Mass loss along the red giant branch of the intermediate stellar populations in NGC 6752 and NGC 2808

Abstract

The morphology of the horizontal branch (HB) in globular clusters (GCs) offers some early evidence that they contain multiple populations of stars. Indeed, the location of each star along the HB depends both on its initial helium content (Y) and on the global average mass loss along the red giant branch (μ). In most GCs, it is generally straightforward to analyse the first stellar population (standard Y) and the most extreme one (largest Y), while it is more tricky to look at the ‘intermediate’ populations (mildly enhanced Y). In this work, we consider this segement for the GCs NGC 6752 and NGC 2808. When possible, the helium abundance for each stellar populations was constrained using independent measurements from the literature. We compared population synthesis models with photometric catalogues from the Hubble Space Telescope Treasury survey to derive the parameters of these HB stars. We find that the location of helium-enriched stars on the HB can be reproduced only by adopting a higher value of μ, with respect to the first -generation stars in all the analysed stellar populations. We also find that μ is correlated with the helium enhancement of the populations. This holds for both clusters. This finding is naturally predicted by the model of ‘pre-main sequence disc early loss’, suggested in the literature. It is also consistent with the findings of multiple-population formation models that foresee the formation of second-generation stars in a cooling flow.