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

TL;DR

This study investigates SMSS J022423.27$-$573705.1, an extremely metal-poor star with the most pronounced weak $r$-process signature to date. Using high-resolution VLT/UVES spectroscopy, the authors measure 26 elements, including $N$, $O$, $V$, $Zn$, and $Ba$, and set upper limits for several heavy species; they find [Zn/Fe] = $+0.88$, strong first-peak enhancements ([Sr/Fe], [Y/Fe], [Zr/Fe]) and a remarkably low [Ba/H] = $-5.25$ (hence [Ba/Fe] = $-1.45$) with [Zr/Ba] = $+2.60$. The neutron-capture pattern shows a sharp decline beyond Zr, disfavoring NSMs and ECSNe, while being reproduced by magneto-rotational supernovae (MRSN) or, less likely, proto-neutron-star winds; the high Zn also supports MRSN as the plausible origin. These results underscore the importance of measuring both light and neutron-capture elements, even at low abundances, to pinpoint weak $r$-process sites and inform models of early Galactic chemical enrichment.

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.

Figures (9)

• Figure 1: Spectrum of SMSS 0224$-$5737 (black points) and synthetic spectra for the CH (top), NH (middle), and OH (bottom) molecular lines. The best-fit synthetic spectrum (red solid line) and synthetic spectra offset by the total errors (red dashed lines) are shown for the NH and OH lines, whereas the synthetic spectrum with an upper limit (red solid line) is shown for the CH line. The blue solid line represents the synthetic spectrum, assuming a zero abundance of each element. The adopted elemental abundances are shown in each panel.
• Figure 2: Same as Figure \ref{['fig:spec_CNO']}, but for the spectrum of SMSS 0224$-$5737 and the synthetic spectra for the Ba2 lines at 4554 Å and 4934 Å. The best-fit abundance is [Ba/H] = $-5.25 \pm 0.23$.
• Figure 3: Same as Figure \ref{['fig:spec_CNO']}, but for the spectrum of SMSS 0224$-$5737 and the synthetic spectra for the Zn1 lines at 3345 Å and 4810 Å. The best-fit abundance is [Zn/H] = $-2.96 \pm 0.20$.
• Figure 4: Spectrum of SMSS 0224$-$5737 (black points) and synthetic spectra at the strongest transitions of the neutron-capture elements (Mo, Ru, Pd, Ag, and Eu). The blue solid lines show the synthetic spectra assuming a zero abundance for each element, whereas the red solid lines represent the synthetic spectra with an upper limit. The orange dashed horizontal lines indicate the $3 \sigma$ detection thresholds estimated from the local signal-to-noise ratio (S/N). No significant lines are detected, and only the upper limits are derived.
• Figure 5: Elemental abundance ratios [X/Fe] as a function of metallicity of elements from C to Eu. The grey points represent the abundance ratios of stars compiled by the SAGA database 2008PASJ...60.1159S. The elemental abundance ratios and upper limits of SMSS 0224$-$5737 are indicated by orange symbols. When the abundance estimates from the two ionization levels are available, the abundance ratios derived from singly-ionized ions are shown as open circles.