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].