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V Urpin - One of the best experts on this subject based on the ideXlab platform.
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rotation and stability of the toroidal magnetic field in Stellar radiation zones
The Astrophysical Journal, 2013Co-Authors: A Bonanno, V UrpinAbstract:The stability of the magnetic field in radiation zones is of crucial importance for mixing and angular momentum transport in the Stellar Interior. We consider the stability properties of stars containing a predominant toroidal field in spherical geometry by means of a linear stability in the Boussinesq approximation taking into account the effect of thermal conductivity. We calculate the growth rate of instability and analyze in detail the effects of stable stratification and heat transport. We argue that the stabilizing influence of gravity can never entirely suppress the instability caused by electric currents in radiation zones. However, the stable stratification can essentially decrease the growth rate of instability.
C. Kayhan - One of the best experts on this subject based on the ideXlab platform.
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fundamental properties of kepler and corot targets iii tuning scaling relations using the first adiabatic exponent
Monthly Notices of the Royal Astronomical Society, 2016Co-Authors: Mutlu Yildiz, Celik Z Orhan, C. KayhanAbstract:So called scaling relations have the potential to reveal the mass and radius of solar-like oscillating stars, based on oscillation frequencies. In derivation of these relations, it is assumed that the first adiabatic exponent at the surface (Gamma_1s) of such stars is constant. However, by constructing Interior models for the mass range 0.8-1.6 Msun, we show that Gamma_1s is not constant at Stellar surfaces for the effective temperature range with which we deal. Furthermore, the well-known relation between large separation and mean density also depends on Gamma_1s. Such knowledge is the basis for our aim of modifying scaling relations. There are significant differences between masses and radii found from modified and conventional scaling relations. However, comparison of predictions of these relations with the non-asteroseismic observations of Procyon A reveals that new scaling relations are effective in determining the mass and radius of stars. In the present study, solar-like oscillation frequencies of 89 target stars (mostly Kepler and CoRoT) were analysed. As well as two new reference frequencies (nu_min1 and nu_min2) found in the spacing of solar-like oscillation frequencies of Stellar Interior models, we also take into account nu_min0. In addition to the frequency of maximum amplitude, these frequencies have very strong diagnostic potential for determination of fundamental properties. The present study involves the application of derived relations from the models to the solar-like oscillating stars, and computes their effective temperatures using purely asteroseismic methods. There are in general very close agreements between effective temperatures from asteroseismic and non-asteroseismic (spectral and photometric) methods. For the Sun and Procyon A, for example, the agreement is almost total.
Q Zhang - One of the best experts on this subject based on the ideXlab platform.
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turbulent convection model in the overshooting region ii theoretical analysis
The Astrophysical Journal, 2012Co-Authors: Q ZhangAbstract:Turbulent convection models (TCMs) are thought to be good tools to deal with the convective overshooting in the Stellar Interior. However, they are too complex to be applied to calculations of Stellar structure and evolution. In order to understand the physical processes of the convective overshooting and to simplify the application of TCMs, a semi-analytic solution is necessary. We obtain the approximate solution and asymptotic solution of the TCM in the overshooting region, and find some important properties of the convective overshooting. (1) The overshooting region can be partitioned into three parts: a thin region just outside the convective boundary with high efficiency of turbulent heat transfer, a power-law dissipation region of turbulent kinetic energy in the middle, and a thermal dissipation area with rapidly decreasing turbulent kinetic energy. The decaying indices of the turbulent correlations k, (u'T-r') over bar, and (T'T') over bar are only determined by the parameters of the TCM, and there is an equilibrium value of the anisotropic degree.. (2) The overshooting length of the turbulent heat flux (u'T-r') over bar is about 1H(k) (H-k = vertical bar dr/d ln k vertical bar). (3) The value of the turbulent kinetic energy at the convective boundary k(C) can be estimated by a method called the maximum of diffusion. Turbulent correlations in the overshooting region can be estimated by using kC and exponentially decreasing functions with the decaying indices.
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turbulent convection model in the overshooting region ii theoretical analysis
arXiv: Solar and Stellar Astrophysics, 2012Co-Authors: Q ZhangAbstract:Turbulent convection models are thought to be good tools to deal with the convective overshooting in the Stellar Interior. However, they are too complex to be applied in calculations of Stellar structure and evolution. In order to understand the physical processes of the convective overshooting and to simplify the application of turbulent convection models, a semi-analytic solution is necessary. We obtain the approximate solution and asymptotic solution of the turbulent convection model in the overshooting region, and find some important properties of the convective overshooting: I. The overshooting region can be partitioned into three parts: a thin region just outside the convective boundary with high efficiency of turbulent heat transfer, a power law dissipation region of turbulent kinetic energy in the middle, and a thermal dissipation area with rapidly decreasing turbulent kinetic energy. The decaying indices of the turbulent correlations $k$, $\bar{u_{r}'T'}$, and $\bar{T'T'}$ are only determined by the parameters of the TCM, and there is an equilibrium value of the anisotropic degree $\omega$. II. The overshooting length of the turbulent heat flux $\bar{u_{r}'T'}$ is about $1H_k$($H_k=|\frac{dr}{dlnk}|$). III. The value of the turbulent kinetic energy at the convective boundary $k_C$ can be estimated by a method called \textsl{the maximum of diffusion}. Turbulent correlations in the overshooting region can be estimated by using $k_C$ and exponentially decreasing functions with the decaying indices.
C Aerts - One of the best experts on this subject based on the ideXlab platform.
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photometric detection of internal gravity waves in upper main sequence stars i methodology and application to corot targets
Astronomy and Astrophysics, 2019Co-Authors: D M Bowman, C Aerts, M G Pedersen, C Johnston, T M Rogers, P V F Edelmann, S Simondiaz, T Van ReethAbstract:Context. Main sequence stars with a convective core are predicted to stochastically excite Internal Gravity Waves (IGWs), which effectively transport angular momentum throughout the Stellar Interior and explain the observed near-uniform Interior rotation rates of intermediate-mass stars. However, there are few detections of IGWs, and fewer still made using photometry, with more detections needed to constrain numerical simulations. Aims. We aim to formalise the detection and characterisation of IGWs in photometric observations of stars born with convective cores (M > 1.5 M$_{\odot}$) and parameterise the low-frequency power excess caused by IGWs. Methods. Using the most recent CoRoT light curves for a sample of O, B, A and F stars, we parameterise the morphology of the flux contribution of IGWs in Fourier space using an MCMC numerical scheme within a Bayesian framework. We compare this to predictions from IGW numerical simulations and investigate how the observed morphology changes as a function of Stellar parameters. Results. We demonstrate that a common morphology for the low-frequency power excess is observed in early-type stars observed by CoRoT. Our study shows that a background frequency-dependent source of astrophysical signal is common, which we interpret as IGWs. We provide constraints on the amplitudes of IGWs and the shape of their detected frequency spectrum across a range of mass, which is the first ensemble study of stochastic variability in such a diverse sample of stars. Conclusions. The evidence of a low-frequency power excess across a wide mass range supports the interpretation of IGWs in photometry of O, B, A and F stars. We also discuss the prospects of observing hundreds of massive stars with the Transiting Exoplanet Survey Satellite (TESS) in the near future.
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forward asteroseismic modeling of stars with a convective core from gravity mode oscillations parameter estimation and Stellar model selection
arXiv: Solar and Stellar Astrophysics, 2018Co-Authors: C Aerts, Geert Molenberghs, M Michielsen, M G Pedersen, R Bjorklund, C JohnstonAbstract:We propose a methodological framework to perform forward asteroseismic modeling of stars with a convective core, based on gravity-mode oscillations. These probe the near-core region in the deep Stellar Interior. The modeling relies on a set of observed high-precision oscillation frequencies of low-degree coherent gravity modes with long lifetimes and their observational uncertainties. Identification of the mode degree and azimuthal order is assumed to be achieved from rotational splitting and/or from period spacing patterns. This paper has two major outcomes. The first is a comprehensive list and discussion of the major uncertainties of theoretically predicted gravity-mode oscillation frequencies based on linear pulsation theory, caused by fixing choices of the input physics for evolutionary models. Guided by a hierarchy among these uncertainties of theoretical frequencies, we subsequently provide a global methodological scheme to achieve forward asteroseismic modeling. We properly take into account correlations amongst the free parameters included in Stellar models. Aside from the Stellar mass, metalicity and age, the major parameters to be estimated are the near-core rotation rate, the amount of convective core overshooting, and the level of chemical mixing in the radiative zones. This modeling scheme allows for maximum likelihood estimation of the Stellar parameters for fixed input physics of the equilibrium models, followed by Stellar model selection considering various choices of the input physics. Our approach uses the Mahalanobis distance instead of the often used $\chi^2$ statistic and includes heteroscedasticity. It provides estimation of the unknown variance of the theoretically predicted oscillation frequencies.
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gravity mode period spacings as a seismic diagnostic for a sample of γ doradus stars from kepler space photometry and high resolution ground based spectroscopy
Astrophysical Journal Supplement Series, 2015Co-Authors: T Van Reeth, C Aerts, A Tkachenko, P I Papics, S A Triana, K ZwintzAbstract:Gamma Doradus stars (hereafter ? Dor stars) are gravity-mode pulsators of spectral type A or F. Such modes probe the deep Stellar Interior, offering a detailed fingerprint of their structure. Four-year high-precision space-based Kepler photometry of ? Dor stars has become available, allowing us to study these stars with unprecedented detail. We selected, analyzed, and characterized a sample of 67 ? Dor stars for which we have Kepler observations available. For all the targets in the sample we assembled high-resolution spectroscopy to confirm their F-type nature. We found fourteen binaries, among which are four single-lined binaries, five double-lined binaries, two triple systems, and three binaries with no detected radial velocity variations. We estimated the orbital parameters whenever possible. For the single stars and the single-lined binaries, fundamental parameter values were determined from spectroscopy. We searched for period spacing patterns in the photometric data and identified this diagnostic for 50 of the stars in the sample, 46 of which are single stars or single-lined binaries. We found a strong correlation between the spectroscopic and the period spacing values, confirming the influence of rotation on ? Dor-type pulsations as predicted by theory. We also found relations between the dominant g-mode frequency, the longest pulsation period detected in series of prograde modes, , and .
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fast core rotation in red giant stars as revealed by gravity dominated mixed modes
Nature, 2012Co-Authors: P G Beck, J Montalban, T Kallinger, Joris De Ridder, C Aerts, R A Garcia, S Hekker, Marcantoine Dupret, Benoit Mosser, P EggenbergerAbstract:When the core hydrogen is exhausted during Stellar evolution, the central region of a star contracts and the outer envelope expands and cools, giving rise to a red giant. Convection takes place over much of the star's radius. Conservation of angular momentum requires that the cores of these stars rotate faster than their envelopes; indirect evidence supports this. Information about the angular-momentum distribution is inaccessible to direct observations, but it can be extracted from the effect of rotation on oscillation modes that probe the Stellar Interior. Here we report an increasing rotation rate from the surface of the star to the Stellar core in the Interiors of red giants, obtained using the rotational frequency splitting of recently detected 'mixed modes'. By comparison with theoretical Stellar models, we conclude that the core must rotate at least ten times faster than the surface. This observational result confirms the theoretical prediction of a steep gradient in the rotation profile towards the deep Stellar Interior.
B W Hindman - One of the best experts on this subject based on the ideXlab platform.
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the spectral amplitude of Stellar convection and its scaling in the high rayleigh number regime
The Astrophysical Journal, 2016Co-Authors: Nicholas A Featherstone, B W HindmanAbstract:Convection plays a central role in the dynamics of any Stellar Interior, and yet its operation remains largely hidden from direct observation. As a result, much of our understanding concerning Stellar convection necessarily derives from theoretical and computational models. The Sun is, however, exceptional in that regard. The wealth of observational data afforded by its proximity provides a unique test bed for comparing convection models against observations. When such comparisons are carried out, surprising inconsistencies between those models and observations become apparent. Both photospheric and helioseismic measurements suggest that convection simulations may overestimate convective flow speeds on large spatial scales. Moreover, many solar convection simulations have difficulty reproducing the observed solar differential rotation owing to this apparent overestimation. We present a series of three-dimensional Stellar convection simulations designed to examine how the amplitude and spectral distribution of convective flows are established within a star's Interior. While these simulations are nonmagnetic and nonrotating in nature, they demonstrate two robust phenomena. When run with sufficiently high Rayleigh number, the integrated kinetic energy of the convection becomes effectively independent of thermal diffusion, but the spectral distribution of that kinetic energy remains sensitive to both of these quantities. A simulation that has converged to a diffusion-independent value of kinetic energy will divide that energy between spatial scales such that low-wavenumber power is overestimated and high-wavenumber power is underestimated relative to a comparable system possessing higher Rayleigh number. We discuss the implications of these results in light of the current inconsistencies between models and observations.
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the spectral amplitude of Stellar convection and its scaling in the high rayleigh number regime
arXiv: Solar and Stellar Astrophysics, 2015Co-Authors: Nicholas A Featherstone, B W HindmanAbstract:Convection plays a central role in the dynamics of any Stellar Interior, and yet its operation remains largely-hidden from direct observation. As a result, much of our understanding concerning Stellar convection necessarily derives from theoretical and computational models. The Sun is, however, exceptional in that regard. The wealth of observational data afforded by its proximity provides a unique testbed for comparing convection models against observations. When such comparisons are carried out, surprising inconsistencies between those models and observations become apparent. Both photospheric and helioseismic measurements suggest that convection simulations may overestimate convective flow speeds on large spatial scales. Moreover, many solar convection simulations have difficulty reproducing the observed solar differential rotation due to this apparent overestimation. We present a series of 3-dimensional (3-D) Stellar convection simulations designed to examine how the amplitude and spectral distribution of convective flows are established within a star's Interior. While these simulations are non-magnetic and non-rotating in nature, they demonstrate two robust phenomena. When run with sufficiently high Rayleigh number, the integrated kinetic energy of the convection becomes effectively independent of thermal diffusion, but the spectral distribution of that kinetic energy remains sensitive to both of these quantities. A simulation that has converged to a diffusion-independent value of kinetic energy will divide that energy between spatial scales such that low-wavenumber power is overestimated, and high-wavenumber power is underestimated relative to a comparable system possessing higher Rayleigh number. We discuss the implications of these results in light of the current inconsistencies between models and observations.