NLTE Effects in Modeling of Supernovae near Maximum Light

Abstract

Supernovae, with their diversity of compositions, velocities, envelope masses, and interactions are good testing grounds for probing the importance of 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 SNe Ia and II. Since the total number of potential line transitions that one has to include is enormous ($\approx 40$ 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, 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 $ε= 0$) is a poor approximation and that an overall mean value of $ε\sim 0.05 - 0.10 $ is a far better approximation. This is true even in SNe Ia, where the continuum absorption optical depth at 5000 Å ($\equiv τ_{\rm std}$) is $<< 1$. We also demonstrate that a detailed treatment of NLTE effects is required to properly determine the ionization states of both abundant and trace elements.