Models for circumstellar nebulae around red and blue supergiants

Abstract

In this thesis, we model the circumstellar medium of stars with initial masses of 8, 12, 18 and 20 solar masses, over their entire life from the main sequence until their supernova explosion. During the post-main-sequence stages, stars can evolve through several blue and red supergiant stages depending on their initial mass, composition and rotation rate. The models considered in the second Chapter have long-lasting RSG stages starting after the MS. In this phase, they develop shells of RSG wind material at the location where the free streaming RSG wind is stalled by the thermal pressure of the hot MS bubble, close to the central star. The RSG shells develop violent Rayleigh-Taylor instabilities. Once these start to grow non-linear, the RSG shell becomes highly structured as clumps form, and shell material mixes with material in the hot bubble. Later, the stars evolve to the BSG stage, during which the RSG shells are completely destroyed. These models return to the RSG stage, and build new RSG shells, which are more massive than those formed earlier. RSG shells are essential for our understanding of bipolar emission nebulae around BSGs. In the third Chapter are shown the results of the wind-wind interaction model of single star with 12 solar masses. On a time scale of a few 10000 yr, a BSG hour-glas shaped nebula expands into the sphere defined by the RSG shell. The faster polar parts of the hour glass hit the inner edge of the RSG shell first. The collision creates a pair of hot and dense polar caps. As time passes, the collision zone moves to lower latitudes of the RSG shell and becomes more confined in latitude. At the same time, the interaction of the BSG wind with the equatorial disk defines a second, ring shaped collision zone in the equatorial plane. These structures are reminiscent of the observed nebulae around the blue supergiant Sher 25. In the Chapter 3 we present calculations that predict the properties of the circumstellar medium for rapidly rotating and slowly rotating blue supergiants. In their post-main-sequence phases, the stars with 10 and 12 solar masses undergo a blue-loop following a first red supergiant stage. Before they explode as supernovae, the stars go back for a second time to the red supergiant stage. In case of a rapidly rotating central star, the collision of an hour-glass shaped shell with a stationary red supergiant shell forms a highly structured rotationally symmetric nebula. In contrast: if the star rotates slowly, a spherical nebula may form. The last Chapter shows preliminary results of possible ways for the formation of the outer rings observed around SN 1987A nebula through wind-wind interaction. At the end of the main sequence stage, our star model goes through a first blue supergiant post-main sequence stage becoming later a red supergiant. Before exploding as a supernova, the progenitor enters a second blue supergiant phase. During this phase the star reaches critical rotation. Due to the non-spherical wind of the first BSG phase, a shell of wind material is broken into two parts, with a high density ring-shaped intersection at mid-latitudes, which has some similarity to the outer circumstellar rings of SN 1987A.