TYPE IA SUPERNOVAE - INFLUENCE OF THE INITIAL COMPOSITION ON THE NUCLEOSYNTHESIS, LIGHT CURVES, AND SPECTRA AND CONSEQUENCES FOR THE DETERMINATION OF OMEGA(M) AND LAMBDA

The influence of the initial composition of the exploding white dwarf on the nucleosynthesis, light curves, and spectra of Type Ia supernova e has been studied in order to evaluate the size of evolutionary effec ts 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 op tical and infrared light curves and of the spectral evolution. The exp losions and light curves are calculated using a one-dimensional Lagran gian radiation-hydro code including a nuclear network. Spectra are com puted for various epochs using the structure resulting from the light- curve code. Our non-LTE code solves the relativistic radiation transpo rt equations in the comoving frame consistently with the statistical e quations and ionization due to gamma-radiation for the most important elements (C, O, Ne, Na, Mg, Si, S, Ca, Fe, Co, Ni). About 10(6) additi onal lines are included assuming LTE-level populations and an equivale nt-two-level approach for the source functions. Changing the initial m etallicity Z from Population I to Population II alters the isotopic co mposition of the outer layers of the ejecta that have undergone explos ive O burning. Especially important is the increase of the Fe-54 produ ction with metallicity. The influence on the resulting rest-frame visu al and blue light curves is found to be small. Detailed analysis of sp ectral evolution should permit a determination of the progenitor metal licity. Mixing Ni-56 into the outer layers during the explosion can pr oduce effects similar to an increased initial metallicity. Mixing can be distinguished from metallicity effects by means of the strong cobal t and nickel lines, by a change of the calcium lines in the optical an d IR spectra and, in principle, by gamma-ray observations. As the C/O ratio of the white dwarf is decreased, the explosion energy and the Ni -56 production are reduced, and the Si-rich layers are more confined i n velocity space. A reduction of the C/O ratio by about 60% gives slow er rise times by about three days, an increased luminosity at maximum light, a somewhat faster postmaximum decline, and a larger ratio betwe en maximum light and Ni-56 tail. A reduction of the C/O ratio has an e ffect on the colors, light-curve shapes and element distribution simil ar 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 ini tial C/O ratio and the transition density is possible for local supern ovae if detailed analyses of both the spectra and light curves are per formed simultaneously. Because the spectra are shifted into different color bands at different redshifts, the effect of metallicity Z on a g iven 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 i n the optical wavelength range by up to approximate to 0.3 mag for z g reater than or equal to 0.2. These variations are comparable to the ef fect of changes of Omega(M) and A at redshifts of 0.5-1.0. The systema tic effects due to changes in composition are expected to remain small up to about z approximate to 0.5 for R-V and up to z approximate to 0 .7 for R-I. We discuss how evolution in the progenitor population can be recognized and taken into account. With proper account of evolution ary 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.