Multiple ring nebulae around blue supergiants

Abstract

Context. In the course of the life of a massive star, wind-wind interaction can give rise to the formation of circumstellar nebulae which are both predicted and observed in nature. Aims. We present generic model calculations to predict the properties of such nebulae for blue supergiants. Methods. From stellar evolution calculations including rotation, we obtain the time dependence of the stellar wind properties and of the stellar radiation field. These are used as input for hydro-calculations of the circumstellar medium throughout the star’s life. Results. Here, we present the results for a rapidly rotating 12 M single star. This star forms a blue loop during its post main sequence evolution, at the onset of which its contraction spins it up close to critical rotation. Due to the consequent anisotropic mass loss, the blue supergiant wind sweeps up the preceding slow wind into an hourglass structure. Its collision with the previously formed spherical red supergiant wind shell forms a short-lived luminous nebula consisting of two polar caps and a central inner ring. With time, the polar caps evolve into mid-latitude rings which gradually move toward the equatorial plane while the central ring fades. These structures are reminiscent of the observed nebulae around the blue supergiant Sher 25 and the supernova 1987A. Conclusions. The simple model of an hourglass colliding with a spherical shell reproduces most of the intriguing nebula geometries discovered around blue supergiants, and suggests that they form an evolutionary sequence. Our results indicate that a binary system is not required to obtain them.