Determination of Primordial Metallicity and Mixing in the Type II-P Supernova 1993W^*

We present the results of a large grid of synthetic spectra and compare them to early spectroscopic observations of SN 1993W. This supernova was discovered close to its explosion date and, at a recession velocity of 5400 km s^-1, is located in the Hubble flow. We focus here on two early spectra that were obtained approximately 5 and 9 days after explosion. We parameterize the outer supernova envelope as a power-law density profile in homologous expansion. In order to extract information on the value of the parameters, a large number of models was required. We show that very early spectra combined with detailed models can provide constraints on the value of the power-law index, the ratio of hydrogen to helium in the surface of the progenitor, the progenitor metallicity, and the amount of radioactive nickel mixed into the outer envelope of the supernova. The spectral fits reproduce the observed spectra exceedingly well. The spectral results combined with the early photometry predict that the explosion date was 4.7 ± 0.7 days before the first spectrum was obtained. The ability to obtain the metallicity from early spectra make SNe II-P attractive probes of chemical evolution in the universe, and by showing that we have the ability to pin down the parameters of the progenitor and mixing during the supernova explosion, it is likely to make SNe II-P useful cosmological distance indicators which are at the same time complementary to SNe Ia.