Abundance stratification in Type Ia supernovae - I. The case of SN 2002bo

The abundance stratification in the ejecta of the normal Type Ia supernova 2002bo is derived by fitting a series of spectra obtained at close time intervals. A Monte Carlo code, modified to include abundance stratification, is used to compute synthetic spectra at 13 epochs in the photospheric phase, starting 13d before B maximum. A description of the abundance distribution above 7600 km/s is thus obtained. Abundances in deeper layers, down to zero velocity, are derived from models of two nebular-phase spectra. Elements synthesized in different stages of burning are significantly, but not completely, mixed in the ejecta. A total 56Ni mass of 0.52M_sun is derived. Evidence for intermediate-mass elements at high velocities (> 18 000 km/s) is found, most clearly in Si II 6355A, in Ca II H&K, and in the Ca II infrared triplet. Carbon lines are not seen at any velocity, with possible implications for the progenitor/explosion scenario. A synthetic bolometric light curve computed using the inferred abundance distribution is in very good agreement with the observed one, providing an independent check. In particular, the fast rise of the light curve is reproduced very well. This is due to outward mixing of 56Ni, which is clearly well determined by the spectroscopic modelling.