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

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 analysed for surface properties and abundances. This paper reports the analyses of four of these stars. PG 1559+048 and FBS 1749+373, having only intermediate helium enrichment, show strong lines of triply ionized lead. PG 1559+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.