SMSS J022423.27$-$573705.1: An Extremely Metal-Poor Star with the Most Pronounced Weak $r$-Process Signature

Abstract

We present the measurement of 26 elemental abundances of SMSS J022423.27$-$573705.1 (SMSS 0224$-$5737), an extremely metal-poor (EMP) star with a weak $r$-process signature. We report the measurements of N, O, V, Zn, and Ba, and the upper limits for Mo, Ru, Pd, Ag, and Eu for the first time. SMSS 0224$-$5737 exhibits low C abundance and high N and O abundances suggesting that C is converted to N by the enhanced mixing during the evolution. The abundance pattern up to the Fe-peak elements is generally in good agreement with the average abundance of EMP stars, although a notable feature is the high [Zn/Fe] ratio ([Zn/Fe] $= +0.88$). We confirm the enhancement of the first-peak neutron-capture elements (Sr, Y, and Zr) and determine a low Ba abundance [Ba/H] $= -5.25$, that is, [Ba/Fe] $= -1.45$. The extremely high ratio of [Zr/Ba] $= +2.60$ makes SMSS 0224$-$5737 the EMP star with the most pronounced weak $r$-process signature observed to date. The abundance pattern of the neutron-capture elements is compared with the yields from $r$-process nucleosynthesis models. The sharp decline in abundances beyond Zr disfavors neutron star merger or electron-capture supernova models, but are reproduced either by proto-neutron star wind models or by magneto-rotational supernova models. Considering the high [Zn/Fe] ratio, a magneto-rotational supernova is the most plausible origin of SMSS 0224$-$5737. This study demonstrates that the abundance measurements of both light and neutron-capture elements, even at low abundances, are crucial for unveiling the astrophysical sites of the weak $r$-process.