Iron, Cobalt, and Nickel in SN 1987A

During the first several weeks after the explosion of SN 1987A, Fe/Co/Ni clumps, containing ∼1% of the mass of the supernova envelope, absorbed most of the energy released by ⁵⁶Ni and ⁵⁶Co decay. As a result, the clumps expanded relative to the substrate, forming a "nickel bubble" of low-density Fe/Co/Ni. Later the clumps captured ∼10% of the radioactive luminosity of gamma rays and positrons. An analysis of the light curves of several infrared emission lines of Fe I, Fe II, Co I, Co II, Ni I, and Ni II in the spectrum of SN 1987A confirms these predictions. Assuming that these elements are not mixed microscopically with other elements, we find that the clumps must occupy > ≲30% of the volume of the emitting region (radial velocity ≲ 2500 km s^-1). We can account for the light curves (for 150 days ≲ t ≲ 2 yr) of all the emission lines of Fe, Co, and Ni with a model consisting of ∼60 to 100 identical clumps. The temperature decreases from T ≈ 4200 K at 200 days to T ≈ 600 K at 800 days. For t ≳ 2 yr, the predicted fluxes are considerably lower than the data. This result indicates that the emission at late times is dominated by an extra source of heating and ionization, most likely photoionization by two-photon continuum from metastable helium in the gas surrounding the clumps. The resulting "frothy" structure, consisting of bubbles of low-density Fe surrounded by higher-density filaments of H, He, and other elements, will persist and may be seen in the spectra and structure of supernova remnants.