Alex Kemp

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.

Context . Due to their proximity, the Pleiades are an important benchmark open cluster. Despite its status, asteroseismic analyses of its members are rare. In particular, the gravity-mode (g-mode) pulsators, which allow near-core stellar properties to be inferred, have not been analysed yet. Aims . We aim to identify and analyse the population of g-mode pulsators in the Pleiades. Our focus lies on the internal rotation as measured from asteroseismology to obtain a well-defined sample of stellar rotation on the early main sequence. Methods . Based on full-frame images from the Transiting Exoplanet Survey Satellite (TESS), we constructed light curves for intermediate-mass Pleiades members and searched for g-mode pulsators among them. For pulsators exhibiting period spacing patterns, we determined their near-core rotation rate and buoyancy periods. For all other g-mode pulsators, we estimated the near-core rotation rate based on the dominant mode frequency to obtain a comprehensive rotation rate distribution. Results . Among our 105 target stars, we find 28 g-mode pulsators distributed across the entire upper main sequence, 19 of which are hybrid pulsators, but only three stars exhibit period spacing patterns in the current TESS data. The near-core rotation rates in A- and early F-type members are distributed between 1 and 3 d ⁻¹ without any clear mass-dependence. This distribution is much broader than the one in the similar open cluster NGC 2516. A comparison of the buoyancy periods shows that the Pleiades and NGC 2516 are of a similar asteroseismic age. Conclusions . With the large population of g-mode and hybrid pulsators, the Pleiades constitute a valuable asteroseismic benchmark cluster, reaffirming its important role in stellar astrophysics.

Context. HR6819 is the first system with a puffed-up low-mass stripped star + a classical Be star whose nature has been confirmed by optical interferometry. The system exhibits the most extreme mass ratio (15.7 ± 1.1), the lowest stripped star mass (0.270 ± 0.056 M _⊙ ), and one of the shortest orbital periods (40.3266 ± 0.0016 days) compared to similar observed binaries. As a post mass transfer candidate, HR6819 offers a unique opportunity to test the physics of binary interaction, especially the fraction of mass accreted by the Be progenitor (the efficiency of the mass transfer) required to produce the system’s extreme mass ratio. Aims. This work aims to reconstruct the possible evolutionary history of HR6819 in the context of stable mass transfer via Roche lobe overflow. We want to explore how the tight constraints on the system’s total mass, mass ratio and orbital period are limiting the range of possible progenitors of the system. Methods. Based on analytical expectations for the orbital evolution, we build grids of MESA simulations designed to match the present-day orbital period and mass ratio of the system, with different mass transfer efficiencies from fully conservative to 50% efficient. Results. We show that evolution via stable mass transfer cannot explain the combined extreme current mass ratio and tight orbital period of the system. There is a limit on how extreme the post-mass transfer mass ratio of the progenitor binary can be at a fixed detachment period, and this limit is dependent on the efficiency of the mass transfer episode: the less efficient the mass transfer episode, the less extreme the mass ratio at detachment. Even in the case of fully conservative mass transfer, the most extreme mass ratio we can produce with binary evolution simulations is q ∼ 11.5 at P ∼ 40 days, which is significantly below the observed value. We also show that the reported luminosities for each component significantly exceed the value expected from their mass. In particular, based on simple stripped star models, we find that the luminosity of the bloated stripped star requires a star with a mass of ∼0.7 M _⊙ , which is over twice the measured value. Conclusions. Our work shows that the post-interaction properties of HR6819, especially its extreme mass ratio and orbital period, cannot be produced by stable mass transfer under standard assumptions.

Context. In the modern era of large-scale photometric time-domain surveys, relatively rare but information-rich eclipsing binary systems can be leveraged at a population level across the Hertzsprung-Russel diagram to improve our knowledge of stellar evolution. This high-precision photometry is also excellent for assessing and exploiting the asteroseismic properties of such stars and results in a powerful synergy that has great potential for shedding light on how stellar interiors and tides affect stellar evolution and mass transfer. Aims. In this work, we seek to characterise a large sample of 14 377 main sequence eclipsing binaries in terms of their stellar, astero-seismic, and orbital properties. Methods. We conducted manual vetting on a sub-set of 4000 targets from our full 14 377 target sample to identify targets with pressure or gravity modes. We inferred stellar properties including mass, the convective core mass, radius, and central H fraction for the primary using the Gaia Data Release 3 effective temperature and luminosity estimates and a grid of asteroseismically calibrated stellar models. We used surface brightness ratio and radius ratio estimates from previous eclipse analyses to study the effect of binarity on our results. Results. Through our manual vetting, we identified 751 candidate g-mode pulsators, 131 p-mode pulsators, and a further 48 hybrid pulsators. The inferred stellar properties of the hybrid and p-mode pulsators are highly correlated, while the orbital properties of the hybrid pulsators align best with the g-mode pulsators. The g-mode pulsators themselves show a distribution that peaks around the classical γ Dor instability region but extends continuously towards higher masses, with no detectable divide between the classical γ Dor and SPB instability regions. There is evidence at the population level for a heightened level of tidal efficiency in stars showing g-mode or hybrid variability. We corrected the primary mass inference for binarity based on eclipse measurements of the surface brightness and radius ratios, resulting in a relatively small shift towards lower masses. Conclusions. This work provides a working initial characterisation of this sample from which more detailed analyses folding in aster-oseismic information can be built. It also provides a foundational understanding of the limitations and capabilities of this kind of rapid, scalable analysis that will be highly relevant in planning the exploitation of future large-scale binary surveys.

We recently identified an upturn in [Na/Fe] for the population of solar-type stars in the Galactic young thick disc ($-0.3 < \mathrm{[Fe/H]} < +0.3$ dex) at supersolar metallicity in GALAH (GALactic Archaeology with HERMES) data. Here, we investigate the cause of this unexplained Na enrichment ($[{\rm Na/Fe}] \approx 0 \!-\!0.6$ dex) using the omega+ Galactic chemical evolution code. We investigate the increase of [Na/Fe] with four combinations of nucleosynthetic yields from the literature, with source contributions from core-collapse supernovae (CC SNe), asymptotic giant branch (AGB) stars, and Type Ia supernovae (SNe Ia). We focus on two possible causes for the Na enhancement: the ‘metallicity effect’ resulting from CC SNe at supersolar metallicity and the contribution of metal-rich AGB stars. We adopt two sets of SN Ia yields with one model assuming only Chandrasekhar-mass explosions, and another assuming only sub-Chandrasekhar-mass explosions. We find that the assumed SN Ia explosion has little effect on the [Na/Fe] Galactic chemical evolution modelling, and all Galactic chemical evolution models tested fail to reproduce the observed [Na/Fe] enrichment in the young thick disc population at supersolar metallicities. Our study indicates a possible ‘underpollution effect’ by SNe Ia, which are the dominant producers of iron, in the Galactic disc’s solar-type star population. These findings provide a step forward towards understanding the origin of the unexplained sodium enrichment at supersolar metallicities in the Galactic disc.

Context. Galactic chemical evolution (GCE) models aim to bring together stellar yields and galactic evolution models to make predictions for the chemical evolution of real stellar environments. Until recently, stellar yields accounting for binary stellar evolution were unavailable, leading to an inability for GCE calculations to account for most binary stellar evolution effects. Fortunately, effective stellar yields that account for binary stellar evolution at a population level can be pre-computed and then used as if they were single yields. Aims. We present a framework for the computation of effective stellar yields that accounts for a mixed population of binary and single stars under an adjustable mix of binary evolution settings: the binary fraction, the accretion efficiencies of winds, Roche-lobe overflow, and supernovae. We emphasise the critical need for more complete yield coverage of the binary nucleosynthesis and evolution, without which the ability to make accurate predictions on the true role of binarity on GCE calculations is hamstrung. We also provide clear guidelines for future stellar modelling works to ensure their results are maximally useful to the wider community. Methods. We compute effective stellar yields using detailed binary stellar yields accounting for binary induced mass-loss from a solar-metallicity donor star. We study the effect of varying the binary mixture and accretion efficiencies, and consider a range of models for the treatment of accreted material on the secondary in detail. Results. In the absence of detailed binary yields for the secondary, we put forth a model for the composition of accreted material that preserves the signature of the primary’s nuclear processing within the post-mass-transfer secondary yields. This model includes special treatment for isotopes of the light elements Li, Be, and B and accreted radioisotopes. Among the binary parameters, we find that the binary fraction, which determines the ratio of binary and single star systems, has the most significant effect on the effective stellar yields, with widespread impact across most isotopes. In contrast, varying the accretion efficiencies produces comparatively minor changes. We also find that the binary fraction has a significant influence on the logarithmic elemental abundance ratios relative to H present in the effective yield; this impact is the largest for the lower-mass primaries. Conclusions. Comprehensive coverage of binary systems is essential for advancing our understanding of the role of binary stellar evolution in galactic chemical evolution. Priority areas include low-mass stellar evolution, binary mergers, and supernova yields coupled with the evolution of their binary progenitors with nuclear post-processing. The low-metallicity regime is also largely unexplored, offering great opportunity for novel and impactful research in this area.

We present a framework for the computation of effective stellar yields that accounts for a mixed population of binary and single stars under an adjustable mix of binary evolution settings: the binary fraction, the accretion efficiencies of winds, Roche-lobe overflow, and supernovae. We emphasise the critical need for more complete yield coverage of the binary nucleosynthesis and evolution, without which the ability to make accurate predictions on the true role of binarity on GCE calculations is hamstrung. We also provide clear guidelines for future stellar modelling works to ensure their results are maximally useful to the wider community. We compute effective stellar yields using detailed binary stellar yields accounting for binary induced mass-loss from a solar-metallicity donor star. We study the effect of varying the binary mixture and accretion efficiencies, and consider a range of models for the treatment of accreted material on the secondary in detail. In the absence of detailed binary yields for the secondary, we put forth a model for the composition of accreted material that preserves the signature of the primary's nuclear processing within the post-mass-transfer secondary yields. Among the binary parameters, we find that the binary fraction, which determines the ratio of binary and single star systems, has the most significant effect on the effective stellar yields, with widespread impact across most isotopes. In contrast, varying the accretion efficiencies produces comparatively minor changes. We also find that the binary fraction has a significant influence on the logarithmic elemental abundance ratios relative to H present in the effective yield; this impact is largest for the lower-mass primaries.

Data Release 3 (DR3) has published for the first time epoch photometry, BP/RP (XP) low-resolution mean spectra, and supervised classification results for millions of variable sources. This extensive dataset offers a unique opportunity to study the variability of these objects by combining multiple data products. In preparation for DR4, we propose and evaluate a machine learning methodology capable of ingesting multiple data products to achieve an unsupervised classification of stellar and quasar variability. A dataset of 4 million DR3 sources was used to train three variational autoencoders (VAEs), which are artificial neural networks (ANNs) designed for data compression and generation. One VAE was trained on XP low-resolution spectra, another on a novel approach based on the distribution of magnitude differences in the band, and the third on folded band light curves. Each source was compressed into 15 numbers, representing the coordinates in a 15-dimensional latent space generated by combining the outputs of these three models. The learned latent representation produced by the ANN effectively distinguishes between the main variability classes present in DR3, as demonstrated through both supervised and unsupervised classification analysis of the latent space. The results highlight a strong synergy between light curves and low-resolution spectral data, emphasising the benefits of combining the different data products. A 2D projection of the latent variables revealed numerous overdensities, most of which strongly correlate with astrophysical properties, showing the potential of this latent space for astrophysical discovery. We show that the properties of our novel latent representation make it highly valuable for variability analysis tasks, including classification, clustering, and outlier detection.

A small fraction of low-mass stars have been found to have anomalously high Li abundances. Although it has been suggested that mixing during the red giant branch phase can lead to Li production, this method of intrinsic Li production cannot explain Li-rich stars that have not yet undergone the first dredge-up. To obtain clues about the origin of such stars, we present a detailed chemical abundance analysis of four unevolved Li-rich stars with −2.1<[Fe/H]<−1.3 and 2.9<A(Li)M≳1.3 M_⊙) nonluminous companions. We show that three of the four stars exhibit abundance patterns similar to the ones of known unevolved Li-rich stars, namely normal abundances in most elements except for Li and Na. These abundance similarities suggest a common origin for the unevolved Li-rich stars and low-mass metal-poor stars with massive compact companions. We also made the first detection of N abundance to unevolved Li-rich stars in D25_6334, and found that it is significantly enhanced ([N/Fe] = 1.3). The observed abundance pattern of D25_6334, spanning from C to Si, indicates that its surface has been polluted by a former intermediate-mass companion star or a nova system that involves a massive ONe white dwarf. Using a population synthesis model, we show that the nova scenario can lead to the observed level of Li enhancement and also provide an explanation for Li-rich stars without companions and those with massive compact companions.