A search for heavy-metal stars: abundance analyses of hot subdwarfs with Subaru

TL;DR

This study investigates heavy-metal enrichment in hot subdwarfs using Subaru telescope data, revealing that extreme heavy-metal surface enrichment is less likely in helium-rich stars, thus informing stellar atmospheric chemistry and evolution.

Contribution

It provides new high-resolution abundance analyses of four hot subdwarfs, showing the relationship between helium enrichment and heavy-metal surface abundance.

Findings

Strong triply ionized lead lines in some subdwarfs.

Heavy-metal enrichment is suppressed in helium-rich stars.

Heavy-metal overabundances vary with helium surface composition.

Abstract

The discovery of extremely zirconium- and lead-rich surfaces amongst a small subgroup of hot subdwarfs has provoked questions pertaining to chemical peculiarity in hot star atmospheres and about their evolutionary origin. With only three known in 2014, a limited search for additional `heavy-metal' subdwarfs was initiated with the Subaru telescope. Five hot subdwarfs having intermediate to high surface enrichment of helium were observed at high-resolution and analyzed for surface properties and abundances. This paper reports the analyses of four of these stars. PG1559+048 and FBS 1749+373, having only intermediate helium enrichment, show strong lines of triply ionized lead. PG1559+048 also shows a strong overabundance of germanium and yttrium. With more helium-rich surfaces, Ton 414 and J17554+5012, do not show evidence of heavy-metal enrichment. This limited survey suggests that extreme enrichment of `heavy metals' by selective radiative levitation in hot subdwarf atmospheres is suppressed if the star is too helium-rich.