Models Of Stellar Convection Research Articles

Stellar evolution models with overshooting based on 3-equation non-local theories

Context.Mixing by convective overshooting has long been suggested to play an important role in the amount of hydrogen available for nuclear burning in convective cores of stars. The best way to model this effect is still debated.Aims.We suggest an improved model for the computation of the dissipation rate of turbulent kinetic energy which can be used in non-local models of stellar convection and can readily be implemented and self-consistently used in 1D stellar evolution calculations.Methods.We review the physics underlying various models to compute the dissipation rate of turbulent kinetic energy,ϵ, in local models of convection in stellar astrophysics and particularly in non-local ones. The different contributions to the dissipation rate and their dependence on local stratification and on non-local transport are analysed and a new method to account for at least some of these physical mechanisms is suggested.Results.We show how the new approach influences predictions of stellar models of intermediate-mass main-sequence stars and how these changes differ from other modifications of the non-local convection model that focus on the ratio of horizontal to vertical (turbulent) kinetic energy.Conclusions.The new model is shown to allow for a physically more complete description of convective overshooting and mixing in massive stars. Dissipation by buoyancy waves is found to be a key ingredient which has to be accounted for in non-local models of turbulent convection.

The shock-heated atmosphere of an asymptotic giant branch star resolved by ALMA

Our current understanding of the chemistry and mass-loss processes in solar-like stars at the end of their evolution depends critically on the description of convection, pulsations and shocks in the extended stellar atmosphere. Three-dimensional hydrodynamical stellar atmosphere models provide observational predictions, but so far the resolution to constrain the complex temperature and velocity structures seen in the models has been lacking. Here we present submillimeter continuum and line observations that resolve the atmosphere of the asymptotic giant branch star W Hya. We show that hot gas with chromospheric characteristics exists around the star. Its filling factor is shown to be small. The existence of such gas requires shocks with a cooling time larger than commonly assumed. A shocked hot layer will be an important ingredient in the models of stellar convection, pulsation and chemistry that underlie our current understanding of the late stages of stellar evolution.

Applications of the k–ω Model in Stellar Evolutionary Models

Abstract The k–ω model for turbulence was first proposed by Kolmogorov. A new k–ω model for stellar convection was developed by Li, which could reasonably describe turbulent convection not only in the convectively unstable zone, but also in the overshooting regions. We revised the k–ω model by improving several model assumptions (including the macro-length of turbulence, convective heat flux, and turbulent mixing diffusivity, etc.), making it applicable not only for convective envelopes, but also for convective cores. Eight parameters are introduced in the revised k–ω model. It should be noted that the Reynolds stress (turbulent pressure) is neglected in the equation of hydrostatic support. We applied it into solar models and 5 M ⊙ stellar models to calibrate the eight model parameters, as well as to investigate the effects of the convective overshooting on the Sun and intermediate mass stellar models.

Study on Properties of the k − ω Model for Stellar Convection

For applying the k − ω model proposed by Li to the general stellar environment, it is necessary to study the physical meanings of the parameters in this model, in order to set a limit for the range of their values. It is indicated by the study that the variation of the parameter clμ impacts all the Péclet number, kinetic energy, characteristic timescale and characteristic length of turbulent flows. Besides, as the model parameter clμ increases, the damping rate of turbulent kinetic energy in the bottom convective overshoot region can be accelerated evidently. Both the model parameter clμ and the equivalent mixing length parameter α are proportional to the effciency of convective heat transfer in the convection zone, and their logarithms have a linear relation. There is also a linear relation between the logarithms of the model parameter clμ and the damping index θ of turbulent kinetic energy in the convective overshoot region. For the Sun, a group of appropriate model parameters are obtained to be: clμ = 0.004, and α = 1.7.

Zonal flow regimes in rotating anelastic spherical shells: An application to giant planets

The surface zonal winds observed in the giant planets form a complex jet pattern with alternating prograde and retrograde direction. While the main equatorial band is prograde on the gas giants, both ice giants have a pronounced retrograde equatorial jet.We use three-dimensional numerical models of compressible convection in rotating spherical shells to explore the properties of zonal flows in different regimes where either rotation or buoyancy dominates the force balance. We conduct a systematic parameter study to quantify the dependence of zonal flows on the background density stratification and the driving of convection.In our numerical models, we find that the direction of the equatorial zonal wind is controlled by the ratio of the global-scale buoyancy force and the Coriolis force. The prograde equatorial band maintained by Reynolds stresses is found in the rotation-dominated regime. In cases where buoyancy dominates Coriolis force, the angular momentum per unit mass is homogenized and the equatorial band is retrograde, reminiscent to those observed in the ice giants. In this regime, the amplitude of the zonal jets depends on the background density contrast with strongly stratified models producing stronger jets than comparable weakly stratified cases. Furthermore, our results can help to explain the transition between solar-like (i.e. prograde at the equator) and the “anti-solar” differential rotations (i.e. retrograde at the equator) found in anelastic models of stellar convection zones.In the strongly stratified cases, we find that the leading order force balance can significantly vary with depth. While the flow in the deep interior is dominated by rotation, buoyancy can indeed become larger than Coriolis force in a thin region close to the surface. This so-called “transitional regime” has a visible signature in the main equatorial jet which shows a pronounced dimple where flow amplitudes notably decay towards the equator. A similar dimple is observed on Jupiter, which suggests that convection in the planet interior could possibly operate in this regime.

On the effects of coherent structures on higher order moments in models of solar and stellar surface convection

Non-local models of stellar convection usually rely on the assumption that the transfer of convective heat flux, turbulent kinetic energy and related quantities can be described as a diffusion process or that the fourth-order moments of velocity and temperature fluctuations follow a Gaussian distribution (quasi-normal approximation). The latter is also assumed in models of solar p-mode excitation. We have used realistic numerical simulations of solar granulation and of granulation in a K dwarf to test the quasi-normal approximation and several alternatives. For the superadiabatic layer of the Sun and for the quasi-adiabatic zone underneath, we find that the hypothesis of quasi-normality is a rather poor approximation. In the superadiabatic layer, it overestimates some of the fourth-order moments of vertical velocity and temperature by up to a factor of 2 while it underestimates them in the quasi-adiabatic layers by up to a factor of 3.5. The model proposed by Gryanik & Hartmann and Gryanik et al. reduces the discrepancies within the quasi-adiabatic zone to typically less than 30 per cent and is partially comparable and partially in better agreement with the simulation data than two earlier models by Grossman & Narayan. Simulation data for the K dwarf confirm these results. However, none of the proposed models works well in the superadiabatic layer nor in the photospheric layers above. For the Sun, we provide evidence that the fourth-order moments of horizontal velocity fields can be estimated to within about 30 per cent with the quasi-normal approximation despite the complexity of the horizontal flow. Comparing our results to those from solar simulations with idealized microphysics and with related studies of geophysical convection zones confirms our conclusions about the quasi-normal approximation and the new models. The improvements come from including the effects of coherent structures (such as granules or plumes), while the limitations are tied to the transition regions or boundaries such as the rapid radiative cooling that occurs at the top of the convection zone. Incorporating the model of Gryanik & Hartmann and Gryanik et al. into non-local convection models may well produce a significant improvement in the modelling of convection or of solar-like p-mode excitation in the quasi-adiabatic part of convection zones. For application to entire convection zones, modifications are necessary which can account for the change in background properties of the convective medium near boundaries or transition regions.

Some open questions concerning the modelling of non-locality in Reynolds stress type models of stellar convection.

AbstractWe discuss some benefits and pitfalls when combining conceptually different types of closure approximations into complete Reynolds stress models of stellar convection.

Probing Reynolds stress models of convection with numerical simulations: III. Compressibility modelling and dissipation

AbstractWe provide results from an extended 3D numerical simulation study of Reynolds stress models of stellar convection and probe the modelling of compressibility, pressure fluctuations, and dissipation of turbulent kinetic energy.

Radiative hydrodynamics models of stellar convection

AbstractIn the quest for a better theoretical understanding of the dynamical processes operating in stellar envelopes, increasingly more realistic hydrodynamical models of stellar convection have been developed over the last 25 years. Based on 3D state-of-the-art simulations performed recently with the radiation hydrodynamics code CO5BOLD, we review the present status of hydrodynamical modeling of stellar surface convection and demonstrate the basic differences between 3D hydrodynamical models and 1D ‘classical’ hydrostatic atmospheres where convection is treated by the mixing-length theory. We briefly discuss some of the many possible applications across the Hertzsprung-Russell diagram, including spectroscopic abundance determinations using 3D stellar atmospheres, particularly with regard to the much debated question of the solar oxygen abundance.

Probing Reynolds stress models of convection with numerical simulations: I. Overall properties: fluxes, mean profiles

AbstractWe introduce an extended 3D numerical simulation study of Reynolds stress models of stellar convection and probe fluxes as well as mean temperature gradient profiles.

Probing Reynolds stress models of convection with numerical simulations: II. Non-locality and third order moments.

AbstractWe provide results from an extended numerical simulation study of Reynolds stress models of stellar convection and probe the modelling of third order moments and non-locality.

Local models of stellar convection

Aims: The Strouhal number (St), which is a nondimensional measure of the correlation time, is determined from numerical models of convection. The Strouhal number arises in the mean-field theories of angular momentum transport and magnetic field generation, where its value determines the validity of certain widely used approximations, such as the first order smoothing (hereafter FOSA). More specifically, the relevant transport coefficients can be calculated by means of a cumulative series expansion if St < St_crit \approx 1. Methods: We define the Strouhal number as the ratio of the correlation and turnover times, which we determine separately, the former from the autocorrelation of velocity, and the latter by following test particles embedded in the flow. Results: We find that the Strouhal numbers are, generally, of the order of 0.1 to 0.4 which is close to the critical value above which deviations from FOSA become significant. Increasing the rotational influence tends to shorten both timescales in such a manner that St decreases. However, we do not find a clear trend as function of the Rayleigh number for the parameter range explored in the present study.

Local models of stellar convection

Local models of stellar convection

Local models of stellar convection:

We study stellar convection using a local three-dimensional MHD model, with which we investigate the influence of rotation and large-scale magnetic fields on the turbulent momentum and heat transport and their role in generating large-scale flows in stellar convection zones. The former is studied by computing the turbulent velocity correlations, known as Reynolds stresses, the latter by calculating the correlation of velocity and temperature fluctuations, both as functions of rotation and latitude. We find that the horizontal correlation, , capable of generating horizontal differential rotation, attains significant values and is mostly negative in the southern hemisphere for Coriolis numbers exceeding unity, corresponding to equatorward flux of angular momentum. This result is also in accordance with solar observations. The radial component is negative for slow and intermediate rotation indicating inward transport of angular momentum, while for rapid rotation, the transport occurs outwards. Parametrisation in terms of the mean-field Λ-effect shows qualitative agreement with the turbulence model of Kichatinov & Rüdiger ([CITE]) for the horizontal part , whereas for the vertical Λ-effect, , agreement only for intermediate rotation exists. The Λ-coefficients become suppressed in the limit of rapid rotation, this rotational quenching being stronger and occurring with slower rotation for the V component than for H. We have also studied the behaviour of the Reynolds stresses under the influence of a large-scale azimuthal magnetic field of varying strength. We find that the stresses are enhanced by the presence of the magnetic field for field strengths up to and above the equipartition value, without significant quenching. Concerning the turbulent heat transport, our calculations show that the transport in the radial direction is most efficient at the equatorial regions, obtains a minimum at midlatitudes, and shows a slight increase towards the poles. The latitudinal heat transport does not show a systematic trend as a function of latitude or rotation.

On application of multi-colour photometry of δ Scuti stars

AbstractIn δ Scuti star models the photometric amplitudes and phases exhibit a strong dependence on convection, which enters through the complex parameter,f, that describes the bolometric flux variation. We present a new method of extracting l and f from multi-colour data and apply it to several δ Scuti stars. The inferred values of f are sufficiently accurate to yield a useful constraint on models of stellar convection.

Constraints on stellar convection from multi-colour photometry ofδScuti stars

In Scuti star models, the calculated amplitude ratios and phase dierences for multi-colour photometry exhibit a strong dependence on convection. These observables are tools for the determination of the spherical harmonic degree,' ,o f the excited modes. The dependence on convection enters through the complex parameter f , which describes bolometric flux perturbation. We present a method of simultaneous determination of f and harmonic degree' from multi-colour data and apply it to three Scuti stars. The method indeed works. Determination of' appears unique and the inferred values of f are suciently accurate to yield a useful constraint on models of stellar convection. Furthermore, the method helps to refine stellar parameters, especially if the identified mode is radial.

Helioseismological test of a new model for stellar convection

We compare the predictions of two solar models with the observed p-mode eigenfrequencies. The two models use the same input microphysics (nuclear reaction rates, opacity, and equation of a state) and the same numerical evolutionary code, but differ in the treatment of turbulent convection. The first model employs the standard mixing-length theory of convection (MLT), while the second model employs a new model of turbulent convection (CM) whose primary goal was that of accounting for the whole spectrum of turbulent eddies so as to avoid the MLT approximation that such a wide spectrum be represented by a single, large eddy. The main result of this paper is that the p-mode eigenfrequencies calculated with the CM model show an overall improvement with respect to those calculated with the standard MLT model.

A Model for Stellar Convection and Spectral Line Asymmetries

A model for stellar convection zones based on linear convective modes using a nonlocal mixing length theory is developed to study the spectral line asymmetries and the line shifts resulting from convective motions in the stellar photospheric region. The amplitudes of these linear convective modes is estimated by requiring the convective flux due to a linear superposition of such modes to reproduce the convective flux in the mixing length model. To study the spectral line asymmetries the convective mode with the largest amplitude in the photospheric line formation region is chosen to represent the stellar velocity field and the accompanying intensity fluctuations. Synthetic spectral line profiles are obtained by summing locally symmetric profiles over the stellar disk according to the local Doppler velocity and intensity fluctuations. The resulting line bisector shapes and the line shifts are compared with observations for α-Cen B. It is found that while the simple model proposed here can explain either the line shifts or the line bisector shape reasonably well, it fails to explain both these characteristics simultaneously.

A model for stellar surface convection and photospheric line asymmetrics

view Abstract Citations (3) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Model for Stellar Surface Convection and Photospheric Line Asymmetries Antia, H. M. ; Pandey, S. K. Abstract A model for stellar convection zones based on linear convective modes using a nonlocal mixing length theory is developed to study the spectral line asymmetries resulting from convective motions in the stellar photospheric region. The amplitudes of these linear convective modes is estimated by demanding that the convective flux due to a linear superposition of such modes should reprodeuce the convective flux required by the mixing length model. The mode with the largest amplitude in the photospheric line formation region is chosen to represent the stellar surface structure. Synthetic spectral line profiles are obtained by summing locally symmetric profiles over the stellar disk according to the local Doppler velocity and intensity fluctuations. Four stars, i.e., the sun, Alpha Cen A, Arcturus, and Procyon, which have characteristically different observed line bisector shapes are chosen for the study. It is found that the simple model considered here can explain the gross features of the observed bisector shapes for these stars of different spectral types. Publication: The Astrophysical Journal Pub Date: June 1989 DOI: 10.1086/167568 Bibcode: 1989ApJ...341.1097A Keywords: Convection; Mixing Length Flow Theory; Photosphere; Stellar Models; Stellar Spectra; Line Spectra; Stellar Structure; Astrophysics; CONVECTION; LINE PROFILES; STARS: ATMOSPHERES; SUN: ATMOSPHERIC MOTIONS full text sources ADS | data products SIMBAD (3)