An AGB star as the source of the abundance pattern of hyper-metal-poor star HE 1327-2326

Abstract

Abridged. HE 1327-2326 (HE 1327), with [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 match its abundance pattern. However, no model satisfactorily explains its entire surface chemical composition. The 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 those asymptotic giant branch (AGB) stars that undergo third dredge-up, hot bottom burning, and s-processing, suggesting that AGBs may have been progenitors of the star. We assume that, where HE 1327 formed, the interstellar medium was well-mixed, and adopt an initial stellar composition based on the observed chemical evolution of the early universe. We calculated models of hyper-metal-poor AGB stars and compared their yields to the observed abundances of HE 1327. Our 3 Msun models match 13 of the 14 measured elements in HE1327. They are also consistent with the seven elements with upper limits. The only discrepancy is O, underproduced by 0.5-1.0 dex. Unlike the SN models, the AGB models also match Sr and Ba. Our model predicts high abundances of P and Pb, which would be useful in testing the AGB scenario. 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. Recent Pop III star formation simulations support their formation, and their strong N 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 may have shown some degree of homogeneity and mixing even at these early epochs.