Can differences in the nickel abundance in Chandrasekhar mass models explain the relation between brightness and decline rate of normal Type Ia Supernovae?

Abstract

The use of Type Ia supernovae as distance indicators relies on the determination of their brightness. This is not constant, but it can be calibrated using an observed relation between the brightness and the properties of the optical light curve (decline rate, width, shape), which indicates that brighter SNe have broader, slower light curves. However, the physical basis for this relation is not yet fully understood. Among possible causes are different masses of the progenitor white dwarfs or different opacities in Chandrasekhar-mass explosions. We parametrise the Chandrasekhar-mass models presented by Iwamoto et al (1999), which synthesize different amounts of Ni, and compute bolometric light curves and spectra at various epochs. Since opacity in SNe Ia is due mostly to spectral lines, it should depend on the mass of Fe-peak elements synthesized in the explosion, and on the temperature in the ejecta. Bolometric light curves computed using these prescriptions for the optical opacity reproduce the relation between brightness and decline rate. Furthermore, when spectra are calculated, the change in colour between maximum and two weeks later allows the observed relation between M_B(Max) and Dm_{15}(B) to be reproduced quite nicely. Spectra computed at various epochs compare well with corresponding spectra of spectroscopically normal SNeIa selected to cover a similar range of Dm_{15}(B) values.