Helium abundance and asymmetry in the wind from the precursor to supernova 1987A

Abstract

TO explain why the progenitor of supernova 1987A exploded as a blue supergiant star, models of stellar evolution require that there must have been extensive mixing into the hydrogen-rich envelope of materials from deeper, helium-rich regions^1–3. Estimates of the helium abundance in the outer layers, which can be obtained from infrared spectroscopy of the stellar wind from the progenitor, can help to determine the extent of the mixing. The initial ultraviolet flash from the surface of supernova 1987A ionized the circumstellar gas to produce narrow emission lines, which were first seen at ultraviolet wavelengths⁴ where the supernova's photospheric emission quickly faded. Later, optical lines were also reported⁵. The ultraviolet lines indicated electron densities of ∼10⁴cm⁻³, far greater than expected for the wind shed by the precursor star, Sk –69 202, in either its blue supergiant or preceding red supergiant phase⁶. However, the interaction of the two winds could produce a shell of the requisite density^7,8. From observations of the infrared helium triplet at 1,083 nm we infer that the red supergiant wind was strongly asymmetric and that part of it had a high helium abundance.