Title:
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Late Spectral Evolution of SN 1987A: II. Line Emission

Author:
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Cecilia Kozma & Claes Fransson (Stockholm Observatory)

Abstract:
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Using the temperature and ionization  calculated in our previous
paper, we model
the spectral evolution of SN 1987A. We find that the temperature
evolution is
directly reflected in the time evolution of the lines. In particular,
the
IR-catastrophe is seen in the metal lines as a transition from thermal
to
non-thermal excitation, most clearly in the [O I]$\wll$6300, 6364
lines. The good agreement with observations clearly confirms the
predicted
optical to IR-transition. Because the line emissivity is independent
of
temperature in the non-thermal phase, this phase has a strong
potential for
estimating the total mass of the most abundant elements. 
The
hydrogen lines arise as a result of recombinations following
ionizations in the
Balmer continuum during the first $\sim$ 500 days, and as a result of
non-thermal ionizations later.

The distribution
of the different zones, and 
therefore the gamma-ray deposition, is determined from the line profiles
of the most important lines, where possible. We find that hydrogen
extends into the core to $\lesssim 700 \kms$. The hydrogen envelope has
a density 
profile close to $\rho \propto V^{-2}$ from $2000 - 5000 ~\kms$. The
total mass of hydrogen-rich gas is $\sim 7.7 \Msun$, of which $\sim
2.2 \Msun$ is mixed within 2000 $\kms$.
The helium mass derived from the line fluxes is sensitive to
assumptions about the degree of redistribution in the line.
The mass of the helium dominated
zone is consistent with $\sim 1.9 \Msun$, with a further $\sim 3.9 \Msun$
of helium residing in the hydrogen component. Most of the oxygen-rich gas
is confined to  400 -- 2000 $\kms$, with a total mass of $\sim 1.9
\Msun$. Because of uncertainties in the modeling of the non-thermal
excitation of the 
[O I] lines, the uncertainty in the oxygen mass is considerable. In
addition, masses of nitrogen, neon, magnesium, iron and nickel
are estimated. 

The dominant contribution to the line luminosity often
originates in a 
different zone from where most of the newly synthesized material
resides. This applies to e.g. carbon, calcium and iron. The [C I]
lines, mainly arising in the helium zone,
indicate a substantially lower abundance of carbon mixed with helium
than stellar evolution models give, and a more extended zone with CNO
processed gas is indicated. The [Fe II] lines
have in most phases a strong contribution from primordial iron, and at
$t \gtrsim 600 - 800$ days this component dominates the [Fe II] lines. The
wings of the [Fe II] lines may therefore come from primordial iron,
rather than synthesized iron mixed to high velocity. 
Lines from ions with low ionization potential indicate that the UV
field below at least $1600 $ \AA~ is severely quenched by dust
absorption and resonance scattering. 

To appear in:
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ApJ (Main Journal); scheduled for April 10, 1998, Vol. 497.