Afterward, the
temperature at the neutrino sphere (the analogue of the photosphere in
ordinary stars) has dropped so much that the detectors are no longer
sensitive to the neutrino flux although the star continues to cool by
surface neutrino emission.
After 10−100 yr the internal temperature has dropped to about
10 9 K ≈ 100 keV where the neutron star becomes entirely transparent
to neutrinos and continues to cool by neutrino volume emission.
After about 10 5 yr it reaches an inner temperature of about 2×10 8 K,