HE 1327-2326: Hyper Metal-Poor Subgiant

• HE 1327-2326 is a hyper metal-poor halo subgiant, marked by an extremely low iron content and unusually high C, N, and O abundances.
• Spectroscopic analyses reveal significant discrepancies between UV Fe II and optical Fe I measurements, highlighting challenges in 1D versus 3D NLTE modeling.
• The star serves as a benchmark for early-Universe nucleosynthesis, informing models ranging from aspherical Pop III explosions to AGB enrichment scenarios.

HE 1327-2326 is a hyper metal-poor halo subgiant and one of the most iron-poor stars known. Its importance derives not simply from the extremity of its iron deficiency, but from a highly non-solar abundance pattern in which Fe is extraordinarily depressed while C, N, O, and neutron-capture elements are strongly enhanced relative to Fe. As a result, it has become a central empirical constraint on first-star nucleosynthesis, early binary pollution, dust-related fractionation, and the line-formation systematics that limit abundance work at the lowest metallicities (Ezzeddine et al., 2018, Bonifacio et al., 2012, Ezzeddine et al., 2019, Gil-Pons et al., 1 Sep 2025).

1. Stellar status and abundance framework

HE 1327-2326 is treated in recent work as a subgiant or unevolved star rather than a giant. Modern abundance analyses commonly adopt atmospheric parameters near $T_{\rm eff}=6180\ \mathrm{K}$ and $\log g=3.7$, with microturbulence around $1.6$-$1.7\ \mathrm{km\,s^{-1}}$; Gaia-based gravity estimates are consistent with a subgiant solution (Ezzeddine et al., 2018, Bonifacio et al., 2012, Ezzeddine et al., 2019). A recent reanalysis further argues that internal depletion is small, of order $\sim 0.2$ dex, so the photospheric composition is often taken to be close to the birth composition (Gil-Pons et al., 1 Sep 2025).

Abundance work on the star uses the standard spectroscopic notation

$$[X/Y] = \log_{10}(N_X/N_Y)_\star - \log_{10}(N_X/N_Y)_\odot$$

and

$A(X)=\log_{10}(N_X/N_H)+12.$

That framework is essential because the star’s significance lies in ratios rather than in any single absolute metallicity estimate (Bonifacio et al., 2012).

A distinctive feature of HE 1327-2326 is that its total metal content is not as low as its Fe abundance alone would suggest. Because the star is strongly enriched in C, N, and O relative to Fe, one analysis gives $\log(Z/Z_\odot)\approx -2.5$, emphasizing that it is chemically peculiar rather than uniformly metal poor (Bonifacio et al., 2012).

2. Spectroscopic determinations and the abundance scale

A persistent issue in the literature is the iron abundance scale itself. UV spectroscopy with COS/HST produced the first Fe II detections in HE 1327-2326, specifically at 2327.39, 2331.30, 2332.79, 2338.00, and 2343.49 Å, together with the first Si I detection at 2124.12 Å. From the Fe II lines, the derived abundance was $[\mathrm{Fe\ II}/\mathrm{H}] = -5.99\pm0.25$ in 1D LTE and $[\mathrm{Fe/H}] = -6.01\pm0.25$ in 1D NLTE. Nevertheless, that study adopted the earlier optical Fe I result $\log g=3.7$0 as the working abundance scale, arguing that the UV Fe II versus optical Fe I discrepancy likely reflects residual 3D effects in the absence of full 3D NLTE Fe calculations (Ezzeddine et al., 2018).

Sulfur has been targeted as a volatile-element diagnostic. CRIRES observations of S I Multiplet 3 near 1045 nm yielded no detection, with a $\log g=3.7$1 equivalent-width upper limit $\log g=3.7$2. The resulting robust abundance constraint is $\log g=3.7$3, corresponding to $\log g=3.7$4 under the abundance scale used in that study (Bonifacio et al., 2012).

Zinc was measured from HST/COS UV spectroscopy via Zn I 2138.57 Å. The line was detected at $\log g=3.7$5 with equivalent width $\log g=3.7$6, giving $\log g=3.7$7, $\log g=3.7$8, and $\log g=3.7$9 when combined with the adopted NLTE iron abundance (Ezzeddine et al., 2019).

Selected published values frequently used in recent analyses are heterogeneous, reflecting different line sets and radiative-transfer treatments:

Quantity	Reported value	Context
Iron abundance	$1.6$0	Adopted final 1D NLTE value from optical Fe I
UV Fe II abundance	$1.6$1	1D LTE result from first UV Fe II detections
Sulfur	$1.6$2; $1.6$3	CRIRES non-detection of S I Multiplet 3
Zinc	$1.6$4	HST/COS UV Zn I detection
C, N, Sr, Ba	$1.6$5; $1.6$6; $1.6$7; $1.6$8	Observed pattern adopted in recent AGB modeling

3. Chemical pattern

The observed abundance structure is dominated by extreme light-element enhancement. A recent synthesis emphasizes three defining empirical features: very high CNO abundances with the ordering $1.6$9, a descending “slide” from Na to Si, and the presence of both Sr and Ba. In that compilation, the adopted values are $1.7\ \mathrm{km\,s^{-1}}$0, $1.7\ \mathrm{km\,s^{-1}}$1, recommended $1.7\ \mathrm{km\,s^{-1}}$2, $1.7\ \mathrm{km\,s^{-1}}$3, and $1.7\ \mathrm{km\,s^{-1}}$4, implying $1.7\ \mathrm{km\,s^{-1}}$5 (Gil-Pons et al., 1 Sep 2025).

The lithium abundance is unusually low for a warm star. Comparative analyses of the ultra-iron-poor dwarfs J0023+0307 and J0815+4729 quote HE 1327-2326 with only upper limits, $1.7\ \mathrm{km\,s^{-1}}$6 in one discussion and $1.7\ \mathrm{km\,s^{-1}}$7 in another. That places it well below the Spite plateau and makes it a standard comparison object in discussions of lithium behavior at $1.7\ \mathrm{km\,s^{-1}}$8 (Frebel et al., 2018, Hernández et al., 2020).

The neutron-capture signature is especially important because it is not restricted to Sr alone. Earlier literature often highlighted HE 1327-2326 primarily as a high-Sr star; the availability of Ba in recent reinterpretations shifts the problem from isolated Sr enhancement to the joint origin of Sr and Ba (Gil-Pons et al., 1 Sep 2025).

4. Interpretive frameworks

One interpretive axis concerns whether the photospheric pattern is primarily natal or substantially modified after formation. The sulfur upper limit was obtained specifically to test dust-gas winnowing against early supernova enrichment. The result, $1.7\ \mathrm{km\,s^{-1}}$9, was judged non-decisive: it is consistent with both fallback-supernova scenarios and dust-related fractionation, and the study explicitly states that dust formation and fallback are not mutually exclusive (Bonifacio et al., 2012).

A prominent nucleosynthetic interpretation links the star to an aspherical Population III explosion. The Zn detection was central to that shift, because large grids of faint quasi-spherical mixing-and-fallback models failed to reproduce $\sim 0.2$0. In the tested framework, the preferred model was a $\sim 0.2$1 Population III progenitor with artificially modified densities and explosion energy $\sim 0.2$2, with $\sim 0.2$3, $\sim 0.2$4, and $\sim 0.2$5 (Ezzeddine et al., 2019).

A different supernova-based family invokes weak explosions of rotating or non-rotating first stars. In that framework, HE 1327-2326 can be reproduced by non-rotating $\sim 0.2$6-$\sim 0.2$7 models or rotating $\sim 0.2$8-$\sim 0.2$9 models, with formal best fits given by a non-rotating $$[X/Y] = \log_{10}(N_X/N_Y)_\star - \log_{10}(N_X/N_Y)_\odot$$0 progenitor and a rotating $$[X/Y] = \log_{10}(N_X/N_Y)_\star - \log_{10}(N_X/N_Y)_\odot$$1 progenitor. The main unresolved tension in that study was the very large nitrogen enhancement, which the standard grid underproduced (Takahashi et al., 2014).

Convective-reactive alternatives replace deep explosive ejecta with shell processing. One proposal invokes a hydrogen-ingestion event in a $$[X/Y] = \log_{10}(N_X/N_Y)_\star - \log_{10}(N_X/N_Y)_\odot$$2 Pop III star, producing i-process neutron densities and yielding a combined He-burning plus i-process pattern that reproduces the general light-element structure of HE 1327-2326, including Ca without co-produced Fe (Clarkson et al., 2017). Related proton-ingestion/core-He-flash models in low-mass extremely metal-poor stars can, after strong dilution or binary mass transfer, reproduce CNO and Sr at the level claimed in those studies, but typically overproduce Ba or depend sensitively on uncertain 1D mixing physics (Campbell et al., 2010, Cruz et al., 2013).

A recent reappraisal moves the problem away from supernovae and toward AGB enrichment. In that work, a $$[X/Y] = \log_{10}(N_X/N_Y)_\star - \log_{10}(N_X/N_Y)_\odot$$3 hyper-metal-poor AGB model with $$[X/Y] = \log_{10}(N_X/N_Y)_\star - \log_{10}(N_X/N_Y)_\odot$$4 and dilution $$[X/Y] = \log_{10}(N_X/N_Y)_\star - \log_{10}(N_X/N_Y)_\odot$$5 matched 13 of the 14 measured elements and all seven upper limits, with oxygen underproduced by $$[X/Y] = \log_{10}(N_X/N_Y)_\star - \log_{10}(N_X/N_Y)_\odot$$6-$$[X/Y] = \log_{10}(N_X/N_Y)_\star - \log_{10}(N_X/N_Y)_\odot$$7 dex. Because that model also reproduces Sr and Ba together, it was used to argue that HE 1327-2326 is better viewed as an N-enhanced CEMP-s star than as a conventional CEMP-no object; the same model predicts strong P and Pb enhancements as direct observational tests (Gil-Pons et al., 1 Sep 2025).

5. Benchmark role in ultra-metal-poor-star research

HE 1327-2326 functions not only as an object of study but also as a benchmark against which newer discoveries are ranked. A Subaru/HDS survey of four extremely metal-poor turn-off stars identifies a new star at $$[X/Y] = \log_{10}(N_X/N_Y)_\star - \log_{10}(N_X/N_Y)_\odot$$8 as “the second most iron-poor star observed with Subaru” after HE 1327-2326, using the latter’s literature metallicity $$[X/Y] = \log_{10}(N_X/N_Y)_\star - \log_{10}(N_X/N_Y)_\odot$$9 as the Subaru reference point. The same study also treats HE 1327-2326 as the comparison case for an “exceptionally high” $A(X)=\log_{10}(N_X/N_H)+12.$0 ratio and as an example of severe Ca abundance discrepancies between the Ca I 4226 Å resonance line and other Ca diagnostics at the lowest metallicities (Suda et al., 23 Jun 2025).

In observational comparisons of other ultra-iron-poor dwarfs, HE 1327-2326 serves as the nearest warm analogue. Studies of J0023+0307 and J0815+4729 repeatedly use it for direct spectral comparison, for lithium benchmarking, and for assessing whether newly found stars belong to the same CNO-enhanced class (Frebel et al., 2018, Hernández et al., 2020).

Its Zn abundance also gives the star leverage beyond stellar archaeology narrowly construed. A 2026 Galactic chemical-evolution study treats HE 1327-2326 as the clearest low-metallicity Zn anchor, arguing that its $A(X)=\log_{10}(N_X/N_H)+12.$1 favors a collapsar contribution, either in a collapsar-dominated local environment or in a mixed CCSN+collapsar environment weighted toward top-heavy, especially $A(X)=\log_{10}(N_X/N_H)+12.$2, collapsars (Leung et al., 26 Feb 2026).

6. Outstanding problems and discriminating tests

Three issues remain central. The first is the iron abundance scale. The present working value $A(X)=\log_{10}(N_X/N_H)+12.$3 is adopted because full 3D NLTE Fe calculations are not yet available, while the UV Fe II detections reveal a large Fe I/Fe II mismatch that 1D analysis alone cannot reconcile (Ezzeddine et al., 2018).

The second is the volatile-versus-refractory question. Sulfur remains undetected, and the limit $A(X)=\log_{10}(N_X/N_H)+12.$4 still allows both natal enrichment and dust-related fractionation. The same study therefore leaves open a hybrid picture in which an unusual supernova yield pattern was later distorted by dust effects (Bonifacio et al., 2012).

The third is the origin of the neutron-capture pattern. AGB models now explain Sr and Ba together and make specific predictions of high P and Pb, whereas earlier proton-ingestion models and several supernova-based fits either struggled with Ba or reproduced only part of the observed structure. This suggests that improved measurements or stringent upper limits for P, Pb, Eu, and oxygen, together with line-aware treatment of Ca and fully consistent 3D NLTE abundance analyses, remain the most direct route to deciding whether HE 1327-2326 is best understood as the record of an aspherical Pop III explosion, an AGB-polluted system, or a chemically composite object in which more than one early-universe process contributed (Gil-Pons et al., 1 Sep 2025, Campbell et al., 2010).