LATE SPECTRAL EVOLUTION OF SN-1987A - I - TEMPERATURE AND IONIZATION

The temperature and ionization of SN 1987A are modeled time-dependentl y in its nebular phase between 200 and 2000 days. We include all impor tant elements, as well as the primary composition zones in the superno va. The energy input is provided by radioactive decay of Co-56, Co-57, and Ti-44. The thermalization of the resulting gamma-rays and positro ns is calculated by solving the Spencer-Fano equation. Both the ioniza tion and the individual level populations are calculated time-dependen tly, Adiabatic cooling is included in the energy equation. Charge tran sfer is important for determining the ionization, and is included with available and estimated rates. Full, multilevel atoms are used for th e 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 ev olution of the temperature and ionization of the various abundance zon es. The metal-rich core undergoes a thermal instability, often referre d to as the IR catastrophe, at 600-1000 days. The hydrogen-rich zones evolve adiabatically after 500-800 days, while in the helium region bo th adiabatic cooling and line cooling are of equal importance after si milar to 1000 days. Freezeout of the recombination is important in the hydrogen and helium zones. Concomitant with the IR catastrophe, the b ulb; 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 import ant in the zones where dust forms. We find that the dust condensation temperatures occur later than similar to 500 days in the oxygen-rich z ones, and that the most favorable zone for dust condensation is the ir on core. The uncertainties introduced by the (in some cases) unknown c harge transfer rates are discussed. Especially for ions with low abund ances, differences can be substantial.