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Astron. Astrophys. 360, 120-132 (2000)

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6. Discussion

We find that the atmospheres of HBB stars in NGC 6752 with [FORMULA] [FORMULA] 11,500 K are enriched in iron ([Fe/H] [FORMULA]0.1) whereas their magnesium abundances are the same as found in cluster giants. Our results are consistent with those of Behr et al. (1999, 2000b) for HBB stars in M 13 and M 15. Using model atmospheres that try to take into account this enrichment in iron (and presumably other heavy elements) reconciles the atmospheric parameters of stars with 11,500 K [FORMULA] [FORMULA] [FORMULA] 15,100 K with canonical expectations as suggested by Grundahl et al. (1999). Also the masses derived from these analyses are in good agreement with canonical predictions within this temperature range. However, we found that even with model atmospheres as metal rich as [M/H] = +0.5 the atmospheric parameters of the hot HBB stars (15,300 K [FORMULA] [FORMULA] [FORMULA] 19,000 K) in NGC 6752 cannot be reconciled with the canonical ZAHB. Both the gravities and masses of these hot HBB stars remain too low. In addition, the masses for the stars below 15,100 K are slightly too high for the super-solar metallicity (the EHB stars are hardly affected at all by changes in the metallicity of the model atmospheres).

Michaud et al. (1983) noted that diffusion will not necessarily enhance all heavy elements by the same amount and that the effects of diffusion vary with effective temperature. Elements that were originally very rare may be enhanced even more strongly than iron (see also Behr et al. 1999, where P and Cr are enhanced to [M/H] [FORMULA]). The question of whether diffusion is the (one and only) solution to the "low gravity" problem cannot be answered without detailed abundance analyses to determine the actual abundances and the use of model atmospheres that allow the use of non-scaled solar abundances (like ATLAS12). We can, however, state that those model atmospheres, which reproduce the u-jump discussed by Grundahl et al. (1999) cannot completely reconcile the atmospheric parameters of hot HB stars with canonical theory. Model atmospheres with abundance distributions that may solve the discrepancy between theoretically predicted and observed atmospheric parameters of hot HB stars may then, in turn, not reproduce the Strömgren u-jump. It is intriguing that the temperature, at which the stars in [FORMULA] seem to return to the ZAHB, is roughly the same at which they start to deviate again from the canonical ZAHB in [FORMULA], [FORMULA] when analysed with metal-rich atmospheres.

The stars between 15,300 K and 19,000 K (when analysed with metal-rich atmospheres) are currently best fit by a moderately mixed ZAHB. However, the fact that their masses are too low cautions against identifying He mixing as the only cause for these low gravities - because in this case the luminosities of the stars would be increased and canonical masses would result.

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© European Southern Observatory (ESO) 2000

Online publication: July 27, 2000