Mass loss along the red giant branch of the intermediate stellar populations in NGC6752 and NGC2808

TL;DR

This study investigates how mass loss on the red giant branch influences the horizontal branch morphology in globular clusters NGC6752 and NGC2808, revealing a correlation between helium enrichment and mass loss.

Contribution

It provides the first detailed analysis of intermediate helium populations' mass loss in these clusters, supporting models linking helium enrichment to increased mass loss.

Findings

Helium-enriched stars require higher mass loss to match observed HB positions.

Mass loss correlates with helium enhancement across multiple populations.

Results support models of early disc loss and second-generation star formation in globular clusters.

Abstract

The morphology of the Horizontal Branch (HB) in Globular Clusters (GC) is among the early evidences 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 ($\mu$). 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 do this for the GCs NGC6752 and NGC2808; wherever possible the helium abundance for each stellar populations is constrained by using independent measurements present in the literature. We compare 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 is reproduced only by adopting a higher value of $\mu$ with respect to the first generation stars in all the analysed stellar populations. We also find that $\mu$ correlates 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'', previously suggested in the literature, and is consistent with the findings of multiple-populations formation models that foresee the formation of second generation stars in a cooling flow.