TYPE-I SUPERNOVAE

The observations and theory of hydrogen-deficient Type I supernovae (SN Ia) are reviewed. Classical Type Ia supernovae are thought to arise in the thermonuclear explosions of degenerate stellar cores, but more recently defined classes, Types Ib and Ic (SN Ib, Ic), may result from core collapse in more massive stars. Binary evolutionary schemes for the origin of SN Ia are described including transfer of hydrogen from a non-degenerate companion and the disruption of a degenerate companion. No scheme adequately accounts for all the observational constraints. The process of degenerate carbon ignition thought to power SN Ia still has many uncertainties including the role of the convective Urca process and questions involving the mechanism of explosion propagation. Subsonic deflagration processes match many of the observational constraints, including the light curve, early- and late-time spectra, and the properties of young extended remnants, but supersonic detonation seems to follow from the physics of the ignition process. Calculations of theoretical spectra of SN Ia at various stages in their evolution are described in some detail and the importance of SN Ia for nucleosynthesis and for determination of the cosmological distance scale is summarized. Less is known about the details of the origin of the other classes of SN I. SN Ib are characterized by the appearance of helium lines near maximum and emission lines of oxygen and calcium at later epochs. Like SN Ib, SN Ic lack the evidence for Si near maximum that characterizes SN Ia, but they have only weak indications of He. They are indistinguishable from SN Ib in the later supernebular phases. There are indications from the light curves that SN Ic may differ in significant ways from SN Ib although the spectra suggest that they are generically related. Circumstantial evidence suggests that SN Ib and Ic arise in the hydrogen-denuded cores of massive stars which may have lost their hydrogen envelopes to a binary companion. The possible connection of SN Ib and Ic events to the 'transition' event, SN 1987K, is also discussed. An appendix describes the operation of the supernova atmosphere code used to calculate early-time spectra of SN I.