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

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Abstract
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 nonthermal excitation, seen most clearly in the [O I] λλ6300, 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 nonthermal 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 ~500 days and later as a result of nonthermal ionizations. 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 700 km s-1. The hydrogen envelope has a density profile close to ρ ∝ V-2 from 2000-5000 km s-1. The total mass of hydrogen-rich gas is ~7.7 M☉, of which ~2.2 M☉ is mixed within 2000 km s-1. 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 ~1.9 M☉, with a further ~3.9 M☉ of helium residing in the hydrogen component. Most of the oxygen-rich gas is confined to 400-2000 km s-1, with a total mass of ~1.9 M☉. Because of uncertainties in the modeling of the nonthermal excitation of the [O I] lines, the uncertainty in the estimated oxygen mass is considerable. Masses of nitrogen, neon, magnesium, iron, and nickel are also estimated. The dominant contribution to the line luminosity often originates in a different zone from that in which most of the newly synthesized material resides. This applies to, e.g., carbon, calcium, and iron. The [C I] lines, arising mainly in the helium zone, indicate a substantially lower abundance of carbon mixed with helium than given by stellar evolution models, and a more extended zone with CNO-processed gas is also indicated. The [Fe II] lines have in most phases a strong contribution from primordial iron, and at t 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 potentials indicate that the UV field below at least 1600 Å is severely quenched by dust absorption and resonance scattering.
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