Red Giant Research Articles

Observations of the Ultraviolet-bright Star III-60 in the Globular Cluster NGC 6723

Abstract We have analyzed archival far-ultraviolet spectra of the UV-bright star III-60 in the globular cluster NGC 6723 obtained with the Far Ultraviolet Spectroscopic Explorer and the Cosmic Origins Spectrograph. We find that the star's photospheric parameters (effective temperature T eff = 44,800 ± 1200 K, surface gravity log g = 4.89 ± 0.18 , and helium abundance log N ( He ) / N ( H ) = − 0.84 ± 0.29 ) are consistent with the values derived from its optical spectrum, suggesting that optically derived values are generally accurate for evolved stars with T eff ≲ 50,000 K. Relative to the cluster's red giant branch stars, III-60 is enhanced in nitrogen and depleted in carbon and oxygen. The star exhibits strong P Cygni profiles in both components of the N v λ1240 doublet, but the resonance lines of other species show no evidence of a stellar wind. The star's effective temperature and luminosity place it on the evolutionary tracks of stars evolving from the blue horizontal branch, but its high mass (∼1.2 M ⊙) indicates that it is the product of a stellar merger. Its helium, carbon, and nitrogen abundances suggest that it is following an evolutionary path similar to that of the low-carbon, intermediate helium-rich hot subdwarfs.

Stellar Evolution and Axion-like Particles: New Constraints and Hints from Globular Clusters in the GAIA DR3 Data

Axion-like particles (ALPs) are hypothetical pseudoscalar bosons, natural in extensions of the Standard Model. Their interactions with ordinary matter and radiation are suppressed, making it challenging to detect them in laboratory experiments. However, these particles, produced within stellar interiors, can provide an additional mechanism for energy loss, potentially influencing stellar evolution. Prominent methods for searching for such effects involve measuring the properties of red giants and helium-burning stars in globular clusters (GCs). Here we use published catalogs of stars selected as members of seven GCs on the basis of parallaxes and proper motions measured by Gaia (Data Release 3). Making use of previously derived theoretical relations and the new data, we find the upper limit on the ALP-electron coupling, g ae < 5.2 × 10–14 (95% CL), and an indication (3.3σ) to nonzero ALP-photon coupling, g aγ = $$(6.5_{{ - 1.3}}^{{ + 1.1}})$$ × 10–11 GeV–1. Given the precision of contemporary observational data, it is imperative to refine ALP constraints through more sophisticated analyses, which will be explored in detail elsewhere.

Testing the Breakdown of the Asteroseismic Scaling Relations in Luminous Red Giants

Abstract Nearly all cool, evolved stars are solar-like oscillators, and fundamental stellar properties can be inferred from these oscillations with asteroseismology. Scaling relations are commonly used to relate global asteroseismic properties—the frequency of maximum power ν max and the large-frequency separation Δν—to stellar properties. Mass, radius, and age can then be inferred with the addition of stellar spectroscopy. There is excellent agreement between seismic radii and fundamental data on the lower red giant branch and red clump. However, the scaling relations appear to breakdown in luminous red-giant stars. We attempt to constrain the contributions of the asteroseismic parameters to the observed breakdown. We test the ν max and Δν scaling relations separately, by using stars of known mass and radius in star clusters and the Milky Way's high-α sequence. We find evidence that the Δν scaling relation contributes to the observed breakdown in luminous giants more than the ν max relation. We test different methods of mapping the observed Δν to the mean density via a correction factor, F Δν , and find a ≈ 1%–3% difference in the radii in the luminous giant regime, depending on the technique used to measure F Δν . The differences between the radii inferred by these two techniques are too small on the luminous giant branch to account for the inflated seismic radii observed in evolved giant stars. Finally, we find that the F Δν correction is insensitive to the adopted mixing length, chosen by calibrating the models to observations of T eff .

TESS Giants Transiting Giants. VII. A Hot Saturn Orbiting an Oscillating Red Giant Star**This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

Abstract We present the discovery of TOI-7041 b (TIC 201175570 b), a hot Saturn transiting a red giant star with measurable stellar oscillations. We observe solar-like oscillations in TOI-7041 with a frequency of maximum power of ν max = 218.50 ± 2.23 μHz and a large frequency separation of Δν = 16.5282 ± 0.0186 μHz. Our asteroseismic analysis indicates that TOI-7041 has a mass of 1.07 ± 0.05(stat) ± 0.02(sys) M ⊙ and a radius of 4.10 ± 0.06(stat) ± 0.05(sys) R ⊙, making it one of the largest stars around which a transiting planet has been discovered with the Transiting Exoplanet Survey Satellite (TESS), and the mission's first oscillating red giant with a transiting planet. TOI-7041 b has an orbital period of 9.691 ± 0.006 days and a low eccentricity of e = 0.04 ± 0.04. We measure a planet radius of 1.02 ± 0.03 R Jup with TESS photometry, and a planet mass of 0.36 ± 0.16 M Jup (114 ± 51 M ⊕) with ground-based radial velocity measurements. TOI-7041 b appears less inflated than similar systems receiving equivalent incident flux, and its circular orbit indicates that it is not undergoing tidal heating due to circularization. The asteroseismic analysis of the host star provides some of the tightest constraints on the stellar properties of a TESS planet host and enables precise characterization of the hot Saturn. This system joins a small number of TESS-discovered exoplanets orbiting stars that exhibit clear stellar oscillations and indicates that extended TESS observations of evolved stars will similarly provide a path to improved exoplanet characterization.

Red Stellar Populations and Dust Extinction toward W3

We explore red stellar populations toward the W3 giant molecular cloud through the use of optical-to-infrared (IR) photometry and Gaia DR 3 data, simultaneously characterizing stellar content and properties of dust in the molecular medium. We use a Rayleigh–Jeans color excess method modified to de-redden stellar observations of both red giants (RGs) and OB stars, and construct an IR Hertzsprung–Russell diagram validated against the Besançon Galactic model. Taking advantage of the near-universal IR interstellar extinction law and precise Gaia measurements, we develop a method for obtaining the spectral classification, foreground extinction, and distance moduli of stars, validated by spectroscopically confirmed OB stars. We constrain the observed parallax and proper motion of OB stars in W3, demonstrating the importance of considering systematic effects in the parallax bias, and assign parallax- and proper-motion-based cloud membership to our stellar samples. While it has been assumed that all spectroscopic OB stars are inside the W3 cloud, we find evidence of seven background B stars and three potential runaway OB stars. The methods developed here, which are based on known stellar populations, enable us to identify 82 new OB candidates that are confidently within the cloud. We quantify several dust-to-dust empirical correlations, in particular the IR color excess E(H − [4.5]) and the optical depth τ 1 of submillimeter dust emission at 1 THz using RGs behind W3, measuring a best fit of E(H−[4.5])=(1.07±0.04)×103τ1,HOTT+(0.00±0.02)mag .

The robustness of inferred envelope and core rotation rates of red giant stars from asteroseismology

Context. Rotation is an important phenomenon influencing stellar structure and evolution, however, it has not been adequately modelled thus far. Therefore, accurate estimates of internal rotation rates are valuable for constraining stellar evolution models. Aims. We aim to assess the accuracy of asteroseismic estimates of internal rotation rates and how they depend on the fundamental stellar parameters. Methods. We applied the recently developed extended-multiplicative optimally localised averages (eMOLA) inversion method, to infer localised estimates of internal rotation rates of synthetic observations of red giants. We searched for suitable reference stellar models, following a grid-based approach, and we assessed the robustness of the resulting inferences with respect to the choice of reference model. Results. We find that matching the mixed-mode pattern between the observation and the reference model is an important criterion for selecting suitable reference models. We propose (i) selecting a set of reference models based on the correlation between the observed rotational splittings and the mode-trapping parameter; (ii) computing the rotation rates for all these models; and (iii) using the average value obtained across the whole set as the estimate of the internal rotation rates. We find that the effect of a near surface perturbation in the synthetic observations on the rotation rates estimated based on the correlation between the observed rotational splittings and the mode-trapping parameter is negligible. Conclusions. We conclude that when using an ensemble of reference models that are selected by matching the mixed-mode pattern, the input rotation rates can be recovered across a range of fundamental stellar parameters such as mass, mixing-length parameter, and composition. Further, red giant rotation rates determined in this way are also independent of any near-surface perturbation of the stellar structure.

Testing the asteroseismic estimates of stellar radii with surface brightness-colour relations and Gaia DR3 parallaxes

Context. A recent investigation highlighted peculiar trends between the radii derived from surface brightness-colour relations (SBCRs) combined with Gaia DR3 parallaxes with respect to asteroseismic scaling relation radii from K2 data. Aims.Kepler data differ from K2 data in many aspects. We investigated on the robustness of the results based on Kepler data. Methods. We cross-matched asteroseismic and astrometric data for over 12 000 red giant branch and red clump stars from the end-of-mission Kepler catalogue with the Gaia DR3 and TESS Input Catalogue v8.2 to obtain precise parallaxes, V- and K-band magnitudes, and E(B − V) colour excesses. Two well-tested SBCRs from the literature were adopted to estimate stellar radii. Results. The analysis confirmed that SBCR and asteroseismic radii agree very well. The overall differences are only 1–2% depending on the adopted SBCR. The dispersion of 7% was about two-thirds of what was found for K2-based data. As a difference from the K2-based investigation, the ratio of SBCRs-to-asteroseismic radii did not depend on the metallicity [Fe/H]. Moreover, the intriguing decreasing trend with [α/Fe] of the radius ratio for massive stars that was observed in K2 data was absent in Kepler data. The SBCR radii are systematically higher than asteroseismic estimates by 5% for stars with masses below 1.0 M⊙. Conclusions. The SBCRs have proven to be a highly effective tool for estimating radii with a precision comparable to that obtained from asteroseismology, but at a significantly lower observational cost. Moreover, the superior concordance of Kepler-derived radii with SBCR measurements and the absence of the discrepancies observed in the K2-derived radii suggest the existence of underlying systematic errors that impact specific mass and metallicity regimes within the K2 dataset.

MAGIS (Measuring Abundances of red super Giants with Infrared Spectroscopy) project

Context. Given their high luminosities (L ≳ 104 L⊙), red supergiants (RSGs) are good tracers of the chemical abundances of the young stellar population in the Milky Way and nearby galaxies. However, previous abundance analyses tailored to RSGs suffer some systematic uncertainties originating in, most notably, the synthesized molecular spectral lines for RSGs. Aims. We establish a new abundance analysis procedure for RSGs that circumvents difficulties faced in previous works, and test the procedure with ten nearby RSGs observed with the near-infrared high-resolution spectrograph WINERED (0.97−1.32 µm, R = 28 000). The wavelength range covered here is advantageous in that the molecular lines contaminating atomic lines of interest are mostly weak. Methods. We first determined the effective temperatures (Teff) of the targets with the line-depth ratio (LDR) method, and calculated the surface gravities (log 𝑔) according to the Stefan-Boltzmann law. We then determined the microturbulent velocities (vmicro) and metallicities ([Fe/H]) simultaneously through the fitting of individual Fe I lines. Finally, we also determined the abundance ratios ([X/Fe] for element X) through the fitting of individual lines. Results. We determined the [X/Fe] of ten elements (Na I, Mg I, Al I, Si I, K I, Ca I, Ti I, Cr I, Ni I, and Y II). We estimated the relative precision in the derived abundances to be 0.04−0.12 dex for elements with more than two lines analyzed (e.g., Fe I and Mg I) and up to 0.18dex for the other elements (e.g., Y II). We compared the resultant abundances of RSGs with the well-established abundances of another type of young star, namely the Cepheids, in order to evaluate the potential systematic bias in our abundance measurements, assuming that the young stars (i.e., both RSGs and Cepheids) in the solar neighborhood have common chemical abundances. We find that the determined RSG abundances are highly consistent with those of Cepheids within <0.1 dex for some elements (notably [Fe/H] and [Mg/Fe]), which means the bias in the abundance determination for these elements is likely to be small. In contrast, the consistency is worse for some other elements (e.g., [Si/Fe] and [Y/Fe]). Nevertheless, the dispersion of the chemical abundances among our target RSGs is comparable with the individual statistical errors on the abundances. Hence, the procedure is likely to be useful to evaluate the relative difference in chemical abundances among RSGs.

Chemical Evolution of R-process Elements in Stars (CERES)

Context. The third r-process peak (Os, Ir, Pt) is poorly understood due to observational challenges, with spectral lines located in the blue or near-ultraviolet region of stellar spectra. These challenges need to be overcome for a better understanding of the r-process in a broader context. Aims. To understand how the abundances of the third r-process peak are synthesised and evolve in the Universe, it is necessary to carry out a homogeneous chemical analysis of metal-poor stars using high-quality data observed in the blue region of the electromagnetic spectrum (<400 nm). We provide a homogeneous set of abundances for the third r-process peak (Os, Ir, Pt) and Hf, increasing their availability in the literature by up to one order of magnitude. Methods. We performed a classical 1D, local thermodynamic equilibrium (LTE) analysis of four elements (Hf, Os, Ir, Pt) using ATLAS model atmospheres to fit synthetic spectra on high signal-to-noise-ratio spectra of 52 red giants observed with UVES/VLT in high resolution (>40,000). Due to the heavy line blending involved, we carefully determined upper limits and uncertainties. The observational results are compared with state-of-the-art nucleosynthesis models. Results. Our sample displays larger abundances of Ir (Z=77) in comparison to Os (Z=76), both of which have been measured in a few stars in the past. The results also suggest decoupling between the abundances of third r-process peak elements with respect to Eu (a rare earth element) in Eu-poor stars. This seems to contradict a co-production scenario of Eu and the third r-process peak elements Os, Ir, and Pt in the progenitors of these objects. Our results are challenging to explain from a nucleosynthetic point of view: the observationally derived abundances indicate the need for an additional early, primary formation channel (or a non-robust r-process).

Decoding the Future of Exoplanets: Asteroseismic Confirmation of Subgiant and Red Giant Hosts

Abstract Asteroseismology has emerged as a powerful tool to unravel the intricate relationships between evolved stars and their planetary systems. In this study, we leverage this technique to investigate the evolutionary stages of five exoplanet host stars, each exhibiting solar-like oscillations. Building on our previous work that identified two host stars as red clump and red giant branch (RGB) stars, this study focuses on a new and broader sample. By precisely measuring asteroseismic parameters such as the period spacing of dipole gravity modes (ΔΠ1), we provide definitive confirmation of these stars’ evolutionary states as subgiants or RGB stars. These results are not only crucial for understanding the internal structures of evolved stars but also for predicting the eventual fate of their planetary companions, which may face engulfment as their host stars expand. This research highlights the profound role of asteroseismology in advancing our knowledge of planetary system evolution and opens new pathways for exploring how stellar evolution impacts planetary survival. Our findings set the stage for future studies on the dynamic fates of exoplanets, providing key insights into the intricate processes of stellar and planetary evolution.

Cosmic ray contributions from rapidly rotating stellar mass black holes: cosmic Ray GeV to EeV proton and anti-proton sources

In Radio Super Novae (RSNe) a magnetic field of (B × r) = 1016.0±0.12 Gauss × cm is observed; these are the same numbers for Blue Super Giant (BSG) star explosions as for Red Super Giant (RSG) star explosions, despite their very different wind properties. The EHT data for M87 as well for low power radio galaxies all show consistency with just this value of the quantity (B × r), key for angular momentum and energy transport, and can be derived from the radio jet data. We interpret this as a property of the near surroundings of a black hole (BH) at near maximal rotation, independent of BH mass. In the commonly used green onion model, in which a 2 π flow changes over to a jet flow we interpret this as a wind emanating from the BH/accretion disk system and its surroundings. Near the BH collisions in the wind can produce a large fraction of anti-protons. In this scenario the cosmic Ray (CR) population from the wind/jet is proposed to be visible as EeV protons and anti-protons in the CR data to EeV energy, with a E−7/3 spectrum. This can be connected to a concept of inner and outer Penrose zones in the ergo-region. The observed numbers for the magnetic field imply the Planck time as the governing time scale: A BH rotating near maximum can accept a proton per log bin of energy in an extended spectrum with the associated pions every Planck time.

Variability in hot sub-luminous stars and binaries: Machine-learning analysis of Gaia DR3 multi-epoch photometry

Hot sub-luminous stars represent a population of stripped and evolved red giants that is located on the extreme horizontal branch. Since they exhibit a wide range of variability due to pulsations or binary interactions, it is crucial to unveil their intrinsic and extrinsic variability to understand the physical processes of their formation. In the Hertzsprung-Russell diagram, they overlap with interacting binaries such as cataclysmic variables (CVs). By leveraging the most recent clustering algorithm tools, we investigate the variability of 1,576 candidate hot subdwarf variables using comprehensive data from Gaia DR3 multi-epoch photometry and Transiting Exoplanet Survey Satellite (TESS) observations. We present a novel approach that uses the t-distributed stochastic neighbour embedding and the uniform manifold approximation and projection dimensionality reduction algorithms to facilitate the identification and classification of different populations of variable hot subdwarfs and CVs in a large dataset. In addition to the publicly available Gaia time-series statistics table, we adopted additional statistical features that enhanced the performance of the algorithms. The clustering results led to the identification of 85 new hot subdwarf variables based on Gaia and TESS light curves and of 108 new variables based on Gaia light curves alone, including reflection-effect systems, HW Vir, ellipsoidal variables, and high-amplitude pulsating variables. A significant number of known CVs (140) distinctively cluster in the 2D feature space among an additional 152 objects that we consider candidates for new CVs. This study paves the way for more efficient and comprehensive analyses of stellar variability from ground- and space-based observations, and for the application of machine-learning classifications of candidate variable stars in large surveys.

The physical mechanism for the formation of lithium-rich red clump stars: Rotation, thermohaline mixing, and internal gravity waves

About 0.2-2 percent of red clump stars are revealed as Lithium-rich stars and thus the surface abundance of lithium clearly increases in some red clump stars. The physical mechanism of the enrichment of lithium on the surface of these stars has not yet been explained satisfactorily by the evolutionary models of single stars. Our aim is to investigate how rotation, thermohaline mixing, and internal gravity waves have an important impact on the surface chemical abundance of lithium-rich red giants. The equations for angular momentum transport and the chemical element diffusion for rotating stars have been implemented in this paper. The diffusion coefficients of rotationally induced instabilities, thermohaline mixing, and internal gravitational waves have been included in the diffusion equation of chemical elements. Rotational mixing, thermohaline mixing, and internal gravity waves have been invoked to explain this feature. Rotation impacts the evolution of the surface abundance of Lithium, but it seems an unlikely explanation for a ubiquitous mixing event occurring between the tip of the red giant star and the red clump star. Thermohaline mixing can explain the observed behaviour of $ C/^ C$ and $ N/^ C$ and lithium in low-mass stars that are more luminous than the red-giant branch bump, and its efficiency is decreasing with the increasing initial stellar mass. The internal gravity wave- (IGW-) induced mixing is located between the hydrogen-burning shell, and the outer convective envelope, and it is mainly triggered by turbulent convective motion. This physical process is beneficial to transfer the large amount of $ Be$ to the cool envelope where it is converted to $ Li$. Therefore, IGW-induced mixing could play a main role in explaining the red clump star with lithium enrichment. Rotation can indirectly increase the above effect by making the core-helium-burning lifetime longer. Thermohaline mixing is much smaller than the one of IGWs during the evolution of red clump stars.

Asteroseismic masses of red giants in the Galactic Globular Clusters M9 & M19

Abstract Asteroseismic masses of globular cluster (GC) stars are invaluable to investigate stellar evolution. Previously, only two GCs have been seismically studied. We present new detections of solar-like oscillations in the clusters M9 and M19, focusing on two key areas: stellar mass loss and GC multiple populations. Using K2 photometry, we detect solar-like oscillations in stars on the red giant branch and early asymptotic giant branch. We measure an integrated mass-loss for M9 of 0.16 ± 0.02(rand)$^{+0.03}_{-0.03}$(sys)M⊙ and M19 of 0.33 ± 0.03(rand)$^{+0.09}_{-0.07}$(sys)M⊙. Comparing these to the mass-loss estimates from previous seismically studied clusters, we derive a preliminary relationship between stellar mass-loss and metallicity for Type I GCs. We find that the mass-loss for M19 – a Type II GC– is significantly larger, suggesting Type II clusters follow a different mass-loss-metallicity trend. We also examine the mass distributions in each evolutionary phase for evidence of a bimodality that could indicate mass differences between sub-populations. While no clear bimodality is observed, there is tentative evidence suggesting the presence of two mass populations. Classification through spectroscopic abundances into the sub-populations is needed to verify these findings. This study reinforces that asteroseismology of GC stars provides an excellent testbed for studying stellar evolution. However, to advance the field we need high-quality photometry of more GCs, a goal that could be realised with the upcoming Roman Telescope.

Revisiting the Activity–Rotation Relation for Evolved Stars

The magnetic dynamo mechanism of giant stars remains an open question, which can be explored by investigating their activity–rotation relations with multiple proxies. By using the data from the LAMOST and Galaxy Evolution Explorer surveys, we carried out a comprehensive study of activity–rotation relations of evolved stars based on Ca ii H and K lines, Hα lines, and near-ultraviolet (NUV) emissions. Our results show that evolved stars and dwarfs obey a similar power law in the unsaturated region of the activity–rotation relation, indicating a common dynamo mechanism in both giants and dwarfs. There is no clear difference in the activity levels between red giant branch stars and red clump stars, nor between single giants and those in binaries. Additionally, our results show that the NUV activity levels of giants are comparable to those of G- and K-type dwarfs and are higher than those of M dwarfs.

Comparison of the Chemical Compositions between the Bright and Faint Red Clumps for the Metal-poor and Metal-rich Populations in the Milky Way Bulge

We examined the double red clump (RC) observed in the Galactic bulge, interpreted as a difference in distance (“X-shaped bulge scenario”) or in chemical composition (“multiple population scenario”). To verify chemical differences between the RC groups, we performed low-resolution spectroscopy for RC and red giant branch (RGB) stars using Gemini-South/GMOS in three fields of the bulge and collected diverse data from the literature. We divided our sample stars not only into bright RC (bRC) and faint RC (fRC) groups, but also into bluer ([Fe/H] < −0.1) and redder ([Fe/H] > −0.1) groups following recent u-band photometric studies. For the metal-poor stars, no statistically significant difference in the CN index was detected between the bright and faint RC groups for all observed fields. However, we found, from crossmatching with high-resolution spectroscopic data, a sign of Na enhancement in the “metal-poor and bright” RC group compared to the “metal-poor and faint” group at (l, b) = (−1°, −8.°5). When the contributions of the RGB stars on the RC regimes are taken into account, the Na abundance difference between genuine RCs would correspond to Δ[Na/Fe] ≃ 0.23 dex, similar to a globular cluster (GC) with multiple populations. In contrast, the metal-rich stars do not show chemical differences between the bright and faint RC groups. This implies that the double RC observed in the metal-poor component of the bulge might be linked to the multiple populations originating from GC-like subsystems, whereas that of the metal-rich component would have been produced by the X-shaped structure. Our results support previous studies suggesting the composite nature of the Milky Way bulge.

The shape of convection in 2D and 3D global simulations of stellar interiors

Context. Theoretical descriptions of convective overshooting in stellar interiors often rely on a basic one-dimensional parameterization of the flow called the filling factor for convection. Several different definitions of the filling factor have been developed for this purpose, based on: (1) the percentage of the volume, (2) the mass flux, and (3) the convective flux that moves through the boundary. Aims. We examine these definitions of the filling factor with the goal of establishing their ability to explain differences between 2D and 3D global simulations of stellar interiors that include fully compressible hydrodynamics and realistic microphysics for stars. Methods. We study convection and overshooting in pairs of identical two-dimensional (2D) and three-dimensional (3D) global simulations of stars produced with MUSIC, a fully compressible, time-implicit hydrodynamics code. We examine pairs of simulations for (1) a 3 M⊙ red giant star near the first dredge-up point, (2) a 1 M⊙ pre-main-sequence star with a large convection zone, (3) the current sun, and (4) a 20 M⊙ main-sequence star with a large convective core. Results. Our calculations of the filling factor based on the volume percentage and the mass flux indicate asymmetrical convection near the surface for each star with an outer convection zone. However, near the convective boundary, convective flows achieve inward-outward symmetry for each star that we study; for 2D and 3D simulations, these filling factors are indistinguishable. A filling factor based on the convective flux is contaminated by boundary-layer-like flows, making a theoretical interpretation difficult. We present two possible new alternatives to these frequently used definitions of a filling factor, which instead compare flows at two different radial points. The first alternative is the penetration parameter of Anders et al. (2022, ApJ, 926, 169). The second alternative is a new statistic that we call the plume interaction parameter. We demonstrate that both of these parameters captures systematic differences between 2D and 3D simulations around the convective boundary.

The evolutionary state of the red giant star L2 Puppis

Context. L2 Puppis (L2 Pup) is a nearby red giant star and an important object in late-type star research because it has a dust disc and potentially a companion. Aims. L2 Pup is often called the second-closest asymptotic giant branch (AGB) star to the sun, second only to R Doradus. However, whether the star is indeed on the AGB or the red giant branch (RGB) is questionable. We review its evolutionary state. Methods. We analysed high-resolution optical archive spectra to search for absorption lines of the third dredge-up indicator technetium (Tc) in L2 Pup. We also compared the star to a sample of well-known AGB stars in terms of luminosity and pulsation properties and placed it in a Gaia-2MASS diagram. Results. L2 Pup is found to be Tc-poor. Thus, it is not undergoing third dredge-up events. The star is fainter than the RGB tip and fainter than all Tc-rich stars in the comparison sample. L2 Pup pulsates in the fundamental mode, similar to Mira variables, but its pulsation properties do not allow us to distinguish between the RGB and AGB stages. Conclusions. In conclusion, L2 Pup could be an RGB or early AGB star, but it is more likely to be an RGB than an AGB star. Our findings are important for a better understanding of the L2 Pup system and its past and future evolution.

Constraints on Common Envelope Ejection from Double Helium White Dwarfs

Double helium white dwarfs (He WDs) are a type of gravitational wave source and are greatly important in studies of binary interaction, particularly in common envelope (CE) ejection physics. Most double He WDs with mass ratios of q ∼ 1 are formed through a particular channel. In this channel, one He WD is initially produced from a red giant (RG) with a degenerate core via stable Roche lobe overflow, and another He WD is formed from an RG with a degenerate core via CE ejection. They may have significant implications for binary evolution processes but have not received specific studies, especially for the CE phase. This paper adopts a semianalytic method and a detailed stellar evolution simulation to model the formation of double He WDs. We find that most double He WDs show mass ratios slightly greater than 1, and their orbital period–mass ratio relations are broadly consistent with observations. There is also a relation between the mass ratios and progenitor masses of the He WDs produced via CE ejection for double He WDs with determined WD masses. Based on this relation, the mass of the He WD progenitor can be inferred from the mass ratio. Then, the CE ejection efficiency can be constrained with the orbital period. In addition, we constrain the CE ejection efficiency for two double He WDs, J1005-2249 and WD0957-666. The results show that the CE ejection efficiencies increase with the WD progenitor masses.

Chemodynamic evolution of Sun-like stars in nearby moving groups

ABSTRACT Sun-like stars are well represented in the solar neighbourhood but are currently underutilized, with many studies of chemical and kinematic evolution focusing on red giants (which can be observed further away) or turn-off stars (which have well-measured ages). Recent surveys (e.g. GALAH) provide spectra for large numbers of nearby Sun-like stars, which provides an opportunity to apply our newly developed method for measuring metallicities, temperatures, and surface gravities – the ${\small EPIC}$ algorithm – which yields improved ages via isochrone fitting. We test this on moving groups, by applying it to the large GALAH DR3 sample. This defines a sample of $72\,288$ solar analogue targets for which the stellar parameter measurements are most precise and reliable. These stars are used to estimate, and test the accuracy and precision of, age measurements derived with the samd isochrone fitting algorithm. Using these ages, we recover the age–metallicity relationships for nearby ($\lesssim 1 \,{\rm kpc}$) moving groups, traced by solar analogues, and analyse them with respect to the stellar kinematics. In particular, we found that the age–metallicity relationships of all moving groups follow a particular trend of young ($\textrm {age}\lt 6 \,{\rm Gyr}$) stars having constant metallicity and older ($\textrm {age} \ge 6 \,{\rm Gyr}$) stars decreasing in metallicity with increasing age. The Hercules stream carries the highest fraction of metal-rich young stars ($\sim 0.1 \,{\rm dex}$) in our sample, which is consistent with a migrating population of stars from the inner Galaxy, and we discuss the possible causes of this migration in the context of our results.