Astron. Astrophys. 364, 455-466 (2000)

5. Summary and conclusions

Based on extensive Strömgren uvby photometry we have investigated four CCD fields, two in the LMC and two in the SMC. Because u-band observations were included it has been possible to study reddening variations on very small ( pc) scales.

From B stars we find that reddenings vary by several hundreths of a magnitude in within a CCD field. The reddening variations are random rather than smooth, so unless the reddening for a particular object is directly measured it will be uncertain by at least 0.035 mag in (HV 12578 field) and up to 0.07 mag (HV 11284 field). This translates directly into metallicity errors for red giants of 0.15 dex and 0.45 dex respectively, unless individual reddenings are available. We thus suggest that the u filter be included in LMC/SMC Strömgren photometry whenever possible. In particular, metallicity studies of young star clusters with a suitable number of B stars for reddening determinations would benefit strongly from the inclusion of band data.

Metallicities have been derived for GK giants. Within the 0.2 dex uncertainties, the average metallicities are largely consistent with those derived from spectroscopic studies of F and G supergiants, about [Fe/H] = -0.3 for the LMC and [Fe/H] = -0.65 for the SMC (Westerlund 1997), but we also see indications that a smaller number of more metal-poor stars with metallicities down to [Fe/H] = -2.0 are present in each of the fields. There are also hints of stars with higher metallicities, but this might be an effect of chemical peculiarities such as enhanced CNO abundances which are known to affect metallicities derived for red giants by means of Strömgren photometry in Galactic globular clusters (Dickens et al. 1979; Richter et al. 1999). Further follow-up spectroscopic studies are needed.

An unexpected byproduct of this study is the peculiar location of some early-type stars in the diagram. Comparison with stellar models shows that this may be partly due to the lower-metallicity environments in the LMC and SMC, combined with the effect of random photometric errors. However, spectroscopy of some of the stars with peculiar indices would be highly desirable in order to check if the early-type stars in the LMC/SMC show other physical differences with respect to stars in the Solar neighbourhood.

© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001
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