Red Giant Research Articles

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

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. 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. We study convection and overshooting in pairs of identical two-dimensional (2D) and three-dimensional (3D) global simulations of stars produced with a fully compressible, time-implicit hydrodynamics code. We examine pairs of simulations for (1) a $3$ odot $ red giant star near the first dredge-up point, (2) a $1$ M$_ odot $ pre-main-sequence star with a large convection zone, (3) the current sun, and (4) a $20$ M$_ odot $ main-sequence star with a large convective core. 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 anders2022stellar . 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.

Finding the unusual red giant remnants of cataclysmic variable mergers

Mergers between helium white dwarfs and main-sequence stars are likely common, producing red giant-like remnants making up roughly a few percent of all low-mass ( ≲2M⊙) red giants. Through detailed modeling, we show that these merger remnants possess distinctive photometric, asteroseismic, and surface abundance signatures through which they may be identified. During hydrogen shell burning, merger remnants reach higher luminosities and possess pulsations which depart from the usual degenerate sequence on the asteroseismic Δν– ΔΠ diagram for red giant branch stars. For sufficiently massive helium white dwarfs, merger remnants undergo especially violent helium flashes which can dredge up a large amount of core material (up to ∼0.1M⊙) into the envelope. Such post-dredge-up remnants are more luminous than normal red clump stars, are surface carbon-, helium-, and possibly lithium-rich, and possess a wider range of asteroseismic g-mode period spacings and mixed-mode couplings. Recent asteroseismically determined low-mass ( ≲0.8M⊙) red clump stars may be core helium-burning remnants of mergers involving lower-mass helium white dwarfs.

Testing the asteroseismic estimates of stellar radii with surface brightness-colour relations and Gaia DR3 parallaxes. Red giants and red clump stars

We compared stellar radii derived from asteroseismic scaling relations with those estimated using two independent surface brightness-colour relations (SBCRs) combined with Gaia DR3 parallaxes. We cross-matched asteroseismic and astrometric data for over 6,400 red giant branch (RGB) and red clump (RC) stars from the APO-K2 catalogue with the TESS Input Catalogue v8.2 to obtain precise $V$ band magnitudes and $E(B-V)$ colour excesses. We then adopted two different SBCRs from the literature to derive stellar radius estimates, denoted as $R^a$ and $R^b$, respectively. We analysed the ratio of these SBCR-derived radii to the asteroseismic radius estimates, $R,$ provided in the APO-K2 catalogue. Both SBCRs exhibited good agreement with asteroseismic radius estimates. On average, $R^a$ was overestimated by 1.2<!PCT!> with respect to $R$, while $R^b$ was underestimated by 2.5<!PCT!>. For stars larger than 20 $R_ sun $, SBCR radii are systematically lower than asteroseismic ones. The dispersion in the radius ratio was similar for the two methods (around 10<!PCT!>). The agreement with asteroseismic radii shows a strong dependence on the parallax. The dispersion is halved for stars with a parallax greater than 2.5 mas. In this subsample, $R^b$ showed perfect agreement with $R$, while $R^a$ remained slightly overestimated, by 3<!PCT!>. A trend with Fe/H was found at a level of 4<!PCT!> to 6<!PCT!> per dex. Additionally, a clear trend with asteroseismic mass is found. For stars less massive than about 0.95 $M_ sun $, SBCR radii were significantly higher than asteroseismic ones, by about 6<!PCT!>. This overestimation correlated with the presence of extended helium cores in these stars' structures relative to their envelopes. Furthermore, radius ratios showed a dichotomous behaviour at higher masses, mainly due to the presence of several RC stars with SBCR radii significantly lower with respect to asteroseismology. This behaviour originates from a different response of asteroseismic scaling relations and SBCR to alpha /Fe abundance ratios for massive stars, both in RGB and RC phases, which is reported here for the first time.

Formation and Evolution of Special Stellar Types: A Comprehensive Study of Special Stars.

This article presents a comprehensive study of the formation, evolution, and distinctive characteristics of special stellar types, including protostars, super-giants, pulsars, red giants, etc. By exploring the life cycles of these stars, we gain a deeper understanding of the nuclear reactions, gravitational influences, and electromagnetic spectra that govern their development and eventual demise. Through a synthesis of observational data, theoretical models, and recent advancements in astrophysics, this research aims to elucidate the intricate processes driving the life cycles of these stellar phenomena. The findings contribute to the broader knowledge of stellar dynamics, offering insights into the mechanisms of stellar evolution and the fundamental forces shaping our universe.

True unicorns and false positives: Simulated probabilities of dark massive companions to bright stars

Abstract Many compact objects (black holes and neutron stars) exist in binaries. These binaries are normally discovered through their interactions, either from accretion as an X-ray binary or collisions as a gravitational wave source. However, the majority of compact objects in binaries should be non-interacting. Recently proposed discoveries have used radial velocities of a bright star (main sequence or evolved) that are indicative of a massive but dark companion, which is inferred to be a compact object. Unfortunately, this burgeoning new field has been hindered by false positives, including the “Unicorn” (V723 Mon) which was initially believed to be a red giant/black hole binary before being refuted. In this work, we investigate the evolution of stellar binary populations over time, using the binary evolution code COSMIC to simulate binary populations and determine the probability of a candidate object being either a “true Unicorn” (actual compact objects in binaries) or a false positive. We find that main sequence stars have a higher true Unicorn probability than red giants or naked helium stars (an exposed core of an evolved star), particularly if the companion is more massive and is ≥3 times less luminous than the MS star. We also find that a top-heavy initial mass function raises the true Unicorn probability further, that super-solar metallicity reduces the probability, and that most true Unicorns are found at periods ≤100 days. Finally, we find that a significant fraction of true Unicorns do not evolve into x-ray binaries during the age of the universe.

Investigation of motion around out-of-plane points in the restricted three-body problem with variable shape and masses

The paper examines out-of-plane equilibrium points (OEPs) of the restricted three-body problem with variable masses and shape. The bigger primary varies it shape as the lengths of the semi-axes vary with time. For the autonomized system, two pair of OEPs L6,7κ and L8,9κ, are obtained and differ from those of the non-autonomous system due to time t. The stability of OEPs of both systems is found to be unstable. Further, numerical illustrations is provided when variations in shape of the bigger primary is, a triaxial prolate, a sphere and a triaxial oblate shape. The positions, stability and zero velocity curves (ZVC) of the particle around the OEPs are explored. It is seen that when the bigger primary is a triaxial prolate body, the OEPs L8,9κ are closer to the primaries than L6,7κ. However, the converse happens when it is a triaxial oblate body. Also, when the bigger primary is a triaxial oblate body, the OEPs are farther away from the primaries than when it is a triaxial prolate. In the case of the ZVC, it is seen that when the bigger primary is a triaxial prolate body, there is a petal around it, and region of allowed motion of the particle increases, while the region reduces when the bigger primary evolves from a sphere to a triaxial oblate body. This study can be used to describe motion of a dust grain in the vicinity of Betelgeuse, a red giant star whose mass and shape changes with time and its stellar companion.

Determination of metallicities of red giant stars using machine learning techniques applied to the narrow and broadband photometry of the S-PLUS survey The catalogue is available at the CDS.

Determination of metallicities of red giant stars using machine learning techniques applied to the narrow and broadband photometry of the S-PLUS survey The catalogue is available at the CDS.

Anomalously low-mass core-He-burning star in NGC 6819 as a post-common-envelope phase product

Precise masses of red giant stars enable a robust inference of their ages, but there are cases where these age estimates are very precise but also very inaccurate. Examples are core-helium-burning (CHeB) stars that have lost more mass than predicted by standard single-star evolutionary models. Members of star clusters in the database represent a unique opportunity to identify such stars because they combine exquisite asteroseismic constraints with independent age information (members of a star cluster share a similar age and chemical composition). We focus on the single metal-rich ($Z Li-rich low-mass CHeB star KIC4937011, which is a member of the open cluster NGC 6819 (turn-off mass of $ i.e. an age of $ 2.4$ Gyr). This star has a lower mass by $ than expected for its age and metallicity, which might be explained by binary interactions or mass loss along the red giant branch (RGB). To infer formation scenarios for this object, we performed a Bayesian analysis by combining the binary stellar evolutionary framework binary\_c v2.2.3 with the dynamic nested-sampling approach contained in the dynesty v2.1.1 package. We find that this star probably is the result of a common-envelope evolution (CEE) phase during the RGB stage of the primary star in which the low-mass ($<0.71$ main-sequence companion does not survive. The mass of the primary star at the zero-age main sequence is in the range with a log-orbital period in the range days )$. During the CEE phase, $ of material is ejected from the system, and the final star reaches the CHeB stage after helium flashes as if it were a single star with a mass of $ which is what we observe today. Although the proposed scenario is consistent with photometric and spectroscopic observations, a quantitative comparison with detailed stellar evolution calculations is needed to quantify the systematic skewness of the radius, luminosity, and effective temperature distributions towards higher values than observations.

From gas to stars: MUSEings on the internal evolution of IC 1613

Context. The kinematics and chemical composition of stellar populations of different ages provide crucial information on the evolution of the various components of a galaxy. Aim. Our aim is to determine the kinematics of individual stars as a function of age in IC 1613, a star-forming, gas-rich, and isolated dwarf galaxy of the Local Group (LG). Methods. We present results of a new spectroscopic survey of IC 1613 conducted with MUSE, an integral field spectrograph mounted on the Very Large Telescope. We extracted ∼2000 sources, from which we separated stellar objects for their subsequent spectral analysis. The quality of the dataset allowed us to obtain accurate classifications (Teff to better than 500 K) and line-of-sight velocities (with average δv ∼ 7 km s−1) for about 800 stars. Our sample includes not only red giant branch (RGB) and main sequence (MS) stars, but also a number of probable Be and C stars. We also obtained reliable metallicities (δ[Fe/H] ∼ 0.25 dex) for about 300 RGB stars. Results. The kinematic analysis of IC 1613 revealed for the first time the presence of stellar rotation with high significance. We found general agreement with the rotation velocity of the neutral gas component. Examining the kinematics of stars as a function of broad age ranges, we find that the velocity dispersion increases as a function of age, with the behaviour being very clear in the outermost pointings, while the rotation-to-velocity dispersion support decreases. On timescales of < 1 Gyr, the stellar kinematics still follow very closely that of the neutral gas, while the two components decouple on longer timescales. The chemical analysis of the RGB stars revealed average properties comparable to other Local Group dwarf galaxies. We also provide a new estimation of the inclination angle using only independent stellar tracers. Conclusions. Our work provides the largest spectroscopic sample of an isolated LG dwarf galaxy. The results obtained seem to support the scenario in which the stars of a dwarf galaxy are born from a less turbulent gas over time.

Elemental Abundances in And XIX from Coadded Spectra

With a luminosity similar to that of Milky Way dwarf spheroidal systems like Sextans, but a spatial extent similar to that of ultra-diffuse galaxies, Andromeda (And) XIX is an unusual satellite of M31. To investigate the origin of this galaxy, we measure chemical abundances for And XIX derived from medium-resolution (R ∼ 6000) spectra from the Deep Extragalactic Imaging Multi-Object Spectrograph on the Keck II telescope. We coadd 79 red giant branch stars, grouped by photometric metallicity, in order to obtain a sufficiently high signal-to-noise ratio to measure 20 [Fe/H] and [α/Fe] abundances via spectral synthesis. The latter are the first such measurements for And XIX. The mean metallicity we derive for And XIX places it ∼2σ higher than the present-day stellar mass–metallicity relation for Local Group dwarf galaxies, potentially indicating it has experienced tidal stripping. A loss of gas and associated quenching during such a process, which prevents the extended star formation necessary to produce shallow [α/Fe]–[Fe/H] gradients in massive systems, is also consistent with the steeply decreasing [α/Fe]–[Fe/H] trend we observe. In combination with the diffuse structure and disturbed kinematic properties of And XIX, this suggests tidal interactions, rather than galaxy mergers, are strong contenders for its formation.

Benchmarking the effective temperature scale of red giant branch stellar models: The case of the metal-poor halo giant HD 122563

Context. There is plenty of evidence in the literature of significant discrepancies between the observations and models of metal-poor red giant branch stars, in particular regarding the effective temperature Teff scale. Aims. We revisit the benchmark star HD 122563 using the most recent observations from Gaia Data Release 3, to investigate if these new constraints may help in resolving this discrepancy. Methods. We review the most recent spectroscopic determinations of the metallicity [Fe/H] of HD 122563, and provide a new assessment of its fundamental parameters, specifically, bolometric luminosity, Teff, surface gravity, plus a photometric determination of its metal content. Using these constraints, we compare the position of the star in the Hertzsprung-Russell (H–R) diagram with various recent sets of stellar evolution tracks. Results. The H-R diagram analysis reveals a significant disagreement between observed and theoretical Teff values, when adopting the most recent spectroscopic estimate of [Fe/H]. On the other hand, by using the photometric determination of [Fe/H], some of the selected sets of stellar tracks appear in fair agreement with observations. The sets with discrepant Teff can be made to agree with observations either by modifying the prescription adopted to calculate the models’ outer boundary conditions, and/or by reducing the adopted value of the mixing length parameter with respect to the solar-calibration. Conclusions. A definitive assessment of whether the Teff scale of metal-poor stellar red giant branch models is consistent with observations requires a more robust determination of the fundamental parameters of HD 122563 and also a larger sample of calibrators. From the theoretical side, it is crucial to minimise the current uncertainties in the treatment (boundary conditions, temperature gradient) of the outer layers of stellar models with convective envelopes.

Element abundances of galactic RGB stars in the APO-K2 catalogue

Aims. We conducted an investigation on the chemical abundances of 4316 stars in the red giant branch (RGB) phase from the recently released APO-K2 catalogue. Our aim was to characterize the abundance trends of the single elements with [α/Fe], mainly focusing on C, N, and O, which are the most relevant for the estimation of stellar ages. Methods. The chemical analysis of the RGB sample involved cross-matching data from the APO-K2 catalogue with individual element abundances from APOGEE DR17. Results. The analysis detected a statistically significant difference in the [(C+N+O)/Fe]–[α/Fe] trend with respect to the simple α-enhancement scenario. This difference remained robust across different choices for the reference solar mixture and potential zero-point calibrations of C and N abundances. The primary discrepancy was a steeper increase in [O/Fe] with [α/Fe], reaching a 0.1 dex difference at [α/Fe] = 0.3. Notably, the impact on the evolutionary timescale of such oxygen over-abundance with respect to the commonly adopted uniform α-enhancement is rather limited. We verified that stellar models computed using an ad hoc O-rich mixture sped up the evolution by only 1% at [α/Fe] = 0.3, due to the counterbalancing effects of O enrichment on both the evolutionary timescale and the Z-to-[Fe/H] relationship.

Exploring the Small-scale Magnetic Fields in the Atmosphere of HD 49385 by Asteroseismic Analysis

Recent asteroseismic studies have shown convincing evidence that magnetic fields may exist in the interior of some pulsating red giants. Inspired by this breakthrough, we explored the effect of small-scale magnetic fields on the p-mode oscillations in an evolved star, HD 49385. We incorporate a modified Eddington T–τ equation that phenomenologically mimics the effect of the magnetic fields in the atmosphere of HD 49385, and calculate the frequencies of p-modes with l = 0, 1, and 2. By comparing the calculated frequencies with the observed ones, we select two best-fit models with either GS98 or A09 chemical composition. Our best-fit models not only fit satisfactorily the observed frequencies but also well reproduce some spectroscopically observed stellar parameters such as effective temperature and log g. Based on the two best-fit models, we have estimated that the small-scale magnetic fields possess a strength of approximately 80 G and spread concentratively at approximately a height of 1850 km in the atmosphere. By selecting the best-fit models with a special requirement on the avoided-crossing mode, we have confirmed that the frequency of the avoided-crossing mode is tightly related to the helium core of the star, and determined the size of the helium core as 0.117 M ⊙ in mass and 0.078 R ⊙ in radius. Based on the improvements of the previous two sides, we can accurately determine the mass of HD 49385 to be 1.25 ± 0.02 M ⊙ with an age of 4.1 Gyr for GS98 composition and 4.5 Gyr for A09 composition.

Asteroseismic signatures of core magnetism and rotation in hundreds of low-luminosity red giants

ABSTRACT Red giant stars host solar-like oscillations which have mixed character, being sensitive to conditions both in the outer convection zone and deep within the interior. The properties of these modes are sensitive to both core rotation and magnetic fields. While asteroseismic studies of the former have been done on a large scale, studies of the latter are currently limited to tens of stars. We aim to produce the first large catalogue of both magnetic and rotational perturbations. We jointly constrain these parameters by devising an automated method for fitting the power spectra directly. We successfully apply the method to 302 low-luminosity red giants. We find a clear bimodality in core rotation rate. The primary peak is at $\delta \nu _{\mathrm{rot}}$ = 0.32 $\mu$Hz, and the secondary at $\delta \nu _{\mathrm{rot}}$ = 0.47 $\mu$Hz. Combining our results with literature values, we find that the percentage of stars rotating much more rapidly than the population average increases with evolutionary state. We measure magnetic splittings of 2$\sigma$ significance in 23 stars. While the most extreme magnetic splitting values appear in stars with masses $\gt $1.1 M$_{\odot }$, implying they formerly hosted a convective core, a small but statistically significant magnetic splitting is measured at lower masses. Asymmetry between the frequencies of a rotationally split multiplet has previously been used to diagnose the presence of a magnetic perturbation. We find that of the stars with a significant detection of magnetic perturbation, 43 per cent do not show strong asymmetry. We find no strong evidence of correlation between the rotation and magnetic parameters.

The Three-phase Evolution of the Milky Way

We illustrate the formation and evolution of the Milky Way over cosmic time, utilizing a sample of 10 million red giant stars with full chemodynamical information, including metallicities and α-abundances from low-resolution Gaia XP spectra. The evolution of angular momentum as a function of metallicity—a rough proxy for stellar age, particularly for high-[α/Fe] stars—displays three distinct phases: the disordered and chaotic protogalaxy, the kinematically hot old disk, and the kinematically cold young disk. The old high-α disk starts at [Fe/H] ≈ −1.0, “spinning up” from the nascent protogalaxy, and then exhibiting a smooth “cooldown” toward more ordered and circular orbits at higher metallicities. The young low-α disk is kinematically cold throughout its metallicity range, with its observed properties modulated by a strong radial gradient. We interpret these trends using Milky Way analogs from the TNG50 cosmological simulation, identifying one that closely matches the kinematic evolution of our galaxy. This halo’s protogalaxy spins up into a relatively thin and misaligned high-α disk at early times, which is subsequently heated and torqued by a major gas-rich merger. The merger contributes a large amount of low-metallicity gas and angular momentum, from which the kinematically cold low-α stellar disk is subsequently born. This simulated history parallels several observed features of the Milky Way, particularly the decisive Gaia–Sausage–Enceladus merger that likely occurred at z ≈ 2. Our results provide an all-sky perspective on the emerging picture of our galaxy’s three-phase formation, impelled by the three physical mechanisms of spinup, merger, and cooldown.

Identifying Uncertainties in Stellar Evolution Models Using the Open Cluster M67

Stellar age estimates are often calculated by interpolating a star’s properties in a grid of models. However, different model grids will give different ages for the same star. We used the open cluster M67 to compare four different model grids: DSEP, GARSTEC, MIST, and YREC (10.5281/zenodo.12775242). Across all model grids, age estimates for main sequence stars were consistently higher than the accepted age of M67, while age estimates for red giant stars were lower. We compared model-generated age and mass values to external constraints as an additional test of the reliability of each model grid. For stars near solar age and metallicity, we recommend using the DSEP model grid to estimate the ages of main sequence stars and the GARSTEC model grid for red giant stars.

Revealing the chemical structure of the magellanic clouds with APOGEE. I. Calculating individual stellar ages of RGB stars in the large magellanic cloud

Abstract Stellar ages are critical for understanding the temporal evolution of a galaxy. We calculate the ages of over 6000 red giant branch stars in the Large Magellanic Cloud (LMC) observed with SDSS-IV / APOGEE-S. Ages are derived using multi-band photometry, spectroscopic parameters ($\rm T_{eff}$, log g, [Fe/H], and [α/Fe]) and stellar isochrones and the assumption that the stars lie in a thin inclined plane to get accurate distances. The isochrone age and extinction are varied until a best match is found for the observed photometry. We perform validation using the APOKASC sample, which has asteroseismic masses and accurate ages, and find that our uncertainties are ∼20% and range from ∼1–3 Gyr for the calculated ages (most reliable below 10 Gyr). Here we present the LMC age map as well as the age-radius relation and an accurate age-metallicity relation (AMR). The age map and age-radius relation reveal that recent star formation in the galaxy was more centrally located and that there is a slight dichotomy between the north and south with the northern fields being slightly younger. The northern fields that cover a known spiral arm have median ages of ≳2 Gyr, which is the time when an interaction with the SMC is suggested to have happened. The AMR is mostly flat especially for older ages although recently (about 2.0–2.5 Gyr ago) there is an increase in the median [Fe/H]. Based on the time frame, this might also be attributed to the close interaction between the LMC and SMC.

Li Evolution among Stars of Low/Intermediate Mass: the Metal-deficient Open Cluster NGC 2204

We have analyzed high-dispersion spectra in the Li 6708 Å region for 167 stars within the anticenter cluster NGC 2204. From 105 probable members, abundance analysis of 45 evolved stars produces [Fe/H] = −0.40 ± 0.12, [Si/Fe] = 0.14 ± 0.12, [Ca/Fe] = 0.29 ± 0.07, and [Ni/Fe] = −0.12 ± 0.10, where quoted errors are standard deviations. With E(B − V) = 0.07 and (m − M)0 = 13.12, appropriate isochrones provide an excellent match from the main sequence through the tip of the giant branch for an age of 1.85 ± 0.05 Gyr. Li spectrum synthesis produces A(Li) below 1.4 at the base of the red giant branch to a detectable value of −0.4 at the tip. Six probable asymptotic giant branch stars and all but one red clump star have only Li upper limits. A rapidly rotating red giant is identified as a possible Li-rich giant, assuming it is a red clump star. Main-sequence turnoff stars have a well-defined A(Li) = 2.83 ± 0.03 (sem) down to the Li-dip wall at the predicted mass of 1.29 M ☉. Despite having the same isochronal age as the more metal-rich NGC 2506, the luminosity distribution of red giants reflects a younger morphology similar to NGC 7789, possibly indicating a deeper impact of metallicity on stellar structure and A(Li) than previously assumed. As in NGC 2506 and NGC 7789, the NGC 2204 turnoff exhibits a broad range of rotation speeds, making abundance estimation impossible for some stars. The place of the cluster within Galactic A(Li) evolution is discussed.

Revealing the Fate of Exoplanet Systems: Asteroseismic Identification of Host Star in the Red Clump or Red Giant Branch

Determining the evolutionary stage of stars is crucial for understanding the evolution of exoplanetary systems. In this context, red giant branch (RGB) and red clump (RC) stars, which formed at stages in the later evolution of stars situated before and after the helium flash, harbor critical clues to unveiling the evolution of planets. The first step in revealing these clues is to confirm the evolutionary stage of the host stars through asteroseismology. However, up to now, host stars confirmed to be RGB or RC stars are extremely rare. In this investigation, we present a comprehensive asteroseismic analysis of two evolved stars, HD 120084 and HD 29399, known to harbor exoplanets, using data from the Transiting Exoplanet Survey Satellite. We have discovered for the first time that HD 120084 is an RC star in the helium-core-burning phase, and confirmed that HD 29399 is an RGB star in the hydrogen-shell burning phase. Through the precise measurement of asteroseismic parameters such as νmax , Δν, and ΔΠ1, we have determined the evolutionary states of these stars and derived their fundamental stellar parameters. The significance of this study lies in the application of automated techniques to measure asymptotic period spacings in red giants, which provides critical insights into the evolutionary outcomes of exoplanet systems. We demonstrate that asteroseismology is a potent tool for probing the internal structures of stars, thereby offering a window into the past and future dynamics of planetary orbits. The presence of a long-period giant planet orbiting HD 120084, in particular, raises intriguing questions about the potential engulfment of inner planets during the host star’s expansion, a hypothesis that warrants further investigation.

Asteroseismic measurement of core and envelope rotation rates for 2006 red giant branch stars

Context. Tens of thousands of red giant stars in the Kepler data exhibit solar-like oscillations. The mixed-mode characteristics of their oscillations enable us to study the internal physics from the core to the surface, such as differential rotation. However, envelope rotation rates have only been measured for about a dozen red giant branch (RGB) stars so far. This limited the theoretical interpretation of angular momentum transport in post-main sequence phases. Aims. We report the measurements of g-mode properties and differential rotation in the largest sample of Kepler RGB stars. Methods. We applied a new approach to calculate the asymptotic frequencies of mixed modes, which accounts for so-called near-degeneracy effects (NDEs) and leads to improved measurements of envelope rotation rates. By fitting these asymptotic expressions to the observations, we obtained measurements of the properties of g modes (period spacing, ΔΠ1, coupling factor, q, g-mode offset term, εg, small separation, δν01) and the internal rotation (mean core, Ωcore, and envelope, Ωenv, rotation rates). Results. Among 2495 stars with clear mixed-mode patterns, we found that 800 show doublets and 1206 show triplets, while the remaining stars do not show any rotational splittings. We measured core rotation rates for 2006 red giants, doubling the size of pre-existing catalogues. This led us to discover an over-density of stars that are narrowly distributed around a well-defined ridge in the plane, showing core rotation rate versus evolution along the RGB. These stars could experience a different angular momentum transport compared to other red giants. With this work, we also increased the sample of stars with measured envelope rotation rates by two orders of magnitude. We found a decreasing trend between envelope rotation rates and evolution, implying that the envelopes slow down with expansion, as expected. We found 243 stars whose envelope rotation rates are significantly larger than zero. For these stars, the core-to-envelope rotation ratios are around Ωcore/Ωenv ∼ 20 and show a large spread with evolution. Several stars show extremely mild differential rotations, with core-to-surface ratios between 1 and 2. These stars also have very slow core rotation rates, suggesting that they go through a peculiar rotational evolution. We also discovered more stars located below the ΔΠ1–Δν degeneracy sequence, which presents an opportunity to study the history of plausible stellar mergers.