Explosive nucleosynthesis in SN 1987A. II. Composition, radioactivities, and the neutron star mass

We utilize the 20 M_⊙ model published by Nomoto and Hashimoto in 1988 with a 6 M_⊙ He core in order to perform explosive nucleosynthesis calculations. The employed explosion energy of 10⁵¹ ergs lies within the uncertainty range inferred from the bolometric light curve. The nucleosythesis processes and their burning products are discussed in detail. The results are compared with abundances from IR observations of SN 1987A and the average nucleosynthesis expected for Type II supernovae in Galactic chemical evolution. We predict the abundances of long-lived radioactive nuclei and their importance for the late light curve and gamma-ray observations. The position of the mass cut between the neutron star and the ejecta is deduced from the total amount of ejected ⁵⁶Ni (0.07±0.01 M_⊙). This requires a neutron star with a baryonic mass M_b = 1.6±0.045 M_⊙, which corresponds to a gravitational mass M_g = 1.43±0.05 M_⊙ after subtracting the binding energy of a nonrotating neutron star. This uncertainty range only covers errors in the observed values of ⁵⁶Ni and the explosion energy; uncertainties of the stellar model could increase this value up to M_b = 1.7 M_⊙ and M_g = 1.52 M_⊙.