Can Differences in the Nickel Abundance in Chandrasekhar‐Mass Models Explain the Relation between the Brightness and Decline Rate of Normal Type Ia Supernovae?

The use of Type Ia supernovae as distance indicators relies on thedetermination of their brightness. This is not constant, but it can becalibrated using an observed relation between the brightness and the propertiesof the optical light curve (decline rate, width, shape), which indicates thatbrighter SNe have broader, slower light curves. However, the physical basis forthis relation is not yet fully understood. Among possible causes are differentmasses of the progenitor white dwarfs or different opacities inChandrasekhar-mass explosions. We parametrise the Chandrasekhar-mass modelspresented by Iwamoto et al (1999), which synthesize different amounts of Ni,and compute bolometric light curves and spectra at various epochs. Sinceopacity in SNe Ia is due mostly to spectral lines, it should depend on the massof Fe-peak elements synthesized in the explosion, and on the temperature in theejecta. Bolometric light curves computed using these prescriptions for theoptical opacity reproduce the relation between brightness and decline rate.Furthermore, when spectra are calculated, the change in colour between maximumand two weeks later allows the observed relation between M_B(Max) andDm_{15}(B) to be reproduced quite nicely. Spectra computed at various epochscompare well with corresponding spectra of spectroscopically normal SNeIaselected to cover a similar range of Dm_{15}(B) values.Comment: 25 pages, including 6 figures. Accepted for publication in Ap