Non-local thermodynamic equilibrium effects in modelling of supernovae near maximum light

Supernovae (SNe), with their diversity of compositions, velocities, envelope masses, and interactions, are good testing grounds for probing the importance of non-local thermodynamic equilibrium (NLTE) in expanding atmospheres. In addition to treating H, He, Li I, O I, Ne I, Na I, and Mg II in NLTE, we use a very large model atom of Fe II to test the importance of NLTE processes in both Type Ia and Type II SNe. Since the total number of potential line transitions that one has to include is enormous (approximate to 40 million), approximations and simplifications are required to treat the problem accurately and in finite computer time. With our large Fe II model atom (617 levels and 13 675 primary NLTE line transitions) we are able to test several assumptions for treating the background opacity that are needed to obtain correct UV line blanketing, which determines the shape of near-maximum light supernova spectra. We find that, due to interactions within the multiplets, treating the background lines as pure scattering (thermalization parameter epsilon = 0) is a poor approximation, and that an overall mean value of epsilon similar to 0.05-0.10 is a far better approximation. This is true even in SNe Ia, where the continuum absorption optical depth at 5000 Angstrom (=tau(std)) is much less than 1. We also demonstrate that a detailed treatment of NLTE effects is required to determine properly the ionization states of both abundant and trace elements.