arXiv subjects

C. Charbonnel

Publications and source records attributed to C. Charbonnel.

At least 19 recordsLinked to original sources

arXiv2026

A universal chromosome map for globular clusters: chemical calibration and environmental regulation of the multiple populations

Chromosome maps (ChMs) are two-dimensional diagrams of UV/optical pseudocolours widely used to diagnose the multiple stellar populations (MPs) phenomenon in globular clusters. Their raw morphology is affected by the metallicity-dependent response of the photometric filters, preventing unbiased comparisons across clusters of different metallicities. We identify a cross-cluster ChM framework that accounts for this dependence, enabling an unbiased investigation of the physical drivers of MP diversity. We analyse ChMs for 23 Galactic globulars and devise a technique to correct the raw maps for the clusters' different metallicities. On the resulting "universal" ChM we define a new photometric enrichment index $S_{\rm ChM,z}$, validated against APOGEE spectroscopy. We compare this index with cluster masses, structural parameters, orbital quantities, and accretion-origin classifications. $S_{\rm ChM,z}$ correlates with the multivariate chemical abundance ranges of the enriched population and with the aluminium spread. Across the sample it increases with initial mass but correlates most strongly with a family of orbital-confinement quantities ($z_{\max}$, vertical action, apocentre, Galactocentric radius, orbital energy). The corrected ChMs provide a chemically meaningful, population-level measure of the enriched sequence. $S_{\rm ChM,z}$ does not trace a single abundance ratio but captures the cluster-to-cluster amplitude of the combined light-element variations, with particular sensitivity to the high-temperature Mg-Al/O component of proton-capture processing. Its dependence on both cluster potential depth and orbital confinement suggests that 2P chemical diversity is shaped by internal enrichment physics together with an environmental imprint, whether inherited at formation, modified by early evolution, or filtered by subsequent orbital survival.

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arXiv2026

A quasi-star is born: formation and evolution of accreting quasi-stars as a pathway to Little Red Dots at non-zero metallicity

The recently discovered Little Red Dots identified by the James Webb Space Telescope are compact high-redshift sources whose properties have motivated models involving black holes embedded within optically thick gaseous envelopes. We investigate their rest-frame optical emission by modeling quasi-stars, i.e. stellar envelopes powered by accretion onto a central black hole, formed from rapidly accreting proto-stars that reach the supermassive star regime ($>10^4$~\Msun) before undergoing general relativistic instability. We compute stellar evolution models with mass gain rates of 0.01, 0.1, and 1~\Msun/yr and metallicities $Z=0$-$0.01$. For accretion rates $\ge0.1$~\Msun/yr, stars remain nearly fully convective with $T_\mathrm{eff}\sim4000$-$9000$~K. General relativistic instability occurs at $M_\star\sim3.5\times10^4$~\Msun\ ($6.8\times10^4$~\Msun) for $\dot{m}=0.1$~\Msun/yr (1~\Msun/yr), at $L\sim10^9$~\Lsun. Assuming the black hole supports the envelope until complete accretion ($M_{\rm BH,max}/M_{\rm QS}=1$), quasi-stars reach maximum lifetimes of $10^7$-$10^8$~yr, $\sim100$-$1000$ times longer than their progenitors. Their formation and evolution are nearly independent of metallicity. Matching our models to Little Red Dots at $z<4.5$ ($L_\mathrm{bol}\sim10^{9.5}$-$10^{11.5}$~\Lsun) implies quasi-star masses of $10^{4.5}$-$10^{6.5}$~\Msun, while the minimum observed luminosity requires progenitor accretion rates $\gtrsim0.1$~\Msun/yr. Our models support quasi-stars as the origin of Little Red Dot optical emission and constrain their masses, lifetimes, progenitor environments, and luminosities. Our models offer a framework supporting quasi-stars as the source of Little Red Dot optical emission, and provide insights into their lifetimes, composition, progenitor's environment as well on their minimum and maximum observed luminosities.

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arXiv2026

Lithium as a probe of stellar and galactic physics

Lithium plays a unique role in astrophysics, as it is a powerful diagnostic for the physics and evolution of low-mass stars, Galactic archaeology, and cosmology. We review the Li observations in stars at different phases of their evolution, the strengths and the limitations of the current theoretical stellar models to explain the Li abundance data, our understanding of the Li sources and of the evolution of Li through- out the Galactic history. Key takeaways from the current state of the research in the field are: 1) Stellar evolution models accounting for fundamental transport processes of chemical species and angular momentum hold the promise of providing a common stellar Li depletion explanation to the Li abundance patterns observed in all Galactic stellar populations, including the dip and the plateau(s). 2) Novae are most probably the main source of Li in the Galaxy, on observational (but not yet theoretically established) grounds. 3) Radial migration of stars in the Galactic disk holds the key to understand many aspects of the Li evolution in the Milky Way.

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arXiv2026

N-emitters as possible sign-posts of GC formation

Based on the finding of unusual chemical abundance ratios of N-emitters, which resemble those of globular cluster (GC) stars, their compactness, high ISM densities and other properties, it has been suggested that N-emitters could indicate the formation sites of globulars. A recent statistical study of the N-emitter population has quantified the frequency $f_N$ of these rare objects and their redshift evolution (Morel et al. 2025). Using these results we here test if N-emitters trace the formation of GCs and use the observed cosmic star-formation rate density evolution to predict the cosmological evolution of the GC population with time, their age distribution, and the total present-day stellar mass density formed in globulars. The predicted age distribution of GCs strongly resembles the typical asymmetric observed distributions in the Galaxy and ellipiticals, with a peak at $\sim 11.5-12$ Gyr and a longer tail extending to younger ages. We derive a total stellar mass density formed in N-emitters down to redshift zero of $(2-7) \times 10^5$ M$_{\odot}$ Mpc$^{-3}$, which matches within a factor $\sim 2$ the observed fraction of stellar mass found in the GC population at $z=0$. These results provide additional indirect arguments supporting the hypothesis that N-emitters could represent sign-posts of a short phase of GC formation.

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arXiv2026

Massive stellar cannibals: How stellar mergers drive mass-loss in extremely massive stars

It has been theorized that the formation of extremely massive and supermassive stars ($>10^3\ {\rm M}_\odot$) could plausibly be the outcome of stellar mergers in low metallicity ($Z<10^{-1}$~Z$_\odot$) and dense ($\gtrsim10^3\ {\rm M}_\odot\ {\rm pc}^{-3}$) stellar environments. These objects remain relevant as they can serve as the progenitors of intermediate-mass black holes and they are also formidable chemical polluter candidates, as evidenced by the peculiar abundances seen across cosmic history. This work investigates merger-induced mass loss in extremely massive stars within a hydrodynamic framework and provides a prescription derived from the simulations to estimate both the mass loss and the outcome of the interaction. We adapted the 1D hydrodynamic, stellar structure, and evolution code MESA to simulate stellar inspirals. In our simulations, we considered stars of $>1000\,\rm M_{\odot}$ with inspiraling companions of $<100$ M$_\odot$; hence, with mass ratios of $<0.1$. As the inspiral progresses, the orbital energy of the system is lost through the hydrodynamic and gravitational drag forces. This energy gets deposited as thermal energy in the extremely massive star's envelope. We find that the total ejected mass is $\sim$10-30$\%$ of the system's mass. Our results point out that most of the energy deposited by the inspiral is used to eject mass. These findings demonstrate that merger-induced mass loss is non-negligible for the considered configurations. Thus, it is an important process to account for when investigating the formation of extremely massive stars and predicting their possible role throughout cosmic history.

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arXiv2025

Discovery of new N-emitters over a wide redshift range

JWST observations have revealed rare galaxies with UV spectra exhibiting intense lines of nitrogen, indicative of super-solar N/O abundances at low metallicity. To better understand these enigmatic objects and provide new constraints on proposed scenarios, we have undertaken a systematic search for galaxies with UV emission lines of nitrogen. Using public JWST NIRSpec data, we have identified 45 N-emitters with robust NIII] or NIV] detections, including 4 previously known objects. We find N-emitters from redshift $z\sim 3-11$ among a broad diversity of galaxies, in terms of morphology, UV magnitude, stellar mass, SFR, metallicity, and rest-optical line strengths. The UV nitrogen lines show typical equivalent widths between $\sim 5-50$ Å. Carbon lines are generally fainter than the N lines. Using strong line calibrations established at high-redshift, we find metallicities $12+log(O/H)\sim 7.15-8.5$, including thus also high metallicities. The H$β$ equivalent width of N-emitters varies strongly, and sources with low EWs show clear signs of a Balmer break, indicative of composite stellar populations combining both young (< 10 Myr) stars responsible of the UV emission lines and an older population contributing to the rest-optical spectrum. Supersolar N/O ratios are found in all N-emitters. C/O abundances are comparable to those of galaxies at the same metallicity, and all N-emitters show high N/C ratios or lower limits ($\log(N/C) > 0.5$), independently of metallicity. The observed abundance ratios are compatible with ejecta from H-burning and do not show signs of Carbon enhancements, even at higher metallicities. Finally, we find that the fraction of N-emitters increases with redshift, and we quantify this evolution. Our study increases the sample of known N-emitters by a factor $\sim 3$, reveals a diversity of properties among N-emitters, and provides new constraints on their nature.

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arXiv2024

He-enriched STAREVOL models for globular cluster multiple populations. Self-consistent isochrones from ZAMS to the TP-AGB phase

A common property of globular clusters (GC) is to host multiple populations characterized by peculiar chemical abundances. Recent photometric studies suggest that the He content could vary between the populations of a GC by up to $Δ$He $\sim$ 0.13, in mass fraction. The initial He content impacts the evolution of low-mass stars by ultimately modifying their lifetimes, luminosity, temperatures, and, more generally, the morphology of post-RGB evolutionary tracks in the Hertzsprung-Russell diagram. We present new physically accurate isochrones with different initial He-enrichments and metallicities, with a focus on the methods implemented to deal with the post-RGB phases. The isochrones are based on tracks computed with the stellar evolution code STAREVOL for different metallicities (Z = 0.0002, 0.0009, 0.002, and 0.008) and with different He-enrichment (from 0.25 to 0.6 in mass fraction). We describe the effect of He-enrichment on the morphology of the isochrones and test these by comparing the predicted number counts of HB and AGB stars with those of selected GCs. Comparing the number ratios, we find that our new theoretical ones agree with the observed values within $1σ$ in most cases. The work presented here sets the ground for future studies on stellar populations in globular clusters, in which the abundances of light elements in He-enhanced models will rely on different assumptions for the causes of this enrichment. The developed methodology permits the computation of isochrones from new stellar tracks with non-canonical stellar processes. The checked number counts ensure that, at least in this reference set, the contribution of the luminous late stages of stellar evolution to the integrated light of a GC is represented adequately.

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arXiv2023

$^{12}$C/$^{13}$C of Kepler giant stars: the missing piece of the mixing puzzle

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. We present the CNO and $^{12}$C/$^{13}$C 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. We have determined the abundances of CNO and more specifically 12C/13C using the FIES Spectrograph on the Nordic Optical Telescope, for 71 giant field stars. In addition, asteroseismology is available for all stars, providing their mass, age as well as the evolutionary states. Finally, astrometry from Gaia data is also available for the majority 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 Galactic evolution and the impact of stellar evolution, we built mock catalogues using the Besancon Galaxy model in which stellar evolution models taking into account the effects of thermohaline instability are included. We confirm that 12C/13C at the surface of core He-burning stars is lower than that of first ascent RGB stars. 12C/13C measured at the surface of the core He-burning stars increases with [Fe/H] and mass while it decreases with age. These trends are all very well explained by the thermohaline mixing that occurs in red giants. We have shown 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.

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arXiv2023

Extreme N-emitters at high-redshift: signatures of supermassive stars and globular cluster or black hole formation in action?

[Abridged] Using the JWST/NIRSpec observations from CEERS we found an extreme N-emitter, CEERS-1019 at z=8.6782 showing intense NIV and NIII emission. From the observed rest-UV and optical lines we conclude that it is compatible with photoionization from stars and we determine accurate abundances for C, N, O, and Ne, relative to H, finding a highly supersolar ratio log(N/O) = -0.18+/-0.11, and normal log(C/O) = -0.75+/-0.11 and log(Ne/O) = -0.63+/-0.07, for its low metallicity, 12+log(O/H)= 7.70+/-0.18. We also analyze other N-emitters from the literature. All show strongly enhanced N/O ratios and two of them normal C/O. Massive star ejecta from WR stars are needed to explain the galaxies with enhanced C/O (Lynx arc and Mrk 996). On the other hand, supermassive stars (>1000 Msun, SMS) in the ``conveyer-belt model'' put forward to explain globular clusters (GCs), predict a high N/O and small changes in C/O, compatible with CEERS-1019, the Sunburst cluster, SMACS2031, and GN-z11. Based on the chemical abundances, possible enrichment scenarios, compactness, and high ISM density, we suggest that CEERS-1019, SMACS2031, and the Sunburst cluster could contain proto-GCs. Finally, we propose that some N-emitters enriched by SMS could also have formed intermediate-mass black holes, and we suggest that this might be the case for GN-z11. Our observations and analysis reinforce the suggested link between some N-emitters and proto-GC formation, which is supported both by empirical evidence and quantitative models. Furthermore, the observations provide possible evidence for the presence of supermassive stars in the early Universe (z>8) and at z~2-3. Our analysis also suggests that the origin and nature of the N-emitters is diverse, including also objects like GN-z11 which possibly host an AGN.

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arXiv2023

N-enhancement in GN-z11: First evidence for supermassive stars nucleosynthesis in proto-globular clusters-like conditions at high redshift ?

Unusually high N/O abundance ratios were recently reported for a very compact, intensively star-forming object GN-z11 at $z=10.6$ from JWST/NIRSpec observations. We present an empirical comparison with the C, N, and O abundance ratios in Galactic globular clusters (GCs) over a large metallicity range. We show that hot hydrogen-burning nucleosynthesis within supermassive stars (SMS) formed through runaway collisions can consistently explain the observed abundances ratio in GN-z11 and in GCs. This suggests that a proto-globular cluster hosting a SMS could be at the origin of the strong N-enrichment in GN-z11. Our model predicts the behavior of N/O, C/O, and Ne/O ratios as a function of metallicity, which can be tested if high-$z$ objects similar to GN-z11 are detected with JWST in the future. Further studies and statistics will help differentiate the proto-GC scenario from the Wolf-Rayet scenario that we quantify with a population synthesis model, and shed more light on this peculiar object.

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arXiv2023

Search strategies for supermassive stars in young clusters and application to nearby galaxies

Supermassive stars (SMS) with masses $M \gtrsim 10^3-10^4 M_{\odot}$, formed by runaway collisions in young, massive, and dense star clusters have been invoked as a possible solution to the puzzles raised by the presence of multiple stellar populations and peculiar abundance patterns observed in globular clusters. However, such objects have not been observed so far. We developed observational strategies to search for SMS hosted within young massive clusters (thought to be the precursors of globular clusters, GCs), which could be applicable in a relatively general fashion, using both photometric and spectroscopic observations. We used theoretical predictions of spectra of SMS and SMS-hosting clusters, together with predictions from standard simple stellar populations to examine their impact on color-color diagrams and on individual optical spectral lines (primarily Hydrogen emission and absorption lines). As a first step, we apply our search strategies to a sample of $\sim 3000$ young star clusters (YSC) from two nearby galaxies with multi-band observations from the HST and optical integral-field spectroscopy obtained with MUSE on the Very Large Telescope. We focus on models for SMS with large radii (corresponding to $ Teff \lesssim 7000$ K), which predict strong Balmer breaks, and construct proper color-color diagrams to select the corresponding SMS-hosting cluster candidates. We show that their spectrophotometric properties are similar to that of normal clusters with ages of a few hundred Myr, which would, however, show signs of composite stellar populations, in particular the presence of nebular lines (H$α$ and others). Examining the photometry, overall SEDs, and the spectra of $\sim 100$ clusters with strong Balmer breaks, we have found several objects with peculiar SEDs, the presence of emission lines, or other peculiar signatures. [abridged]

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arXiv2022

Lithium, masses, and kinematics of young Galactic dwarf and giant stars with extreme [$α$/Fe] ratios

Recent spectroscopic explorations of large Galactic stellar samples stars have revealed the existence of red giants with [$α$/Fe] ratios that are anomalously high, given their relatively young ages. We revisit the GALAH DR3 survey to look for both dwarfs and giants with extreme [$α$/Fe] ratios, that is, the upper 1% in the [$α$/Fe]-[Fe/H] plane over the range in [Fe/H] between -1.1 and +0.4 dex. We refer to these outliers as "ex$α$fe" stars. We used the GALAH DR3 data and their value-added catalog to trace the properties (abundances, masses, ages, and kinematics) of the ex$α$fe stars. We investigated the effects of secular evolution and the magnitude limitations of GALAH to understand the mass and metallicity distributions of the sample stars. We also discuss the corresponding biases in previous studies of stars with high [$α$/Fe] in other surveys. We find both dwarf and giant ex$α$fe stars younger than 3 Gyr, which we refer to as "y-ex$α$fe" stars. Dwarf y-ex$α$fe stars exhibit lithium abundances similar to those of young [$α$/Fe]-normal dwarfs at the same age and [Fe/H]. In particular, the youngest and most massive stars of both populations exhibit the highest Li abundances, A(Li)~3.5 dex (i.e., a factor of 2 above the protosolar value), while cooler/older stars exhibit the same Li depletion patterns increasing with both decreasing mass and increasing age. In addition, the [Fe/H] and mass distributions of both the dwarf and giant y-ex$α$fe stars do not differ from those of their [$α$/Fe]-normal counterparts found in the thin disk and they share the same kinematic properties. We conclude that y-ex$α$fe dwarfs and giants are indeed young, their mass distribution shows no peculiarity, and they differ from young [$α$/Fe]-normal stars by their extreme [$α$/Fe] content only. However, their origin remains unclear.

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arXiv2022

CORALIE radial-velocity search for companions around evolved stars (CASCADES). I. Sample definition and first results: Three new planets orbiting giant stars

Context. Following the first discovery of a planet orbiting a giant star in 2002, we started the CORALIE radial-velocity search for companions around evolved stars (CASCADES). We present the observations of three stars conducted at the 1.2 m Leonard Euler Swiss telescope at La Silla Observatory, Chile, using the CORALIE spectrograph. Aims. We aim to detect planetary companions to intermediate-mass G- and K- type evolved stars and perform a statistical analysis of this population. We searched for new planetary systems around the stars HD22532 (TIC 200851704), HD64121 (TIC 264770836), and HD69123 (TIC 146264536). Methods. We have followed a volume-limited sample of 641 red giants since 2006 to obtain high-precision radial-velocity measurements. We used the Data & Analysis Center for Exoplanets (DACE) platform to perform a radial-velocity analysis to search for periodic signals in the line profile and activity indices, to distinguish between planetary-induced radial-velocity variations and stellar photospheric jitter, and to search for significant signals in the radial-velocity time series to fit a corresponding Keplerian model. Results. In this paper, we present the survey in detail, and we report on the discovery of the first three planets of the sample around the giant stars HD22532, HD64121, and HD69123.

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arXiv2021

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

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, edr3. We select a sample of main sequence, sub-giant, and giant stars in which Li abundance is measured by the Gaia-ESO survey, belonging to 57 open clusters with ages from 120~Myr to about 7 Gyr and to Milky Way fields, covering a range in [Fe/H] between -1.0 and +0.5dex. We study the behaviour of the Li abundances as a function of stellar parameters. We compare 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. The comparison with stellar evolution models confirms that classical models cannot reproduce the lithium abundances observed 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 to constrain stellar mixing processes in low-mass stars. For stars with well-determined masses, we find a better agreement between observed surface abundances and models with rotation-induced and thermohaline mixings, the former dominating during the main sequence and the first phases of the post-main sequence evolution and the latter after the bump in the luminosity function.

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arXiv2021

The behaviour of lithium at high metallicity in the Milky Way -- Selection effects in the samples and the possible role of atomic diffusion

We revisit large spectroscopic data sets for field stars from the literature to derive the upper Li envelope in the high metallicity regime in our Galaxy. We take advantage of Gaia EDR3 data and state-of-the-art stellar models to precisely determine the position of the sample dwarf stars in the Hertzsprung-Russell diagram. The highest Li abundances are found in field metal-rich warm dwarfs from the GALAH survey, located on the hot side of the Li-dip. Their mean Li value agrees with what was recently derived for warm dwarfs in metal-rich clusters, pointing towards a continuous increase of Li up to super-solar metallicity. However, if only cool dwarfs are considered in GALAH, as done in the other literature surveys, it is found that the upper Li envelope decreases at super-solar metallicities, blurring the actual Li evolution picture. We confirm the suggestion that field and open cluster surveys that found opposite Li behaviour in the high metallicity regime do not sample the same types of stars: The first ones, with the exception of GALAH, miss warm dwarfs that can potentially preserve their original Li content. Although we can discard the bending of the Li upper envelope at high metallicity derived from the analysis of cool star samples, we still need to evaluate the effects of atomic diffusion on warm, metal-rich early-F and late-A type dwarfs before deriving the actual Li abundance at high metallicity.

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arXiv2021

Pollux: A weak dynamo-driven dipolar magnetic field and implications for its probable planet

Context: Pollux is considered as an archetype of a giant star hosting a planet. We then discovered a weak magnetic field at its surface using spectropolarimetry. Aims and Methods: We followed up our investigations on Pollux first using ESPaDOnS at CFHT and then Narval at TBL to obtain Stokes I and Stokes V spectra to study their variations for a duration of 4.25 years, that is, for more than two periods of about 590 d of the RV variations. We used the least-squares deconvolution (LSD) profiles to measure the longitudinal magnetic field and to perform a Zeeman Doppler imaging (ZDI) investigation. Results: The longitudinal magnetic field of Pollux is found to vary with a sinusoidal behavior and a period similar to that of the RV variations. From the ZDI investigation a rotation period of Pollux is determined to be equal to 660+/-15 days and possibly different than the period of variations of the RV. As to the magnetic topology, the poloidal component is dominant and almost purely dipolar with an inclination of 10.5° of the dipole with respect to the rotation axis. The mean strength of the surface magnetic field is 0.44 G. Conclusions: As to the origin of the magnetic field of Pollux, we favor the hypothesis that it is maintained through contemporaneous dynamo action. Pollux appears as the representative of a class of slowly rotating and weakly magnetic G-K red giants. To explain the sinusoidal RV variations of Pollux, two scenarios are proposed. If the RV period is different from the rotation period, the observed periodic RV variations are due to the hosted planet and the contribution of Pollux magnetic activity is not significantly detected. In the peculiar case in which the two periods are equal, we cannot discard the possibility that the activity of Pollux could explain the total RV variations and that the planet hypothesis would appear unnecessary.

astro-ph.SR↗

arXiv2020

Lithium depletion and angular momentum transport in solar-type stars

Transport processes occurring in the radiative interior of solar-type stars are evidenced by the surface variation of light elements, in particular Li, and the evolution of their rotation rates. For the Sun, inversions of helioseismic data indicate that the radial profile of angular velocity in its radiative zone is nearly uniform, which implies the existence of angular momentum transport mechanisms. While there are many independent transport models for angular momentum and chemical species, there is a lack of self-consistent theories that permit stellar evolution models to simultaneously match the present-day observations of solar lithium abundances and radial rotation profiles. We explore how additional transport processes can improve the agreement between evolutionary models of rotating stars and observations. We constrain the resulting models by simultaneously using the evolution of the surface rotation rate and Li abundance in the solar-type stars of open clusters, and the solar surface and internal rotation profile as inverted from helioseismology. We show the relevance of penetrative convection for the depletion of Li. The rotational dependence of the depth of penetrative convection yields an anti-correlation between the initial rotation rate and Li depletion in our models of solar-type stars that is in agreement with the observed trend. Simultaneously, the addition of an ad hoc vertical viscosity leads to efficient transport of angular momentum between the core and the envelope. We also self-consistently compute for the first time the thickness of the tachocline and find that it is compatible with helioseismic estimations. However, the main sequence depletion of Li in solar-type stars is only reproduced when adding a parametric turbulent mixing below the convective envelope. The need for additional transport processes in stellar evolution models is confirmed.

astro-ph.SR↗

arXiv2020

The Gaia-ESO Survey: an extremely Li-rich giant in the globular cluster NGC 1261

Lithium rich stars in globular clusters are rare. In fact, only 14 have been found so far, in different evolutionary phases from dwarfs to giants. Different mechanisms have been proposed to explain this enhancement, but it is still an open problem. Using spectra collected within the Gaia-ESO Survey, obtained with the GIRAFFE spectrograph at the ESO Very Large Telescope, we present the discovery of the first Li-rich star in the cluster NGC 1261, the second star known in the red giant branch bump phase. The star shows an extreme Li overabundance of A(Li)_LTE=3.92\pm0.14, corresponding to A(Li)_NLTE=3.40 dex. We propose that the Li enhancement is caused by fresh Li production through an extra mixing process (sometimes referred to as {\em cool bottom burning}) or could be a pre-existing Li overabundance resulting from binary mass transfer, likely from a red giant branch star, because of the low barium abundance. To unambiguously explain the Li enhancement in globular cluster stars, however, a reliable determination of the abundance of key species like Be, 6Li, 12C/13C, and several s-process elements is required, as well as detailed modeling of chromospheric activity indicators.

astro-ph.SR↗