SIMBAD references

The colour magnitude diagram of M 15 shows a gap along the blue horizontal branch (BHB), separating the "classical" BHB from a long, almost vertical, blue tail. In order to clarify the nature of this blue tail, low and intermediate resolution optical spectra of stars above and below this gap are analysed for the atmospheric parameters T_eff and logg and spectroscopic masses are derived using ATLAS9 models of Kurucz. The blue tail extends to temperatures as high as 20000K, which is still considerably cooler than the sdB stars found in NGC 6752. The blue tail of M 15, thus, does not consist of sdB stars but of B stars similar to the Horizontal Branch B-type (HBB) stars in the field. A careful comparison with evolutionary models for the horizontal branch reveals that the surface gravities are systematically lower than predicted by canonical HB models even when luminosity evolution is accounted for. Also the resulting masses lie in general significantly below the values predicted by canonical HB theory. Non-canonical HB theory (i.e. the white dwarf merger scenario) is also found to be inconsistent with our results. A comparison with data for HB stars in other clusters shows that the contradictory results are not limited to M 15 but represent a common phenomenon. Systematic errors in photometric data and/or distances are highly unlikely to cause the observed discrepancies. An increase of the gravity by about 0.2dex would erase the discrepancies both for the logg-T_eff diagram as well as for the masses. However, an investigation into the analysis techniques as well as possible deficits in the model atmospheres for any systematic effects on the gravity determination was unsuccessful. We conclude that the stellar parameters derived with present day analysis techniques and model atmospheres lead for an as yet unknown reason to low surface gravities and thus low masses for globular cluster BHB stars.