Simon W. Campbell

We present a systematic test of formalisms for common envelope evolution by forward modeling observable post-common-envelope binaries (PCEBs). We compare predictions from the α -formalism and the Two-stage and SCATTER formalisms against observed PCEBs, including wide binaries with ultramassive white dwarfs and central binaries of planetary nebulae. The angular-momentum-based SCATTER formalism does not predict populations that match the complete observed population, even with adjustments to its parameters. We take this as indicative of fundamental challenges with using the orbital angular momentum balance to predict common envelope outcomes. The energy-based α - and hybrid Two-stage formalisms both well replicate the observed population. α _CE ∼ 0.2–0.3 can match current observations, in agreement with previous works. Recombination energy is necessary, but only a fraction of it (∼10%–40%) can contribute in order to predict IK Peg–like binaries with ultramassive white dwarfs at the correct orbital periods. Our work suggests energy-based formalisms remain the most accurate for predicting common envelope outcomes, but more observations can constrain the recombination contribution and how these outcomes systematically vary with the donor mass.

Aims. We report the serendipitous discovery of two RR Lyrae stars that exhibit significant s-process element enrichment, a rare class previously represented solely by TY Gruis. Our goal is to characterise these objects chemically and dynamically, and explore their origins and evolutionary histories. Methods. Using high-resolution spectroscopy from HERMES@AAT and UVES@VLT, we derived detailed chemical abundances of key s-process elements (Y, Ba, La, Ce, Nd, and Eu) and carbon, along with α elements (Ca, Mg, and Ti). We also employed Gaia Data Release 3 astrometric data to analyse their kinematics, orbital properties, and classify their Galactic population membership. We compared observational results with theoretical asymptotic giant branch (AGB) nucleosynthesis models to interpret their enrichment patterns. Results. Both stars exhibit clear signatures of s-process enrichment, with significant overabundances in second-peak elements such as Ba and La compared to first-peak Y and Zr. Comparison with AGB nucleosynthesis models suggests their progenitors experienced pollution of s-process-rich material, consistent with early binary interactions. However, notable discrepancies in dilution factors highlight the need for more refined low-metallicity AGB models. We also explore and discuss alternative scenarios, including sub-luminous post-AGB-like evolution or double episodes of mass transfer. In the latter case, the star initially undergoes a mass transfer when it is on the main sequence, accreting material from a former AGB companion. Subsequently, as the star evolves along the red giant branch, it may again transfer mass to its companion before becoming an RR Lyrae star. Conclusions. Our findings confirm the existence of s-process-enhanced RR Lyrae stars and demonstrate the importance of combining chemical and dynamical diagnostics to unveil their complex evolutionary pathways. Future detailed binary evolution modelling and long-term orbital monitoring are essential to resolve their formation scenarios and assess the role of binarity in the evolution of pulsating variables.

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.

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

Galactic globular clusters (GCs) are now known to harbour multiple stellar populations, which are chemically distinct in many light element abundances. It is becoming increasingly clear that asymptotic giant branch (AGB) stars in GCs show different abundance distributions in light elements compared to those in the red giant branch (RGB) and other phases, skewing towards more primordial, field-star-like abundances, which we refer to as subpopulation one (SP1). As part of a larger programme targeting giants in GCs, we obtained high-resolution spectra for a sample of 106 RGB and 15 AGB stars in Messier 4 (NGC 6121) using the 2dF+HERMES facility on the Anglo–Australian Telescope. In this Letter, we report an extreme paucity of AGB stars with [Na/O] >−0.17 in M4, which contrasts with the RGB that has abundances up to [Na/O] =0.55. The AGB abundance distribution is consistent with all AGB stars being from SP1. This result appears to imply that all subpopulation two stars (SP2; Na-rich, O-poor) avoid the AGB phase. This is an unexpected result given M4's horizontal branch morphology – it does not have an extended blue horizontal branch. This is the first abundance study to be performed utilizing the HERMES spectrograph.

Stellar models provide a vital basis for many aspects of astronomy and astrophysics. Recent advances in observational astronomy -- through asteroseismology, precision photometry, high-resolution spectroscopy, and large-scale surveys -- are placing stellar models under greater quantitative scrutiny than ever. The model limitations are being exposed and the next generation of stellar models is needed as soon as possible. The current uncertainties in the models propagate to the later phases of stellar evolution, hindering our understanding of stellar populations and chemical evolution. Here we give a brief overview of the evolution, importance, and substantial uncertainties of core helium burning stars in particular and then briefly discuss a range of methods, both theoretical and observational, that we are using to advance the modelling.