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. The IR-catastrophe is seen in the metal lines as a transition from thermal to non-thermal excitation, most clearly in the [O I] 6300, 6364 lines. 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 the total mass of hydrogen-rich gas to be ~7.7 Msun. 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 Msun, with a further ~3.9 Msun of helium residing in the hydrogen component. 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 > 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.

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