Annu. Rev. Astron. Astrophys. 1992. 30: 359-89 Copyright © 1992 by Annual Reviews. All rights reserved

3.5 Nickel-Cobalt Radioactivity

The recognition around 1980 that SN Ia light curves are powered by the radioactive decay of ⁵⁶Ni and ⁵⁶Co (Woosley & Weaver 1986 and references therein) presented another new opportunity to derive distances to SNe Ia. An approximate but useful early rule was provided by Arnett (1982b), who predicted on the basis of an analytical model and reasonable assumptions that the SN Ia maximum luminosity is equal to the instantaneous decay luminosity of the nickel and cobalt. In this case the maximum luminosity can be expressed in terms of just the ejected nickel mass and the rise time to maximum light. Owing to uncertainties in the physics of the nuclear burning front that explodes the white dwarf (e.g. Woosley 1990) the ejected nickel mass cannot yet be predicted accurately by theory. As outlined by Sutherland & Wheeler (1984) and Arnett et al (1985), however, limits to the nickel mass can be inferred from the SN Ia spectra and light curves. Doppler shifts in the spectrum and the decay rate of the light curve constrain the explosion kinetic energy. Assuming that the white dwarf disrupts completely, the nuclear fusion energy must be the sum of the kinetic energy and the net binding energy of the immediate preexplosion white dwarf. Taking into account the fraction of the nuclear energy that comes from the synthesis of isotopes other than ⁵⁶Ni, Arnett et al (1985) used this line of reasoning to argue that the nickel mass must be in the range 0.4-1.4 M, with a most likely value of 0.6 M.

The effect of recent observational and theoretical developments on the radioactivity method have been reviewed by Branch (1992). On the assumptions that SNe Ia are the complete disruptions of carbon-oxygen white dwarfs near the Chandrasekhar mass, that their light curves are powered entirely by nickel-cobalt radioactivity, and that scatter in their maximum luminosities can be disregarded, the absolute blue magnitude is estimated. Combining a rise time to maximum blue and bolometric light of 19 ± 2 days, an ejected nickel mass M_Ni = 0.6(+0.2, -0.1) M, a ratio of maximum bolometric luminosity to instantaneous radioactivity luminosity of 1.2 ± 0.2, as found in light-curve calculations with realistic opacities (Harkness 1991, Höflich et al 1991), and a bolometric correction M_B - M_bol = -0.28 obtained from an observed maximum-light flux distribution, gives M_B = -19.4 ± 0.3. Possibilities of external error include the ejection of less than a Chandrsekhar mass (Shigeyama et al 1992) and effects associated with shape asymmetries such as have been detected in SN 1987A or with small-scale clumping of ejected matter (Muller & Arnett 1986).