SMSS J022423.27-573705.1: Weak r-Process Signature

• The paper demonstrates that SMSS J022423.27-573705.1 exhibits an extreme weak r-process signature, with a record [Zr/Ba] ratio of +2.60, based on high-resolution spectroscopy.
• Methodologies combining SMSS photometry and Gaia astrometry with detailed spectral synthesis robustly determine its atmospheric parameters despite noted photometric-spectroscopic discrepancies in [Fe/H].
• Its unique abundance pattern, featuring enhanced Sr, Y, Zr, and Zn, supports the scenario of a jet-driven magneto-rotational supernova as a key early nucleosynthetic event in the Galactic halo.

SMSS J022423.27$-$573705.1 (also SMSS 0224$-$5737) is a chemically peculiar extremely metal-poor star in the Galactic halo, notable for exhibiting the most pronounced signature of the weak $r$-process among known metal-poor stars. Identified through the SkyMapper Southern Survey (SMSS) and cross-matched with Gaia DR3, the star has been the subject of extensive photometric, astrometric, and high-dispersion spectroscopic follow-up. Its exquisite abundance pattern and atmospheric parameters offer key insights into the early nucleosynthetic processes of the Milky Way and the nature of the first explosive transients (Huang et al., 12 Mar 2025, Okada et al., 30 Nov 2025).

1. Astrometry, Photometry, and Survey Identifiers

SMSS J022423.27$-$573705.1 is cataloged in SMSS DR4 and Gaia DR3 as follows:

Survey	Identifier	Value
SMSS	SMSS J022423.27$-$573705.1	Short name: SMSS 0224$-$5737
Gaia DR3	Source ID	4786099460583745920
ICRS (J2000)	RA	02h 24m 23.27s
ICRS (J2000)	Dec	$-57^\circ$ 37' 05.1''

Photometry (corrected for $E(B-V) = 0.045$) from SMSS DR4 yields: $u_0 = 16.674 \pm 0.020$, $v_0 = 16.512 \pm 0.020$, $g_0 = 15.741 \pm 0.010$, $r_0 = 15.413 \pm 0.010$, $i_0 = 15.219 \pm 0.010$, $z_0 = 15.154 \pm 0.010$ (all on the re-calibrated SMSS/AB scale) (Huang et al., 12 Mar 2025). Gaia astrometry gives parallax $\varpi = 0.123 \pm 0.020$ mas and proper motions $\mu_{\alpha*} = -5.42 \pm 0.03$ mas yr$^{-1}$, $\mu_\delta = +2.21 \pm 0.03$ mas yr$^{-1}$ (Huang et al., 12 Mar 2025).

2. Stellar Atmospheric and Physical Parameters

Atmospheric parameters from high-resolution VLT/UVES LTE analyses and SMSS+Gaia photometric pipelines are broadly consistent. The star's established properties are:

Parameter	Recent High-Resolution Spectroscopy (Okada et al., 30 Nov 2025)	SMSS DR4+Gaia DR3 (Huang et al., 12 Mar 2025)
$T_{\rm eff}$ (K)	$5039$	$5100 \pm 100$
$\log g$ (cgs)	$1.96$	$2.0$ (photometric/isochoric)
Fe/H	$-3.80$ (Fe I/II, LTE)	$-2.30 \pm 0.12$ (photometric)
Distance (kpc)	—	$\sim 8$ (from Gaia parallax)

Photometric metallicity ([Fe/H]) derived from polynomial fits to SMSS+Gaia photometry consistently classifies the star as "very metal-poor" (VMP: $[\mathrm{Fe/H}] \leq -2.0$) (Huang et al., 12 Mar 2025). High-resolution spectroscopic analysis, however, places it among the "extremely metal-poor" (EMP: $[\mathrm{Fe/H}] \leq -3.0$) subset, reflecting the well-known offset between photometric and spectroscopic iron abundance scales in halo giants.

3. Chemical Abundance Pattern and Weak $r$-Process Signature

Comprehensive abundance analysis with VLT/UVES blue and red spectra ($R \sim 40,000$--$50,000$) yields measurements for 26 elements (Okada et al., 30 Nov 2025). The defining features of SMSS 0224$-$5737's abundance pattern are:

• Carbon: $A(\mathrm{C}) < 4.33$, implying $[\mathrm{C}/\mathrm{Fe}] < -0.31$
• Nitrogen: $[\mathrm{N}/\mathrm{Fe}] = +1.03$
• Oxygen: $[\mathrm{O}/\mathrm{Fe}] = +0.86$
• Zinc: Significantly enhanced, $[\mathrm{Zn}/\mathrm{Fe}] = +0.88$
• First-peak neutron-capture elements enhanced: $[\mathrm{Sr}/\mathrm{Fe}] = +1.06$, $[\mathrm{Y}/\mathrm{Fe}] = +0.67$, $[\mathrm{Zr}/\mathrm{Fe}] = +1.15$
• Second-peak neutron-capture elements sharply depressed: $[\mathrm{Ba}/\mathrm{Fe}] = -1.45$, $[\mathrm{Eu}/\mathrm{Fe}] < +0.74$

The [Zr/Ba] ratio $= +2.60$ is the most extreme weak $r$-process signature identified in any EMP star, establishing SMSS 0224$-$5737 as the current archetype for this nucleosynthetic pathway (Okada et al., 30 Nov 2025). Upper limits for Mo, Ru, Pd, Ag, and Eu indicate a sharp abundance cutoff beyond $Z = 40$ (Zr).

4. Methodologies: Spectroscopy, Photometric Pipelines, and Quality Control

Elemental abundances were extracted by combining equivalent width (EW) analysis via MOOG and full-spectrum synthesis with Turbospectrum under 1D LTE assumptions, as implemented in the iSpec interface. Atmospheric models employed the MARCS spherical grid; solar abundances adopted Asplund et al. (2009) (Okada et al., 30 Nov 2025). Systematic uncertainties were evaluated by varying $T_{\rm eff}$, $\log g$, [Fe/H], and microturbulent velocity within plausible ranges. Photometric parameters are based on polynomial calibrations (Huang et al. 2022, "Paper I") tied to external spectroscopy, and reinforced by robustness checks against other surveys (GALAH DR4, etc.) (Huang et al., 12 Mar 2025).

Quality assessment in all catalog releases includes photometric cuts (e.g., class_star $\geq 0.6$), detection flags, and reddening corrections with the SFD98 map for high-latitude ($|b| > 10^\circ$) objects.

5. Nucleosynthetic Interpretation and Candidate $r$-Process Sites

The observed abundance pattern, with strong first-peak/weak second-peak $r$-process elements, rules out main $r$-process sources such as neutron star mergers (NSMs; which generically produce both peaks unless post-merger ejecta dominates), or electron-capture supernovae (which fail to synthesize observed Ba) (Okada et al., 30 Nov 2025). Proto-neutron-star wind models can approach the extreme [Zr/Ba] ratio for a narrow range of proto-NS masses ($\sim 1.8$–$2.0\, M_\odot$), but require rare evolutionary circumstances.

The most plausible progenitor, given both the weak $r$-process pattern and high [Zn/Fe], is a jet-driven magneto-rotational supernova (MRSN). MRSN yields from axisymmetric MHD simulations (Nishimura 2017) match the steep decline beyond $Z = 40$ and simultaneously account for $\alpha$-rich freezeout products such as Zn, in contrast to the yields from NSMs (Okada et al., 30 Nov 2025).

6. Astrophysical Significance and Context in Galactic Archaeology

SMSS 0224$-$5737 provides direct evidence for a rare enrichment channel in the early Galaxy, likely representing the chemical yield of a single explosion. Its halo kinematics, extremely low metallicity, and nucleosynthetic fingerprint support a scenario in which early MRSNe were responsible for seeding the nascent Milky Way with light $r$-process elements and high-entropy Fe-peak products. Its discovery in wide-field photometric surveys followed by deep blue/UV spectroscopy demonstrates the efficacy of combined SMSS and Gaia datasets to systematically identify chemically primitive stars at the lowest metallicities (Huang et al., 12 Mar 2025).

The frequency of such stars, when folded with nucleosynthetic model yields, constrains the initial mass function, explosion rates, and mixing timescales in Galactic chemical evolution models, and sharpens the contrast between canonical main and weak $r$-process astrophysical sites.

7. Uncertainties, Cross-Survey Comparison, and Future Prospects

Uncertainties on $T_{\rm eff}$ are of order 100 K (photometric/spectroscopic spread), while $\sigma_{\mathrm{[Fe/H]}}$ ranges from $0.12$ dex (photometric) to $0.26$ dex (high-resolution spectroscopy, focusing on EMP stars). Cross-method comparisons (e.g., SMSS DR2/DR4, SPar algorithm, Gaia geometric distances) show agreement to within 0.1 dex in [Fe/H] and 100 K in $T_{\rm eff}$, validating robust statistical treatments for stellar parameter estimation in large samples (Huang et al., 12 Mar 2025, Sun et al., 29 Mar 2025). Quality control flags and multi-parameter Bayesian sampling frameworks mitigate biases from extinction, photometric errors, and parallax systematics.

Future work leveraging increasingly sensitive blue/UV spectrographs on large telescopes, and expanded EMP star samples from next-generation surveys, will refine the census and physical interpretation of weak $r$-process objects such as SMSS J022423.27$-$573705.1, further elucidating the intertwined histories of stars, supernovae, and chemical elements in the early Milky Way (Okada et al., 30 Nov 2025).