Stellar evolution with rotation
V. Changes in all the outputs of massive star models
G. Meynet and
Received 15 March 2000 / Accepted 16 June 2000
Grids of models for rotating stars are constructed in the range of 9 to 120 at solar metallicity. The following effects of rotation are included: shellular rotation, new structure equations for non-conservative case, surface distortions, increase of mass loss with rotation, meridional circulation and interaction with horizontal turbulence, shear instability and coupling with thermal effects, advection and diffusion of angular momentum treated in the non-stationary regime, transport and diffusion of the chemical elements.
Globally we find that for massive stars the effects of rotation have an importance comparable to those of mass loss. Due to meridional circulation the internal rotation law rapidly converges, in 1-2% of the MS lifetime, towards a near equilibrium profile which then slowly evolves during the MS phase. The circulation shows two main cells. In the deep interior, circulation rises along the polar axis and goes down at the equator, while due to the Gratton-Öpik term it is the inverse in outer layers. This external inverse circulation grows in depth as evolution proceeds. We emphasize that a stationary approximation and a diffusive treatment of meridional circulation would be inappropriate. After the MS phase, the effects of core contraction and envelope expansion dominate the evolution of the angular momentum.
The surface velocities decrease very much during the MS evolution of the most massive stars, due to their high mass loss, which also removes a lot of angular momentum. This produces some convergence of the velocities, but not necessarily towards the break-up velocities. However, stars with masses below 12 with initially high rotation may easily reach the break-up velocities near the end of the MS phase, which may explain the occurrence of Be-stars. Some other interesting properties of the rotational velocities are pointed out.
For an average rotation, the tracks in the HR diagram are modified like a moderate overshoot would do. In general, an average rotation may increase the MS lifetime up to about 30%; for the helium-burning phase the effects are smaller and amount to at most 10%. From plots of the isochrones, we find that rotation may increase the age estimate by about 25% in general. However, for stars with M 40 and fast rotation, a bluewards "homogeneous-like" track, with important He- and N-enrichments, may occur drastically affecting the age estimates for the youngest clusters. Rotation also introduces a large scatter in the mass-luminosity relation: at the same and , differences of masses by 30% may easily occur, thus explaining what still remains of the alleged mass discrepancy.
Rotation also brings significant surface He- and N-enhancements, they are higher for higher masses and rotation. While it is not difficult to explain very fast rotators with He- and N-excesses, the present models also well account for the many OB stars exhibiting surface enrichments and moderate or low rotation, (cf. Herrero et al. 1992, 2000). These stars likely result from initially fast rotators, which experienced mixing and lost a lot of angular momentum due to enhanced mass loss. The comparison of the N-excesses for B- and A-type supergiants supports the conclusion by Venn (1995a, 1999), that these enrichments mostly result from mixing during the MS phase, which is also in agreement with the results of Lyubimkov (1996).
Key words: stars: rotation stars: evolution stars: abundances stars: early-type
Send offprint requests to: G. Meynet (Georges.Meynet@obs.unige.ch)
© European Southern Observatory (ESO) 2000
Online publication: September 5, 2000