The Phillips relation of 'Normal' type Ia Supernovae

The use of Type Ia Supernovae (SNe Ia) as distance indicators relies on the calibration of their brightness using the observed property that brighter SNe have broader, slower light curves. Among possible causes are different masses of the progenitor white dwarfs or different opacities. We construct Chandrasekhar-mass models which synthesize different amounts of (56)Ni, and compute their bolometric light curves and spectra. Since opacity in SNe Ia at early times is due mostly to spectral lines, and Fe-peak ions have many more lines than lighter ions, the opacity depends on the synthesized (56)Ni mass and on the temperature in the ejecta. Bolometric light curves computed using these prescriptions reproduce the relation between brightness and decline rate. The synthetic spectra compare well with spectra of spectroscopically normal SNe Ia which cover a similar Deltam(15)(B) range. The change in colour of the spectra between maximum and two weeks later allows the observed relation between M(B)(Max) and Deltam(15)(B) to be reproduced.