SIMBAD references

Core helium burning is the dominant source of energy of extreme horizontal branch stars, as the hydrogen envelope is too small to contribute to the nuclear energy output. The evolution of each mass in the HR diagram occurs along vertical tracks that, when the core helium is consumed, evolve to higher T_eff and then to the white dwarf stage. The larger is the mass, the smaller is the T_eff of the models, so that the zero age horizontal branch (ZAHB) is `horizontal'. In this paper we show that, if the helium mass fraction (Y) of the envelope is larger than Y ∼ 0.5, the shape of the tracks changes completely: the hydrogen burning becomes efficient again also for very small envelope masses, thanks to the higher molecular weight and to the higher temperatures of the hydrogen shell. The larger is Y, the smaller is the envelope mass that provides strong H shell burning. These tracks have a curled shape, are located at a T<sub>eff</sub>following the approximate relation T<sub>eff</sub>= 8090+32900Y and become more luminous for larger envelope masses. Consequently, the ZAHB of the very high helium models is `vertical' in the HR diagram. Synthetic models based on these tracks nicely reproduce the location and shape of the `blue hook' in the globular cluster ω Cen, best fit by a very high T_eff(bluer) sequence with Y = 0.80 and a cooler (redder) one with Y = 0.65. Although these precise values of Y may depend on the colour-T_eff conversions, we know that the helium content of the progenitors of the blue hook stars cannot be larger than Y ∼ 0.38-0.40, if they are descendants of the cluster blue main sequence. Consequently, this interpretation implies that all these objects must in fact be progeny of the blue main sequence, but they have all suffered further deep mixing, that has largely and uniformly increased their surface helium abundance, during the red giant branch evolution. A late helium flash cannot be the cause of this deep mixing, as the models we propose have hydrogen-rich envelopes much more massive than those required for a late flash. We discuss different models of deep mixing proposed in the literature, and conclude that our interpretation of the blue hook cannot be ruled out, but requires a much deeper investigation before it can be accepted.