An asymptotic giant branch star as the source of the abundance pattern of hyper-metal-poor star HE 1327-2326

¹ EETAC, Universitat Politècnica de Catalunya, CBL, 08840 Castelldefels, Spain
² School of Physics and Astronomy, Monash University, Clayton, Victoria 3800, Australia
³ ARC Centre of Excellence for All Sky Astrophysics in Three Dimensions (ASTRO-3D), Melbourne, Australia
⁴ Institut d’Estudis Espacials de Catalunya IEEC, Barcelona, Spain
⁵ Konkoly Observatory, HUN-REN Research Centre for Astronomy and Earth Sciences, MTA Centre for Excellence, Konkoly Thege Miklós ut 15-17, Budapest 1121, Hungary
⁶ ELTE Eötvös Loránd University, Institute of Physics, Budapest, 1117 Pázmány Péter sétány 1/A, Hungary

^⋆ Corresponding author: pilar.gil@upc.edu

Received: 11 July 2025
Accepted: 4 September 2025

Abstract

Context. Understanding the most metal-poor objects is key to interpreting the nature of the first stars. HE 1327-2326 (HE 1327), with metallicity [Fe/H] = −5.2, is one of the most metal-poor stars detected and a candidate to be the offspring of the first stars. Numerous efforts have been made to reproduce its abundance pattern, especially with high-mass stars undergoing supernova (SN) explosions. However, no model satisfactorily explains its entire surface chemical composition.

Aims. The characteristic high CNO pattern with [N/Fe] > [C/Fe] > [O/Fe], the light element ‘slide’ (between Na and Si), and the presence of Sr and Ba in HE 1327 is reminiscent of asymptotic giant branch (AGB) stars that undergo third dredge-up, hot-bottom burning, and s-processing – suggesting that these stars were the source of the chemistry of the star. We tested this hypothesis.

Methods. We assumed that, where HE 1327 formed, the interstellar medium was well mixed, and adopted an initial stellar composition based on the observed chemical evolution of the early Universe. Zinc, which is enhanced in HE 1327, is well matched by this initial composition, as are the α-elements. We calculated models of hyper-metal-poor AGB stars and compared the predicted chemical yields to the observed chemical pattern of HE 1327.

Resulst. We find our 3 M_⊙ models match 13 of the 14 measured elements in HE1327, more than any model thus far. They are also consistent with the seven elements with known upper limits. The only discrepancy is oxygen, which is underproduced by 0.5 − 1.0 dex. For elements up to Zn, the match is comparable to that of the best-fitting, finely tuned SN models. Unlike the SN models, the AGB models also match Sr and Ba. We stress that the AGB scenario requires only standard stellar evolution, without invoking exotic scenarios. Our model predicts high abundances of P and Pb; thus, observations of these elements would be useful in testing the AGB scenario.

Conclusions. We propose that HE 1327 is the oldest known object that shows nucleosynthetic evidence of the first AGB stars. With lifetimes as short as 200 Myr, these stars may have formed and polluted the Universe very early on. Recent Pop III star formation simulations support this hypothesis, and their strong nitrogen production is qualitatively consistent with recent JWST observations showing high N/O ratios just 440 Myr after the Big Bang. Importantly, our results also suggest that the interstellar medium experienced some degree of homogeneity and mixing at these early epochs.