Red giants and supergiants with degenerate neutron cores.

Results are presented for calculations of spherically symmetric stellar models with degenerate neutron cores which bear the same relationship to neutron stars as ordinary red giants have the white dwarfs. The external appearance of the models is shown to be similar to that of an ordinary M-type supergiant, while their evolutionary lifetime is ten times as long. The models are relativistic and nonrotating, containing a degenerate neutron core of 1 solar mass (radius of 10 km) surrounded by a nondegenerate massive diffuse envelope, with the core and envelope separated by a thin energy-producing layer. Effective temperatures and radii are calculated, and two families of models (red giants and supergiants) are described which are distinguished by their luminosities. Uncertainties and possible instabilities in the models are discussed. Possible formation mechanisms for such stars are hypothesized, including collapse of the degenerate electron core of a 'normal' giant or supergiant, supercritical accretion onto a neutron star, and spiralling of a neutron star in a close binary system into its companion's interior.