References of "Samadi, R."
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See detailHelioseismic Spectral Diagnostics - An Update
Leibacher, John W; Baudin, F.; Belkacem, Kevin ULg et al

in AAS/Solar Physics Division Meeting (2009, May 01)

In order to test models of the excitation of p modes and their behavior in the visible atmosphere, we calculate time sequences of Fraunhofer absorption line profiles of the Ni, Fe, K, Na, and Ca lines ... [more ▼]

In order to test models of the excitation of p modes and their behavior in the visible atmosphere, we calculate time sequences of Fraunhofer absorption line profiles of the Ni, Fe, K, Na, and Ca lines used in helioseismology, focusing on Sun-as-a-star observations in this initial work. The atmospheric models, which give rise to the line profiles, are based on the VAL-C, perturbed by realistic p-mode eigenfunctions. The time sequences of line profiles are analyzed as for various instruments, to compare predicted and observed mode amplitudes as a function of temporal frequency [less ▲]

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See detailCoRot observations of active giants: preliminary results
Gondoin, P.; Fridlund, M.; Goupil, M. J. et al

in American Institute of Physics Conference Series (2009, February 01)

We have analysed rotation modulated light-curves of active giants observed with CoRot using spots model. Preliminary results suggest an increase of the surface spot coverage with decreasing rotation ... [more ▼]

We have analysed rotation modulated light-curves of active giants observed with CoRot using spots model. Preliminary results suggest an increase of the surface spot coverage with decreasing rotation period. A maximum of the surface spot coverage seems to occur on giants with effective temperature around 5100 K. Confirmation and interpretation of these preliminary results require groundbased follow-up observations to measure activity indicators, to identify binary systems, and to determine the stellar parameters and evolutionary status of the sample giants. [less ▲]

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See detailStochastic excitation of nonradial modes. II. Are solar asymptotic gravity modes detectable?
Belkacem, Kevin ULg; Samadi, R.; Goupil, M. J. et al

in Astronomy and Astrophysics (2009), 494

Context: Detection of solar gravity modes remains a major challenge to our understanding of the inner parts of the Sun. Their frequencies would enable the derivation of constraints on the core physical ... [more ▼]

Context: Detection of solar gravity modes remains a major challenge to our understanding of the inner parts of the Sun. Their frequencies would enable the derivation of constraints on the core physical properties, while their amplitudes can put severe constraints on the properties of the inner convective region. Aims: Our purpose is to determine accurate theoretical amplitudes of solar g modes and estimate the SOHO observation duration for an unambiguous detection of individual modes. We also explain differences in theoretical amplitudes derived from previous works. Methods: We investigate the stochastic excitation of modes by turbulent convection, as well as their damping. Input from a 3D global simulation of the solar convective zone is used for the kinetic turbulent energy spectrum. Damping is computed using a parametric description of the nonlocal, time-dependent, convection-pulsation interaction. We then provide a theoretical estimation of the intrinsic, as well as apparent, surface velocity. Results: Asymptotic g-mode velocity amplitudes are found to be orders of magnitude higher than previous works. Using a 3D numerical simulation from the ASH code, we attribute this to the temporal-correlation between the modes and the turbulent eddies, which is found to follow a Lorentzian law rather than a Gaussian one, as previously used. We also find that damping rates of asymptotic gravity modes are dominated by radiative losses, with a typical life time of 3 × 10[SUP]5[/SUP] years for the ell=1 mode at nu=60 muHz. The maximum velocity in the considered frequency range (10-100 muHz) is obtained for the ell=1 mode at nu=60 muHz and for the ell=2 at nu=100 muHz. Due to uncertainties in the modeling, amplitudes at maximum i.e. for ell=1 at 60 muHz can range from 3 to 6 mm s[SUP]-1[/SUP]. The upper limit is too high, as g modes would have been easily detected with SOHO, the GOLF instrument, and this sets an upper constraint mainly on the convective velocity in the Sun. [less ▲]

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See detailFirst asteroseismic results from CoRoT
Michel, Eric; Baglin, A.; Weiss, W. W. et al

in Communications in Asteroseismology (2008), 156

About one year after the end of the first observational run and six months after the first CoRoT data delivery, we comment the data exploitation progress for different types of stars. We consider first ... [more ▼]

About one year after the end of the first observational run and six months after the first CoRoT data delivery, we comment the data exploitation progress for different types of stars. We consider first results to illustrate how these data of unprecedented quality shed a new light on the field of stellar seismology. [less ▲]

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See detailModeling stochastic excitation of acoustic modes in stars: present status and perspectives
Samadi, R.; Belkacem, Kevin ULg; Goupil, M*-J et al

in Communications in Asteroseismology (2008), 157

Solar-like oscillations have now been detected for more than ten years and their frequencies measured for a still growing number of stars with various characteristics (e.g. mass, chemical composition ... [more ▼]

Solar-like oscillations have now been detected for more than ten years and their frequencies measured for a still growing number of stars with various characteristics (e.g. mass, chemical composition, evolutionary stage ...). Excitation of such oscillations is attributed to turbu- lent convection and takes place in the uppermost part of the convective envelope. Since the pioneering work of Goldreich & Keely (1977), more sophisticated theoretical models of stochastic excitation were developed, which differ from each other both by the way turbulent convection is modeled and by the assumed sources of excitation. We briefly review here the different underlying approximations and assumptions of those models. A second part shows that computed mode excitation rates crucially depend on the way time-correlations between eddies are described but also on the surface metal abundance of the star. [less ▲]

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See detailRadial velocities with the Gaia RVS spectrometer
Viala, Y. P.; Blomme, R.; Damerdji, Yassine ULg et al

in Charbonnel, C.; Combes, F.; Samadi, R. (Eds.) SF2A-2008; Annual Meeting of the French Society of Astronomy (2008, November 01)

Four different method are used to derive radial velocities from spectra observed by the Gaia Radial Velocity Spectrometer (RVS). They are briefly presented here together with very preliminary results.

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See detailModeling the excitation of acoustic modes in alpha Centauri A
Samadi, R.; Belkacem, Kevin ULg; Goupil, M. J. et al

in Astronomy and Astrophysics (2008), 489

From different seismic observations we infer the energy supplied per unit of time by turbulent convection to the acoustic modes of alpha Centauri A (HD 128620), a star that is similar but not identical to ... [more ▼]

From different seismic observations we infer the energy supplied per unit of time by turbulent convection to the acoustic modes of alpha Centauri A (HD 128620), a star that is similar but not identical to the Sun. The inferred rates of energy supplied to the modes (i.e. mode excitation rates) are found to be significantly higher than in the Sun. They are compared with those computed with an excitation model that includes two sources of driving, the Reynolds stress contribution and the advection of entropy fluctuations. The model also uses a closure model, the Closure Model with Plumes (CMP hereafter), that takes the asymmetry between the up- and down-flows (i.e. the granules and plumes, respectively) into account. Different prescriptions for the eddy-time correlation function are also compared to observational data. Calculations based on a Gaussian eddy-time correlation underestimate excitation rates compared with the values derived from observations for alpha Centauri A. On the other hand, calculations based on a Lorentzian eddy-time correlation lie within the observational error bars. This confirms results in the solar case. Compared to the helioseismic data, those obtained for alpha Centauri A constitute an additional support for our model of excitation. We show that mode masses must be computed taking turbulent pressure into account. Finally, we emphasize the need for more accurate seismic measurements in order to distinguish between the CMP closure model and the quasi-normal approximation in the case of alpha Centauri A, as well as to confirm or not the need to include the excitation by the entropy fluctuations. [less ▲]

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See detailHD 203608, a quiet asteroseismic target in the old galactic disk
Mosser, B.; Deheuvels, S.; Michel, Eric et al

in Astronomy and Astrophysics (2008), 488

Context: We conducted a 5-night observing run with the spectrometer harps at the ESO 3.6-m telescope in August 2006, to continue exploring the asteroseismic properties of F-type stars. In fact, Doppler ... [more ▼]

Context: We conducted a 5-night observing run with the spectrometer harps at the ESO 3.6-m telescope in August 2006, to continue exploring the asteroseismic properties of F-type stars. In fact, Doppler observations of F-type on the main sequence are demanding and remain currently limited to a single case (HD 49933). Comparison with photometric results obtained with the CoRoT space mission (Convection, Rotation and planetary Transits) on similar stars will be possible with an enhanced set of observations. Aims: We selected the 4th magnitude F8V star HD 203608 to investigate the oscillating properties of a low-metallicity star of the old galactic disk. Methods: We reduced the spectra with the on-line data reduction software provided by the instrument. We developed a new statistical approach for extracting the significant peaks in the Fourier domain. Results: The oscillation spectrum shows a significant excess power in the frequency range [1.5, 3.0 mHz]. It exhibits a large spacing of about 120.4 muHz at 2.5 mHz. Variations of the large spacing with frequency are clearly identified, which require an adapted asymptotic development. The modes identification is based on the unambiguous signature of 15 modes with l = 0 and 1. Conclusions: These observations show the potential diagnostic of asteroseismic constraints. Including them in the stellar modeling significantly enhances the precision on the physical parameters of HD 203608, resulting in a much more precise position in the HR diagram. The age of the star is now determined in the range 7.25±0.07 Gyr. Based on observations obtained with the harps échelle spectrometer mounted on the 3.6-m telescope at ESO-La Silla Observatory (Chile), programme 077.D-0720. Data corresponding to Figs. [see full textsee full textsee full text], [see full textsee full textsee full text], and Table [see full textsee full textsee full text] are available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/488/635 [less ▲]

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See detailStochastic excitation of non-radial modes. I. High-angular-degree p modes
Belkacem, Kevin ULg; Samadi, R.; Goupil, Marie-José et al

in Astronomy and Astrophysics (2008), 478

Context: Turbulent motions in stellar convection zones generate acoustic energy, part of which is then supplied to normal modes of the star. Their amplitudes result from a balance between the efficiencies ... [more ▼]

Context: Turbulent motions in stellar convection zones generate acoustic energy, part of which is then supplied to normal modes of the star. Their amplitudes result from a balance between the efficiencies of excitation and damping processes in the convection zones. Aims: We develop a formalism that provides the excitation rates of non-radial global modes excited by turbulent convection. As a first application, we estimated the impact of non-radial effects on excitation rates and amplitudes of the high-angular-degree modes that are observed on the Sun. Methods: A model of stochastic excitation by turbulent convection was developed to compute the excitation rates and then successfully applied to solar radial modes. We generalise this approach to the case of non-radial global modes. This enables us to estimate the energy supplied to high-(l) acoustic modes. Qualitative arguments, as well as numerical calculations, are used to illustrate the results. Results: We find that non-radial effects for p modes are non-negligible: - For high-n modes (i.e. typically n > 3) and for high values of l, the power supplied to the oscillations depends on the mode inertia. - For low-n modes, independent of the value of l, the excitation is dominated by the non-radial components of the Reynolds stress term. Conclusions: Our numerical investigation of high-l p modes shows that the validity of the present formalism is limited to l < 500 due to the spatial separation of scale assumption. Thus, a model for very high-l p-mode excitation rates calls for further theoretical developments; however, the formalism is valid for solar g modes, which will be investigated in a paper in preparation. [less ▲]

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See detailFirst asteroseismic results from CoRoT
Michel, Eric; Baglin, Annie; Weiss, W.W. et al

in Communication in Asteroseismology (2008), 157

About one year after the end of the first observational run and six months after the first CoRoT data delivery, we comment the data exploitation progress for different types of stars. We consider first ... [more ▼]

About one year after the end of the first observational run and six months after the first CoRoT data delivery, we comment the data exploitation progress for different types of stars. We consider first results to illustrate how these data of unprecedented quality shed a new light on the field of stellar seismology. [less ▲]

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See detailTwo-scale mass-flux closure models for turbulence: p-mode amplitudes in solar-like stars
Belkacem, Kevin ULg; Samadi, R.; Goupil, Marie-José et al

in Communications in Asteroseismology (2007), 150

A new closure model has been developed, which takes into account both the skewness of the velocity distribution induced by the presence of two flows in the convection zone, and the effects of turbulence ... [more ▼]

A new closure model has been developed, which takes into account both the skewness of the velocity distribution induced by the presence of two flows in the convection zone, and the effects of turbulence onto each flow (Belkacem et al. 2006a). Applied to the formalism of p-mode excitation, it has been possible to validate this theoretical model by a comparison with the observational excitation rates in the solar case using GOLF data (see Belkacem et al. 2006b). The next step is to consider alpha Cen A for which observations of the mode-damping rates are available. [less ▲]

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See detailSolar-like oscillation amplitudes and line-widths as a probe for turbulent convection in stars
Samadi, R.; Belkacem, Kevin ULg; Goupil, M*-J et al

in IAU Symposium (2007, May 01)

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 ... [more ▼]

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. [less ▲]

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See detailThe Seismology Programme of CoRoT
Michel, Eric; Baglin, A.; Auvergne, M. et al

in Proceedings of "The CoRoT Mission Pre-Launch Status - Stellar Seismology and Planet Finding (2006, November 01)

We introduce the main lines and specificities of the CoRoT Seismology Core Programme. The development and consolidation of this programme has been made in the framework of the CoRoT Seismology Working ... [more ▼]

We introduce the main lines and specificities of the CoRoT Seismology Core Programme. The development and consolidation of this programme has been made in the framework of the CoRoT Seismology Working Group. With a few illustrative examples, we show how CoRoT data will help to address various problems associated with present open questions of stellar structure and evolution. [less ▲]

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See detailRotational Splittings with CoRoT, Expected Number of Detections and Measurement Accuracy
Goupil, M. J.; Lochard, J.; Samadi, R. et al

in ESA Special Publication (2006, November 01)

One of the main goal of the CoRoT experiment is to determine the internal rotation of stars. A seismic measure of rotation requires the detection and an accurate measurement of rotational splittings. Our ... [more ▼]

One of the main goal of the CoRoT experiment is to determine the internal rotation of stars. A seismic measure of rotation requires the detection and an accurate measurement of rotational splittings. Our ability to achieve this goal with CoRoT observations depends on the properties of the target star (in short: spectral type and distance) and will be discussed. [less ▲]

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See detailWhy Bothering to Measure Stellar Rotation with CoRoT?
Goupil, Marie-José; Moya, A.; Suarez, J. C. et al

in ESA Special Publication (2006, November 01)

One important goal of the CoRoT experiment is to obtain information about the internal rotation of stars, in particular the ratio of central to surface rotation rates. This will provide constraints on the ... [more ▼]

One important goal of the CoRoT experiment is to obtain information about the internal rotation of stars, in particular the ratio of central to surface rotation rates. This will provide constraints on the modelling of transport mechanisms of angular momentum acting in radiative (rotationally induced turbulent) and convective zones (plumes, extension beyond convectively instable regions). Relations between the surface rotation period and age, magnetic activity, mass loss and other stellar characteristics can also be studied with a statistically significant set of data as will be provided by Corot. We present various theoretical efforts performed over the past years in order to develope the theoretical tools which will enable us to study rotation with Corot. [less ▲]

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See detailTheoretical damping rates and phase-lags for solar-like oscillations
Dupret, Marc-Antoine ULg; Barban, C.; Goupil, Marie-José et al

in Proceedings of SOHO 18/GONG 2006/HELAS I, Beyond the spherical Sun (2006, October 01)

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See detailA non-local MLT treatment fitting 3D simulations
Dupret, Marc-Antoine ULg; Goupil, M*-J; Samadi, R. et al

in Proceedings of SOHO 18/GONG 2006/HELAS I, Beyond the spherical Sun (2006, October 01)

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See detailNon-local time-dependent treatments of convection in A-G type stars
Dupret, Marc-Antoine ULg; Samadi, R.; Grigahcene, A. et al

in Communications in Asteroseismology (2006), 147

Time-Dependent Convection (TDC) models obtained by combining the local treatment of Gabriel (1996) and Grigahcene et al. (2005) and the non-local prescriptions of Spiegel (1963) are presented. We show ... [more ▼]

Time-Dependent Convection (TDC) models obtained by combining the local treatment of Gabriel (1996) and Grigahcene et al. (2005) and the non-local prescriptions of Spiegel (1963) are presented. We show that in the stationary unperturbed case, these non-local treatments can be constrained by the results of 3D hydrodynamic simulations (Stein & Nordlund 1998). We consider here the case of solar-type stars with a large convective envelope and A-F type stars with two very thin surface convection zones. [less ▲]

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See detailAsteroseismology of delta Scuti Stars: Problems and Prospects
Goupil, Marie-José; Dupret, Marc-Antoine ULg; Samadi, R. et al

in Journal of Astrophysics & Astronomy (2005), 26

We briefly outline the state-of-the-art seismology of delta Scuti stars from a theoretical point of view: why is it so difficult a task? The recent theoretical advances in the field that these ... [more ▼]

We briefly outline the state-of-the-art seismology of delta Scuti stars from a theoretical point of view: why is it so difficult a task? The recent theoretical advances in the field that these difficulties have influenced are also discussed. [less ▲]

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