ANALYSIS OF THE TYPE IA SUPERNOVA SN 1994D

We present an analysis of the observed light curves and spectra of the Type Ia supernova SN 1994D in the galaxy NGC 4526. The sensitivity of theoretical light curves and spectra to the underlying hydrodynamical model is discussed. The calculations are consistent with respect to t he explosion mechanism, the optical and infrared light curves, and the spectral evolution, leaving the description of the nuclear-burning fr ont and the structure of the white dwarf as the only free parameters. The explosions are calculated using a one-dimensional Lagrangian code including a nuclear network (Khokhlov 1991). Subsequently, the light c urves are constructed. Spectra are computed for several instants of ti me using the density, chemical, and luminosity structure resulting fro m the light-curve code. Our non-LTE (NLTE) code solves the relativisti c radiation transport equations in a comoving frame consistently with the statistical equations and ionization due to gamma-radiation for th e most important elements (He, C, O, Ne, Na, Mg, Si, S, Ca, Fe). About 300,000 additional lines are included, assuming LTE-level populations and an equivalent-two-level approach for the source functions. We fin d that the classical two-level approach underestimates thermalization processes by several orders of magnitude. Besides models already discu ssed in previous papers, a new series of delayed detonations has been included with a Ni-56 production ranging from similar or equal to 0.2 up to 0.7 M. depending on the density at which the transition from a d eflagration to a detonation occurs. The visual magnitude at maximum li ght M(V) ranges from approximate to -18.4 to approximate to -19.5 mag. Only one model with M(V) = -19.39 mag shows good agreement with the o bservations of SN 1994D both for B, V, R, and I colors and the spectra l evolution. The deflagration velocity is dose to the laminar deflagra tion (upsilon = 0.03c(s)), and the transition from the deflagration to the detonation occurs at rho(tr) = 2 x 10(7) g cm(-3). The initial ce ntral density of the white dwarf is 2.7 x 10(9) g cm(-3) i.e., about 2 0% lower than in our delayed detonation models previously considered. The lower density may de understood in terms of a higher accretion rat e on the progenitor. During the explosion, 0.6 M. of Ni-56 produced. T he need to reduce Ti in the outer layers becomes evident from the spec tral fits. This may be explained by small-scale density fluctuations d uring the explosion or by different primordial metallicity in the expl oding white dwarf. The distance to SN 1994D is determined to 16.2 +/- 2 Mpc. The explosion took place between 1994 March 3 and 4.