Discovery of extremely lead-rich subdwarfs: does heavy metal signal the formation of subdwarf B stars?

TL;DR

This study reports the discovery of extremely lead-rich subdwarfs with unique surface chemistries, suggesting a transitional phase in the evolution of hot subdwarf stars and highlighting the role of heavy metals in their formation.

Contribution

The paper presents the first optical detection of near ten-thousand times solar lead abundance in hot subdwarfs, revealing heavily stratified atmospheres and new insights into their evolutionary processes.

Findings

Discovery of stars with unprecedented lead abundance

Identification of zirconium and yttrium in some subdwarfs

Evidence for atmospheric stratification and chemical transition

Abstract

Hot subdwarfs represent a group of low-mass helium-burning stars formed through binary-star interactions and include some of the most chemically-peculiar stars in the Galaxy. Stellar evolution theory suggests that they should have helium-rich atmospheres but, because radiation causes hydrogen to diffuse upwards, a majority are extremely helium poor. Questions posed include: when does the atmosphere become chemically stratified and at what rate? The existence of several helium-rich subdwarfs suggests further questions; are there distinct subgroups of hot subdwarf, or do hot subdwarfs change their surface composition in the course of evolution? Recent analyses have revealed remarkable surface chemistries amongst the helium-rich subgroup. In this paper, we analyse high-resolution spectra of nine intermediate helium-rich hot subdwarfs. We report the discovery that two stars, HE 2359-2844 and HE 1256-2738, show an atmospheric abundance of lead which is nearly ten thousand times that seen in the Sun. This is measured from optical Pb IV absorption lines never previously seen in any star. The lead abundance is ten to 100 times that measured in normal hot subdwarf atmospheres from ultraviolet spectroscopy. HE 2359-2844 also shows zirconium and yttrium abundances similar to those in the zirconium star LS IV-14 116. The new discoveries are interpreted in terms of heavily stratified atmospheres and the general picture of a surface chemistry in transition from a new-born helium-rich subdwarf to a normal helium-poor subdwarf.