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13 Publications visible to you, out of a total of 13
Testing a non-local 1-equation turbulent convection model: A solar model

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Context: Turbulent convection models treat stellar convection more physically than standard mixing-length theory by including non-local effects. We recently successfully applied the Kuhfuss version to … convective cores in main sequence stars. Its usefulness for convective envelopes remains to be tested. Aims: The solar convective envelope constitutes a viable test bed for investigating the usefulness of the 1-equation Kuhfuss turbulent convection model. Methods: We used the one-dimensional stellar evolution code GARSTEC to calculate a standard solar model with the 1-equation Kuhfuss turbulent convection model, and compared it to helioseismic measurements and a solar model using standard mixing-length theory. Additionally, we investigated the influence of the additional free parameters of the convection model on the solar structure. Results: The 1-equation Kuhfuss model reproduces the sound-speed profile and the lower boundary of the convective region less well than the mixing-length model, because the inherent non-local effects overestimate the amount of convective penetration below the Schwarzschild boundary. We trace this back to the coupling of the temperature gradient to the convective flux in the 1-equation version of the Kuhfuss theory. Conclusions: The temperature stratification of the solar convective envelope is not well modelled by the 1-equation Kuhfuss turbulent convection model, and the more complex 3-equation version is needed to improve the modelling of convection in the envelopes of 1D stellar evolution models.

Authors: T. A. M. Braun, F. Ahlborn, A. Weiss

Date Published: 1st Sep 2024

Publication Type: Journal

Effect of nuclear reactions rates and core boundary mixing on the seismology of red clump stars

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Context: Modeling of the stars in the red clump (RC), that is, core helium-burning stars that have gone through a He flash, is challenging because of the uncertainties associated with the physical … processes in their core and during the helium flash. By probing the internal stellar structure, asteroseismology allows us to constrain the core properties of RC stars and eventually, to improve our understanding of this evolutionary phase. Aims: We aim to quantify the impact on the seismic properties of the RC stars of the two main core modeling uncertainties: core boundary mixing, and helium-burning nuclear reaction rates. Methods: Using the MESA stellar evolution code, we computed models with different core boundary mixing as well as different 3α and 12C(α, γ)16O nuclear reaction rates. We investigated the impact of these parameters on the period spacing ΔΠ, which is a probe of the region around the core. Results: We find that different core boundary mixing schemes yield significantly different period spacings, with differences of 30 s between the maximum ΔΠ value computed with semiconvection and maximal overshoot. We show that an increased rate of 12C(α, γ)16O lengthens the core helium-burning phase, which extends the range of ΔΠ covered by the models during their evolution. This results in a difference of 10 s between the models computed with a nominal rate and a rate multiplied by 2, which exceeds the observational uncertainties. The effect of changing the 3α reaction rate is comparatively small. Conclusions: The core boundary mixing is the main source of uncertainty in the seismic modeling of RC stars. Moreover, the effect of the 12C(α, γ)16O is non-negligible, even though it is difficult to distinguish from the effect of the mixing. This degeneracy could be seen more frequently in the future in the new seismic data from the PLATO mission and through theoretical constraints from numerical simulations.

Authors: Anthony Noll, Sarbani Basu, Saskia Hekker

Date Published: 1st Mar 2024

Publication Type: Journal

Asteroseismic Inversions for Internal Sound Speed Profiles of Main-sequence Stars with Radiative Cores

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The theoretical oscillation frequencies of even the best asteroseismic models of solar-like oscillators show significant differences from observed oscillation frequencies. Structure inversions seek to … use these frequency differences to infer the underlying differences in stellar structure. While used extensively to study the Sun, structure inversion results for other stars have so far been limited. Applying sound speed inversions to more stars allows us to probe stellar theory over a larger range of conditions, as well as look for overall patterns that may hint at deficits in our current understanding. To that end, we present structure inversion results for 12 main-sequence solar-type stars with masses between 1 and 1.15M⊙. Our inversions are able to infer differences in the isothermal sound speed in the innermost 30% by radius of our target stars. In half of our target stars, the structure of our best-fit model fully agrees with the observations. In the remainder, the inversions reveal significant differences between the sound speed profile of the star and that of the model. We find five stars where the sound speed in the core of our stellar models is too low and one star showing the opposite behavior. For the two stars in which our inversions reveal the most significant differences, we examine whether changing the microphysics of our models improves them and find that changes to nuclear reaction rates or core opacities can reduce, but do not fully resolve, the differences.

Authors: Lynn Buchele, Earl P. Bellinger, Saskia Hekker, Sarbani Basu, Warrick Ball, Jørgen Christensen-Dalsgaard

Date Published: 26th Jan 2024

Publication Type: Journal

New red giants in NGC 6791 and NGC 6819 using Kepler superstamps

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Context: Stars that are members of stellar clusters are assumed to be formed at the same time and place from material with the same initial chemical composition. These additional constraints on the … ensemble of cluster stars make these stars suitable as benchmarks. Aims: We aimed (1) to identify previously unknown red giants in the open clusters NGC 6791 and NGC 6819, (2) to extract their asteroseismic parameters, and (3) to determine their cluster membership. Methods: We followed a dedicated method based on difference imaging to extract the light curves of potential red giants in NGC 6791 and NGC 6819 from Kepler superstamp data. We extracted the asteroseismic parameters of the stars that showed solar-like oscillations. We performed an asteroseismic membership study to identify which of these stars are likely to be cluster members. Results: We found 149 red giant stars within the Kepler superstamps, 93 of which are likely cluster members. We were able to find 29 red giants that are not primary targets of Kepler, and therefore, their light curves had not been released previously. Five of these previously unknown red giants have a cluster membership probability greater than 95%.

Authors: A. Covelo-Paz, N. Themeßl, F. Espinoza-Rojas, S. Hekker

Date Published: 1st Nov 2023

Publication Type: Journal

Impact of central mixing scheme and nuclear reaction network on the extent of convective cores

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Convective cores are the hydrogen reservoirs of main sequence stars that are more massive than around 1.2 solar masses. The characteristics of the cores have a strong impact on the evolution and structure … of the star. However, such results rely on stellar evolution codes, in which simplistic assumptions are often made on the physics in the core. Indeed, mixing is commonly considered to be instantaneous and the most basic nuclear networks assume beryllium at its equilibrium abundance. Those assumptions lead to significant differences in the central composition of the elements for which the timescale to reach nuclear equilibrium is lower than the convective timescale. In this work, we show that those discrepancies impact the nuclear energy production and, therefore, the size of convective cores in models computed with overshoot. We find that cores computed with instantaneous mixing are up to 30% bigger than those computed with diffusive mixing. Similar differences are found when using basic nuclear networks. Additionally, we observed an extension of the duration of the main sequence due to those core size differences. We then investigated the impact of those structural differences on the seismic modeling of solar-like oscillators. Modeling two stars observed by Kepler, we find that the overshoot parameter of the best models computed with a basic nuclear network is significantly lower, compared to models computed with a full nuclear network. This work is a necessary step in improving the modeling of convective cores, which is key to determining accurate ages in the framework of future space missions such as Plato.

Authors: Anthony Noll, Sébastien Deheuvels

Date Published: 1st Aug 2023

Publication Type: Journal

Exoplanet interior retrievals: core masses and metallicities from atmospheric abundances

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ABSTRACT The mass and distribution of metals in the interiors of exoplanets are essential for constraining their formation and evolution processes. Never the less, with only masses and radii measured, … the determination of exoplanet interior structures is degenerate, and so far simplified assumptions have mostly been used to derive planetary metallicities. In this work, we present a method based on a state-of-the-art interior code, recently used for Jupiter, and a Bayesian framework, to explore the possibility of retrieving the interior structure of exoplanets. We use masses, radii, equilibrium temperatures, and measured atmospheric metallicities to retrieve planetary bulk metallicities and core masses. Following results on the giant planets in the Solar system and recent development in planet formation, we implement two interior structure models: one with a homogeneous envelope and one with an inhomogeneous one. Our method is first evaluated using a test planet and then applied to a sample of 37 giant exoplanets with observed atmospheric metallicities from the pre-JWST era. Although neither internal structure model is preferred with the current data, it is possible to obtain information on the interior properties of the planets, such as the core mass, through atmospheric measurements in both cases. We present updated metal mass fractions, in agreement with recent results on giant planets in the Solar system.

Authors: S Bloot, Y Miguel, M Bazot, S Howard

Date Published: 1st Aug 2023

Publication Type: Journal

Testing angular momentum transport processes with asteroseismology of solar-type main-sequence stars

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Context: Thanks to the so-called photometry revolution with the space-based missions CoRoT, Kepler, and TESS, asteroseismology has become a powerful tool to study the internal rotation of stars. The … rotation rate depends on the efficiency of the angular momentum (AM) transport inside the star, and its study allows to constrain the internal AM transport processes, as well as improve our understanding of their physical nature. Aims: We compared the ratio of the rotation rate predicted by asteroseismology and starspot measurements of solar-type stars considering different AM transport prescriptions and investigated whether some of these prescriptions can be ruled out observationally. Methods: We conducted a two-step modelling procedure of four main-sequence stars from the Kepler LEGACY sample, which consists of an asteroseismic characterisation that serves as a guide for a modelling with rotating models, including a detailed and coherent treatment of the AM transport. The rotation profiles derived with this procedure were used to estimate the ratio of the mean asteroseismic rotation rate with the surface rotation rate from starspot measurements for each AM transport prescriptions. Comparisons between the models were then conducted. Results: In the hotter part of the Hertzsprung-Russell (HR) diagram (masses typically above ∼1.2 M⊙ at solar metallicity), models with only hydrodynamic transport processes and models with additional transport by magnetic instabilities are found to be consistent with previous measurements that observed a low degree (below 30%) of radial differential rotation between the radiative and convective zones. For these stars, which constitute a significant fraction of the Kepler LEGACY sample, a combination of asteroseismic constraints from the splitting of pressure modes and of the surface rotation rate does not allow us to conclude that an efficient AM transport is required in addition to transport by meridional circulation and shear instability alone. Even a model assuming local AM conservation cannot be ruled out. In the colder part of the HR diagram, the situation is different because of the efficient braking of the stellar surface by magnetised winds. We find a clear disagreement between the rotational properties of models that only include hydrodynamic processes and asteroseismic constraints, while models with magnetic fields correctly reproduce the observations, similarly to the solar case. Conclusions: This shows the existence of a mass regime corresponding to main-sequence F-type stars for which it is difficult to constrain the AM transport processes, unlike for hotter, Gamma Dor stars or colder, less massive solar analogues. The comparison between asteroseismic measurements and surface rotation rates enables us to easily rule out models with an inefficient transport of AM in the colder part of the HR diagram.

Authors: J. Bétrisey, P. Eggenberger, G. Buldgen, O. Benomar, M. Bazot

Date Published: 1st May 2023

Publication Type: Journal

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