Astron. Astrophys. 346, L1-L4 (1999)

4. Conclusions

Our conclusions can be summarized as follows:

1. We have obtained new optical spectra of 32 hot HB stars in NGC 6752 with 11,000 K 25,000. When these spectra (together with older spectra of hotter stars) are analysed using model atmospheres with the cluster metallicity ([Fe/H] = -1.5), they show the same "low-gravity" anomaly with respect to canonical HB models, that has been observed in several other clusters (Moehler 1999).

2. For 18 stars with 11,500 K, we estimate a mean iron abundance of [Fe/H] 0.1, whereas magnesium is consistent with the cluster metallicity. The hot HB stars in NGC 6752 thus show an abundance pattern similar to that observed in M 13 (Behr et al. 1999), which presumably arises from radiative levitation of iron (Grundahl et al. 1999).

3. When the hot HB stars are analysed using model atmospheres with an appropriately high iron abundance, the size of the gravity anomaly with respect to canonical HB models is significantly reduced. Whether the remaining differences between observations and canonical theory can be attributed to levitation effects on elements other than iron remains to be investigated by detailed modeling of the diffusion processes in the stellar atmospheres. With presently available models, the derived gravities for HB stars hotter than  K are best fit by non-canonical HB models which include deep mixing of helium on the RGB (Sweigart 1999).

© European Southern Observatory (ESO) 1999

Online publication: May 6, 1999
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