Thermohaline Mixing Research Articles

Isopycnal Eddy Stirring Dominates Thermohaline Mixing in the Upper Subpolar North Atlantic

AbstractThe Atlantic Meridional Overturning Circulation entails vigorous thermohaline transformations in the subpolar North Atlantic Ocean (SPNA). There, warm and saline waters originating in the (sub)tropics are converted into cooler and fresher waters by a combination of surface fluxes and sub‐surface mixing. Using microstructure measurements and a small‐scale variance conservation framework, we quantify the diapycnal and isopycnal contributions –by microscale turbulence and mesoscale eddies, respectively– to thermohaline mixing within the eastern SPNA. Isopycnal stirring is found to account for the majority of thermal (65%) and haline (84%) variance dissipation in the upper 400 m of the eastern SPNA. A simple dimensional analysis suggests that isopycnal stirring could account for (5–10) Sv of diahaline volume flux, suggesting an important role of such stirring in regional water‐mass transformations. Our mixing measurements are thus consistent with recent indirect estimates in highlighting the importance of isopycnal stirring for North Atlantic overturning.

Enrichment of Chemical Element 7Li in the Rotating Red Clump Star

Abstract About 2 percent of red clump stars are found to be the lithium-rich and thus the surface lithium increases obviously in some red clump stars. The physical mechanism of the lithium enrichment in these stars has not yet been explained satisfactorily by the evolutionary models of single stars. The flash induced internal gravity wave mixing (i.e., IGW) could play a primary role in explaining the red clump star with lithium enrichment and it has a very significant impact on the internal structure and surface compositions of a star. Rotation can significantly increase the mixing efficiency of the internal gravity wave because the timescale for the enrichment event has been enlarged. Thermohaline mixing can explain the observed behavior of lithium on red giant stars that are more luminosity than the RGB bump. However, it has a very small effect on the diffusion of elements because its diffusion coefficient is much smaller than the one of IGW induced mixing after the core helium flash.

12C/13C of Kepler giant stars: The missing piece of the mixing puzzle

Context. Despite a rich observational background, few spectroscopic studies have dealt with the measurement of the carbon isotopic ratio in giant stars. However, it is a key element in understanding the mixing mechanisms that occur in the interiors of giant stars. Aims. We present the CNO and 12C/13C abundances derived for 71 giant field stars. Then, using this new catalogue and complementary data from the Kepler and Gaia satellites, we study the efficiency of mixing occurring in the giant branch as a function of the stellar properties of the stars (e.g. mass, age, metallicity). Methods. We determined the abundances of CNO and more specifically the carbon isotopic ratio using the high-resolution FIbre-fed Echelle Spectrograph on the Nordic Optical Telescope, for 71 giant field stars. In addition, asteroseismology from the Kepler satellite is available for all stars, providing the stellar masses, ages, and evolutionary states. Finally, astrometry from the Gaia data is also available for most of the sample. We compare these new determinations with stellar evolution models taking into account the effects of transport processes. To exploit the complete potential of our extensive catalogue, and considering both the Milky Way evolution and the impact of stellar evolution, we built mock catalogues using the Besançon Galaxy model in which stellar evolution models taking into account the effects of thermohaline instability are included. Results. We confirm that the carbon isotopic ratio at the surface of core He-burning stars is lower than that of first-ascent RGB stars. The carbon isotopic ratio measured at the surface of the core He-burning stars increases with [Fe/H] and stellar mass, while it decreases with stellar age. These trends are all nicely explained by the thermohaline mixing that occurs in red giants. We show that our models can explain the behaviour of 12C/13C versus N/O, although the observations seem to show a lower N/O than the models. We also note that more constraints on the thick disc core He-burning stars are needed to understand this difference. Conclusions. Overall, the current model including thermohaline mixing is able to reproduce very well the 12C/13C with the stellar metallicity and with the stellar mass and age.

Fluid Mixing during Phase Separation in Crystallizing White Dwarfs

Accurate models of cooling white dwarfs must treat the energy released as their cores crystallize. This phase transition slows the cooling by releasing latent heat and also gravitational energy, which results from phase separation: liquid C is released from the solid C/O core, driving an outward carbon flux. The Gaia color–magnitude diagram provides striking confirmation of this theory by revealing a mass-dependent overdensity of white dwarfs, indicating slowed cooling at the expected location. However, the observed overdensity is enhanced relative to the models. Additionally, it is associated with increased magnetism, suggesting a link between crystallization and magnetic field generation. Recent works aimed at explaining an enhanced cooling delay and magnetic field generation employ a uniform mixing prescription that assumes large-scale turbulent motions; we show here that these calculations are not self-consistent. We also show that thermohaline mixing is most likely efficient enough to provide the required chemical redistribution during C/O phase separation, and that the resulting velocities and mixing lengths are much smaller than previous estimates. These reduced fluid motions cannot generate measurable magnetic fields, suggesting any link with crystallization needs to invoke a separate mechanism. Finally, this mixing alters the chemical profiles, which in turn affects the frequencies of the pulsation modes.

An investigation of non-canonical mixing in red giant stars using APOGEE 12C/13C ratios observed in open cluster stars

ABSTRACT Standard stellar evolution theory poorly predicts the surface abundances of chemical species in low-mass, red giant branch (RGB) stars. Observations show an enhancement of p–p chain and CNO cycle products in red giant envelopes, which suggests the existence of non-canonical mixing that brings interior burning products to the surface of these stars. The 12C/13C ratio is a highly sensitive abundance metric used to probe this mixing. We investigate extra RGB mixing by examining: (1) how 12C/13C is altered along the RGB, and (2) how 12C/13C changes for stars of varying age and mass. Our sample consists of 43 red giants, spread over 15 open clusters from the Sloan Digital Sky Survey’s APOGEE DR17, that have reliable 12C/13C ratios derived from their APOGEE spectra. We vetted these 12C/13C ratios and compared them as a function of evolution and age/mass to the standard mixing model of stellar evolution, and to a model that includes prescriptions for RGB thermohaline mixing and stellar rotation. We find that the observations deviate from standard mixing models, implying the need for extra mixing. Additionally, some of the abundance patterns depart from the thermohaline model, and it is unclear whether these differences are due to incomplete observations, issues inherent to the model, our assumption of the cause of extra mixing, or any combination of these factors. Nevertheless, the surface abundances across our age/mass range clearly deviate from the standard model, agreeing with the notion of a universal mechanism for RGB extra mixing in low-mass stars.

Reinterpreting the Polluted White Dwarf SDSS J122859.93+104032.9 in Light of Thermohaline Mixing Models: More Polluting Material from a Larger Orbiting Solid Body

The polluted white dwarf (WD) system SDSS J122859.93+104032.9 (SDSS J1228) shows variable emission features interpreted as originating from a solid core fragment held together against tidal forces by its own internal strength, orbiting within its surrounding debris disk. Estimating the size of this orbiting solid body requires modeling the accretion rate of the polluting material that is observed mixing into the WD surface. That material is supplied via sublimation from the surface of the orbiting solid body. The sublimation rate can be estimated as a simple function of the surface area of the solid body and the incident flux from the nearby hot WD. On the other hand, estimating the accretion rate requires detailed modeling of the surface structure and mixing in the accreting WD. In this work, we present MESA WD models for SDSS J1228 that account for the thermohaline instability and mixing in addition to heavy element sedimentation to constrain accurately the sublimation and accretion rate necessary to supply the observed pollution. We derive a total accretion rate of , several orders of magnitude higher than the estimate obtained in earlier efforts. The larger mass accretion rate implies that the minimum estimated radius of the orbiting solid body is = 72 km, which, although significantly larger than prior estimates, still lies within the upper bounds (a few hundred kilometers) for which the internal strength could no longer withstand the tidal forces from the gravity of the WD.

Evidence of extra mixing in field giants as traced by the lithium and carbon isotope ratio

Context.Although not predicted by standard stellar evolution, the surface abundance of light elements, such as lithium (Li), carbon, and nitrogen, changes during the red giant branch (RGB) as a result of extra mixing. This is usually associated with thermohaline mixing acting after the RGB bump. Peculiar Li-enriched RGB stars might also be related to either enhanced mixing or pollution from external sources.Aims.We measure the Li abundance and carbon isotopic ratio12C/13C in a sample of 166 field red giants with −0.3 ≤ [Fe/H] ≤ 0.2, targeted by the EXPRESS radial velocity program to analyze the effects of extra mixing.Methods.We measured the abundances with spectral synthesis using high-quality spectra. Multiple-epoch observations needed for exoplanet detection were used to decrease the effects of telluric contamination in12C/13C measurements.Results.Due to the prevalence of upper limits, the Li abundance pattern is complicated to interpret, but the comparison between RGB and core He-burning giants shows effects of mixing consistent with thermohaline. The most Li-enriched giant in the sample, classified as a RGB star close to the RGB bump, has low12C/13C. Given that the12C/13C should not be affected by planet engulfment, this does not seem to be the source of the high Li. There is a decreasing correlation between mass and12C/13C in the RGB and an increasing correlation in the horizontal branch, which, once again, is consistent with thermohaline mixing. Our data also show a correlation between12C/13C and [Fe/H]. There is no evident impact of binarity either on Li or on12C/13C.Conclusions.Our sample shows behavior consistent with additional mixing acting after the RGB bump. The12C/13C adds new clues which can be used to describe extra mixing, and it could well be the best tool to study mixing in giants. Additional measurements of12C/13C in field stars would greatly improve our ability to compare data with models and understand mixing mechanisms.

Distribution and Evolution of the Li Abundance in Red Clump Stars Can Be Explained by Internal Gravity Waves

The study of Li phenomena in red clump (RC) stars can give us a deeper understanding of the structure and evolution of stars. Chanamé et al. explained the RC Li abundance distributions naturally using only standard post-main-sequence (MS) Li evolution models when the distribution of progenitor masses and the depletion of Li during the MS observed in MS stars were considered, thus neither an extra Li depletion nor Li creation mechanism is required. Nevertheless, it is interesting to consider the effects of mixing caused by some extra mechanisms. By constructing different models, we find that the mixing caused by internal gravity waves can explain the observed Li abundances of RC stars with low-mass progenitors. To explain this, we rely on the extra mixing induced by internal gravity waves that are excited at the bottom of the convective envelope during the red giant branch (RGB) stage. During the RGB stage, introducing internal gravity waves can improve the diffusion coefficient and strengthen the mixing effect. The effective enrichment of Li occurs during the late RGB stage and requires the diffusion coefficient of the H-burning shell to reach ∼108 cm2 s−1. Our models predict that the Li abundance decreases from ∼1.5 to ∼0.0 dex at the end of the core He-burning stage, thereby revealing ∼99% of the observed Li abundance distribution. Thermohaline mixing regulates the Li abundance of RGB stars, which combined with internal gravity waves can explain the Li abundances of most giants.

Planet engulfment signatures in twin stars

ABSTRACT Planet engulfment can be inferred from enhancement of refractory elements in the photosphere of the engulfing star following accretion of rocky planetary material. Such refractory enrichments are subject to stellar interior mixing processes, namely thermohaline mixing induced by an inverse mean-molecular-weight gradient between the convective envelope and radiative core. Using mesa stellar models, we quantified the strength and duration of engulfment signatures following planet engulfment. We found that thermohaline mixing dominates during the first ∼5–45 Myr post-engulfment, weakening signatures by a factor of ∼2 before giving way to depletion via gravitational settling on longer time-scales. Solar metallicity stars in the 0.5–1.2 M⊙ mass range have observable signature time-scales of ∼1 Myr–8 Gyr, depending on the engulfing star mass and amount of material engulfed. Early type stars exhibit larger initial refractory enhancements but more rapid depletion. Solar-like stars (M = 0.9–1.1 M⊙) maintain observable signatures (>0.05 dex) over time-scales of ∼20 Myr–1.7 Gyr for nominal 10 M⊕ engulfment events, with longer-lived signatures occurring for low-metallicity and/or hotter stars (1 M⊙, ∼2–3 Gyr). Engulfment events occurring well after the zero-age main sequence produce larger signals due to suppression of thermohaline mixing by gravitational settling of helium (1 M⊙, ∼1.5 Gyr). These results indicate that it may be difficult to observe engulfment signatures in solar-like stars that are several Gyr old.

Characterizing Observed Extra Mixing Trends in Red Giants using the Reduced Density Ratio from Thermohaline Models

Observations show an almost ubiquitous presence of extra mixing in low-mass upper giant branch stars. The most commonly invoked explanation for this is thermohaline mixing. One-dimensional stellar evolution models include various prescriptions for thermohaline mixing, but the use of observational data directly to discriminate between thermohaline prescriptions has thus far been limited. Here, we propose a new framework to facilitate direct comparison: using carbon-to-nitrogen measurements from the Sloan Digital Sky Survey-IV APOGEE survey as a probe of mixing and a fluid parameter known as the reduced density ratio from one-dimensional stellar evolution programs, we compare the observed amount of extra mixing on the upper giant branch to predicted trends from three-dimensional fluid dynamics simulations. Using this method, we are able to empirically constrain how mixing efficiency should vary with the reduced density ratio. We find the observed amount of extra mixing is strongly correlated with the reduced density ratio and that trends between reduced density ratio and fundamental stellar parameters are robust across choices for modeling prescription. We show that stars with available mixing data tend to have relatively low density ratios, which should inform the regimes selected for future simulation efforts. Finally, we show that there is increased mixing at low reduced density ratios, which is consistent with current hydrodynamical models of thermohaline mixing. The introduction of this framework sets a new standard for theoretical modeling efforts, as validation for not only the amount of extra mixing, but trends between the degree of extra mixing and fundamental stellar parameters is now possible.

Li-rich and super Li-rich giants produced by element diffusion

Context.About 0.2−2% of giant stars are Li rich, that is to say their lithium abundance (A(Li)) is higher than 1.5 dex. Nearly 6% of these Li-rich giant stars are super Li rich, with an A(Li) exceeding 3.2 dex. Meanwhile, the formation mechanism of these Li-rich and super Li-rich giants is still under debate.Aims.Considering the compact He core of red giants, attention is paid to the effect of element diffusion on A(Li). In particular, when the He-core flash occurs, element diffusion makes the thermohaline mixing zone extend inward and connect to the inner convection region of stars. Then, a large amount of7Be produced by the He flash can be transferred to the stellar surface, finally turning into7Li. Thus, the goal of this work is to propose the mechanism of A(Li) enrichment and achieve consistency between the theoretical and observation data.Methods.Using the Modules for Experiments in Stellar Astrophysics (MESA) stellar evolution code, we simulated the evolution of low-mass stars, considering the effects of element diffusion on the Li abundances. The timescale ratio of Li-rich giants to normal giants was estimated using the population synthesis method. Then we obtained the theoretical value of A(Li) and made a comparison with observations.Results.Considering the influence of element diffusion in the model results in the increase of the lithium abundance up to about 1.8 dex, which can reveal Li-rich giants. Simultaneously, introducing high constant diffusive mixing coefficients (Dmix) with the values from 1011to 1015cm2s−1in the model allows the A(Li) to increase from 2.4 to 4.5 dex, which can explain most of the Li-rich and super Li-rich giant stars. The population synthesis method reveals that the amount of Li-rich giants is about 0.2−2% of all giants, which is consistent with observation estimated levels.Conclusions.In our models the element diffusion, mainly triggered by the gravity field, changes the mean molecular weight at the junction zone between the stellar envelope and the He core, which makes the thermohaline mixing region expand to the inner convection region of stars. A transport channel, efficiently transporting7Be in the hydrogen-burning region of the star to the convective envelope where7Be decays into7Li, is formed. Combining high constant diffusive mixing coefficients, the transport channel can explain the origin of Li-rich and super Li-rich giants, even the most super Li-rich giants.

Long-term lithium abundance signatures following planetary engulfment

ABSTRACT Planetary engulfment events can occur while host stars are on the main sequence. The addition of rocky planetary material during engulfment will lead to refractory abundance enhancements in the host star photosphere, but the level of enrichment and its duration will depend on mixing processes that occur within the stellar interior, such as convection, diffusion, and thermohaline mixing. We examine engulfment signatures by modelling the evolution of photospheric lithium abundances. Because lithium can be burned before or after the engulfment event, it produces unique signatures that vary with time and host star type. Using mesa stellar models, we quantify the strength and duration of these signatures following the engulfment of a 1, 10, or 100 M⊕ planetary companion with bulk Earth composition, for solar-metallicity host stars with masses ranging from 0.5 to 1.4 M⊙. We find that lithium is quickly depleted via burning in low-mass host stars ($\lesssim 0.7 \, {\rm M}_\odot$) on a time-scale of a few hundred Myrs, but significant lithium enrichment signatures can last for Gyrs in G-type stars ($\sim \! 0.9 \, {\rm M}_{\odot }$). For more massive stars (1.3−1.4 M⊙), engulfment can enhance internal mixing and diffusion processes, potentially decreasing the surface lithium abundance. Our predicted signatures from exoplanet engulfment are consistent with observed lithium-rich solar-type stars and abundance enhancements in chemically inhomogeneous binary stars.

Is Thermohaline Mixing the Full Story? Evidence for Separate Mixing Events near the Red Giant Branch Bump

The abundances of mixing-sensitive elements including lithium, [C/N], and 12C/13C are known to change near the red giant branch bump. The explanation most often offered for these alterations is double diffusive thermohaline mixing in the stellar interior. In this analysis, we investigate the ability of thermohaline mixing to explain the observed timing of these chemical depletion events. Recent observational measurements of lithium and [C/N] show that the abundance of lithium decreases before the abundance of [C/N], whereas numerical simulations of the propagation of the thermohaline-mixing region computed with MESA show that the synthetic abundances drop simultaneously. We therefore conclude that thermohaline mixing alone cannot explain the distinct events of lithium depletion and [C/N] depletion, as the simultaneity predicted by simulations is not consistent with the observation of separate drops. We thus invite more sophisticated theoretical explanations for the observed temporal separation of these chemical depletion episodes as well as more extensive observational explorations across a range of masses and metallicities.

Surface C, N, O, and Na abundances of RR Lyrae variables implying the nature of internal mixing in low-mass stars

ABSTRACT Photospheric abundances of C, N, O, and Na were determined by applying the synthetic spectrum-fitting technique to 34 snap-shot high-dispersion spectra of 22 RR Lyr stars covering a metallicity range of $-1.8 \lesssim$ [Fe/H] $\lesssim 0.0$, with an aim of investigating the mixing mechanism in the interior of low-mass giant stars by examining the abundance anomalies of these elements possibly affected by the evolution-induced dredge-up of nuclear burning products. Special attention was paid to check the recent theoretical stellar evolution simulations indicating the importance of thermohaline mixing in low-mass stars ($M \lesssim$ 1 M⊙), which is expected to be more significant as the metallicity is lowered. By inspecting the resulting abundances in comparison with those of unevolved metal-poor dwarfs at the same metallicity, the deficiency in C as well as enrichment in N was confirmed (while O is almost unchanged), the extent of peculiarities tending to increase with a decrease in [Fe/H]. Accordingly, the [C/N] ratio turned out to progressively decrease towards lower metallicity from ∼0 ([Fe/H] ∼0) to ∼−1 ([Fe/H] ∼−1.5), which is reasonably consistent with the theoretical prediction in the presence of thermohaline mixing. However, these RR Lyr stars do not show any apparent Na anomaly (i.e. essentially the same [Na/Fe] versus [Fe/H] trends as those of dwarfs), despite that metallicity-dependent overabundance in Na is theoretically expected for the case of non-canonical mixing. This inconsistency between C/N and Na may suggest a necessity of further improvement in the current theory.

New simulations of accreting DA white dwarfs: Inferring accretion rates from the surface contamination

Context.A non-negligible fraction of white dwarf stars show the presence of heavy elements in their atmospheres. The most accepted explanation for this contamination is the accretion of material coming from tidally disrupted planetesimals, which forms a debris disk around the star.Aims.We provide a grid of models for hydrogen-rich white dwarfs accreting heavy material. We sweep a 3D parameter space that has different effective temperatures, envelope hydrogen contents, and accretion rates. The grid is appropriate for determining accretion rates in white dwarfs that show the presence of heavy elements.Methods.Full evolutionary calculations of accreting white dwarfs were computed including all relevant physical processes, particularly the fingering (thermohaline) convection, a process neglected in most previous works, which has to be considered to obtain realistic estimations. Accretion is treated as a continuous process, and bulk-Earth composition is assumed for the accreted material.Results.We obtain final (stationary or near-stationary) and reliable abundances for a grid of models that represent hydrogen-rich white dwarfs of different effective temperatures and hydrogen contents, which we apply to various accretion rates.Conclusions.Our results provide estimates of accretion rates, accounting for thermohaline mixing, to be used for further studies on evolved planetary systems.

NGC 6124: a young open cluster with anomalous- and fast-rotating giant stars

ABSTRACT We present the results of a chemical analysis of fast and anomalous rotator giants members of the young open cluster NGC 6124. For this purpose, we carried out abundances of the mixing sensitive species such as Li, C, N, Na, and 12C/13C isotopic ratio, as well as other chemical species for a sample of four giants among the seven observed ones. This study is based on standard spectral analysis technique using high-resolution spectroscopic data. We also performed an investigation of the rotational velocity ($v\, \sin \, i$) once this sample exhibits abnormal values – giant stars commonly present rotational velocities of few km s−1. In parallel, we have performed a membership study, making use of the third data release from ESA Gaia mission. Based on these data, we estimated a distance of d = 630 pc and an age of 178 Myr through isochrone fitting. After that procedure, we matched all the information raised and investigated the evolutionary stages and thermohaline mixing model through of spectroscopic Teff and $\log \, g$ and mixing tracers, such as 12C/13C and Na, of the studied stars. We derived a low mean metallicity of [Fe/H] = −0.13 ± 0.05 and a modest enhancement of the elements created by the s-process, such as Y, Zr, La, Ce, and Nd, which is in agreement with what has already been reported in the literature for young clusters. The giants analysed have homogeneous abundances, except for lithium abundance [$\log \, \epsilon$(Li)NLTE = 1.08 ± 0.42] and this may be associated with a combination of mechanisms that act increasing or decreasing lithium abundances in stellar atmospheres.

Formation, diffusion, and accreting pollution of DB white dwarfs

Context. Over 1500 DBZ or DZ white dwarfs (WDs) have been observed so far, and polluted atmospheres with metal elements have been found among these WDs. The surface heavy element abundances of known DBZ or DZ WDs show an evolutionary sequence. Cooling, diffusion, and accretion are important physical processes in WD evolution which can alter the element abundances of the WD surface. Aims. Using the stellar evolutionary code, we investigated the DB WD formation and the effects of input parameters −, including the mixing length parameter (αMLT), thermohaline mixing efficiency (αth), and the metallicity (Z) −, on the structures of these DB WDs. The impacts of the convective zone mass (Mcvz), cooling timescales, diffusive timescales (τdiff), and the mass-accretion rate (Ṁa) on the element abundances of the WDs’ surfaces are discussed. By comparing the theoretical model results with observations, we try to understand the evolutionary sequence of the heavy element abundance on DBZ WD surfaces. Methods. By using Modules for Experiments in Stellar Evolution, we created DB WDs, and simulated the element diffusion due to high gravitational fields and the metal-rich material accretion coming from the planet disrupted by the WD. Then, we calculated the element abundances of these DB WDs for a further comparison with observations. Results. In our models, the input parameters (αMLT, αth, and Z) have a very weak effect on DB WD structures, including interior temperatures, chemical profiles, and convective zones. They hardly affect the evolution of the heavy elements on the surface of DB WDs. The mass-accretion rate and the effective temperature of DB WDs determine the abundances of heavy elements. The evolutionary sequence of the Ca element for about 1500 observed DB or DBZ WDs cannot be explained by the model with a constant mass-accretion rate, but it is very consistent with the model in which the mass-accretion rate decreases by one power law when Teff > 10 kK and it slightly increases by another power law when Teff < 10 kK. Conclusions. The observed DB WD evolutionary sequence of heavy element abundances originates from WD cooling and the change in the mass-accretion rate.

NLTE CNO abundances in a sample of nine field RR Lyr type stars

AbstractFor the first time, a direct non‐local thermodynamic equilibrium (NLTE) analysis of carbon and nitrogen lines in the spectra of nine RR Lyrae stars was carried out. We have determined the abundances of these elements together with oxygen, and have shown that the nitrogen content is increased in metallicity‐deficient program stars. We conclude that this is a sign of the first dredge up, which occurred at the previous stage of the red giant branch, and brought material processed in an incomplete carbon, nitrogen, oxygen (CNO) cycle to the surface of the star. This effect is significantly enhanced by thermohaline (extra‐) mixing, which is more effective for metal‐poor RR Lyrae stars. This is clearly seen in the plot showing that C/N ration in our sample of stars gradually decreases as metallicity decreases from about −0.2 to −2. Oxygen abundance depends on metallicity in a similar way to what we see in the Population II stars.

The Gaia-ESO survey: Mixing processes in low-mass stars traced by lithium abundance in cluster and field stars

Aims. We aim to constrain the mixing processes in low-mass stars by investigating the behaviour of the Li surface abundance after the main sequence. We take advantage of the data from the sixth internal data release of Gaia-ESO, IDR6, and from the Gaia Early Data Release 3, EDR3s. Methods. We selected a sample of main-sequence, sub-giant, and giant stars in which the Li abundance is measured by the Gaia-ESO survey. These stars belong to 57 open clusters with ages from 130 Myr to about 7 Gyr and to Milky Way fields, covering a range in [Fe/H] between ∼​ − 1.0 and ∼​ + 0.5 dex, with few stars between ∼​ − 1.0 and ∼​ − 2.5 dex. We studied the behaviour of the Li abundances as a function of stellar parameters. We inferred the masses of giant stars in clusters from the main-sequence turn-off masses, and for field stars through comparison with stellar evolution models using a maximum likelihood technique. We compared the observed Li behaviour in field giant stars and in giant stars belonging to individual clusters with the predictions of a set of classical models and of models with mixing induced by rotation and thermohaline instability. Results. The comparison with stellar evolution models confirms that classical models cannot reproduce the observed lithium abundances in the metallicity and mass regimes covered by the data. The models that include the effects of both rotation-induced mixing and thermohaline instability account for the Li abundance trends observed in our sample in all metallicity and mass ranges. The differences between the results of the classical models and of the rotation models largely differ (up to 2 dex), making lithium the best element with which to constrain stellar mixing processes in low-mass stars. We discuss the nature of a sample of Li-rich stars. Conclusions. We demonstrate that the evolution of the surface abundance of Li in giant stars is a powerful tool for constraining theoretical stellar evolution models, allowing us to distinguish the effect of different mixing processes. For stars with well-determined masses, we find a better agreement of observed surface abundances and models with rotation-induced and thermohaline mixing. Rotation effects dominate during the main sequence and the first phases of the post-main-sequence evolution, and the thermohaline induced mixing after the bump in the luminosity function.

Enhancement of lithium in red clump stars by the additional energy loss induced by new physics

ABSTRACT Since 7Li is easily destroyed in low temperatures, the surface lithium abundance decreases as stars evolve. This is supported by the lithium depletion observed in the atmosphere of most red giants. However, recent studies show that almost all of red clump stars have high lithium abundances A(Li) > −0.9, which are not predicted by the standard theory of the low-mass stellar evolution. In order to reconcile the discrepancy between the observations and the model, we consider additional energy loss channels that may come from physics beyond the Standard Model. A(Li) slightly increases near the tip of the red giant branch even in the standard model with thermohaline mixing because of the 7Be production by the Cameron–Fowler mechanism, but the resultant 7Li abundance is much lower than the observed values. We find that the production of 7Be becomes more active if there are additional energy loss channels, because themohaline mixing becomes more efficient and a heavier helium core is formed.