COUPLED MASS AND ANGULAR-MOMENTUM LOSS OF MASSIVE MAIN-SEQUENCE STARS

We investigate the interaction of mass loss and rotation during core h ydrogen burning in massive stars. We compute their main sequence evolu tion assuming rigid rotation, and carry angular momentum as a passive quantity in the stellar interior but incorporate its effect on the ste llar mass loss rate. We consider the example of a 60M. star assuming v arious initial rotation rates. We show that rotation may substantially enhance the total main sequence mass loss of massive stars. Furthermo re, we argue that the surface layers of rotating massive main sequence stars may reach the limit of hydrostatic stability (''Omega-limit'') by achieving a considerable fraction of their Eddington luminosity. We show that this process is not catastrophic for the star, but rather t hat the coupling of mass and angular momentum loss limits the mass los s rate (M)over dot(Omega) of main sequence stars at the Omega-limit. ( M)over dot(Omega) is determined through the angular momentum loss impo sed by the R-limit rather than by atomic physics. For our 60M. sequenc es, it is (M)over dot(Omega) similar or equal to 10(-5) M. yr(-1). We find a convergence of the rotational velocities of main sequence stars of a given initial mass at the Omega-limit, but a strong dependance o f their mass at core hydrogen exhaustion from the initial rotation rat e. Since then also the post-main sequence evolution depends on the ini tial amount of angular momentum, we argue that this is a third indepen dent initial parameter for a the evolution of massive stars, as import ant as initial mass and metallicity. We briefly discuss observable con sequences of the coupling of mass and angular momentum loss, e.g. a si gnificant decline of the projected rotational velocity v sin i towards the cool end of the main sequence, a period of strongly enhanced and aspherical mass loss, disks or rings in the equatorial plane of the st ar reminiscent of B [e]-stars, and highly bipolar circumstellar struct ures.