SUPERNOVAE, SUPERNEBULAE, AND NUCLEOSYNTHESIS

Supernova atmosphere calculations continue to show that variants of previously calculated carbon-deflagration models provide a good representation ofthe maximum light spectra of classical type la supernovae including the ultraviolet deficit. Careful consideration ofthe conditions leading to central thermonuclear runaway of degenerate carbon shows that runaway can, however, lead to detonation and direct conflict with observations. A month or more after maximum light a variety of supernovae enter a nebular phase, a supernebula, characterized by large velocities and often great enhancements of heavy elements energized by gamma rays from radioactive decay. In particular, type Ib supernovae show a dramatic transition beginning about 60 days after maximum light from a spectrum dominated by neutral helium in a strongly non-LTE state to one dominated by strong emission lines of [O I]. These supernovae give our first direct glimpse into the exploding innards of a massive star and open a whole new avenue for exploration. As witnessed by the spectra of type Ib supernovae, massive stars are expected to be the primary source of oxygen. Estimates ofthe absolute production of oxygen in massive stars suggest that if all stars more massive than ~ 12 m⊙ explode as supernovae, oxygen would be overproduced in the solar neighborhood, an effect exacerbated by the recent increase in the reaction rate for ¹²C(α, γ) ¹⁶O.