The circumstellar matter around many stars shows a remarkable axial symmetry. Famous examples comprise Supernova 1987A (Plait et al. 1995; Burrows et al. 1995), the Homunculus nebula around Carina and other nebulae around so called Luminous Blue Variables (Nota et al. 1995), and many planetary nebulae (Schwarz et al. 1992). A less spectacular example are B[e] stars, blue supergiants showing properties which might be well explained by a circumstellar disk (Gummersbach et al. 1995; Zickgraf et al. 1996). Many of these axisymmetric structures have been explained in terms of interacting winds of rotating stars (cf. Martin & Arnett 1995; Langer et al. 1998; García-Segura et al. 1998), which may be axisymmetric when the stars rotate with a considerable fraction of the break-up rate (Ignace et al. 1996; Owocki et al. 1996). However, up to now only little information is available about the evolution of the surface rotational velocity of stars with time, in particular for their post-main sequence phases.
Single stars which evolve into red giants or supergiants may be subject to a significant spin-down (Endal & Sofia 1979; Pinsonneault et al. 1991). Their radius increases strongly, and if the specific angular momentum were conserved in their surface layers (which may not be the case; see below) they would not only spin down but they would also evolve further away from critical rotation. Moreover, they may lose angular momentum through a stellar wind. Therefore, it may appear doubtful at first whether post-red giant or supergiant single stars can retain enough angular momentum to produce aspherical winds due to rotation.
However, by investigating the evolution of rotating massive single stars, we found that red supergiants, when they evolve off the Hayashi line toward the blue part of the Hertzsprung-Russell (HR) diagram may spin up dramatically, much stronger than expected from local angular momentum conservation. In the next section, we describe the spin-up mechanism and its critical ingredients. In Sect. 3 we present the results of evolutionary calculations for a rotating star, which provides a quantitative example for the spin-up. In Sect. 4 we discuss the relevance of our results for various types of stars, and we present our conclusions in Sect. 5.
© European Southern Observatory (ESO) 1998
Online publication: May 12, 1998