Late Spectral Evolution of SN 1987A. I. Temperature and Ionization

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Abstract
The temperature and ionization of SN 1987A are modeled time-dependently in its nebular phase between 200 and 2000 days. We include all important elements, as well as the primary composition zones in the supernova. The energy input is provided by radioactive decay of 56Co,57Co, and 44Ti. The thermalization of the resulting gamma-rays and positrons is calculated by solving the Spencer-Fano equation. Both the ionization and the individual level populations are calculated time-dependently. Adiabatic cooling is included in the energy equation. Charge transfer is important for determining the ionization, and is included with available and estimated rates. Full, multilevel atoms are used for the observationally important ions. As input models for the calculations we use explosion models for SN 1987A calculated by Woosley et al. and Nomoto et al. The most important result in this paper concerns the evolution of the temperature and ionization of the various abundance zones. The metal-rich core undergoes a thermal instability, often referred to as the IR catastrophe, at 600-1000 days. The hydrogen-rich zones evolve adiabatically after 500-800 days, while in the helium region both adiabatic cooling and line cooling are of equal importance after ~1000 days. Freezeout of the recombination is important in the hydrogen and helium zones. Concomitant with the IR catastrophe, the bulk of the emission shifts from optical and near-IR lines to the mid- and far-IR. After the IR catastrophe, the cooling is mainly due to far-IR lines and adiabatic expansion. Dust cooling is likely to be important in the zones where dust forms. We find that the dust condensation temperatures occur later than ~500 days in the oxygen-rich zones, and that the most favorable zone for dust condensation is the iron core. The uncertainties introduced by the (in some cases) unknown charge transfer rates are discussed. Especially for ions with low abundances, differences can be substantial.
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