References of "Grigahcène, A"
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See detailTowards Precise Asteroseismology of Solar-Like Stars
Grigahcène, A.; Dupret, Marc-Antoine ULg; Sousa, S. G. et al

in Astrophysics and Space Science Proceedings series (2013), 31

Adiabatic modeling of solar-like oscillations cannot exceed a certain level of precision for fitting individual frequencies. This is known as the problem of near-surface effects on the mode physics. We ... [more ▼]

Adiabatic modeling of solar-like oscillations cannot exceed a certain level of precision for fitting individual frequencies. This is known as the problem of near-surface effects on the mode physics. We present a theoretical study which addresses the problem of frequency precision in non-adiabatic models using a time-dependent convection treatment. We find that the number of acceptable model solutions is significantly reduced and more precise constraints can be imposed on the models. Results obtained for a specific star (β Hydri) lead to very good agreement with both global and local seismic observables. This indicates that the accuracy of model fitting to seismic data is greatly improved when a more complete description of the interaction between convection and pulsation is taken into account. [less ▲]

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See detailSuccessful Asteroseismology for a Better Characterization of the Exoplanet HAT-P-7b
Oshagh, M.; Grigahcène, A.; Benomar, O. et al

in Astrophysics and Space Science Proceedings (2013), 31

It is well known that asteroseismology is the unique technique permitting the study of the internal structure of pulsating stars using their pulsational frequencies, which is per se very important. It ... [more ▼]

It is well known that asteroseismology is the unique technique permitting the study of the internal structure of pulsating stars using their pulsational frequencies, which is per se very important. It acquires an additional value when the star turns out to be a planet host. In this case, the asteroseismic study output may be a very important input for the study of the planetary system. With this in mind, we use the large time-span of the Kepler public data obtained for the star system HAT-P-7, first to perform an asteroseismic study of the pulsating star using Time-Dependent-Convection (TDC) models. Secondly, we make a revision of the planet properties in the light of the asteroseismic study. [less ▲]

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See detailTowards an effective asteroseismology of solar-like stars: time-dependent convection effects on pulsation frequencies
Grigahcène, A.; Dupret, Marc-Antoine ULg; Sousa, S. G. et al

in Monthly Notices of the Royal Astronomical Society (2012), 422

Since the early days of helioseismology, adiabatic models have shown their limits for a precise fitting of individual oscillation frequencies. This discrepancy, which also exists for solar-type stars, is ... [more ▼]

Since the early days of helioseismology, adiabatic models have shown their limits for a precise fitting of individual oscillation frequencies. This discrepancy, which also exists for solar-type stars, is known to originate near the surface superadiabatic convective region where the interaction between oscillations and convection is likely to have a large effect on the frequencies. We present an asteroseismic study to address the adequacy of time-dependent convection (TDC) non-adiabatic models to better reproduce the observed individual frequencies. We select, for this purpose, three solar-like stars, in addition to the Sun, to which we fit the observed frequencies in a grid of TDC non-adiabatic models. The best model selection is done by applying a maximum likelihood method. The results are compared to pure adiabatic and near-surface corrected adiabatic models. We show that, first, TDC models give very good agreement for the mode frequencies and average lifetimes. In the solar case, the frequency discrepancy is reduced to <1.75 μHz over 95 per cent of the modes considered. Secondly, TDC models give an asteroseismic insight into the usually unconstrained ad hoc stellar parameters, such as the mixing-length parameter α[SUB]MLT[/SUB]. [less ▲]

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See detailTheoretical seismic properties of pre-main sequence γ Doradus pulsators
Bouabid, M*-P; Montalban Iglesias, Josefa ULg; Miglio, Andrea ULg et al

in Astronomy and Astrophysics (2011), 531

Context. The late A and F-type γ Doradus (γ Dor) stars pulsate with high-order gravity modes (g-modes). The existence of different evolutionary phases crossing the γ Dor instability strip raises the ... [more ▼]

Context. The late A and F-type γ Doradus (γ Dor) stars pulsate with high-order gravity modes (g-modes). The existence of different evolutionary phases crossing the γ Dor instability strip raises the question whether pre-main sequence (PMS) γ Dor stars exist. <BR /> Aims: We intend to study the differences between the asteroseismic behaviour of PMS and main sequence (MS) γ Dor pulsators as predicted by the current theory of stellar evolution and stability. <BR /> Methods: We explore the adiabatic and non-adiabatic properties of high-order g-modes in a grid of PMS and MS models covering the mass range 1.2 M[SUB]&sun;[/SUB] < M[SUB]∗[/SUB] < 2.5 M[SUB]&sun;[/SUB]. <BR /> Results: We have derived the theoretical instability strip (IS) for the PMS γ Dor pulsators. This IS covers the same effective temperature range as the MS γ Dor one. Nevertheless, the frequency domain of unstable modes in PMS models with a fully radiative core is greater than in MS models, even if they present the same number of unstable modes. Moreover, the differences between MS and PMS internal structures are reflected in the average values of the period spacing, as well as in the dependence of the period spacing on the radial order of the modes, opening the window to determination of the evolutionary phase of γ Dor stars from their pulsation spectra. [less ▲]

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See detailKepler observations: Light shed on the hybrid γ Doradus - δ Scuti pulsation phenomenon
Grigahcène, A.; Uytterhoeven, K.; Antoci, V. et al

in Astronomische Nachrichten (2010), 331

Through the observational study of stellar pulsations, the internal structure of stars can be probed and theoretical models can be tested. The main sequence γ Doradus (Dor) and δ Scuti (Sct) stars with ... [more ▼]

Through the observational study of stellar pulsations, the internal structure of stars can be probed and theoretical models can be tested. The main sequence γ Doradus (Dor) and δ Scuti (Sct) stars with masses 1.2-2.5 M[SUB]ȯ[/SUB] are particularly interesting for asteroseismic study. The γ Dor stars pulsate in high-order gravity (g) modes, with pulsational periods of order of one day. The δ Sct stars, on the other hand, show low-order g and pressure (p) modes with periods of order of 2 hours. Theory predicts the existence of `hybrid' stars, i.e. stars pulsating in both types of modes, in an overlap region between the instability strips of γ Dor and δ Sct stars in the Hertzsprung-Russell diagram. Hybrid stars are particularly interesting as the two types of modes probe different regions of the stellar interior and hence provide complementary model constraints. Before the advent of Kepler, only a few hybrid stars had been confirmed. The {{Kepler}} satellite is providing a true revolution in the study of and search for hybrid stars. Analysis of the first 50 days of {{Kepler}} data of hundreds of γ Dor and δ Sct candidates reveals extremely rich frequency spectra, with most stars showing frequencies in both the δ Sct and γ Dor frequency range. As these results show that there are practically no pure δ Sct or γ Dor pulsators, a new observational classification scheme is proposed by \cite{Grig10}. We present their results and characterize 234 stars in terms of δ Sct, γ Dor, δ Sct/γ Dor or γ Dor/δ Sct hybrids. [less ▲]

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See detailTheoretical study of γ Doradus pulsations in pre-main sequence stars
Bouabid, M*-P; Montalban Iglesias, Josefa ULg; Miglio, Andrea ULg et al

in Astronomische Nachrichten (2010), 331

The question of the existence of the pre-main sequence (PMS) γ Doradus (γ Dor) pulsators has been raised by observations of young clusters such as NGC 884 hosting γ Dor members. We have explored the ... [more ▼]

The question of the existence of the pre-main sequence (PMS) γ Doradus (γ Dor) pulsators has been raised by observations of young clusters such as NGC 884 hosting γ Dor members. We have explored the properties of γ Dor-type pulsations with a grid of PMS models covering the mass range {1.2 < M_*/M_ȯ < 2.5} and we derive the theoretical instability strip (IS) for the PMS γ Dor pulsators. We explore the possibility of distinguishing between PMS and MS γ Dor by the behaviour of the period spacing of their high order gravity modes (g-modes). [less ▲]

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See detailTheoretical amplitudes and lifetimes of non-radial solar-like oscillations in red giants
Dupret, Marc-Antoine ULg; Belkacem, Kevin ULg; Samadi, Réza et al

in Astronomy and Astrophysics (2009), 506

Context: Solar-like oscillations have been observed in numerous red giants from ground and from space. An important question arises: could we expect to detect non-radial modes probing the internal ... [more ▼]

Context: Solar-like oscillations have been observed in numerous red giants from ground and from space. An important question arises: could we expect to detect non-radial modes probing the internal structure of these stars? <BR />Aims: We investigate under what physical circumstances non-radial modes could be observable in red giants; what would be their amplitudes, lifetimes and heights in the power spectrum (PS)? <BR />Methods: Using a non-radial non-adiabatic pulsation code including a non-local time-dependent treatment of convection, we compute the theoretical lifetimes of radial and non-radial modes in several red giant models. Next, using a stochastic excitation model, we compute the amplitudes of these modes and their heights in the PS. <BR />Results: Distinct cases appear. Case A corresponds to subgiants and stars at the bottom of the ascending giant branch. Our results show that the lifetimes of the modes are mainly proportional to the inertia I, which is modulated by the mode trapping. The predicted amplitudes are lower for non-radial modes. But the height of the peaks in the PS are of the same order for radial and non-radial modes as long as they can be resolved. The resulting frequency spectrum is complex. Case B corresponds to intermediate models in the red giant branch. In these models, the radiative damping becomes high enough to destroy the non-radial modes trapped in the core. Hence, only modes trapped in the envelope have significant heights in the PS and could be observed. The resulting frequency spectrum of detectable modes is regular for â =0 and 2, but a little more complex for â =1 modes because of less efficient trapping. Case C corresponds to models of even higher luminosity. In these models the radiative damping of non-radial modes is even larger than in the previous case and only radial and non-radial modes completely trapped in the envelope could be observed. The frequency pattern is very regular for these stars. The comparison between the predictions for radial and non-radial modes is very different if we consider the heights in the PS instead of the amplitudes. This is important as the heights (not the amplitudes) are used as detection criterion. CIFIST Marie Curie Excellence Team. [less ▲]

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See detailHybrid γ Doradus/δ Scuti Stars: Comparison Between Observations and Theory
Bouabid, M-P; Montalban Iglesias, Josefa ULg; Miglio, Andrea ULg et al

in AIP Conference Proceedings (2009), 1170

γ Doradus (γ Dor) are F-type stars pulsating with high order g-modes. Their instability strip (IS) overlaps the red edge of the δ Scuti δ Sct) one. This observation has led to search for objects in this ... [more ▼]

γ Doradus (γ Dor) are F-type stars pulsating with high order g-modes. Their instability strip (IS) overlaps the red edge of the δ Scuti δ Sct) one. This observation has led to search for objects in this region of the HR diagram showing p and g-modes simultaneously. The existence of such hybrid pulsators has recently been confirmed [10] and the number of candidates is increasing (e.g. [17]). From a theoretical point of view, non-adiabatic computations including a time-dependent treatment of convection (TDC) predict the existence of γ Dor/δ Sct hybrid pulsators ([5], [8]). Our aim is to confront the properties of the observed hybrid candidates with the theoretical predictions from non-adiabatic computations of non-radial pulsations including the convection-pulsation interaction. [less ▲]

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See detailPreliminary seismic study of the gamma Doradus COROT target HD 49434
Bouabid, M.-P.; Uytterhoeven, K.; Miglio, Andrea ULg et al

in Communications in Asteroseismology (2008), 157

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See detailTime-dependent convection study of the driving mechanism in the DBV white dwarfs
Dupret, Marc-Antoine ULg; Quirion, P. O.; Fontaine, G. et al

in Journal of Physics: Conference Series (2008), 118

We apply for the first time time-dependent convection (TDC) models to the study of the driving mechanism of the Pulsating DB (V777 Herculis) white dwarfs. From the blue to the red edge of the instability ... [more ▼]

We apply for the first time time-dependent convection (TDC) models to the study of the driving mechanism of the Pulsating DB (V777 Herculis) white dwarfs. From the blue to the red edge of the instability strip of these stars, TDC appears to play a central role in the driving. Around the blue edge, the convection adapts quasi-instantaneously to the oscillations, so that TDC must be included in the models. For the first time, we show that the red edge of the DB instability strip is successfully obtained with a TDC treatment, especially thanks to the terms due to the turbulent pressure variations, while it is not reproduced with frozen convection models. [less ▲]

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See detailDriving and damping mechanisms in hybrid pressure-gravity modes pulsators
Dupret, Marc-Antoine ULg; Miglio, Andrea ULg; Montalban Iglesias, Josefa ULg et al

in Journal of Physics: Conference Series (2008), 118

We study the energetic aspects of hybrid pressure-gravity modes pulsations. The case of hybrid beta Cephei-SPB pulsators is considered with special attention. In addition to the already known sensitivity ... [more ▼]

We study the energetic aspects of hybrid pressure-gravity modes pulsations. The case of hybrid beta Cephei-SPB pulsators is considered with special attention. In addition to the already known sensitivity of the driving mechanism to the heavy elements mixture (mainly the iron abundance), we show that the characteristics of the propagation and evanescent regions play also a major role, determining the extension of the stable gap in the frequency domain between the unstable low order pressure and high order gravity modes. Finally, we consider the case of hybrid delta Sct-gamma Dor pulsators. [less ▲]

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See detailHydrogen-Deficient Compact Pulsators: The GW Virginis Stars and the Variable DB White Dwarfs
Quirion, P*-O; Dupret, Marc-Antoine ULg; Fontaine, G. et al

in Hydrogen-Deficient Stars (2008, July 01)

We review briefly the basic properties of the GW Vir stars and of the V777 Her stars. We describe the classical kappa-mechanism operating in the GW Vir stars and the effects of the chemical composition ... [more ▼]

We review briefly the basic properties of the GW Vir stars and of the V777 Her stars. We describe the classical kappa-mechanism operating in the GW Vir stars and the effects of the chemical composition and of the interaction between diffusion and mass loss on the boundaries of the instability domain of these objects in the log g - T[SUB]eff[/SUB] diagram. Because of the presence of an extensive superficial convection zone in pulsating DB (V777 Her) white dwarfs, oscillation modes are not excited through a similar classical kappa-mechanism in those stars but, instead, involve pulsation-convection interactions. We describe the effects of a time-dependent convection (TDC) treatment on the driving mechanism of the V777 Her stars. We show how convection deeply affects the excitation of modes via the entropy transport mechanism or S-mechanism. Provisional blue and red edges are calculated for the V777 Her stars and are found at T[SUB]eff[/SUB] ~= 28,500 K and ~= 20,500 K, respectively, for a 0.6 M[SUB]o[/SUB] star under the assumption of ML2 convection. [less ▲]

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See detailA procedure for modelling asymptotic g-mode pulsators: The case of gamma Doradus stars
Moya, A.; Suárez, J. C.; Martín-Ruiz, S. et al

in Astronomische Nachrichten (2008), 329

Mode identification is one of the first and main problems we encounter in trying to develop the complete potential of asteroseismology. In the particular case of {g}-mode pulsators, this is still an ... [more ▼]

Mode identification is one of the first and main problems we encounter in trying to develop the complete potential of asteroseismology. In the particular case of {g}-mode pulsators, this is still an unsolved problem, from both the observational and theoretical points of view. Nevertheless, in recent years, some observational and theoretical efforts have been made to find a solution. In this work we use the latest theoretical and computational tools to understand asymptotic {g}-mode pulsators: 1) the Frequency Ratio Method, and 2) Time Dependent Convection. With these tools, a self-consistent procedure for mode identification and modelling of these {g}-mode pulsators can be constructed. This procedure is illustrated using observational information available for the gamma Doradus star 9 Aurigae. [less ▲]

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See detailThe field high-amplitude SX Phoenicis variable BL Camelopardalis: results from a multisite photometric campaign. I. Pulsation
Rodríguez, E.; Fauvaud, S.; Farrell, J. A. et al

in Astronomy and Astrophysics (2007), 471

Context: BL Cam is an extreme metal-deficient field high-amplitude SX Phe-type variable where a very complex frequency spectrum is detected, with a number of independent nonradial modes excited, unusual ... [more ▼]

Context: BL Cam is an extreme metal-deficient field high-amplitude SX Phe-type variable where a very complex frequency spectrum is detected, with a number of independent nonradial modes excited, unusual among the high-amplitude pulsators in the Lower Classical Instability Strip. Aims: An extensive and detailed study has been carried out to investigate the pulsational content and properties of this object. Methods: The analysis is based on 283 h of CCD observations obtained in the Johnson V filter, during a long multisite photometric campaign carried out along the Northern autumn-winter of 2005-2006. Additionally, multicolour BI photometry was also collected to study the phase shifts and amplitude ratios, between light curves obtained in different filters, for modal discrimination of the main excited modes. Results: The detailed frequency analysis revealed a very rich and dense pulsational content consisting of 25 significant peaks, 22 of them corresponding to independent modes: one is the already known main periodicity f[SUB]0[/SUB] = 25.5765 cd[SUP]-1[/SUP] (Delta V = 153 mmag) and the other 21 are excited modes showing very small amplitudes. Some additional periodicities are probably still remaining in the residuals. This represents the most complex spectrum ever detected in a high-amplitude pulsator of this type. The majority of the secondary modes suspected from earlier works are confirmed here and, additionally, a large number of new peaks are detected. The amplitude of the main periodicity f[SUB]0[/SUB] seems to be stable during decades, but the majority of the secondary modes show strong amplitude changes from one epoch to another. The suspected fundamental radial nature of the main periodicity of BL Cam is confirmed, while the secondary peak f[SUB]1[/SUB] = 25.2523 cd[SUP]-1[/SUP] is identified as a nonradial mixed mode g[SUB]4[/SUB] with l = 1. The radial double-mode nature, claimed by some authors for the main two frequencies of BL Cam, is not confirmed. Nevertheless, the frequency f[SUB]6[/SUB] = 32.6464 cd[SUP]-1[/SUP] could correspond to the first radial overtone. [less ▲]

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See detailAn asteroseismic Main Sequence model for the delta Scuti star 44 Tau
Garrido, R.; Suárez, J. C.; Grigahcène, A. et al

in Communications in Asteroseismology (2007), 150

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See detaillambda Boo stars among the gamma Dor-type pulsators: the cases of HD 218427 and HD 239276
Rodríguez, E.; Suárez, J. C.; Moya, A. et al

in Communications in Asteroseismology (2007), 150

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See detailAsteroseismology and mode driving of the Herbig Ae star HD 104237
Dupret, Marc-Antoine ULg; Théado, S.; Böhm, T. et al

in Communications in Asteroseismology (2007), 150

Eight pulsation frequencies were detected in the Herbig Ae star HD 104237 during two observational campaigns in 1999 -- 2000 (Böhm et al. 2004). Moreover, Böhm et al. (in preparation) detected recently in ... [more ▼]

Eight pulsation frequencies were detected in the Herbig Ae star HD 104237 during two observational campaigns in 1999 -- 2000 (Böhm et al. 2004). Moreover, Böhm et al. (in preparation) detected recently in their data two independent signatures of a signal at 95 hr that corresponds probably to rotational modulation. We present here a seismic study of this Pre-Main Sequence star based on these observations. Different possible interpretations of the pulsation spectrum are considered. The driving of the pulsation modes is not explained by standard models, the observed frequencies being too high for delta Scuti-type pulsations. We consider the effect of He accumulation in its partial ionization zones as a possible explanation for this driving. [less ▲]

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See detailOscillating blue stragglers, gamma Doradus stars and eclipsing binaries in the open cluster NGC 2506
Arentoft, T.; De Ridder, J.; Grundahl, F. et al

in Astronomy and Astrophysics (2007), 465

Context: This is the first step in a project to combine studies of eclipsing binaries and oscillating stars to probe the interior of Blue Stragglers (BS). This may imply a way to discriminate ... [more ▼]

Context: This is the first step in a project to combine studies of eclipsing binaries and oscillating stars to probe the interior of Blue Stragglers (BS). This may imply a way to discriminate observationally between different birth mechanisms of BS stars. Aims: We study the open cluster NGC 2506 which contains oscillating BS stars and detached eclipsing binaries for which accurate parameters can be derived. This will tightly constrain the cluster isochrone and provide an absolute mass, radius and luminosity-scale for the cluster stars along with the cluster age, metallicity and distance. The present work focuses on obtaining the light curves of the binaries and determine their orbital periods, on obtaining power spectra of the oscillating BS stars to select targets for follow-up studies, and on searching for gamma Doradus type variables which are also expected to be present in the cluster. Methods: With a two-colour, dual-site photometric campaign we obtained 3120 CCD-images of NGC 2506 spread over four months. We analysed the BI time-series of the oscillating stars and used simulations to derive statistical uncertainties of the resulting frequencies, amplitudes and phases. A preliminary mode-identification was performed using frequency ratios for the oscillating BS stars, and amplitude ratios and phase differences for a population of newly detected gamma Doradus stars. Results: We quadrupled the number of known variables in NGC 2506 by discovering 3 new oscillating BS stars, 15 gamma Doradus stars and four new eclipsing binaries. The orbital periods of 2 known, detached eclipsing binaries were derived. We discovered a BS star with both p-mode and g-mode variability and we confronted our gamma Doradus observations with state-of-the-art seismic models, but found significant discrepancy between theory and observations. Conclusions: . NGC 2506 is an excellent target for asteroseismic tests of stellar models, as strong external constraints can be imposed on the models of a population of more than 20 oscillating stars of different types. Based on observations obtained at the Flemish Mercator telescope on La Palma, Spain, the Danish 1.5-m telescope at ESO, La Silla, Chile, and on observations collected at the European Southern Observatory, Paranal, Chile (ESO Programme 075.D-0206(B)). Catalog of individual photometry measurements for all variables is only 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/465/965 [less ▲]

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See detailTheoretical aspects of g-mode pulsations in $\gamma$ Doradus stars
Dupret, Marc-Antoine ULg; Miglio, Andrea ULg; Grigahcène, A. et al

in Communications in Asteroseismology (2007), 150

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See detailTime Dependent Convection vs. Frozen Convection Approximations
Grigahcène, A.; Dupret, Marc-Antoine ULg; Garrido, R.

in EAS Publications Series (2007)

The approximation known as Frozen Convection has been widely used in the context of stellar oscillations due to the lack of a Time Dependent Convection theory. This approximation supposes to neglect the ... [more ▼]

The approximation known as Frozen Convection has been widely used in the context of stellar oscillations due to the lack of a Time Dependent Convection theory. This approximation supposes to neglect the interaction between convection and oscillations and there are different ways to do it. The choice can affect theoretical predictions concerning the non-adiabatic treatment of the stellar oscillations. In fact, fictitious excitation or damping of the pulsation modes can be produced depending on the approximation used. [less ▲]

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