Thermal oscillations during carbon burning in an electron-degenerate stellar core

The response of the electron-degenerate carbon-oxygen core of an intermediate-mass star (3 to 8 solar masses) to the initiation of carbon-burning reactions and to the resulting Urca neutrino losses is investigated. It is shown that the core oscillates thermally between two unstable states: (1) a heating phase, during which heating by nuclear reactions and by the release of Fermi energy in response to electron capture exceeds cooling by Urca neutrinos, carries the core from a low-temperature to a high-temperature state and (2) a cooling phase, during which the reverse is the case, carries the core back to the low-temperature state. The time dependence of stellar structure variables and energetics during the thermal oscillations is examined, along with stellar structure and nuclear reaction rates near the beginning and the end of a heating phase, the nature of the convective Urca process at its peak, and the behavior of the convective core when the central density approaches the threshold for electron capture on Ne-21.