Pollution versus diffusion: Abundance patterns of blue horizontal branch stars in globular clusters NGC6388, NGC6397, and NGC6752

Abstract: Context: The metal-rich bulge globular cluster NGC6388 shows a blue horizontal branch (HB). Helium (He) enrichment, which is correlated with changes in other light elements, might explain this feature. The hot HB stars in the metal-poor globular clusters NGC6397 and NGC6752, instead, show high abundances of heavy elements caused by radiative levitation. Aims: I want to use the abundances of cool blue HB stars in NGC6388 to look for He-enrichment. To exclude effects of radiative levitation for NGC6388 and to investigate the abundance changes caused by radiative levitation, I analysed the blue HB stars in NGC6397 and NGC6752. Methods: I determined effective temperatures and surface gravities from UV-optical photometry and high-resolution spectra together with LTE model spectra. The results were used together with equivalent widths by the GALA program, to provide consistent atmospheric parameters and abundances. Results: For NGC6397 and NGC6752, only moderately hot HB stars were suitable for analysis with GALA. When including literature data, a large scatter is seen at the onset of radiative levitation, followed by increasing abundances up to about 13500 K (Si, Fe), then turning to a plateau (Si) and a forking (Fe) for higher temperatures. NGC6388-4113 shows variations in radial velocity, which may indicate binarity. The metal abundances of the remaining three blue HB stars are consistent with the products of hot hydrogen burning. The data were too noisy to determine He abundances directly. Conclusions: The presence of hot hydrogen burning products in the blue HB stars in NGC6388 could indicate He enrichment. The abundance variations with temperature in moderately hot HB stars in NGC6397 and NGC6752 suggest an influence of parameters beyond rotation and effective temperature.