Late-time optical spectra of SN1987A

Abstract

Optical study of late-time nebular emission provided the first direct evidence for the presence of radioactive ⁵⁶Co in the ejecta of Type Ia supernovae¹ and yielded conclusive evidence that Type Ib events constitute a physically distinct class of supernovae². Hydrogen-rich Type II supernovae have not been studied in detail in the nebular phase, but SN1987A provides an unprecedented opportunity to probe the structure and composition of the ejecta by modelling the nebular spectrum. Here we investigate the late-time nebular spectra of SN1987A by computing the emergent spectra from model atmospheres. The new features of this calculation are the inclusion of a complex model hydrogen atom and the self-consistent treatment of an opaque lower boundary which approximates the observed continuum. We adopt hydrodynamic models reported in the literature, which are known to reproduce the observed light curve and γ-ray and X-ray spectra, and we obtain qualitative agreement with observed optical spectra. By examining the effect of mixing core material into the envelope, we conclude that the metals in the hydrogen-rich envelope make a substantial contribution to the optical nebular lines even for models with no outward mixing of the core material; however, a significant fraction of the energy emitted in ⁵⁶Co decay must be deposited in the envelope to produce the observed linewidths and hydrogen line strengths.