A possible formation channel for blue hook stars in globular cluster

Abstract

The formation mechanism for blue hook (BHk) stars in globular clusters (GCs) is still unclear. Following one of the possible scenario, named late hot flash scenario, we proposed that tidally enhanced stellar wind in binary evolution may provide the huge mass loss on the red giant branch (RGB) and produce BHk stars. Employing the detailed stellar evolution code, Modules for Experiments in Stellar Astrophysics (MESA), we investigated the contributions of tidally enhanced stellar wind as a possible formation channel for BHk stars in GCs. We evolved the primary stars with different initial orbital periods using the binary module in MESA (version 6208) from zero age main-sequence (ZAMS) to post horizontal branch (HB) stage, and obtained their evolution parameters which are compared with the observation. The results are consistent with observation in the color-magnitude diagram (CMD) and the logg-Teff plane for NGC 2808, which is an example GC hosting BHk stars. However, the helium abundance in the surface for our models is higher than the one obtained in BHk stars. This discrepancy between our models and observation is possibly due to the fact that gravitational settling and radiative levitation which are common processes in hot HB stars are not considered in the models as well as the fact that the flash mixing efficiency may be overestimated in the calculations. Our results suggested that tidally enhanced stellar wind in binary evolution is able to naturally provide the huge mass loss on the RGB needed for late hot flash scenario and it is a possible and reasonable formation channel for BHk stars in GCs.