Could edge-lit type Ia supernovae be standard candles?

The progenitors of Type la supernovae (SNe Ia) have not been identified. Though they are no longer fashionable, we investigate the consequences if a significant number of SNe la were edge-lit detonations (ELDs) of carbon/oxygen white dwarfs that have accreted a critical mass of helium. Our best understanding of the Phillips relation between light curve speed and peak luminosity assigns both these phenomena to the amount of Ni-56 produced. In ELDs there are two sites of Ni-56 synthesis. If the peak luminosity is determined primarily by the CO ratio in the core it is primarily a function of the initial main-sequence mass of the progenitor of the CO white dwarf. If the light curve decay speed is determined by the total mass of iron group elements ejected this is a function of the total mass of the ELD at the time of explosion because both the CO core and the He envelope are substantially converted to Ni-56. In general, binary star evolution ensures that these two masses are correlated and an empirical relation between peak luminosity and light curve shape can be expected. However when we perform population synthesis for progenitors of different metallicities we find a systematic shift in this relation that would make distant ELD SNe la fainter than those nearby. The abundances of alpha-rich isotopes, such as Ca-44, in the solar system indicate that only about 40 per cent of SNe la could be edge-lit so any systematic effect that could be present would be correspondingly diluted. A systematic effect is still expected even if we examine only the small subset of ELDs that accrete from a naked helium star, rather than a He white dwarf. (C) 2002 Elsevier Science B.V. All rights reserved.