arXiv subjects

R. Samadi

Publications and source records attributed to R. Samadi.

123 records · Page 7Linked to original sources

arXiv2006

A closure model with plumes I. The solar convection

Oscillations of stellar p modes, excited by turbulent convection, are investigated. We take into account the asymmetry of the up and downflows created by turbulent plumes through an adapted closure model. In a companion paper, we apply it to the formalism of excitation of solar p modes developed by Samadi & Goupil 2001. Using results from 3D numerical simulations of the upper most part of the solar convection zone, we show that the two-scale-mass-flux model (TFM) is valid only for quasi-laminar or highly skewed flows (Gryanik & Hartmann 2002). We build a generalized-Two-scale-Mass-Flux Model (GTFM) model which takes into account both the skew introduced by the presence of two flows and the effects of turbulence in each flow. In order to apply the GTFM to the solar case, we introduce the plume dynamics as modelled by Rieutord & Zahn (1995) and construct a Closure Model with Plumes (CMP). When comparing with 3D simulation results, the CMP improves the agreement for the fourth order moments, by approximatively a factor of two compared with the use of the quasi-normal approximation or a skewness computed with the classical TFM. The asymmetry of turbulent convection in the solar case has an important impact on the vertical-velocity fourth-order moment which has to be accounted for by models. The CMP is a significant improvement and is expected to improve the modelling of solar p-mode excitation.

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arXiv2006

Solar-like oscillation amplitudes and line-widths as a probe for turbulent convection in stars

Excitation of solar-like oscillations is attributed to turbulent convection and takes place at the upper-most part of the outer convective zones. Amplitudes of these oscillations depend on the efficiency of the excitation processes as well as on the properties of turbulent convection. We present past and recent improvements on the modeling of those processes. We show how the mode amplitudes and mode line-widths can bring information about the turbulence in the specific cases of the Sun and Alpha Cen A.

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arXiv2006

Excitation of solar-like oscillations across the HR diagram

We extend semi-analytical computations of excitation rates for solar oscillation modes to those of other solar-like oscillating stars to compare them with recent observations. Numerical 3D simulations of surface convective zones of several solar-type oscillating stars are used to characterize the turbulent spectra as well as to constrain the convective velocities and turbulent entropy fluctuations in the uppermost part of the convective zone of such stars. These constraints, coupled with a theoretical model for stochastic excitation, provide the rate 'P' at which energy is injected into the p-modes by turbulent convection. These energy rates are compared with those derived directly from the 3D simulations. The excitation rates obtained from the 3D simulations are systematically lower than those computed from the semi-analytical excitation model. We find that Pmax, the excitation rate maximum, scales as (L/M)^s where s is the slope of the power law and L and M are the mass and luminosity of the 1D stellar model built consistently with the associated 3D simulation. The slope is found to depend significantly on the adopted form of the eddy time-correlation ; using a Lorentzian form results in s=2.6, whereas a Gaussian one gives s=3.1. Finally, values of Vmax, the maximum in the mode velocity, are estimated from the computed power laws for Pmax and we find that Vmax increases as (L/M)^sv. Comparisons with the currently available ground-based observations show that the computations assuming a Lorentzian eddy time-correlation yield a slope, sv, closer to the observed one than the slope obtained when assuming a Gaussian. We show that the spatial resolution of the 3D simulations must be high enough to obtain accurate computed energy rates.

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arXiv2006

A closure model with plumes II. Application to the stochastic excitation of stellar p modes

Our goal is to improve the theoretical modelling of stochastic excitation of p modes by turbulent convection. With the help of the closure model with plume (CMP) developed in a companion paper, we refine the theoretical description of the excitation by the turbulent Reynolds stress term. The CMP is generalized for two-point correlation products so as to apply it to the formalism developed by Samadi & Goupil (2001). The excitation source terms are then computed with this improvement, and a comparison with solar data from the GOLF instrument is performed. The present model provides a significant improvement when comparing absolute values of theoretical ampplitudes with observational data. It gives rise to a frequency dependence of the power supplied to solar p modes, which agrees with GOLF observations. It is shown that the asymmetry of the turbulent convection zone (up- and downflows) plays a major role in the excitation processes. Despite an increase in the Reynolds stress term contribution due to our improved description, an additional source of excitation, identified as the entropy source term, is still necessary for reproducing the observational data. Theoretical excitation rates in the frequency range [2.5 mHz, 4 mHz] now are in agreement with the observational data from the GOLF instrument. However, at lower frequencies, it exhibits small discrepancies at the maximum level of a few per cent. Improvements are likely to come from a better physical description of the excitation by entropy fluctuations in the superadiabatic zone.

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arXiv2005

Influence of local treatments of convection upon solar p mode excitation rates

We compute the rates P at which acoustic energy is injected into the solar radial p modes for several solar models. The solar models are computed with two different local treatments of convection: the classical mixing-length theory (MLT hereafter) and Canuto et al (1996)'s formulation (CGM hereafter). Among the models investigated here, our best models reproduce both the solar radius and the solar luminosity at solar age and the observed Balmer line profiles. For the MLT treatment, the rates P do depend significantly on the properties of the atmosphere whereas for the CGM's treatment the dependence of P on the properties of the atmosphere is found smaller than the error bars attached to the seismic measurements. The excitation rates P for modes associated with the MLT models are significantly underestimated compared with the solar seismic constraints. The CGM models yield values for P closer to the seismic data than the MLT models. We conclude that the solar p-mode excitation rates provide valuable constraints and according to the present investigation clearly favor the CGM treatment with respect to the MLT, although neither of them yields values of P as close to the observations as recently found for 3D numerical simulations.

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arXiv2004

Excitation rates of p modes: mass luminosity relation across the HR diagram

We compute the rates P at which energy is injected into the p modes for a set of 3D simulations of outer layers of stars. We found that Pmax - the maximum in P - scales as (L/M)^s where s is the slope of the power law, L and M are the luminosity and the mass of the 1D stellar models associated with the simulations. The slope is found to depend significantly on the adopted representation for the turbulent eddy-time correlation function, chi_k. According to the expected performances of COROT, it will likely be possible to measure Pmax as a function of L/M and to constrain the properties of stellar turbulence as the turbulent eddy time-correlation.

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arXiv2004

Effect of local treatments of convection upon the solar p-mode excitation rates

We compute, for several solar models, the rates P at which the solar radial p modes are expected to be excited. The solar models are computed with two different local treatments of convection : the classical mixing-length theory (MLT hereafter) and Canuto, Goldmann and Mazzitelli(1996, CGM hereafter)'s formulation. For one set of solar models (EMLT and ECGM models), the atmosphere is gray and assumes Eddington's approximation. For a second set of models (KMLT and KCGM models), the atmosphere is built using a T(tau) law which has been obtained from a Kurucz's model atmosphere computed with the same local treatment of convection. The mixing-length parameter in the model atmosphere is chosen so as to provide a good agreement between synthetic and observed Balmer line profiles, while the mixing-length parameter in the interior model is calibrated so that the model reproduces the solar radius at solar age. For the MLT treatment, the rates P do depend significantly on the properties of the atmosphere. On the other hand, for the CGM treatment, differences in P between the ECGM and the KCGM models are very small compared to the error bars attached to the seismic measurements. The excitation rates P for modes from the EMLT model are significantly under-estimated compared with the solar seismic constraints. The KMLT model results in intermediate values for P and shows also an important discontinuity in the temperature gradient and the convective velocity. On the other hand, the KCGM model and the ECGM model yield values for P closer to the seismic data than the EMLT and KMLT models. We conclude that the solar p-mode excitation rates provide valuable constraints and according to the present investigation cleary favor the CGM treatment with respect to the MLT.

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arXiv2004

Oscillation power spectra of the Sun and of Alpha Cen A: observations versus models

Hydrodynamical, 3D simulations of the outer layers of the Sun and Alpha Cen A are used to obtain constraints on the properties of turbulent convection in such stars. These constraints enable us to compute - on the base of a theoretical model of stochastic excitation - the rate P at which p modes are excited by turbulent convection in those two stars. Results are then compared with solar seismic observations and recent observations of Alpha Cen A. For the Sun, a good agreement between observations and computed P is obtained. For Alpha Cen A a large discrepancy is obtained which origin cannot be yet identified: it can either be caused by the present data quality which is not sufficient for our purpose or by the way the intrinsic amplitudes and the life-times of the modes are determined or finally attributed to our present modelling. Nevertheless, data with higher quality or/and more adapted data reductions will likely provide constraints on the p-mode excitation mechanism in Alpha Cen A.

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arXiv2003

Inferring information about rotation from stellar oscillations

The first part of this paper aims at illustrating the intense scientific activity in the field of stellar rotation although, for sake of shortness, we cannot be exhaustive nor give any details. The second part is devoted to the rotation as a pertubation effect upon oscillation frequencies. The discussion focuses on one specific example: the p-modes frequency small separation which provides information about properties of the stellar inner layers. It is shown that the small separation can be affected by rotation at the level of 0.1-0.2 microHz for a 1.4 Mo model rotating with an equatorial velocity of 20 km/s at the surface. This is of the same order of magnitude as the expected precision on frequencies with a 3 months observation and must therefore be taken into account. We show however that it is possible to recover the small separation free of these contaminating effects of rotation, provided enough high quality data are available as will be with space seismic missions such as Eddington.

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arXiv2003

Numerical 3D constraints on convective eddy time-correlations : consequences for stochastic excitation of solar p modes

A 3D simulation of the upper part of the solar convective zone is used to obtain information on the frequency component, chi_k, of the correlation product of the turbulent velocity field. This component plays an important role in the stochastic excitation of acoustic oscillations. A time analysis of the solar simulation shows that a gaussian function does not correctly reproduce the nu-dependency of chi_k inferred from the 3D simuation in the frequency range where the acoustic energy injected into the solar p modes is important (nu ~ 2 - 4 mHz). The nu-dependency of chi_k is fitted with different analytical functions which can then conveniently be used to compute the acoustic energy supply rate P injected into the solar radial oscillations. With constraints from a 3D simulation, adjustment of free parameters to solar data is no longer necessary and is not performed here. The result is compared with solar seismic data. Computed values of P obtained with the analytical function which fits best chi_k are found ~ 2.7 times larger than those obtained with the gaussian model and reproduce better the solar seismic observations. This non-gaussian description also leads to a Reynolds stress contribution of the same order as the one arising from the advection of the turbulent fluctuations of entropy by the turbulent motions. Some discrepancy between observed and computed P values still exist at high frequency and possible causes for this discrepancy are discussed.

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arXiv2003

Numerical constraints on the model of stochastic excitation of solar-type oscillations

Analyses of a 3D simulation of the upper layers of a solar convective envelope provide constraints on the physical quantities which enter the theoretical formulation of a stochastic excitation model of solar p modes, for instance the convective velocities and the turbulent kinetic energy spectrum. These constraints are then used to compute the acoustic excitation rate for solar p modes, P. The resulting values are found ~5 times larger than the values resulting from a computation in which convective velocities and entropy fluctuations are obtained with a 1D solar envelope model built with the time-dependent, nonlocal Gough (1977) extension of the mixing length formulation for convection (GMLT). This difference is mainly due to the assumed mean anisotropy properties of the velocity field in the excitation region. The 3D simulation suggests much larger horizontal velocities compared to vertical ones than in the 1D GMLT solar model. The values of P obtained with the 3D simulation constraints however are still too small compared with the values inferred from solar observations. Improvements in the description of the turbulent kinetic energy spectrum and its depth dependence yield further increased theoretical values of P which bring them closer to the observations. It is also found that the source of excitation arising from the advection of the turbulent fluctuations of entropy by the turbulent movements contributes ~ 65-75 % to the excitation and therefore remains dominant over the Reynolds stress contribution. The derived theoretical values of P obtained with the 3D simulation constraints remain smaller by a factor ~3 compared with the solar observations. This shows that the stochastic excitation model still needs to be improved.

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arXiv2002

Consequences of the non gaussian character of the stochastic excitation for solar-type oscillations

Our recent study, based on a 3D numerical simulation, suggested that the stochastic excitation of solar p-modes has a non gaussian nature and a non gaussian model was proposed which results in a good agreement between the computed rate $P$ at which p-modes are excited and the solar seismic observational constraints. In the present work, we study some consequences of this non-gaussian model on the rate P at which p-modes are excited in intermediate mass stars (1 Mo < M < 2 Mo). The non gaussian model changes substantially the spectrum of P at high frequency compared with the commonly assumed gaussian model. The largest effects on P are found for stars in the mass range ~ 1.5-1.6 Mo. They are found large enough that observatins in solar-like oscillating stars observed with space based seismology missions (e.g. COROT) will be able to confirm or reject the gaussian nature of stochastic excitation.

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arXiv2002

Seismic diagnostics on stellar convection treatment from oscillation amplitudes of p-modes

The excitation rate P of solar p-modes is computed with a model of stochatic excitation which involves constraints on the averaged properties of the solar turbulence. These constraints are obtained from a 3D simulation. Resulting values for P are found ~9 times larger than when the calculation assumes properties of turbulent convection which are derived from an 1D solar model based on Gough (1977)'s formulation of the mixing-length theory (GMLT). This difference is mainly due to the assumed values for the mean anisotropy of the velocity field in each case.Calculations based on 3D constraints bring the P maximum closer to the observational one.We also compute P for several models of intermediate mass stars (1Mo < M < 2Mo). Differences in the values of P_max between models computed with the classical mixing-length theory and GMLT models are found large enough for main sequence stars to suggest that measurements of P in this mass range will be able to discriminate between different models of turbulent convection.

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arXiv2002

Characterizing the dynamic properties of the solar turbulence with 3-D simulations: Consequences in terms of p-mode excitation

A 3D simulation of the upper part of the solar convective zone is used to derive constraints about the averaged and dynamic properties of solar turbulent convection. Theses constraints are then used to compute the acoustic energy supply rate P(nu) injected into the solar radial oscillations according to the theoretical expression in Samadi & Goupil (2001). The result is compared with solar seismic data. Assuming, as it is usually done, a gaussian model for the frequency (nu) component chi_k(nu) of the model of turbulence, it is found that the computed P(nu) is underestimated compared with the solar seismic data by a factor ~ 2.5. A frequency analysis of the solar simulation shows that the gaussian model indeed does not correctly model chi_k(nu) in the frequency range where the acoustic energy injected into the solar p-modes is important (nu ~ 2 - 4 mHz). One must consider an additional non-gaussian component for chi_k(nu) to reproduce its behavior. Computed values of P obtained with this non-gaussian component reproduce better the solar seismic observations. This non-gaussian component leads to a Reynolds stress contribution of the same order than the one arising from the advection of the turbulent fluctuations of entropy by the turbulent motions.

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arXiv2002

Solar-like oscillations in Delta Scuti stars

Model computations of Delta Scuti stars, located in the vicinity of the red edge of the classical instability strip, suggest amplitudes of solar-like oscillations larger than in cooler models located outside the instability strip. Solar-like amplitudes in our Delta Scuti models are found to be large enough to be detectable with ground-based instruments provided they can be distinguished from the opacity-driven large-amplitude pulsations. There would be advantages in observing simultaneously opacity-driven and stochastically excited modes in the same star. We anticipate their possible detection in the context of the planned asteroseismic space missions, such as the French mission COROT (COnvection ROtation and planetary Transits). We propose known Delta Scuti stars as potential candidates for the target selection of these upcoming space missions.

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