The Curious Radial Distributions of Horizontal Branch Stars in NGC 6441

TL;DR

This study investigates the spatial and color distribution of horizontal branch stars in globular cluster NGC 6441, revealing multiple stellar populations with distinct distributions and potential evolutionary implications.

Contribution

It provides new insights into the distributions of different stellar populations within NGC 6441, highlighting the complexity of horizontal branch star arrangements and their possible origins.

Findings

Brighter, bluer stars are more centrally concentrated.

Blue horizontal branch stars show a complex, less concentrated distribution.

Evidence suggests a helium-enriched second-generation population.

Abstract

NGC 6441 is one of the most massive and most metal-rich globular clusters in the galaxy, and is noted for an unusual extended horizontal branch which reaches past the instability strip. We find evidence that there are two different populations of stars within the heavily-populated red clump. Once a differential reddening correction is applied, a large but compact group of stars is found at the faint red end of the clump in the color magnitude diagram. Brighter, bluer stars in the clump are found to be more centrally concentrated within the cluster at very high level of significance. Curiously, the blue horizontal branch stars show a more complex distribution, and are not more centrally concentrated than the brighter red clump stars. The spatial distributions of clump stars is in agreement with the idea that the the brighter bluer part of the clump is a helium-enriched second generation. The blue horizontal branch stars may be showing evidence that they are being dynamically evaporated.