Metal Abundances of Subdwarf B Stars from SPY - a Pattern Emerges

TL;DR

This study analyzes the chemical compositions of 68 subdwarf B stars using high-resolution spectra, revealing patterns of elemental enrichment and depletion that inform models of their atmospheric processes and formation scenarios.

Contribution

It provides the largest detailed abundance analysis of sdB stars to date, identifying general trends and peculiar cases that challenge existing diffusion models.

Findings

Most heavy elements increase with temperature

Light elements like C, O, N are depleted regardless of temperature

Some stars show enrichment indicating nuclear processing

Abstract

The formation of sdBs is still puzzling, as is the chemical composition of their atmospheres. While helium and other light elements are depleted relative to solar values, heavy elements are highly enriched. Diffusion processes in the hot, radiative atmosphere of these stars are the most likely explanation. Although several attempts were made, it was not yet possible to model all the observed features of sdB atmospheres. A setback of most prior studies was the small sample size. We present a detailed abundance analysis of 68 sdBs. From high resolution spectra obtained with the VLT/UVES instrument in the course of the ESO Supernova Progenitor Survey (SPY) we measured elemental abundances of up to 24 different ions per star. A general trend of enrichment was found with increasing temperature for most of the heavier elements. The lighter elements like carbon, oxygen and nitrogen are depleted irrespective of the temperature. Although there is considerable scatter from one star to another, the general abundance patterns in most sdBs are similar. An interplay between gravitational settling, radiative levitation and weak winds is most likely responsible. About 6 % of the analysed stars show an enrichment in carbon and helium which cannot be explained in the framework of diffusion alone. Nuclear processed material must have been transported to the surface. The late hot-flasher scenario may provide a possible explanation for this effect.