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G. Buldgen

Publications and source records attributed to G. Buldgen.

At least 19 recordsLinked to original sources

Investigating the impact of Solar Fusion III reaction rates on helioseismic constraints and solar neutrino fluxes

Nuclear reaction rates are crucial ingredients of solar and stellar models, they directly impact the duration of the life of stars and the energy they produce. In the solar case, the tight observational constraints put on models (Mass, Radius, Luminosity and chemical composition) coupled to our capabilities to probe the solar interior thanks to helioseismology and solar neutrinos provide an exquisite testbed for such physical ingredients. With the recent publication of the Solar Fusion III reaction rates, a new generation of solar models may be computed and put to the test. We aim to investigate the impact of the new Solar Fusion III reaction rates on solar models, both standard and non-standard, as well as the impact of the current uncertainties on some key solar reactions on the predicted neutrino fluxes for boron and the so-called CNO cycle. We compute various theoretical standard solar models as well as non-standard models reproducing the depletion of lithium and beryllium for various abundances, nuclear reaction rates and opacities. We focus on the impact of the solar fusion III reaction rates on both helioseismic inversion results and neutrino fluxes. We find that using the Solar Fusion III reaction rates significantly impact the agreement of solar models both with helioseismic constraints and neutrino flux measurements. While for helioseismic constraints it seems that there is a slight improvement, for neutrino fluxes, the use of the SFIII reaction rates induces a lowering of the beryllium, boron and CNO neutrino fluxes. When investigating the impact of changes on the rates of key reactions within their quoted uncertainties, we find that these changes are far from sufficient to reconcile models and observations, potentially hinting at other processes (e.g. planetary formation as found in other studies).

astro-ph.SR

Characterizing six seismic solar analogs observed by Kepler, K2, and HERMES

Solar analogs, stars that closely match the fundamental properties of the Sun, provide key benchmarks for testing stellar structure and evolution across different ages and activity levels. Their detailed characterization helps place the Sun in context within the broader population of solar-like stars. This study presents the characterization of six seismic solar analogs observed by the NASA Kepler and K2 missions. Combining asteroseismic constraints from space-based photometry with high-resolution spectroscopy and \textit{Gaia} astrometry, we derived their fundamental parameters and assessed their resemblance to the Sun. Global seismic properties and individual oscillation modes were extracted from the photometric light curves, while atmospheric parameters were obtained from data collected by the HERMES spectrograph at the Mercator telescope. Stellar modeling using seven independent stellar evolution codes yielded consistent masses, radii, and ages. These stars have masses between 0.91 and 1.04~$\mathrm{M}_\odot$, radii between 0.95 and 1.08~$\mathrm{R}_\odot$, and ages from about 1.8 to 9.1~Gyr, with typical systematic uncertainties of $\pm$ 0.02~$\mathrm{M}_\odot$, $\pm$ 0.01~$\mathrm{R}_\odot$, and $\pm$ 0.7~Gyr, respectively. One star, EPIC~206064678, exhibits properties very similar to those of the Sun, with $M = 1.016 \pm 0.033\,\mathrm{M}_\odot$, $R = 0.990 \pm 0.011\,\mathrm{R}_\odot$, and an age of $5.40 \pm 0.12$\,Gyr. It can therefore be considered a close solar twin, although it is slightly older and more metal-rich ($0.25 \pm 0.07$\,dex). Four targets display binarity signatures and all exhibit very low chromospheric activity. This work broadens the sample of well-characterized seismic solar analogs towards a larger sample of metallicities and ages, providing new references for comparative stellar studies and future asteroseismic investigations.

astro-ph.SR

A data-driven estimate of the protosolar helium mass fraction

The protosolar helium mass-fraction is a key ingredient of solar, planetary models and enrichment laws. However, the assumed values often rely on simplified descriptions of the transport of chemicals in solar models. They are also based on the inferred helium mass fraction in the solar convective envelope, which is itself sensitive to uncertainties in the solar equation of state. We update the reference protosolar helium abundance by including the effects of macroscopic mixing at the base of the convective zone and more recent determinations of the helium mass fraction in the convective envelope. We combine results from our inversions to spectroscopic abundances, as well as literature values to provide a robust interval of the current helium mass fraction in the convective zone. We combine this measurement to models including light element depletion to provide an udpated protosolar helium abundance. We show that macroscopic mixing at the base of the envelope is key to infer protosolar helium. We find a revised interval of primordial helium mass fraction of 0.27575 +- 0.00315 slightly lower than previous estimates when combining our latest estimate of surface helium mass fraction and spectroscopic abundances. We find that the effects of macroscopic mixing are partially compensated by an increase in the inferred solar helium mass fraction in recent studies. We also derive more precise estimates based on various reference works in the litterature. Using the usual surface helium mass fraction, the primordial helium mass fraction drops to 0.2669 +- 0.00415 due to the inclusion of macroscopic mixing. The dominant source of uncertainty is found the surface helium abundance inferred from helioseismic constraints and more specifically, the impact on the equation of state of the solar material on this inference result.

astro-ph.SR

Eating planets makes you younger: The magnetic dynamo rejuvenation of GJ 504 by planetary engulfment

With the discovery of a few thousand exoplanets, questions have been raised regarding star-planet interactions and whether the presence of a companion may affect stellar properties. GJ 504 is an evolved (2 Gyr) Sun-like star with a short rotation period (3.4 d) and an intense magnetic activity, which is in stark contrast with what would be expected at such an evolutionary stage. One possible explanation is that a close-in, Jupiter-mass planet was pushed starwards by the action of stellar tides, inducing a stellar spin-up and ultimately a rejuvenation of the stellar magnetic dynamo. By characterising the large-scale magnetic field and magnetised wind of GJ 504, we aim to provide additional observational constraints to test such scenario. We analysed spectropolarimetric observations of GJ 504 collected with ESPaDOnS. Using Zeeman-Doppler imaging, we found a large-scale, dipolar, non-axisymmetric magnetic field with an average strength of 5.3 G, similar to that of evolved early-G type stars. We fed the magnetic field information into our 3D MHD simulation of the stellar wind and space environment of GJ 504, from which we constrained the wind-driven angular momentum loss ($\rm \dot{J}$). We then compared $\rm \dot{J}$ to rotational evolutionary tracks of GJ 504 for two scenarios: evolution with and without the engulfment of a close-in, Jupiter-mass companion. Between the two scenarios, only the planet engulfment can explain the observational constraints obtained previously in the literature, such as the stellar rotation and X-ray luminosity, and the $\rm \dot{J}$ we derived and rescaled to account for underestimated magnetic field strength. Although there are many other stars with similar masses and rotation periods whose rotation evolution does not require planet engulfment, we also identified HD 75332 as a candidate for planet engulfment, suggesting that GJ 504 may not be an isolated case.

astro-ph.SR

The stellar activity-rotation-age relationship under the lens of asteroseismology

In low-mass stars, the connection between magnetic activity, rotation period, and age provides key insights into the functioning of dynamos. Fully understanding the activity-rotation-age relationship requires stars with precise fundamental parameters, measured rotation periods, and reliable magnetic activity indicators (e.g. X-ray luminosity). Thanks to space-based photometry, asteroseismology is now the leading method for determining stellar parameters with unprecedented precision and accuracy. The best-characterized solar-like stars compose the Kepler LEGACY sample, with highest-quality asteroseismic data for 66 stars, most of which have measured rotation periods. In the X-ray band, these stars were observed by the ROentgen Survey with an Imaging Telescope Array (eROSITA) telescope on the Russian Spektrum-Roentgen-Gamma (SRG) satellite in the course of its all-sky survey. We reviewed different components of the stellar activity-rotation-age relationship using the largest sample of solar-like stars with highly accurate fundamental parameters from asteroseismology, along with measured rotation periods and X-ray luminosities. We cross-correlated the Kepler LEGACY sample with the SRG/eROSITA source catalogue, finding X-ray detections for 13 of them. We derived their fundamental parameters using the Forward and Inversion COmbination procedure and revisited widely studied activity-age and activity-rotation relationships by consistently incorporating our 13-star subsample with literature samples. By implementing revised activity-rotation-age relationships in a Star-Planet Interaction code to compute X-ray luminosity tracks and comparing the results with observations, we found improved agreement for 7 stars of our subsample. We explored the effect of the revised relationships on the mass loss of planets in the radius valley, finding a modest impact on planet size distributions.

astro-ph.SR

Detailed theoretical analysis of core Helium-burning stars: Mixed mode patterns I. Impact of the He-flash discontinuity and of induced semi-convection

Space missions like CoRoT, Kepler, and TESS have made asteroseismology a powerful probe of stellar interiors. Red giants are key targets thanks to their rich mixed-mode oscillation spectra, which reveal properties of both core and envelope. However, current models of core helium-burning red giants still fail to fully reproduce observed oscillation patterns, largely due to uncertainties in mixing processes such as overshooting and semi-convection. This motivates the need for better seismic constraints to refine stellar models. We investigate how internal structural features shape asteroseismic signatures in core helium-burning stars, focusing on the links between seismic properties and internal chemical profiles. Using an updated version of the Liege stellar evolution code and its adiabatic oscillation code, we compute and analyse mixed-mode patterns for a range of stellar models. Our results show that sharp chemical gradients and central overshooting strongly influence the mixed-mode spectra. Changes in overshooting modify the extent of the semi-convective region, altering the local Brunt-Vaisala frequency and thus the observed period spacing. Variations in overshooting are compensated by shifts in semi-convective layers, keeping the total mixed-core size nearly constant across models. As a result, stellar evolution is only mildly affected, while the seismic signatures, especially the Brunt-Vaisala frequency profile, are highly sensitive to these internal adjustments.

astro-ph.SR

Asteroseismic investigation of HD 140283: The Methuselah star

HD 140283 is a well-studied metal-poor subgiant and a Gaia benchmark star, often used for testing stellar models due to its proximity, brightness, and low metallicity ([Fe/H] = -2.3 dex). Here we present the first asteroseismic analysis of HD 140283, providing improved constraints on its fundamental properties. The star was observed by TESS in 20-second cadence during Sector 51. We extracted a custom light curve and performed a frequency analysis, revealing a rich spectrum of solar-like oscillations including mixed modes. These were combined with parameters from the literature to provide constraints on our model inference performed with BASTA. Using a dense grid of models, we find a mass of $0.75 \pm 0.01 \ \mathrm{M}_\odot$, a radius of $2.078 \substack{+0.012\\-0.011} \ \mathrm{R}_\odot$, and an age of $14.2 \pm 0.4$ Gyr, in agreement with the upper limit set by the age of the Universe within $1\sigma$. The observed frequency of maximum power, $\left(\nu_\mathrm{max}\right)_\mathrm{obs} = 611.3 \pm 7.4 \ \mu\mathrm{Hz}$, is significantly higher than predicted from standard scaling relations ($\left(\nu_\mathrm{max}\right)_\mathrm{mod} = 537.2 \substack{+2.9\\-1.8} \ \mu\mathrm{Hz}$), extending known deviations into the metal-poor regime. To our knowledge, the oscillations in HD 140283 have the highest $\nu_\mathrm{max}$ of any metal-poor star to date, which will help to advance our understanding of oscillations in metal-poor stars in general. The results demonstrate the value of asteroseismology for precise age determination in old halo stars and taking custom abundances and opacities into account during the modelling is probably important for further improving models of such stars. In addition, a detailed characterisation of metal-poor stars, such as HD 140283, will also help advance our understanding of Population III stars and their impact on future stellar generations.

astro-ph.SR

Asteroseismic modelling of Kepler Legacy stars including lithium depletion

Context. The Kepler Legacy sample is, to this day, the sample of solar-like oscillators with the most exquisite asteroseismic data. In this work, we carry out a detailed modelling of a subsample of these stars for which the surface lithium abundance has also been observed by the LAMOST survey and a photometric surface rotation as been measured. Aims. We aim at studying the impact of additional mixing processes on the asteroseismic modelling of Kepler Legacy G and F-type stars. We also investigate whether a single process can be invoked to reproduce the lithium depletion and asteroseismic constraints at the same time. Methods. We use detailed asteroseismic modelling techniques combining global and local minimization techniques. We start by using standard models and then aim at improving this solution by the addition of extra-mixing at the border of convective regions using either convective penetration or turbulence in radiative layers. Results. We find that lower mass models ($\sim 1M_\odot$) have no problem in reproducing the observed lithium depletion using only turbulence in the radiative zone, similarly to solar models. F-type stars, having a shallower convective envelope, are unaffected by additional turbulence at the BCZ, but require significant convective penetration values to actually reproduce the observed lithium depletion. The extent of this penetration is however incompatible with the frequency separation ratios. Conclusions. We conclude that the impact of extra-mixing is moderate for solar-type stars of the Kepler Legacy sample and well within the requirements of the PLATO mission. For more massive stars ($\sim 1.5M_\odot$), we conclude that the behaviour of the frequency separation ratios must be further investigated, as even models with large convective penetration at the base of their convective envelope are unable to reproduce them.

astro-ph.SR

Entropy proxy inversions as tracers of the evolution of physical conditions at the base of the solar convective envelope

The Sun is an important calibrator for the theory of stellar structure and evolution. However, the accuracy of our solar evolution models is tightly linked to the physical ingredients that enter their computations. This include, amongst other, the equation of state, the opacities, the transport of chemicals and the modelling of turbulent convection. Deriving model-independent probes of these ingredients is therefore crucial to further test the quality of these ingredients and potentially reveal their shortcomings using observational data. We aim at providing additional constraints on the thermodynamical properties of the solar plasma at the base of the solar convective zone using a revised helioseismic indicator mimicking the properties of the specific entropy in the envelope. We derive a revised entropy proxy for the solar convective envelope, directly accessible using helioseismic structure inversions. We then use solar evolutionary models with various modifications of input physics to study the properties of proxy of the entropy in the convective envelope. We find that the entropy proxy for the solar convective envelope allows to invalidate adiabatic overshooting as a solution to the solar modelling problem and strongly points towards the need for revised opacities. Our results show that this new indicator is a strong diagnostic of the overall evolution of the thermodynamical conditions at the base of the convective zone. The new entropy proxy indicator allows for a more accurate characterisation of the conditions at the base of the solar convective zone. While it already allows to rule out overshooting as a solution to the solar modelling problem, its sensitivity to the shape of the opacity modification and the evolution of the properties at the base of the convective zone makes it a powerful helioseismic diagnostic for solar models.

astro-ph.SR

Advancing accuracy in age determinations of old-disk stars using an oscillating red giant in an eclipsing binary

The study of resonant oscillation modes in low-mass red giant branch stars enables their ages to be inferred with exceptional ($\sim$10%) precision, unlocking the possibility to reconstruct the temporal evolution of the Milky Way at early cosmic times. Ensuring the accuracy of such a precise age scale is a fundamental yet difficult challenge. Since the age of red giant branch stars primarily hinges on their mass, an independent mass determination for an oscillating red giant star provides the means for such assessment. We analyze the old eclipsing binary KIC10001167, which hosts an oscillating red giant branch star and is a member of the thick disk of the Milky Way. Of the known red giants in eclipsing binaries, this is the only member of the thick disk that has asteroseismic signal of high enough quality to test the seismic mass inference at the 2% level. We measure the binary orbit and obtain fundamental stellar parameters through combined analysis of light curve eclipses and radial velocities, and perform a detailed asteroseismic, photospheric, and Galactic kinematic characterization of the red giant and binary system. We show that the dynamically determined mass $0.9337\pm0.0077 \rm\ M_{\odot}$ (0.8%) of this 10 Gyr-old star agrees within 1.4% with the mass inferred from detailed modelling of individual pulsation mode frequencies (1.6%). This is now the only thick disk stellar system, hosting a red giant, where the mass has been determined both asteroseismically with better than 2% precision, and through a model-independent method at 1% precision, and we hereby affirm the potential of asteroseismology to define an accurate age scale for ancient stars to trace the Milky Way assembly history.

astro-ph.SR

Detailed seismic study of Gemma (KIC11026764) using EGGMiMoSA: Unveiling the probing potential of mixed modes for subgiant stars

Context. When leaving the main sequence (MS) for the red-giant branch (RGB), subgiant stars undergo fast structural changes. Consequently, their observed oscillation spectra mirror these changes, constituting key tracers of stellar structure and evolution. However, the complexity of their spectra makes their modelling an arduous task, which few authors have undertaken. Gemma (KIC11026764) is a young subgiant with $45$ precise oscillation modes observed with Kepler, making it the ideal benchmark for seismic modelling. Aims. This study is aimed at modelling the subgiant Gemma, taking advantage of most of the precise seismic information available. This approach enables us to pave the way for the seismic modelling of evolved solar-like stars and provide the relevant insights into their structural evolution. Methods. Using our Levenberg-Marquardt stellar modelling tool, we built a family of models representative of Gemma's measured seismic indicators obtained via our seismic tool, EGGMiMoSA. We studied the structural information these indicators hold by carefully varying stellar parameters. We also complemented the characterisation with information held by \who indicators and non-seismic data. Results. From the extensive set of models we built and using most of the seismic information at hand, including two $\ell=1$ and one $\ell=2$ mixed modes, we were able to probe the chemical transition at the hydrogen-burning shell. Indeed, we have demonstrated that among our models, only the ones with the sharpest chemical gradient are able to reproduce all the seismic information considered. One possibility to account for such a gradient is the inclusion of a significant amount of overshooting, namely $\alpha_{\textrm{ov}}=0.17$, which is unexpected for low-mass stars such as Gemma (expected mass of about $1.15~M_{\odot}$).

astro-ph.SR

Is planetary inward migration responsible for GJ 504's fast rotation and bright X-ray luminosity? New constraints from eROSITA

The discovery of an increasing variety of exoplanets in very close orbits around their host stars raised many questions about how stars and planets interact, and to which extent host stars' properties may be influenced by the presence of close-by companions. Understanding how the evolution of stars is impacted by the interactions with their planets is fundamental to disentangle their intrinsic evolution from Star-Planet Interactions (SPI)-induced phenomena. GJ 504 is a promising candidate for a star that underwent strong SPI. Its unusually short rotational period (3.4 days), while being in contrast with what is expected by single-star models, could result from the inward migration of a close-by, massive companion, pushed starward by tides. Moreover, its brighter X-ray luminosity may hint at a rejuvenation of the dynamo process sustaining the stellar magnetic field, consequent to the SPI-induced spin-up. We aim to study the evolution of GJ 504 and establish whether by invoking the engulfment of a planetary companion we can better reproduce its rotational period and X-ray luminosity. We simulate the past evolution assuming two different scenarios: 'Star without close-by planet', 'Star with close-by planet'. In the second scenario, we investigate how inward migration and planetary engulfment driven by tides spin up the stellar surface and rejuvenate its dynamo. We compare our tracks with rotational period and X-ray data collected from the all-sky surveys of the ROentgen Survey with an Imaging Telescope Array (eROSITA) on board the Russian Spektrum-Roentgen-Gamma mission (SRG). Despite the very uncertain stellar age, we found that the second evolutionary scenario is in better agreement with the short rotational period and the bright X-ray luminosity of GJ 504, thus strongly favouring the inward migration scenario over the one in which close-by planets have no tidal impact on the star.

astro-ph.EP

Constraints on the properties of macroscopic transport in the Sun from combined lithium and beryllium depletion

Context. The Sun is a privileged laboratory of stellar evolution, thanks to the quality and complementary nature of available constraints. Using these observations, we are able to draw a detailed picture of its internal structure and dynamics which form the basis of the successes of solar modelling. Amongst such constraints, the depletion of lithium and beryllium are key tracers of the required efficiency and extent of macroscopic mixing just below the solar convective envelope. Thanks to revised determinations of these abundances, we may use them in conjunction with other existing spectroscopic and helioseismic constraints to study in detail the properties of macroscopic transport. Aims. We aim at constraining the efficiency of macroscopic transport at the base of the convective envelope and determining the compatibility of the observations with a suggested candidate linked with the transport of angular momentum in the solar radiative interior. Methods. We use recent spectroscopic observations of lithium and beryllium abundance and include them in solar evolutionary model calibrations. We test the agreement of such models in terms of position of the convective envelope, helium mass fraction in convective zone, sound speed profile inversions and neutrino fluxes. Results. We constrain the required efficiency and extent of the macroscopic mixing at the base of the solar convective envelope, finding that a power law of density with an index n between 3 and 6 would reproduce the data, with efficiencies at the base of the envelope of about 6000 cm2 /s, depending on the value of n. We also confirm that macroscopic mixing worsens the agreement with neutrino fluxes and that the current implementations of the magnetic Tayler instability are unable to explain the observations.

astro-ph.SR

The age of the Methuselah star in light of stellar evolution models with tailored abundances

Context. HD140283, or the Methuselah star, is a well-known reference object in stellar evolution. Its peculiar chemical composition, proximity and absence of reddening makes it an interesting case-study of Pop II stars. Thanks to recent observational efforts, we now have precise interferometric and spectroscopic constraints, as well as revised astrometric parallaxes from the Gaia mission. Aims. We aim at determining the age of HD140283 with these lastest constraints, as well as quantifying the impact of systematics from physical inaccuracies in the stellar evolution models. Methods. Using recent spectroscopic abundances from the literature, including 3D non-LTE values for C, O, and Fe, we compute opacity tables specific to HD140283. We then use them in grids of stellar evolution models coupled to a Markov Chain Monte Carlo tool to determine the age of HD140283. Results. With our tailored models we find an age of 12.3Gy. Using a solar-scaled mixture instead results in an age value of 14Gy, in tension with the age of the universe ($13.77\pm0.06$Gy). We also find that reducing the mixing length parameter from its solar calibrated value will lead to an even lower age, in agreement with other recent studies. However, we find no direct evidence to favour a lower mixing length parameter value from our modelling. Conclusions. Taking into account the specific elemental abundances is crucial for the modelling of HD140283, as it leads to significant differences in the inferred age. However, this effect is degenerate with a lowering of the mixing length parameter. In this respect, asteroseismic constraints might play a key role in accurately deriving the mass of HD140283, therefore strongly constraining its age.

astro-ph.SR

Heavy Elements Abundances Inferred from the First Adiabatic Exponent in the Solar Envelope

The first adiabatic exponent profile, noted $\Gamma_1$, computed along adiabatic coordinates $(T, \rho)$ is in the focus of our study. Under conditions of almost fully ionized hydrogen and helium, the $\Gamma_1$ profile is quite sensitive to heavy elements ionization. $\Gamma_1$ decreases in regions where an element is partially ionized. The recent helioseismic structural inversion is obtained with an accuracy better than $10^{-4}$ in the most of the adiabatic convective zone that allows to study ionization variations. The aim is to determine the major heavy elements content in the solar convective zone. The method of our research is synthesis of the $\Gamma_1$ profile which is based on a linear combination of the contributions of individual heavy elements. The idea of the approach was proposed and justified by Baturin et al. (Astron. Astrophys., 660, A125, 2022). We find the best approximation of the inverted profile $\Gamma_1$ adjusting the abundances of major elements (C, N, O, Ne), meanwhile the abundances of elements heavier than neon are fixed. We synthesize the theoretical $\Gamma_1$ profile using the SAHA-S equation of state, and are able to reproduce the inverted profiles with an accuracy of $(1-2)\cdot 10^{-5}$. Total mass fraction of heavy elements found by this method is $Z=0.0148\pm 0.0004$. The oxygen logarithmic abundance is $8.70\pm 0.03$, carbon $8.44\pm 0.04$, nitrogen $8.12\pm 0.08$, and neon $8.17\pm 0.09$. The obtained estimations of oxygen and carbon agree with spectroscopic abundances by Asplund et al. (Astron. Astrophys., 653, A141, 2021).

astro-ph.SR

The solar beryllium abundance revisited with 3D non-LTE models

The present-day abundance of beryllium in the solar atmosphere provides clues about mixing mechanisms within stellar interiors. However, abundance determinations based on the Be II 313.107 nm line are prone to systematic errors due to imperfect model spectra. These errors arise from missing continuous opacity in the UV, a significant unidentified blend at 313.102 nm, departures from local thermodynamic equilibrium (LTE), and microturbulence and macroturbulence fudge parameters associated with one-dimensional (1D) hydrostatic model atmospheres. Although these factors have been discussed in the literature, no study has yet accounted for all of them simultaneously. To address this, we present 3D non-LTE calculations for neutral and ionised beryllium in the Sun. We used these models to derive the present-day solar beryllium abundance, calibrating the missing opacity on high resolution solar irradiance data and the unidentified blend on the centre-to-limb variation. We find a surface abundance of 1.21 $\pm$ 0.05 dex, which is significantly lower than the value of 1.38 dex that has been commonly adopted since 2004. Taking the initial abundance via CI chondrites, our result implies that beryllium has been depleted from the surface by an extra 0.11 $\pm$ 0.06 dex, or 22 $\pm$ 11%, on top of any effects of atomic diffusion. This is in tension with standard solar models, which predict negligible depletion, as well as with contemporary solar models that have extra mixing calibrated on the abundances of helium and lithium, which predict excessive depletion. These discrepancies highlight the need for further improvements to the physics in solar and stellar models.

astro-ph.SR

MCMC inversions of the internal rotation of Kepler subgiants

The measurement of the internal rotation of post-main sequence stars using data from space-based photometry missions has demonstrated the need for an efficient angular momentum transport in stellar interiors. So far, no clear solution has emerged and explaining the observed trends remain a challenge for stellar modellers. We aim at constraining both the shape of the internal rotation profile of six Kepler subgiants studied in details in 2014 and the properties of the missing angular momentum transport process acting in stellar interiors from MCMC inversions of the internal rotation. We apply a new MCMC inversion technique to existing Kepler subgiant targets and test various shapes of the internal rotation profile of all six original subgiants observed in 2014. We also constrain the limitations on the number of free parameters that can be used in the MCMC inversion, showing the limitations in the amount of information in the seismic data. First, we show that large-scale fossil magnetic fields are not able to explain the internal rotation of subgiants, similarly to what was determined from detailed studies of Kepler red giants. We are also able to constrain the location of the transition in the internal rotation profile for the most evolved stars in the available set of subgiants. We find that some of them exhibit a transition located close to the border of the helium core while one clearly does not. We conclude that it might be possible that various processes might be at play to explain our observations, but that revealing the physical nature of the angular momentum process will require a consistent detailed modelling of all subgiants available, particularly the least evolved. In addition, increasing the number of stars for which such inferences are possible (e.g. with the future PLATO mission) is paramount given the key role they play in validating transport process candidates.

astro-ph.SR

In-depth analysis of solar models with high-metallicity abundances and updated opacity tables

Due to the high quality constraints available for the Sun, we can carry out combined analyses using neutrino, spectroscopic and helioseismic observations. Such studies lay the ground for future improvements of key physical components of solar and stellar models, such as the equation of state, radiative opacities or prescriptions for macroscopic transport processes of chemicals which are then used to study other stars in the Universe. We study the existing degeneracies in solar models using the recent high-metallicity spectroscopic abundances by comparing them to helioseismic and neutrino data and discuss how their properties are impacted by changes in various physical ingredients. We carry out a detailed study of solar models computed with a high-metallicity composition from the literature based on averaged-3D models that was claimed to solve the solar problem. The properties of the solar models are significantly affected by using the recent OPLIB opacities and the inclusion of macroscopic transport. The properties of the standard solar models computed using the OPAL opacities are similar to those using the OP opacities. We show that a modifying the temperature gradient just below the base of the convective zone is required to erase the discrepancies in solar models, particularly in the presence of macroscopic mixing. This can be simulated by a local increase of opacity of a few percent. We conclude that the existing degeneracies and issues in solar modelling are not erased by an increase in the solar metallicity in contradiction to was suggested in recent papers. Therefore, standard solar models cannot be used as an argument for a high metallicity composition. While further work is required to improve solar models, we note that direct helioseismic inversions indicate a low metallicity in the convective envelope, in agreement with spectroscopic analyses based on full 3D models.

astro-ph.SR