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

Authors:
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Cecilia Kozma & Claes Fransson (Stockholm Observatory)

Abstract:
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The temperature and ionization of SN 1987A  is modeled between 200 and
2000 days in its nebular phase, using a
time-dependent model. We include all important elements, as well as
the primary composition zones in the supernova.
The energy input is provided by radioactive decay of \isotope{56}{Co},
\isotope{57}{Co}, and \isotope{44}{Ti}. 
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 to the calculations we use
explosion models
for SN 1987A calculated by Woosley \etal and Nomoto \etal

The most important result in this paper refers to 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 $\sim 1000$ days. Freeze-out 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 $\sim
500$ days in the oxygen-rich zones, and 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 of low abundances differences
can be substantial.

To appear in:
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ApJ (Main Journal); scheduled for April 1, 1998, Vol. 496.