Hubble's constant and exploding carbon–oxygen white dwarf models for Type I supernovae

Abstract

The immediate progenitor of a Type I supernova (SN I) is thought to be a mass-accreting carbon–oxygen (C–O) white dwarf in a binary system. When the mass of the white dwarf approaches the Chandrasekhar mass (1.4 M_⊙) the C–O nuclear fuel ignites, part of the star is incinerated to radioactive ⁵⁶Ni, and the thermonuclear energy completely disrupts the star. The optical luminosity results from the trapping and thermalization of the γ rays and positrons emitted by the decay of ⁵⁶Ni through ⁵⁶Co to stable ⁵⁶Fe. The amount of ⁵⁶Ni synthesized, M_Ni, and the corresponding peak luminosity, L_max, can be used with the observed Hubble diagram for SN I to determine the value of Hubble's constant, H₀. We argue here that if this model is correct, M_Ni is in the range 0.4–1.4 M_⊙, the best estimate being 0.6 M_⊙, and that H₀ is in the range 39–73 km s⁻¹ Mpc⁻¹ with a best estimate of 59 km s⁻¹ Mpc⁻¹. This line of reasoning does not require knowledge of the temperature of the supernova and, therefore, is not subject to the uncertainties associated with attempts to determine supernova luminosities and distances by the Baade method¹. It relies on the physical correctness of the model, which is subject to independent tests.