The Most Metal-Poor Stars. II. Chemical Abundances of 190 Metal-Poor Stars Including 10 New Stars With [Fe/H] < -3.5

TL;DR

This study provides a homogeneous analysis of 16 elements in 190 metal-poor stars, including 10 newly identified stars with [Fe/H] < -3.5, revealing chemical peculiarities and potential multiple nucleosynthetic processes at low metallicity.

Contribution

It presents the largest homogeneous chemical abundance analysis of metal-poor stars, identifying chemically peculiar stars and suggesting multiple nucleosynthetic processes at the lowest metallicities.

Findings

Identification of chemically peculiar stars including CEMP-no objects

Evidence for two distinct nucleosynthetic processes at low metallicity

Detection of trends between [X/Fe] and Teff indicating non-LTE and 3D effects

Abstract

We present a homogeneous chemical abundance analysis of 16 elements in 190 metal-poor Galactic halo stars (38 program and 152 literature objects). The sample includes 171 stars with [Fe/H] < -2.5, of which 86 are extremely metal poor, [Fe/H] < -3.0. Our program stars include ten new objects with [Fe/H] < -3.5. We identify a sample of "normal" metal-poor stars and measure the trends between [X/Fe] and [Fe/H], as well as the dispersion about the mean trend for this sample. Using this mean trend, we identify objects that are chemically peculiar relative to "normal" stars at the same metallicity. These chemically unusual stars include CEMP-no objects, one star with high [Si/Fe], another with high [Ba/Sr], and one with unusually low [X/Fe] for all elements heavier than Na. The Sr and Ba abundances indicate that there may be two nucleosynthetic processes at lowest metallicity that are distinct from the main r-process. Finally, for many elements, we find a significant trend between [X/Fe] versus Teff which likely reflects non-LTE and/or 3D effects. Such trends demonstrate that care must be exercised when using abundance measurements in metal-poor stars to constrain chemical evolution and/or nucleosynthesis predictions.