Surface effects and turbulent pressure

Belkacem, K.; Kupka, F.; Philidet, J.; Samadi, R.

doi:10.1051/0004-6361/202040259

Cited by 10 publications

(6 citation statements)

References 43 publications

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“…While these values of the dispersion are smaller than the reference values, they are by no means negligible in the case of the radius and age. Thus, this result calls for an improvement of the modelling of the surface layers of stars, both in what concerns the structure and the pulsations (Mosumgaard et al 2020;Belkacem et al 2021;Jørgensen et al 2021).…”

Section: Discussionmentioning

confidence: 99%

PLATO Hare-and-Hounds exercise: Asteroseismic model fitting of main-sequence solar-like pulsators

Cunha,

Roxburgh,

Børsen-Koch

et al. 2021

Preprint

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Asteroseismology is a powerful tool to infer fundamental stellar properties. The use of these asteroseismic-inferred properties in a growing number of astrophysical contexts makes it vital to understand their accuracy. Consequently, we performed a hare-and-hounds exercise where the hares simulated data for 6 artificial main-sequence stars and the hounds inferred their properties based on different inference procedures. To mimic a pipeline such as that planned for the PLATO mission, all hounds used the same model grid. Some stars were simulated using the physics adopted in the grid, others a different one. The maximum relative differences found (in absolute value) between the inferred and true values of the mass, radius, and age were 4.32 per cent, 1.33 per cent, and 11.25 per cent, respectively. The largest systematic differences in radius and age were found for a star simulated assuming gravitational settling, not accounted for in the model grid, with biases of -0.88 per cent (radius) and 8.66 per cent (age). For the mass, the most significant bias (-3.16 per cent) was found for a star with a helium enrichment ratio outside the grid range. Moreover, a ∼7 per cent dispersion in age was found when adopting different prescriptions for the surface corrections or shifting the classical observations by ±1𝜎. The choice of the relative weight given to the classical and seismic constraints also impacted significantly the accuracy and precision of the results. Interestingly, only a few frequencies were required to achieve accurate results on the mass and radius. For the age the same was true when at least one 𝑙 = 2 mode was considered.

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Section: Discussionmentioning

confidence: 99%

PLATO Hare-and-Hounds exercise: Asteroseismic model fitting of main-sequence solar-like pulsators

Cunha,

Roxburgh,

Børsen-Koch

et al. 2021

Preprint

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“…Some aspects, however, are very complicated to model, and existing models make use of assumptions that can barely be justified, if at all. For instance, turbulent pressure modulations are usually described in the Gas-Γ 1 (GGM) or reduced-Γ 1 (RGM) approximations (Rosenthal et al 1999), which amounts to neglecting the effects of turbulent dissipation and buoyancy on the mode (Belkacem et al 2021). Another problem is the use of time-dependent mixing-length formalisms (Unno 1967;Gough 1977) to account for modal surface effects (e.g Gabriel et al 1975;Houdek 1996;Grigahcène et al 2005;Sonoi et al 2017;Houdek et al 2017Houdek et al , 2019.…”

Section: Introductionmentioning

confidence: 99%

Coupling between turbulence and solar-like oscillations: a combined Lagrangian PDF/SPH approach. I -- The stochastic wave equation

Philidet,

Belkacem,

Goupil

2021

Preprint

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Context. The development of space-borne missions such as CoRoT or Kepler now provides us with numerous and precise asteroseismic measurements, which allows us to put better constraints on our theoretical knowledge of the physics of stellar interiors. In order to utilise the full potential of these measurements, however, we need a better theoretical understanding of the coupling between stellar oscillations and turbulent convection. Aims. This series of papers aims at building a new formalism specifically tailored to study the impact of turbulence on the global modes of oscillation in solar-like stars. In building this formalism, we circumvent some fundamental limitations inherent to the more traditional approaches, in particular the need for separate equations for turbulence and oscillations, and the reduction of the turbulent cascade to a unique length and time scale. This first paper aims at deriving a linear wave equation that directly and consistently contains the turbulence as an input to the model, and therefore naturally contains the information on the coupling between the turbulence and the modes, through the stochasticity of the equations. Methods. We use a Lagrangian stochastic model of turbulence based on Probability Density Function methods to describe the evolution of the properties of individual fluid particles through stochastic differential equations. We then transcribe these stochastic differential equations from a Lagrangian frame to an Eulerian frame, more adapted to the analysis of stellar oscillations. We combine this method with Smoothed Particle Hydrodynamics, where all the mean fields appearing in the Lagrangian stochastic model are estimated directly from the set of fluid particles themselves, through the use of a weighting kernel function allowing to filter the particles present in any given vicinity. The resulting stochastic differential equations on Eulerian variables are then linearised. As a first step, the gas is considered to follow a polytropic relation, and the turbulence is assumed anelastic. Results. We obtain a stochastic, linear wave equation governing the time evolution of the relevant wave variables, while at the same time containing the effect of turbulence. The wave equation generalises the classical, unperturbed propagation of acoustic waves in a stratified medium (which reduces to the exact deterministic wave equation in the absence of turbulence) to a form that, by construction, accounts for the impact of turbulence on the mode in a consistent way. The effect of turbulence consists in a non-homogeneous forcing term, responsible for the stochastic driving of the mode, and a stochastic perturbation to the homogeneous part of the wave equation, responsible for both the damping of the mode and the modal surface effects. Conclusions. The stochastic wave equation obtained here represents our baseline framework to properly infer properties of turbulenceoscillation coupling, and can therefore be used to constrain the properties of the turbulence itself with the help of asteros...

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“…uncertainties surrounding the near-surface region are to be faced, namely surface effects (Christensen-Dalsgaard et al 1988). As a result of theoretical developments (Canuto 1997;Belkacem et al 2021) and 3D hydrodynamical simulations (Belkacem et al 2019;Schou & Birch 2020), our understanding of the mechanisms involved in these effects (as well as their contributions) has definitely improved. However, the more costly procedures (Mosumgaard et al 2020) are generally avoided in favour of ad hoc corrections of the oscillation frequencies (Kjeldsen et al 2008;Ball & Gizon 2014;Sonoi et al 2015), which may affect stellar parameter estimates (Nsamba et al 2018).…”

Section: Introductionmentioning

confidence: 99%

Properties of the ionisation glitch

Houdayer

Reese

Goupil

et al. 2021

A&A

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Context. Determining the properties of solar-like oscillating stars can be subject to many biases. A particularly important example is the helium-mass degeneracy, where the uncertainties regarding the internal physics can cause a poor determination of both the mass and surface helium content. Accordingly, an independent helium estimate is needed to overcome this degeneracy. A promising way to obtain such an estimate is to exploit the so-called ionisation glitch, that is, the deviation from the asymptotic oscillation frequency pattern caused by the rapid structural variation in the He ionisation zones. Aims. Although it is progressively becoming more sophisticated, the glitch-based approach faces problems inherent to its current modelling such as the need for calibration using realistic stellar models. This requires a physical model of the ionisation region that explicitly involves the parameters of interest, such as the surface helium abundance, Ys. Methods. Through a thermodynamic treatment of the ionisation region, an analytical approximation for the first adiabatic exponent Γ1 is presented. Results. The induced stellar structure is found to depend on only three parameters, including the surface helium abundance Ys and the electron degeneracy ψCZ in the convective region. The model thus defined allows a wide variety of structures to be described, and it is in particular able to approximate a realistic model in the ionisation region. The modelling work we conducted enables us to study the structural perturbations causing the glitch. More elaborate forms of perturbations than those that are usually assumed are found. It is also suggested that there might be a stronger dependence of the structure on the electron degeneracy in the convection zone and on the position of the ionisation region rather than on the amount of helium itself. Conclusions. When analysing the ionisation glitch signature, we emphasise the importance of having a relation that can take these additional dependences into account.

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Surface effects and turbulent pressure

Cited by 10 publications

References 43 publications

PLATO Hare-and-Hounds exercise: Asteroseismic model fitting of main-sequence solar-like pulsators

PLATO Hare-and-Hounds exercise: Asteroseismic model fitting of main-sequence solar-like pulsators

Coupling between turbulence and solar-like oscillations: a combined Lagrangian PDF/SPH approach. I -- The stochastic wave equation

Properties of the ionisation glitch

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