Sanjay Reddy -- Physics of Neutron Stars: A Nuclear Theory Perspective

Sanjay provided a broadly interesting overview of the nuclear physics properties of neutron stars. I'll focus here on one aspect which Sanjay has been championing, and which is very interesting:

A high nuclear symmetry energy can favor large proton fraction at and above nuclear density, and allow direct URCA cooling -- which is extremely efficient. A large symmetry energy will also produce neutron stars with larger radii (>12 km). However, a small symmetry energy will produce a low proton fraction, and will not permit direct URCA cooling, requiring that cooling be dominated by the slow process called "modified URCA"; this will also permit small radius neutron stars (<12 km).

Thus, neutron star radius measurements, cooling, and the symmetry energy are all related. "It is difficult to reconcile a rapid cooling neutron star with mass of 1.4 solar masses, and a radius of < 12 km." This, Sanjay tells us, would require some new physics.