SUPERNOVA EXPLOSIONS

Type Ia Supernovae

Interpretation: Thought to result from the explosion of massive binary stars. There are spectroscopic and photometric indications that the progenitor stars of type Ia supernovae are white dwarfs that are composed of C + O with strongly degenerate electrons. Such stars are formed from intermediate mass stars, < 8 M¤, ### "White dwarfs composed of C+O are formed from intermediate mass stars (M < 8 Mo, where Mo is the mass of our sun), undergo cooling, and eventually become dark matter as they evolve towards fainter luminosities. In a close binary system, the white dwarf evolves differently because the companion star expands to transfer matter to the white dwarf; the accreting white dwarfs are rejuvenated and, in certain cases, undergo thermonuclear explosions to give rise to SNe Ia. Theoretically, the Ch [Chandrasekhar mass] white dwarf models and the sub-Ch models have been considered to explain the origin of SNe Ia [Branch et al. 1995; Renzini 1996]. Various evolutionary scenarios have been proposed, including (i) merging of double C+O white dwarfs with a combined mass exceeding the Ch limit (a DD scenario) [ref. 8 - not on my hardcopy; check PDF] and (ii) accretion of H or He by mass transfer from a binary companion at a relatively high rate (an SD scenario) [refs. 7 & 9 - not on my hardcopy; check PDF]" (Nomoto et al., pp. 1378-1379). Thermonuclear reactions power the expansion of the core and eventual disruption of the star, but not the luminosity of the expanding gas. The energy source for the latter is provided by the slow radioactive decay sequence 56Ni ® 56Co ® 56Fe (Gamezo et al. 2002, p. 77). The supernova occurs when the white dwarf has accreted sufficient mass from its companion to trigger an explosion. However, the progenitor systems and hydrodynamical models are still controversial. (After Nomoto et al. 1997.) Type Ia supernova explosions are caused by the complete thermonuclear disruption of a white dwarf. Model studies attempting to elucidate the explosion mechanism have been limited to spherically symmetric one-dimensional models. Recently, Gamezo et al. (2002, p. 77) developed a three-dimensional model which may herald a new era of model complexity in supernova studies.