Illuminating a red supergiant wind around SN 1987A

Abstract

A recently discovered light echo at a radius of about 9" from SN 1987A is interpreted as the result of a circumstellar shell at a radius of 4.5 pc from the supernova. The shell may have its origin in the deceleration of the presupernova red supergiant wind by the pressure of the surrounding medium. The required pressure is p/k ~ 10^3^ cm^-3^ K. The echo is predicted to expand and contract over the next 30 yr with a declining surface brightness and a broadening of its width. Colors of the echo compared to those of the larger echoes may show differences in the wavelength dependence of scattering of the circumstellar grains compared to interstellar grains. The evolution of the flux from the echo will give information on the average scattering phase function of the grains. A significant linear polarization of the echo light is expected, and the evolution of the polarization will give information on the dependence of polarization on scattering angle. The model predicts the presence of an undisturbed red supergiant wind inside of the echo shell. Supernova light scattered in the wind is expected to have half-intensity radius of 2" at present and may have been detected. The wind echo should expand as (t - t_e_), where t_e_ is the time of maximum light, and its flux should decline as (t - t_e_)^-2^.