Helium enhancements in globular cluster stars from AGB pollution

Amanda Karakas

Some globular clusters such as NGC 2808 and M13 have extended blue horizontal branches that possibly result from the bluest, hottest stars having an enhanced amount of He (0.28 to 0.40) compared to the bulk of the stars in the cluster which have a primordial He abundance. The He is hypothesized to come from a low-metallicity population of intermediate mass AGB stars with initial masses in the range 4 to 6 solar masses.

We test this hypothesis using a chemical evolution model originally designed for NGC 6752 that uses AGB yields tailor made for this cluster. We follow the He, CNO and heavy element content of the gas and obtain He mass fractions of 0.29 but no higher. We test our model using AGB yields from Ventura et al. (2002) and modify the initial mass function to increase the number of stars with masses around 5 solar masses. We present the results from these various tests and discuss the uncertainties and implications of our findings.