Direct observation of radioactive cobalt decay in Supernova 1987A

The presence and decay of radioactive cobalt in the ejecta of supernova 1987A are inferred from the [Co II] 1.547-μm line which was observed between days 255 and 576 after the explosion. The cobalt/iron abundance ratio and its temporal behaviour are obtained by examination of the relative strengths of near-infrared spectral lines. This has the advantages of being relatively insensitive to uncertainties in temperature, A-values and optical depths. The presence of both 56Co and 57Co is revealed. This work provides the earliest evidence of 57Co in the ejecta of SN 1987A. The abundance ratios are found to be consistent with the predictions of explosive nucleo-synthesis calculations, and constrain the ultimate 57Fe/56Fe abundance ratio to between one and two times solar. (author)