The main result is that the relative fraction of Be stars with respect to all B stars in the spectral interval O9 to B3 is increasing for lower metallicities Z. This fact together with the much larger N-excesses observed in A-type supergiants of the SMC than in galactic supergiants strongly supports the suggestion that there are more fast rotators among massive stars at lower Z. Of course, direct observations of large numbers of in LMC and SMC clusters are very much needed in order to further substantiate the above results.
We may wonder about the origin of the possible higher rotation velocities at low metallicities. This origin is likely related to some metallicity effects in the process of star formation. There are many possibilities, for example we may notice that a lower Z implies less dust and ions in star forming regions. The coupling of the magnetic field to the matter is then weaker, the ambipolar diffusion of the magnetic field should proceed faster, thus leading to less angular momentum losses by the central contracting body. Another possibility is that during pre-main sequence evolution, the weaker opacities at lower Z are leading to an earlier disappearance of the external convective zones and thus to less magnetic coupling between the forming star and its surroundings. Also the importance and the survival lifetime of the accretion disk may increase with metallicity, thus favouring the dissipation of angular momentum at higher Z. Of course, numerical models are needed to examine these various tentative suggestions.
Looking ahead we may also mention that the consequence of faster rotation at lower Z may be considerable, with large differences in evolutionary tracks, lifetimes and nucleosynthesis. In particular we know that current models are unable to account for the occurrence of the large quantity of red supergiants in the SMC for which more mixing would be needed (cf. Langer & Maeder 1995). In the context of nucleosynthesis the possible formation of primary nitrogen at low Z is interesting in relation with the suggestion that primary nitrogen is needed in the early chemical evolution of galaxies (cf. Matteucci 1986; Pagel 1998). Appropriate stellar models would bring new insight into the interpretation of star populations in low Z galaxies, like blue compact galaxies or galaxies at cosmological distances like in the Hubble Deep Field.
© European Southern Observatory (ESO) 1999
Online publication: May 21, 1999