Nickel, argon and cobalt in the infrared spectrum of SN1987A: the core becomes visible

Abstract

Infrared spectra of supernova 1987A taken in April and November 1987 from the Kuiper Airborne Observatory (KAO) show two distinctly different stages in the evolution of the expanding gas shell. In April both the optical and infrared spectrum originated from the hydrogen envelope, the still opaque remnant of the outer regions of the progenitor star's atmosphere. This region of the star was presumably of near solar chemical composition (although the elements heavier than helium are expected to have had an abundance from two to four times less than the Sun). Our April spectrum shows weak hydrogen lines rising above a 5,000-K photospheric continuum, in good agreement with this picture. By November, however, the spectrum of SN1987A had changed dramatically. Strong emission lines from heavy elements as well as many lines from highly excited levels of hydrogen dominated the spectrum with peak flux levels in the lines at or slightly above the level of the continuum in April. The intensity of these lines and the abundances of the heavy elements inferred from them demonstrate that the inner regions of the supernova are just now becoming visible at infrared wavelengths. These regfons are expected to contain heavy elements produced by advanced nuclear burning stages in the progenitor star (Sk–69 202) and in the shock wave that ejected all material external to the iron core. They are, of course, very deficient in hydrogen.