Astron. Astrophys. 324, 435-448 (1997)

6. Summary and conclusion

In Fig. 7, the mean [X/Fe] abundance for the six stars of our sample is plotted against atomic number (open circles joined by a solid line), together with the Arcturus values, joined by dotted lines. The error bar corresponds to the one sigma dispersion of the sample of the six SMC stars. The main effects which can be seen from this work are the following:
1. The mean iron abundance of the young population is confirmed to be around -0.7 dex. This value is compatible with previous studies of warmer supergiants (F stars) and with the analysis of B stars by Rolleston et al. (1993). 2. The scatter around the mean iron abundance is comparable with the expected error due to uncertainties in the effective parameters of the stars, so that there is no evidence of spread in abundance across various regions of the Cloud. This small dispersion holds for most elements. The data are even compatible with a very uniform metallicity for the K supergiants, implying a thorough mixing of the matter which formed the supergiants. A similar result was obtained for F supergiants in the LMC (Hill et al. 1995). Further, this is in agreement with the small abundance scatter among the H II  regions within each Cloud (Pagel et al. 1978; Russell & Dopita 1990)
3. Sodium is not strongly enhanced in these stars and seems to vary from star to star.
4. At variance with metal-poor stars in our Galaxy (represented here by Arcturus), the so-called -elements do not seem to be enhanced: only Mg could be slightly enhanced. This also confirms what was observed by previous studies of F supergiants: -elements are more abundant (relative to iron) in the LMC than in the SMC.
5. Nickel displays a noticeable depletion with respect to iron ([Ni/Fe] -0.3 dex), at variance with what is found in the SMC F supergiants and in Arcturus.
6. Except for PMMR 144, the s and r process elements heavier than Ba are strongly enhanced in all of our stars ( =+0.41 0.04 dex where stands for the mean of La, Ce, Nd and Eu), as previously found from F stars in both Clouds (enhanced by respectively 0.3 and 0.5 dex in the LMC and SMC).

Fig. 7. The mean abundance (relative to iron) of the nine SMC supergiant stars of our sample is plotted versus atomic number (open circles), for all the measured elements. The error bar is the one-sigma dispersion over the sample. Arcturus is also plotted as a Galactic reference, represented by filled squares and joined with dotted lines.

Since the abundance determination in supergiants is always a difficult process owing to the very extreme atmospheres one is dealing with (possible NLTE effects, etc...), it is indeed very nice to find that the analysis of stars with different temperature (namely F from previous studies and K in the present paper) yields the same results concerning both the absolute iron abundance and the abundance ratios characteristic of the Magellanic Clouds (such as heavy elements enhancement, -elements). The abundance of the light elements C, N, and O will be published elsewhere (Hill & Barbuy 1996) with further discussion of the implications of the relative abundances of elements on chemical evolution.

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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