Type Ia Supernovae: Influence of the Initial Composition on the Nucleosynthesis, Light Curves, and Spectra and Consequences for the Determination of Ω_M and Λ

The influence of the initial composition of the exploding white dwarf on the nucleosynthesis, light curves, and spectra of Type Ia supernovae has been studied in order to evaluate the size of evolutionary effects on cosmological timescales, how the effects can be recognized, and how one may be able to correct for them.

The calculations are based on a set of delayed detonation models that give a good account of the optical and infrared light curves and of the spectral evolution. The explosions and light curves are calculated using a one-dimensional Lagrangian radiation-hydro code including a nuclear network. Spectra are computed for various epochs using the structure resulting from the light-curve code. Our non-LTE code solves the relativistic radiation transport equations in the comoving frame consistently with the statistical equations and ionization due to γ-radiation for the most important elements (C, O, Ne, Na, Mg, Si, S, Ca, Fe, Co, Ni). About 10⁶ additional lines are included assuming LTE-level populations and an equivalent-two-level approach for the source functions.

Changing the initial metallicity Z from Population I to Population II alters the isotopic composition of the outer layers of the ejecta that have undergone explosive O burning. Especially important is the increase of the ⁵⁴Fe production with metallicity. The influence on the resulting rest-frame visual and blue light curves is found to be small. Detailed analysis of spectral evolution should permit a determination of the progenitor metallicity.

Mixing ⁵⁶Ni into the outer layers during the explosion can produce effects similar to an increased initial metallicity. Mixing can be distinguished from metallicity effects by means of the strong cobalt and nickel lines, by a change of the calcium lines in the optical and IR spectra and, in principle, by γ-ray observations.

As the C/O ratio of the white dwarf is decreased, the explosion energy and the ⁵⁶Ni production are reduced, and the Si-rich layers are more confined in velocity space. A reduction of the C/O ratio by about 60% gives slower rise times by about three days, an increased luminosity at maximum light, a somewhat faster postmaximum decline, and a larger ratio between maximum light and ⁵⁶Ni tail. A reduction of the C/O ratio has an effect on the colors, light-curve shapes and element distribution similar to a reduction in the deflagration to detonation transition density. However, for the same light-curve shape, the absolute brightness is larger for smaller C/O ratios. An independent determination of the initial C/O ratio and the transition density is possible for local supernovae if detailed analyses of both the spectra and light curves are performed simultaneously.

Because the spectra are shifted into different color bands at different redshifts, the effect of metallicity Z on a given observed color is a strong function of redshift. A change of Z by a factor of 3 or of the C/O ratio by 33% alters the peak magnitudes in the optical wavelength range by up to ≈ 0.3 mag for z ≥ 0.2. These variations are comparable to the effect of changes of Ω_M and Λ at redshifts of 0.5-1.0. The systematic effects due to changes in composition are expected to remain small up to about z ≈ 0.5 for R-V and up to z ≈ 0.7 for R-I.

We discuss how evolution in the progenitor population can be recognized and taken into account. With proper account of evolutionary corrections, supernovae will provide a valuable tool to determine the cosmological parameters of the universe, and they will provide new insight into its chemical evolution.