SN 1987A: Optical Spectrophotometry 130–900 Days After Core Collapse

Abstract

SN 1987A in the Large Magellanic Cloud has provided an unprecedented opportunity to study the spectral characteristics and evolution of a Type II supernova at epochs >6 months after core collapse. The initial brilliance of type II supernovae at optical wavelengths is dictated in great measure by the compactness of the hydrogen envelope of the progenitor. Thus, the unusually low luminosity of SN 1987A during the first month or so after outburst was due primarily to the fact that its progenitor, Sk-69°202, was a blue rather than a red supergiant. For the same reason, the spectral evolution of SN 1987A at these early epochs was much faster than normal. However, within a few weeks of outburst, energy released from the radioactive decay of ⁵⁶Co began to power a long steady rise in the bolometric light curve, culminating in a broad maximum which was reached around day 90. By the beginning of the exponential decline phase in the bolometric light curve that followed, SN 1987A was no longer subluminous, indicating that the amount of mass 56 material synthesized in the explosion of Sk -69°202 must have been comparable to that produced in “normal” type II supernovae. Thus, in spite of the peculiarities early on, it is reasonable to expect that the late-time spectral evolution of SN 1987A should be representative of type II supernovae in general.