Nebular Spectra of Type IA Supernovae as Probes for Extragalactic Distances, Reddening, and Nucleosynthesis

We investigate the feasability of extracting basic information contained in the nebular spectra of supernovae: the amount of Ni synthesized in the explosion, the intervening reddening, and the distance to the parent galaxy. These physical quantities have correlated effects on the observed nebular spectra. Nevertheless, discrimination is possible by the use of least-squares theory, which allows one to find the best estimates of the model parameters, their statistical errors, and their mutual correlations. A spectral synthesis code for Type Ia supernovae (SNs Ia) in the nebular phase has been developed that includes the basic physics needed to reproduce the spectra. Intrinsic characteristics of nebular spectra of SNs Ia, such as the dependence of the observed luminosity of Fe III and Fe II forbidden lines at different wavelengths on the initial ^56^Ni mass and on the reddening, provide the basis for the multiparameter determination. It is shown that this procedure can yield estimates of the distance to the host galaxy, of the total reddening, and of the Ni mass synthesized in the explosion. The method, when applied to nebular spectra of SN 1986G in Cen A, gives as best values for the model parameters a production of 0.38 +/- 0.03 M_sun_ of ^56^Ni, a distance to Cen A (and thus to the Centaurus Group) of 3.3 +/- 0.3 Mpc, and a reddening E(B- V) of 1.09 +/- 0.02. These results are compared with determinations of the same quantities by traditional methods. In addition, the least-squares fitting indicates the presence of 0.039 +/- 0.003 M_sun_ of stable isotopes of Ni. The ^56^Ni mass derived for SN 1986G is a clue to the understanding of the peculiar properties of this supernova, suggesting that this event was intrinsically fainter than other SNs Ia. For comparison purposes, the case of SN 1972E is also discussed, although the least-squares technique is not applied.