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The standard cosmological model predicts that galaxies are built through hierarchical assembly on cosmological timescales 1 , 2 . The Milky Way, like other disk galaxies, underwent violent mergers and accretion of small satellite galaxies in its early history. Owing to Gaia Data Release 2 3 and spectroscopic surveys 4 , the stellar remnants of such mergers have been identified 5 – 7 . The chronological dating of such events is crucial to uncover the formation and evolution of the Galaxy at high redshift, but it has so far been challenging due to difficulties in obtaining precise ages for these oldest stars. Here we combine asteroseismology—the study of stellar oscillations—with kinematics and chemical abundances to estimate precise stellar ages (~11%) for a sample of stars observed by the Kepler space mission 8 . Crucially, this sample includes not only some of the oldest stars that were formed inside the Galaxy but also stars formed externally and subsequently accreted onto the Milky Way. Leveraging this resolution in age, we provide compelling evidence in favour of models in which the Galaxy had already formed a substantial population of its stars (which now reside mainly in its thick disk) before the infall of the satellite galaxy Gaia-Enceladus/Sausage 5 , 6 around 10 billion years ago. Leveraging asteroseismology, stellar abundances and kinematics to derive precise ages for a sample of 95 stars, Montalbán et al. determine that the Milky Way was already host to a substantial population of stars when it was just 3.8 billion years old, at the time of the Gaia-Enceladus accretion event.

Red giants are stars in the late stages of stellar evolution. Because they have exhausted the supply of hydrogen in their core, they burn the hydrogen in the surrounding shell . Once the helium in the core starts fusing, the star enters the clump phase, which is identified as a striking feature in the color-magnitude diagram. Since clump stars share similar observational properties, they are heavily used in astrophysical studies, as probes of distance, extinction through the galaxy, galaxy density, and stellar chemical evolution. In this work, we perform the detailed observational characterization of the deepest layers of clump stars using asteroseismic data from Kepler. We find evidence for large core structural discontinuities in about 6.7% of the stars in our sample, implying that the region of mixing beyond the convective core boundary has a radiative thermal stratification. These stars are otherwise similar to the remaining stars in our sample, which may indicate that the building of the discontinuities is an intermittent phenomenon. Red giant stars enter the clump phase as the helium in the cores start fusing. Here, the authors show evidence for large core structural discontinuities in 7% of Kepler satellite clump star data implying that the mixing region beyond the convective core boundary has a radiative thermal stratification.

The Kepler space mission has observed many solar-like pulsators, and helped to decipher their fundamental parameters (e.g: mass, radius, rotation). Most of the achievements recently obtained in that domain result from the analysis of the mode frequencies. However, unique information on non-adiabatic physics derives from the height and width of the modes. In this study, we aim at measuring the mode widths of the pressure modes in thousands of Kepler red giants and to analyze their variations in function of stellar parameters. To achieve that, we used a peakbagging technique on the star radial modes. The results show a relation between the radial mode linewidth and the effective temperature of the star as theoretically predicted. We also unveil a clear dependence with mass and stellar evolution for the radial mode width. This means that the mode damping depends on the evolutionary status of the stars.

When main-sequence stars expand into red giants, they are expected to engulf close-in planets 1 – 5 . Until now, the absence of planets with short orbital periods around post-expansion, core-helium-burning red giants 6 – 8 has been interpreted as evidence that short-period planets around Sun-like stars do not survive the giant expansion phase of their host stars 9 . Here we present the discovery that the giant planet 8 Ursae Minoris b 10 orbits a core-helium-burning red giant. At a distance of only 0.5  au from its host star, the planet would have been engulfed by its host star, which is predicted by standard single-star evolution to have previously expanded to a radius of 0.7  au . Given the brief lifetime of helium-burning giants, the nearly circular orbit of the planet is challenging to reconcile with scenarios in which the planet survives by having a distant orbit initially. Instead, the planet may have avoided engulfment through a stellar merger that either altered the evolution of the host star or produced 8 Ursae Minoris b as a second-generation planet 11 . This system shows that core-helium-burning red giants can harbour close planets and provides evidence for the role of non-canonical stellar evolution in the extended survival of late-stage exoplanetary systems. The giant planet 8 Ursae Minoris b seems to have avoided engulfment by its giant host star through a stellar merger that either affected the evolution of the host star or produced 8 Ursae Minoris b as a second-generation planet.

Over the course of its history, the Milky Way has ingested multiple smaller satellite galaxies 1 . Although these accreted stellar populations can be forensically identified as kinematically distinct structures within the Galaxy, it is difficult in general to date precisely the age at which any one merger occurred. Recent results have revealed a population of stars that were accreted via the collision of a dwarf galaxy, called Gaia–Enceladus 1 , leading to substantial pollution of the chemical and dynamical properties of the Milky Way. Here we identify the very bright, naked-eye star ν Indi as an indicator of the age of the early in situ population of the Galaxy. We combine asteroseismic, spectroscopic, astrometric and kinematic observations to show that this metal-poor, alpha-element-rich star was an indigenous member of the halo, and we measure its age to be 11.0 ± 0.7 (stat) ± 0.8 (sys) billion years. The star bears hallmarks consistent with having been kinematically heated by the Gaia–Enceladus collision. Its age implies that the earliest the merger could have begun was 11.6 and 13.2 billion years ago, at 68% and 95% confidence, respectively. Computations based on hierarchical cosmological models slightly reduce the above limits. Bright star ν Indi shows elevated levels of alpha-process elements, suggesting great age, and is kinematically heated, probably from the merger of a dwarf galaxy with the Milky Way. Chaplin et al. make a case for ν Indi being an accurate indicator of the timing for the Gaia–Enceladus merger.

The space-borne missions CoRoT and Kepler have provided seismic data of unprecedented quality. Among the observed stars, red giants show a complex oscillation pattern exhibiting pressure modes as well as mixed modes. The latter carry information on the radiative region properties of these stars. The very high precision of Kepler data provide enough accuracy to decipher the complex structure of the mixed-mode pattern and, therefore, deduce precise information on the structure of the stellar core. In this work, we studied the precise influence of the core structural discontinuities on the mixed-mode pattern. These phenomena have indeed an influence on the gravity waves that propagate inside the core of the star and, therefore, on the mixed-mode pattern. In this study, we aim to investigate the impact of core structural discontinuities on the observed mixed-mode pattern and measure their properties for several red giants. We identified several objects showing evidence of deviations in their mixed-mode frequency pattern that are characteristic of core structural discontinuities. We fitted these deviations and showed that they likely correspond to the influence of the inner convective core of these stars.

Red giants are stars in the late stages of stellar evolution. Because they have exhausted the supply of hydrogen in their core, they burn the hydrogen in the surrounding shell. Once the helium in the core starts fusing, the star enters the clump phase, which is identified as a striking feature in the color-magnitude diagram. Since clump stars share similar observational properties, they are heavily used in astrophysical studies, as probes of distance, extinction through the galaxy, galaxy density, and stellar chemical evolution. In this work, we perform the detailed observational characterization of the deepest layers of clump stars using asteroseismic data from Kepler. We find evidence for large core structural discontinuities in about 6.7% of the stars in our sample, implying that the region of mixing beyond the convective core boundary has a radiative thermal stratification. These stars are otherwise similar to the remaining stars in our sample, which may indicate that the building of the discontinuities is an intermittent phenomenon.

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 \\(\\nu_{max}\\) and the large frequency separation \\(\\Delta \\nu\\), 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 \\(\\nu_{max}\\) and \\(\\Delta \\nu\\) scaling relations separately, by using stars of known mass and radius in star clusters and the Milky Way's high-\\(\\alpha\\) sequence. We find evidence that the \\(\\Delta \\nu\\)-scaling relation contributes to the observed breakdown in luminous giants more than the \\(\\nu_{max}\\) relation. We test different methods of mapping the observed \\(\\Delta \\nu\\) to the mean density via a correction factor, \\(F_{\\Delta \\nu}\\) and find a \\(\\approx 1 - 3\\%\\) difference in the radii in the luminous giant regime depending on the technique used to measure \\(F_{\\Delta \\nu}\\). 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_{\\Delta \\nu}\\) correction is insensitive to the adopted mixing length, chosen by calibrating the models to observations of \\(T_{eff}\\).

We report novel observational evidence on the evolutionary status of lithium-rich giant stars by combining asteroseismic and lithium abundance data. Comparing observations and models of the asteroseismic gravity-mode period spacing \\(\\Delta\\Pi_{1}\\), we find that super-Li-rich giants (SLR, A(Li)~\\(> 3.2\\)~dex) are almost exclusively young red-clump (RC) stars. Depending on the exact phase of evolution, which requires more data to refine, SLR stars are either (i) less than \\(\\sim 2\\)~Myr or (ii) less than \\(\\sim40\\)~Myr past the main core helium flash (CHeF). Our observations set a strong upper limit for the time of the inferred Li-enrichment phase of \\(< 40\\)~Myr post-CHeF, lending support to the idea that lithium is produced around the time of the CHeF. In contrast, the more evolved RC stars (\\(> 40\\)~Myr post-CHeF) generally have low lithium abundances (A(Li)~\\(<1.0\\)~dex). Between the young, super-Li-rich phase, and the mostly old, Li-poor RC phase, there is an average reduction of lithium by about 3 orders of magnitude. This Li-destruction may occur rapidly. We find the situation to be less clear with stars having Li abundances between the two extremes of super-Li-rich and Li-poor. This group, the `Li-rich' stars (\\(3.2 >\\)~A(Li)~\\(> 1.0\\)~dex), shows a wide range of evolutionary states.

Robust age estimates of red giant stars are now possible thanks to the precise inference of their mass based on asteroseismic constraints. However, there are cases where such age estimates can be highly precise yet very inaccurate. An example is giants that have undergone mass loss or mass transfer events that have significantly altered their mass. In this context, stars with \"apparent\" ages significantly higher than the age of the Universe are candidates as stripped stars, or stars that have lost more mass than expected, most likely via interaction with a companion star, or because of the poorly understood mass-loss mechanism along the red-giant branch. In this work we identify examples of such objects among red giants observed by \\(\\textit{Kepler}\\), both at low ([Fe/H] \\( \\lesssim -0.5\\)) and solar metallicity. By modelling their structure and pulsation spectra, we find a consistent picture confirming that these are indeed low-mass objects consisting of a He core of \\(\\approx 0.5 \\, M_\\odot\\) and an envelope of \\(\\approx 0.1 - 0.2 \\, M_\\odot\\). Moreover, we find that these stars are characterised by a rather extreme coupling (\\(q \\gtrsim 0.4\\)) between the pressure-mode and gravity-mode cavities, i.e. much higher than the typical value for red clump stars, providing thus a direct seismic signature of their peculiar structure. The complex pulsation spectra of these objects, if observed with sufficient frequency resolution, hold detailed information about the structural properties of likely products of mass stripping, hence can potentially shed light on their formation mechanism. On the other hand, our tests highlight the difficulties associated with measuring reliably the large frequency separation, especially in shorter datasets, with impact on the reliability of the inferred masses and ages of low-mass Red Clump stars with e.g. K2 or TESS data.