Explosive Ignition of Carbon in Stars of Intermediate Mass

Abstract

Hoyle and Fowler1 have suggested that type I supernovae are the result of an explosion following the ignition of degenerate nuclear fuel in an evolved star. The purpose of this communication is to place this idea within the context of current stellar evolutionary calculations and to examine the development of the instability. The growth of carbon–oxygen core as a result of helium shell burning in stars of intermediate mass has been studied by Hayashi, Hoshi and Sugimoto2,3, Kippenhahn, Thomas and Weigert4, and Weigert5. For a carbon–oxygen core of mass Mc ∼ 0.7 M⊙, these authors find that inclusion of energy loss by neutrino emission prevents the carbon flash from occurring, and that a cold degenerate core of potentially explosive carbon and oxygen slowly grows. If energy loss by neutrino emission is included, a carbon star cannot ignite by the 12C + 12C reaction if its mass is less than about 1 M⊙ (unpublished results of T. Murai, D. Sugimoto, R. Hoshi and C. Hayashi, and of G. Beaudet and E. E. Salpeter). This implies that all stars which have a less massive carbon–oxygen core at the onset of the helium shell burning stage will not ignite carbon burning by the comparatively gentle carbon flash. In what follows I consider how this ignition might occur.