The Star That Reveals a Hidden Cosmic Process: SMSS J0224−5737 and the Weak r-Process

In “SMSS J022423.27−573705.1: An Extremely Metal-Poor Star with the Most Pronounced Weak r-Process Signature,” Okada and collaborators investigate one of the oldest and most chemically primitive stars known. These “extremely metal-poor” (EMP) stars formed soon after the Big Bang and preserve records of the universe’s earliest chemical processes. The study focuses on how the elements heavier than iron were formed through the rapid neutron-capture process, or r-process. While the “main” r-process produces both light and heavy elements, a rarer and less-understood weak r-process creates only the lighter ones, such as strontium (Sr), yttrium (Y), and zirconium (Zr). SMSS J0224−5737 provides the clearest example of this weak r-process seen so far.

Observations and Data Analysis

The team used the European Southern Observatory’s Very Large Telescope (VLT) with the UVES spectrograph to collect high-resolution spectra of the star. These spectra act like fingerprints of elements, revealing which atoms are present through their light absorption lines. Using advanced computer models (including the iSpec software and Turbospectrum radiative transfer code), Okada measured the strengths of 26 elements in the star’s atmosphere. They also determined the star’s temperature (about 5040 K), surface gravity, and chemical composition under the assumption of local thermodynamic equilibrium. These details are essential for translating light patterns into accurate element abundances.

Results: A Unique Chemical Signature

SMSS J0224−5737 shows remarkably low carbon but high nitrogen and oxygen levels, implying that internal mixing inside the star has converted carbon into nitrogen. Up to the “iron peak” elements (like Fe, Co, and Ni), its composition looks typical for other EMP stars. However, the [Zn/Fe] ratio is unusually high (+0.88), indicating enrichment by a highly energetic supernova. The most striking feature is in the neutron-capture elements: Sr, Y, and Zr are strongly enhanced, while barium (Ba) and europium (Eu) are extremely low. The measured [Zr/Ba] = +2.60 is the highest ever recorded for such a star. This pattern, a steep drop-off after Zr, defines a weak r-process signature more extreme than in any previously studied star.

Discussion: Testing Cosmic Origin Theories

To understand where these elements came from, the authors compared their measurements with computer models of different astrophysical explosions. Neutron star mergers, though known r-process sites, were ruled out because they produce too many heavy elements. Electron-capture supernovae, which occur in smaller stars, also failed to match because they stop before forming barium. Two models remained: proto-neutron star (PNS) winds and magneto-rotational supernovae (MRSNe). The latter, a type of supernova powered by both rotation and magnetic fields, best reproduced SMSS J0224−5737’s element ratios, particularly its high zinc and light r-process elements.

The Role of Mixing and Stellar Evolution

The unusual nitrogen and oxygen abundances suggest that the star’s internal layers have mixed, converting earlier carbon into nitrogen during its evolution on the red giant branch. This mixing process is typical of “mixed” EMP stars, and helps explain why the star appears carbon-poor. When accounting for this, Okada finds that the total sum of carbon and nitrogen is consistent with other EMP stars, indicating that the star’s chemical fingerprint largely reflects the composition of the gas it was born from, not later contamination.

Conclusion: A Clue to the Early Universe’s Chemistry

Okada’s study establishes SMSS J0224−5737 as the clearest-known example of an EMP star shaped by the weak r-process. Its extraordinary [Zr/Ba] ratio and high zinc content point to an origin in a magneto-rotational supernova, where a rapidly spinning, magnetized star exploded in a jet-like burst. This work highlights how even faint chemical signals in ancient stars can reveal which types of stellar explosions enriched the early universe. As Okada concludes, measuring light and heavy element abundances, even at the limits of detectability, remains essential for uncovering where and how the first heavy elements were born.

Source: Okada