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 is reproduced either by protoneutron star wind models or by magneto-rotational supernova models. Considering the high [Zn/Fe] ratio, a magnetorotational 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.