The Light Curve of SN 1987A Revisited: Constraining Production Masses of Radioactive Nuclides

We revisit the evidence for the contribution of the long-lived radioactive nuclides ⁴⁴Ti, ⁵⁵Fe, ⁵⁶Co, ⁵⁷Co, and ⁶⁰Co to the UVOIR light curve of SN 1987A. We show that the V-band luminosity constitutes a roughly constant fraction of the bolometric luminosity between 900 and 1900 days, and we obtain an approximate bolometric light curve out to 4334 days by scaling the late time V-band data by a constant factor where no bolometric light curve data is available. Considering the five most relevant decay chains starting at ⁴⁴Ti, ⁵⁵Co, ⁵⁶Ni, ⁵⁷Ni, and ⁶⁰Co, we perform a least squares fit to the constructed composite bolometric light curve. For the nickel isotopes, we obtain best fit values of M(⁵⁶Ni) = (7.1 ± 0.3) × 10^-2 M _⊙ and M(⁵⁷Ni) = (4.1 ± 1.8) × 10^-3 M _⊙. Our best fit ⁴⁴Ti mass is M(⁴⁴Ti) = (0.55 ± 0.17) × 10^-4 M _⊙, which is in disagreement with the much higher (3.1 ± 0.8) × 10^-4 M _⊙ recently derived from INTEGRAL observations. The associated uncertainties far exceed the best fit values for ⁵⁵Co and ⁶⁰Co and, as a result, we only give upper limits on the production masses of M(⁵⁵Co) < 7.2 × 10^-3 M _⊙ and M(⁶⁰Co) < 1.7 × 10^-4 M _⊙. Furthermore, we find that the leptonic channels in the decay of ⁵⁷Co (internal conversion and Auger electrons) are a significant contribution and constitute up to 15.5% of the total luminosity. Consideration of the kinetic energy of these electrons is essential in lowering our best fit nickel isotope production ratio to [⁵⁷Ni/⁵⁶Ni] = 2.5 ± 1.1, which is still somewhat high but is in agreement with gamma-ray observations and model predictions.