3. Interpretation of the results
There are various possibilities to interpret the above results.
The first possibility is very unlikely. In the Milky Way Be stars have generally been observed over a longer period than in the LMC or SMC. Therefore, the intermittent Be star have a higher chance to be detected in the Milky Way. Consequently, the correction of this possible effect would increase the trend observed in Fig. 2 rather than reduce it. Indeed, in the SMC cluster NGC 330 the Be phenomenon has been found to be intermittent in a number of stars re-observed over the period of several years (Grebel 1995, Keller et al. 1999). The second hypothesis also seems difficult to sustain, because at higher metallicities the ejected shell and circumstellar material would be more opaque and contain more dust, thus their visibility through emission lines as well as by their infrared continuum would be enhanced. Thus, the correction of such an effect (if feasible) would increase the observed trend.
The same kind of remark holds for the third hypothesis. Models of rotating stars with mass loss (Maeder 1999a) show two effects: a) polar ejection is favoured in rotating stars by the higher at the pole; b) equatorial ejection of a shell is favoured by larger opacities. Thus, at lower Z, it is not expected that shell ejection is favoured in the equatorial regions of a rotating star, because the opacities are lower. The possible effect of metallicity on the terminal velocities, which may follow from the wind-compressed disk model (Bjorkman & Cassinelli 1993; Grebel 1997), is found to be negligible (Maeder 1999a).
Thus we are left with the possibility that the higher fraction of Be stars at lower Z is the signature of more fast rotators at lower Z. Obviously, direct measurements in nearby galaxies are very needed and envisaged in order to confirm or reject this suggestion. For NGC 330 measurements of projected rotational velocities () by Mazzali et al. (1996) and Keller & Bessell (1998) show values of up to 400 km s-1. Furthermore, a recent study of nitrogen abundances done by Venn (1995) offers a strong support to the possibility of faster rotation in NGC 330 of the SMC.
Indeed, several authors have found that B- and A-type supergiants in the Galaxy, LMC and SMC show nitrogen enhancements not predicted by standard evolutionary models (cf. Lennon et al. 1996; Fitzpatrick & Bohannan 1993; Venn 1995; Venn et al. 1998). The current interpretation of these surface enhancements requires some additional mixing processes, possibly rotational mixing (cf. Langer 1998; Meynet 1998; Maeder & Zahn 1998). New rotating models show consistently some N-enrichments already present for medium rotation from the end of the MS onwards (Meynet 1998). A remarkable point is that part of the N-enhancement observable in B- and A-type supergiants could be of primary origin, due to the fact that some new 12C resulting from the 3 burning is rotationally diffused into the H-burning shell, where the CNO cycle converts this "new 12C" into 14N, which is thus of primary origin.
Another interesting fact recently found by Venn et al. (1998) is that the degree of N enhancement for A supergiants is much higher in the SMC than in the Milky Way. For galactic supergiants the typical enhancement of [N/H] is about a factor of 2, while in the SMC it reaches a factor of 10-20 with respect to the standard local average [N/H] in the SMC. As a matter of fact, converting the 12C entirely into 14N by the end of the CN cycle (which is dominant) would lead to an increase of [N/H] by a factor of 4-5, but not as high as 20. Thus the high [N/H] enhancement observed for SMC supergiants might be a sign of primary nitrogen. We do not know, the only way to clarify is to measure the sum of CNO elements and check whether it is the same or not in supergiants as in MS stars. Anyhow, whether this is primary or secondary nitrogen, the point for now is that this higher [N/H] is the signature of much more mixing in the SMC than in the Galaxy, a fact which is quite consistent with the above suggestion of faster rotation for massive stars in the SMC. The possibility of faster rotation in the SMC has also been mentioned by Venn et al. (1998) and the results presently available on the Be star fractions as a function of Z clearly support this view.
© European Southern Observatory (ESO) 1999
Online publication: May 21, 1999