Three faint, extremely metal-poor stars observed with Subaru include one with an unusually high magnesium-to-iron ratio. The star, SDSS J074238.68+470537.0, was classified as a CEMP-s star, a category marked here by high carbon and elevated barium. Its magnesium-to-iron ratio was 1.85 dex, which the authors describe as the highest value measured in a turn-off CEMP-s star. Its carbon abundance was 7.5, and its barium-to-iron ratio was reported at +1.37 dex. But the study covers only three stars, and the evidence does not settle how that chemical pattern formed or whether the star has a companion.
A small sample, examined in detail
The work was an exploratory chemical and kinematic study of three Sloan Digital Sky Survey candidates. All three were faint turn-off stars observed with the High Dispersion Spectrograph on Subaru. Their spectra covered 4084 to 6892 Å at a resolving power of about 40,000. For the motion analysis, the researchers used Gaia DR3 positions and proper motions, then calculated Galactic actions and integrated the stars' orbits for 1 Gyr.
To estimate the elements in each star, the team used a classical one-dimensional, local-thermodynamic-equilibrium analysis. It compared synthetic spectra with the observed lines using OSMARCS model atmospheres and Turbospectrum. The researchers also estimated departures from that equilibrium assumption for sodium, magnesium, strontium and barium; the reported corrections ranged from -0.2 to +0.2 dex.
All three were classified as genuine extremely metal-poor stars. The abstract reports iron-to-hydrogen metallicities from -3.02 to -3.57 dex, where dex is the logarithmic unit used for abundance ratios. Lithium was detected in SDSS J012630.30+073029.3 at 2.05 dex, while the other two stars had only upper limits.
One star stands out
The standout star's carbon measurement put it in the paper's high-carbon band. The other two could not be classified as either CEMP or carbon-normal because the available carbon measurements yielded only upper limits. This makes J0742 the only star in the trio for which the paper gives a definite CEMP classification.
The wider abundance pattern included calcium-to-iron ratios from +0.3 to 0.5 dex across the three stars. Strontium was detected in two stars, at -0.20 and +0.30 dex. Barium was +1.37 dex in J0742, while the other two yielded only upper limits.
The chemistry points to several possibilities
That pattern did not point to a single settled explanation. When the authors compared J0742 with yields from a primordial-star model, the best fit was a 60-solar-mass model selected from a 10-to-100-solar-mass range. The comparison used only five elements, however, and ordinary AGB models did not reproduce the star's very high magnesium abundance. The authors therefore present the model result as a comparison, not proof of a specific progenitor.
Another calculation examined whether mass transfer from a companion could account for the chemistry. Under that assumed binary scenario, the observed surface abundance is treated as a mixture of accreted material and the star's initial material. The authors estimated a dilution factor of about 0.9 and an accreted mass of about 10−4 solar masses. Those values depend on the adopted one-dimensional AGB and dilution assumptions.
Radial velocities offer a hint, but not a conclusion. The available measurements for J0742 differed by about 24 km/s, against a typical reported error of about 3 km/s. That gap is suggestive of a companion, yet the observations were too sparse to establish binarity or constrain the star's orbital parameters.
The stars' Galactic paths
The stars' Galactic paths also produced a split result. SDSS J214340.08−002835.5 was classified as a thick-disc star, with orbital eccentricity 0.19 and confinement to about ±4 kiloparsecs around the Galactic disc. J012630.30+073029.3 and J074238.68+470537.0 were classified as halo stars and candidate Gaia-Sausage-Enceladus members.
Those halo assignments remain kinematic candidates rather than chemically confirmed memberships. The paper does not treat the proposed Gaia-Sausage-Enceladus connections as settled.
A finding with clear limits
The study provides a detailed record of three stars, but it does not establish a population-wide abundance pattern. It does not establish that J0742 is a binary, prove that the two halo stars belong to Gaia-Sausage-Enceladus, determine the other two stars' carbon classification, or show that a 60-solar-mass progenitor produced J0742's pattern. Each interpretation remains open to further observations and model testing.
The clearest next test for the case that drew the most attention is long-term radial-velocity monitoring to determine whether J0742 is binary and constrain its orbit. The analysis also points to the need for more strontium and barium measurements in main-sequence and turn-off stars. For now, the most defensible conclusion is narrower: one of three Subaru-observed, extremely metal-poor candidates has an unusually magnesium-rich CEMP-s pattern, and both its origin and Galactic setting remain uncertain.
Paper data and sources
Original title: Abundance analysis of three new extremely metal-poor stars, including one CEMP-s star
Authors: P. François, T. Suda, S. Wanajo et al.
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-09-29
DOI: Not available
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