On the radial distribution of horizontal branch stars in ngc2808

TL;DR

This study uses extensive ultraviolet and optical photometry to analyze the distribution of horizontal branch stars in NGC 2808, finding minimal radial variation among HB subpopulations but a significant radial gradient in red giant stars.

Contribution

It provides a detailed, minimally biased analysis of HB star distribution and radial gradients in NGC 2808 using combined ground-based and HST data.

Findings

No significant change in HB subpopulation ratios with radius.

A notable increase in the ratio of HB to RG stars from center to outskirts.

Absence of strong radial differentiation among helium-rich stellar populations.

Abstract

We present accurate new ultraviolet and optical BVI photometry for the Galactic globular cluster ngc2808, based on both ground-based and archival HST imagery. From this we have selected a sample of ~2,000 HB stars; given the extensive wavelength range considered and the combination of both high-angular-resolution and wide-field photometric coverage, our sample should be minimally biased. We divide the HB stars into three radial bins and find that the relative fractions of cool, hot and extreme HB stars do not change radically when moving from the center to the outskirts of the cluster: the difference is typically smaller than ~2sigma. These results argue against the presence of strong radial differentiation among any stellar subpopulations having distinctly different helium abundances. The ratio between HB and RG stars brighter than the ZAHB steadly increases when moving from the innermost to the outermost cluster regions. The difference is larger than ~4sigma and indicates a deficiency of bright RGs in the outskirts of the cluster.