2025A&A...693A.136M

Context. The metal-rich ([M/H] = -0.48) bulge globular cluster NGC 6388 displays a blue horizontal branch (HB). Helium enrichment, which is correlated with changes seen among other light elements, might explain this feature. Conversely, the hot HB stars in the metal-poor globular clusters NGC 6397 and NGC 6752 display high abundances of heavy elements caused by radiative levitation.
Aims. I want to determine the abundances of cool blue HB stars in NGC 6388 to verify whether they are helium-enriched. To exclude the effects of radiative levitation for NGC 6388 and to investigate the abundance changes caused by radiative levitation, I analysed the blue HB stars in NGC 6397 and NGC 6752.
Methods. I obtained high-resolution spectra for all three clusters. I then determined the effective temperatures and surface gravities from UV-optical photometry (NGC 6388) and the spectra (NGC 6397, NGC 6752) together with local thermodynamic equilibrium (LTE) model spectra. The results were used together with equivalent widths by the GALA program, which provides consistent atmospheric parameters and abundances.
Results. For NGC 6397 and NGC 6752, only moderately hot HB stars were suitable for analysis with GALA. When including the literature data, a large scatter is seen at the onset of radiative levitation, followed by increasing abundances up to about 13 500 K (Si, Fe), then turning to a plateau (Si) and a forking (Fe) for higher temperatures. In NGC 6388, the star 4113 shows variations in radial velocity, which may indicate binarity. For the remaining three blue HB stars, the metal abundances are consistent with the products of hot hydrogen burning. The data were too noisy to allow for the helium abundances to be measured.
Conclusions. The presence of hot hydrogen burning products in the blue HB stars in NGC 6388 could indicate helium enrichment. The abundance variations with temperature in moderately hot HB stars in NGC 6397 and NGC 6752 suggest an influence of parameters beyond rotation and effective temperature.