Could the Ultra Metal-poor Stars be Chemically Peculiar and Not Related to the First Stars?

TL;DR

This paper investigates whether ultra metal-poor stars are genuinely primitive or if their peculiar elemental abundances are due to stellar surface effects, comparing their patterns to known chemically peculiar stars.

Contribution

It provides a comparative analysis suggesting that ultra metal-poor stars' abundances may reflect surface effects rather than primordial composition, challenging their status as first-generation stars.

Findings

Ultra metal-poor stars' abundances relate to condensation temperature.

Their true metallicities may be higher than observed, around [X/H] ~ -2 to -4.

Chemical peculiarities are likely due to dust-gas separation effects.

Abstract

Chemically peculiar stars define a class of stars that show unusual elemental abundances due to stellar photospheric effects and not due to natal variations. In this paper, we compare the elemental abundance patterns of the ultra metal-poor stars with metallicities [Fe/H] $\sim -5 $ to those of a subclass of chemically peculiar stars. These include post-AGB stars, RV Tauri variable stars, and the Lambda Bootis stars, which range in mass, age, binarity, and evolutionary status, yet can have iron abundance determinations as low as [Fe/H] $\sim -5$. These chemical peculiarities are interpreted as due to the separation of gas and dust beyond the stellar surface, followed by the accretion of dust depleted-gas. Contrary to this, the elemental abundances in the ultra metal-poor stars are thought to represent yields of the most metal-poor supernova and, therefore, observationally constrain the earliest stages of chemical evolution in the Universe. The abundance of the elements in the photospheres of the ultra metal-poor stars appear to be related to the condensation temperature of that element; if so, then their CNO abundances suggest true metallicities of [X/H]~ -2 to -4, rather than their present metallicities of [Fe/H] < -5.