The peculiar Horizontal Branch of NGC 2808

TL;DR

This study analyzes the complex Horizontal Branch of NGC 2808, revealing multiple sub-populations with different helium abundances and identifying the presence of hot-flasher stars at the hot end of the distribution.

Contribution

It provides a detailed observational and theoretical analysis of NGC 2808's HB, highlighting the need for multiple helium populations and identifying hot-flasher stars, which previous models could not explain.

Findings

Multiple HB sub-populations with different helium abundances

Canonical models cannot explain the hottest HB stars

Identification of hot-flasher stars at the blue-hook region

Abstract

We present an accurate analysis of the peculiar Horizontal Branch (HB) of the massive Galactic globular cluster NGC 2808, based on high-resolution far-UV and optical images of the central region of the cluster obtained with HST. We confirm the multimodal distribution of stars along the HB: 4 sub-populations separated by gaps are distinguishable. The detailed comparison with suitable theoretical models showed that (i) it is not possible to reproduce the luminosity of the entire HB with a single helium abundance, while an appropriate modeling is possible for three HB groups by assuming different helium abundances in the range 0.24 < Y < 0.4 that are consistent with the multiple populations observed in the Main Sequence; (ii) canonical HB models are not able to properly match the observational properties of the stars populating the hottest end of the observed HB distribution, the so called "blue-hook region". These objects are probably "hot-flashers" , stars that peel off the red giant branch before reaching the tip and ignite helium at high effective temperatures. Both of these conclusions are based on the luminosity of the HB in the optical and UV bands and do not depend on specific assumptions about mass loss.