The Be stars are shown to be redder in V-I than main-sequence stars of similar MV. Furthermore, the displacement to the red increases with increasing H emission line strength. Some suggested causes for the redness of Be stars were examined using the widely accepted model that Be stars are main-sequence stars with a surrounding disk. Our photometric observations suggest that the redness of the stars is due to emission from the circumstellar envelope, in line with the results of detailed spectrophotometric studies based upon galactic samples.
The fraction of main-sequence stars that are Be stars varies significantly between the various clusters from . The average value is close to the average fraction of 0.17 seen in the Galaxy, and the maximum value is also similar to the maximum Galactic value, which occurs at spectral type B1.
It was noted that the cluster with the maximum Be star fraction, NGC 330, is distinctly elliptical, suggesting that the stars in it formed out of rapidly rotating material and that there is a connection between rapid rotation and the Be phenomenon.
No connection between Be star fraction and age or metallicity was evident. In some clusters, the Be stars tend to be concentrated toward the main-sequence turnoff. This is consistent with evolutionary models which predict that stars which form with rotation velocities more than 50% of the breakup value spin up as they evolve, reaching the breakup velocity over a moderate fraction of the main-sequence lifetime.
We thank Alistair Walker for making available his photometry prior to publication. SCK acknowledges the support of an Australian Postgraduate Award scholarship. We thank our referee Eva Grebel for her useful comments.
Copyright The European Southern Observatory (ESO)