I can divide my interests in several areas:
As a posdoc in the
Columbia University Astronomy Department
I went on studying the cooling of neutron stars in their various flavors.
A bitter one is the strange star which is a "neuton star" without neutrons: made
entirely of quark matter except, possibly, for a thin outer layer of normal
matter. These strange stars also cool very fast, like neutron stars with a kaon
condensate, but the fast cooling of the core affects the surface much sooner
than in the case of a neutron star: a few years instead of a few
decades because its crust is very thin [a].
Detection of thermal emission from the neutron that has been formed by SN 1987A
may, or may not, give us a definite evidence for the existence of strange stars
(but there may be no neutron star left there, i.e., it may have collapsed into
a black hole).
In 1991 I heard from M. Prakash the big new that `One can fool all the people
all the time': the most efficient neutrino emission process possible in neutron stars,
the direct Urca Process, which was considered to be forbidden by momentum
non-conservation, can occur in many cases. This is the most obvious process but
because it had been stated in the early 60's that it is forbidden nobody had checked
this again for 25 years (except Boguta but apparently nobody cared about his paper).
The occurrence of the direct Urca process has a dramatic effect on the cooling
[4] and I also took advantage of this opportunity
to emphasize the importance of nucleon pairing which reduces strongly the neutrino
emission (something that I had shown in my Ph.d. dissertation but people didn't
notice it because kaons looked too `exotic').
After Halpern & Holt demonstrated the Geminga is a neutron star,
I applied my cooling models to this new one
([b] & [5])
showing that extensive pairing in its core is necessary to obtain temperatures
in agreement with the observed one.
That was the best, maybe even the only, evidence for the presence of extensive
pairing in the core of neutron star.
As things are, it turns out that this `evidence' for baryon pairing is now gone: I had ingeniously assumed (as everybody else) that the surface, and the envelope, of a neutron star is made of iron. This is fine for a textbook neutron star, but real ones may be dirtier. The late post-supernova accretion may deposit light elements at the surface and this strongly affects the transport of heat, i.e., it change dramatically the relationship between the interior temperature and the surface temperature, as shown recently by Potekhin Chabrier and Yakovlev. When applying this to the cooling it turns out that you can `explain everything' without requiring pairing [13]. So, neutron star cooling is presently (early 1997) in a state of total confusion [A].