Red giants and supergiants with degenerate neutron cores

Abstract

A new type of stellar model is constructed. It is related to neutron stars as ordinary red giants are related to white dwarfs. Its external appearance is similar to that of an ordinary M supergiant, but its evolutionary lifetime is 10 times longer. Our models are constrained to be relativistic but nonrotating, to constrain a degenerate neutron core of mass 1 M⊙ and radius 10 km, surrounded by a nondegenerate, massive, diffuse envelope. The core and envelope turn out to be separated by a thin (~40 m) energy-generation layer. The envelope convects from this layer all the way out to the photosphere. The effective temperatures and radii are ~2700 K and ~1000 R⊙. Within a fairly narrow range of effective temperatures and radii, two families of models were found: "red giants" and "red supergiants" with luminosities and masses less than and greater than ~65,000 L⊙ and ~10 M⊙, respectively. The luminosity of a giant comes 97 percent from gravitational contraction and 3 percent from nuclear burning. That of a supergiant is 5 percent from gravitational contraction and 95 percent from hydrogen burning by nonequilibrium, hot CNO reactions. The CNO reaction products are convected directly from the hydrogen-burning shell out to the photosphere of the supergiant, where they should be observable.

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