References of "Dupret, Marc-Antoine"
     in
Bookmark and Share    
See detailTheoretical Instability Domains of Massive Stars
Godart, Mélanie ULg; Dupret, Marc-Antoine ULg; Noels-Grötsch, Arlette ULg et al

in ASP Conference Proceeding, Vol. 462, 27 (2012, September 01)

Massive stars are characterized by a large radiation over gas pressure ratio. With increasing stellar initial mass, they suffer stronger stellar winds, and the induced mass-loss affects the evolution and ... [more ▼]

Massive stars are characterized by a large radiation over gas pressure ratio. With increasing stellar initial mass, they suffer stronger stellar winds, and the induced mass-loss affects the evolution and internal structure on the main sequence and on the post-main sequence. Recent ground-based observations and space missions have shown the presence of pulsations in massive stars, such as acoustic and gravity modes excited by the κ-mechanism and even solar-like oscillations. Strange modes could also be excited in the most massive stars (Aerts et al. 2010). We computed evolutionary tracks and non-adiabatic frequencies for initial masses ranging from 15 to 70 M[SUB]&sun;[/SUB] on the main sequence and on the post-main sequence taking mass loss into account and we discuss in this paper the results for 25 M[SUB]&sun;[/SUB] models. [less ▲]

Detailed reference viewed: 12 (2 ULg)
See detailPulsations of an Evolved Self-consistently Distorted Star
Ouazzani, Rhita-Maria ULg; Dupret, Marc-Antoine ULg; Roxburgh, I. W. et al

in ASP Conference Proceeding, Vol. 462, 402 (2012, September 01)

A new two-dimensional (2D) non-perturbative method to compute accurate oscillation modes of rapidly rotating stars is presented. The 2D calculations fully take into account the centrifugal distortion of ... [more ▼]

A new two-dimensional (2D) non-perturbative method to compute accurate oscillation modes of rapidly rotating stars is presented. The 2D calculations fully take into account the centrifugal distortion of the star while the non-perturbative method includes the full influence of the Coriolis acceleration, and are used to compute oscillation modes of rapid rotators — high-order p-modes in δ Scuti stars, as well as low-order p- and g-modes in β Cephei stars. We compare the oscillation spectra obtained for centrifugally distorted polytropes with those of Reese et al. (2006), and give the first results for a realistic 2D model of a rapidly rotating 2 M[SUB]&sun;[/SUB] evolved star computed with the method developed by Roxburgh (2006). [less ▲]

Detailed reference viewed: 7 (1 ULg)
Full Text
Peer Reviewed
See detailAmplitudes of solar-like oscillations in red giant stars. Evidence for non-adiabatic effects using CoRoT observations
Samadi, R.; Belkacem, K.; Dupret, Marc-Antoine ULg et al

in Astronomy and Astrophysics (2012), 543

Context. A growing number of solar-like oscillations has been detected in red giant stars thanks to the CoRoT and Kepler space-crafts. In the same way as for main-sequence stars, mode driving is ... [more ▼]

Context. A growing number of solar-like oscillations has been detected in red giant stars thanks to the CoRoT and Kepler space-crafts. In the same way as for main-sequence stars, mode driving is attributed to turbulent convection in the uppermost convective layers of those stars. <BR /> Aims: The seismic data gathered by CoRoT on red giant stars allow us to test the mode driving theory in physical conditions different from main-sequence stars. <BR /> Methods: Using a set of 3D hydrodynamical models representative of the upper layers of sub- and red giant stars, we computed the acoustic mode energy supply rate ({p_max}). Assuming adiabatic pulsations and using global stellar models that assume that the surface stratification comes from the 3D hydrodynamical models, we computed the mode amplitude in terms of surface velocity. This was converted into intensity fluctuations using either a simplified adiabatic scaling relation or a non-adiabatic one. <BR /> Results: From L and M (the luminosity and mass), the energy supply rate {p_max} is found to scale as (L/M)[SUP]2.6[/SUP] for both main-sequence and red giant stars, extending previous results. The theoretical amplitudes in velocity under-estimate the Doppler velocity measurements obtained so far from the ground for red giant stars by about 30%. In terms of intensity, the theoretical scaling law based on the adiabatic intensity-velocity scaling relation results in an under-estimation by a factor of about 2.5 with respect to the CoRoT seismic measurements. On the other hand, using the non-adiabatic intensity-velocity relation significantly reduces the discrepancy with the CoRoT data. The theoretical amplitudes remain 40% below, however, the CoRoT measurements. <BR /> Conclusions: Our results show that scaling relations of mode amplitudes cannot be simply extended from main-sequence to red giant stars in terms of intensity on the basis of adiabatic relations because non-adiabatic effects for red giant stars are important and cannot be neglected. We discuss possible reasons for the remaining differences. [less ▲]

Detailed reference viewed: 9 (1 ULg)
Full Text
See detailNon-radial, non-adiabatic solar-like oscillations in RGB and HB stars
Grosjean, Mathieu ULg; Dupret, Marc-Antoine ULg; Belkacem, Kevin et al

Poster (2012, July)

Corot and Kepler observations of red giants reveal rich spectra of non-radial solar-like oscillations allowing to probe their internal structure. We compare the theoretical spectrum (amplitudes and life ... [more ▼]

Corot and Kepler observations of red giants reveal rich spectra of non-radial solar-like oscillations allowing to probe their internal structure. We compare the theoretical spectrum (amplitudes and life- times) of two red giants in the same region of the HR diagram but in different evolutionary phases. The lifetimes are obtained by computing theoretical non-adiabatic non-radial solar-like oscillations for mixed modes in the two models. Thanks to this, we have been able to compute the oscillation amplitudes through a stochastic excitation model. We present here our first results on the inertia, damping rates and amplitudes of the oscillations in the two stars and discuss the trapping, the visibilities and the am- plitudes of the different modes. The differences in the spectra of the two stars are also investigated. As already known, the period spacings in the two models are very different. Moreover, we find significant differences in amplitudes and lifetimes between the two models. [less ▲]

Detailed reference viewed: 25 (7 ULg)
Full Text
Peer Reviewed
See detailTesting the effects of opacity and the chemical mixture on the excitation of pulsations in B stars of the Magellanic Clouds
Salmon, Sébastien ULg; Montalban Iglesias, Josefa ULg; Morel, Thierry ULg et al

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

The B-type pulsators known as β Cephei and slowly pulsating B (SPB) stars present pulsations driven by the κ mechanism, which operates thanks to an opacity bump due to the iron-group elements. In low ... [more ▼]

The B-type pulsators known as β Cephei and slowly pulsating B (SPB) stars present pulsations driven by the κ mechanism, which operates thanks to an opacity bump due to the iron-group elements. In low-metallicity environments such as the Magellanic Clouds, β Cep and SPB pulsations are not expected. Nevertheless, recent observations show evidence for the presence of B-type pulsator candidates in both galaxies. We seek an explanation for the excitation of β Cep and SPB modes in those galaxies by examining basic input physics in stellar modelling: (i) the specific metal mixture of B-type stars in the Magellanic Clouds and (ii) the role of a potential underestimation of stellar opacities. We first derive the present-day chemical mixtures of B-type stars in the Magellanic Clouds. Then, we compute stellar models for that metal mixture and perform a non-adiabatic analysis of these models. In the second approach, we simulate parametric enhancements of stellar opacities due to different iron-group elements. We then study their effects in models of B stars and their stability. We find that adopting a representative chemical mixture of B stars in the Small Magellanic Cloud cannot explain the presence of B-type pulsators there. An increase of the opacity in the region of the iron-group bump could drive B-type pulsations, but only if this increase occurs at the temperature corresponding to the maximum contribution of Ni to this opacity bump. We recommend an accurate computation of the Ni opacity to understand B-type pulsators in the Small Magellanic Cloud, as well as the frequency domain observed in some Galactic hybrid β Cep–SPB stars. [less ▲]

Detailed reference viewed: 33 (16 ULg)
Full Text
Peer Reviewed
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 ▲]

Detailed reference viewed: 9 (3 ULg)
Full Text
Peer Reviewed
See detailDamping rates of solar-like oscillations across the HR diagram. Theoretical calculations confronted to CoRoT and Kepler observations
Belkacem, K.; Dupret, Marc-Antoine ULg; Baudin, F. et al

in Astronomy and Astrophysics (2012), 540

The space-borne missions CoRoT and Kepler are providing a rich harvest of high-quality constraints on solar-like pulsators. Among the seismic parameters, mode damping rates remains poorly understood and ... [more ▼]

The space-borne missions CoRoT and Kepler are providing a rich harvest of high-quality constraints on solar-like pulsators. Among the seismic parameters, mode damping rates remains poorly understood and are thus barely used to infer the physical properties of stars. Nevertheless, thanks to the CoRoT and Kepler spacecrafts it is now possible to measure damping rates for hundreds of main-sequence and thousands of red-giant stars with unprecedented precision. By using a non-adiabatic pulsation code including a time-dependent convection treatment, we compute damping rates for stellar models that are representative of solar-like pulsators from the main-sequence to the red-giant phase. This allows us to reproduce the observations of both CoRoT and Kepler, which validates our modeling of mode damping rates and thus the underlying physical mechanisms included in the modeling. By considering the perturbations of turbulent pressure and entropy (including the perturbation of the dissipation rate of turbulent energy into heat) by the oscillation in our computation, we succeed in reproducing the observed relation between damping rates and effective temperature. Moreover, we discuss the physical reasons for mode damping rates to scale with effective temperature, as observationally exhibited. Finally, this opens the way for the use of mode damping rates to probe turbulent convection in solar-like stars. [less ▲]

Detailed reference viewed: 7 (1 ULg)
Full Text
Peer Reviewed
See detailThe instability strip of ZZ Ceti white dwarfs I. Introduction of time-dependent convection
Van Grootel, Valérie ULg; Dupret, Marc-Antoine ULg; Fontaine, Gilles et al

in Astronomy and Astrophysics (2012), 539

Aims. The determination of the location of the theoretical ZZ Ceti instability strip in the log g − Teff diagram has remained a challenge over the years due to the lack of a suitable treatment for ... [more ▼]

Aims. The determination of the location of the theoretical ZZ Ceti instability strip in the log g − Teff diagram has remained a challenge over the years due to the lack of a suitable treatment for convection in these stars. For the first time, a full nonadiabatic approach including time-dependent convection is applied to ZZ Ceti pulsators, and we provide the appropriate details related to the inner work- ings of the driving mechanism at work. Methods. We used the nonadiabatic pulsation code MAD with a representative evolutionary sequence of a 0.6 M⊙ DA white dwarf. This sequence is made of state-of-the-art models that include a detailed modeling of the feedback of convection on the atmospheric structure. The assumed convective efficiency in these models is the so-called ML2/α = 1.0 version. We also carried out, for comparison purposes, nonadiabatic computations within the frozen convection approximation, as well as calculations based on models with standard grey atmospheres. Results. We find that pulsational driving in ZZ Ceti stars is concentrated at the base of the superficial H convection zone, but at depths, near the blue edge of the instability strip, somewhat larger than those obtained with the frozen convection approach. Despite the fact that this approach is formally invalid in such stars, particularly near the blue edge of the instability strip, the predicted boundaries are not dramatically different in both cases. The revised blue edge for a 0.6 M⊙ model is found to be around Teff = 11,970 K, some 240 K hotter than the value predicted within the frozen convection approximation, in rather good agreement with the empirical value. On the other hand, our predicted red edge temperature for the same stellar mass is only about 5600 K (80 K hotter than with the frozen convection approach), much lower than the observed value. Conclusions. We correctly understand the development of pulsational instabilities of a white dwarf as it cools at the blue edge of the ZZ Ceti instability strip. Our current implementation of time-dependent convection however still lacks important ingredients to fully account for the observed red edge of the strip. We will explore a number of possibilities in the future papers of this series. [less ▲]

Detailed reference viewed: 19 (4 ULg)
Full Text
See detailPulsations of rapidly rotating evolved stars
Ouazzani, Rhita-Maria ULg; Roxburgh, I. W.; Dupret, Marc-Antoine ULg

in proceedings of the 20th Stellar Pulsation Conference Series, Granada sept 2011 (2012), 1301

A new two dimensional non-perturbative code to compute accurate oscillation modes of rapidly rotating stars is presented. The 2D calculations fully take into account the centrifugal distorsion of the star ... [more ▼]

A new two dimensional non-perturbative code to compute accurate oscillation modes of rapidly rotating stars is presented. The 2D calculations fully take into account the centrifugal distorsion of the star while the non perturbative method includes the full influence of the Coriolis acceleration. This 2D non-perturbative code is used to study pulsational spectra of highly distorted evolved models of stars. 2D models of stars are obtained by a self consistent method which distorts spherically averaged stellar models a posteriori. We are also able to compute gravito-acoustic modes for the first time in rapidly rotating stars. We present the dynamics of pulsation modes in such models, and show regularities in their frequency spectra. [less ▲]

Full Text
Peer Reviewed
See detailFast core rotation in red-giant stars as revealed by gravity-dominated mixed modes
Beck, Paul G; Montalban Iglesias, Josefa ULg; Kallinger, Thomas et al

in Nature (2012), 481

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 ... [more ▼]

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. [less ▲]

Detailed reference viewed: 22 (8 ULg)
See detailEnergetic Aspects of Non-Radial Solar-Like Oscillations in Red Giants
Dupret, Marc-Antoine ULg; Belkacem, Kevin; Montalbán, Josefina et al

in Astrophysics and Space Science Proceedings (2012)

The non-radial oscillations discovered by CoRoT (see e.g. de Ridder et al. (2009)) and by Kepler(see e.g. Bedding et al. (2010)) in thousands of red giants constitute a wonderful mine of information to ... [more ▼]

The non-radial oscillations discovered by CoRoT (see e.g. de Ridder et al. (2009)) and by Kepler(see e.g. Bedding et al. (2010)) in thousands of red giants constitute a wonderful mine of information to determine their global characteristics and probe their internal structure. A. Miglio and J. Montalbán have presented in detail in this conference the seismic structure of red giants, the information hold by their oscillation frequencies, and how it can be used. An adiabatic analysis of the oscillations was sufficient at this level as the frequencies are mainly determined by the deep layers were the oscillatons are quasi-adiabatic. We consider here energetic aspects of non-radial oscillations in red-giants. Non-adiabatic models of solar-like oscillations are required to determine the theoretical amplitude and lifetimes of the modes. These parameters allow us to determine how power spectra are expected to look like, depending on the structure of the red giant. Comparison with the observed measures gives thus additional constraints on the models. [less ▲]

Detailed reference viewed: 12 (3 ULg)
Full Text
Peer Reviewed
See detailSlowing down atomic diffusion in subdwarf B stars: mass loss or turbulence?
Hu, Haili; Tout, C. A.; Glebbeek, E. et al

in Monthly Notices of the Royal Astronomical Society (2011), 418

Subdwarf B (sdB) stars show chemical peculiarities that cannot be explained by diffusion theory alone. Both mass loss and turbulence have been invoked to slow down atomic diffusion in order to match ... [more ▼]

Subdwarf B (sdB) stars show chemical peculiarities that cannot be explained by diffusion theory alone. Both mass loss and turbulence have been invoked to slow down atomic diffusion in order to match observed abundances. The fact that some sdB stars show pulsations give upper limits on the amount of mass loss and turbulent mixing allowed. Consequently, non-adiabatic asteroseismology has the potential to decide which process is responsible for the abundance anomalies. We compute for the first time seismic properties of sdB models with atomic diffusion included consistently during the stellar evolution. The diffusion equations with radiative forces are solved for H, He, C, N, O, Ne, Mg, Fe and Ni. We examine the effects of various mass-loss rates and mixed surface masses on the abundances and mode stability. It is shown that the mass-loss rates needed to simulate the observed He abundances (?) are not consistent with observed pulsations. We find that for pulsations to be driven the rates should be ?. On the other hand, weak turbulent mixing of the outer 10[SUP]-6[/SUP] M[SUB]&sun;[/SUB] can explain the He abundance anomalies while still allowing pulsations to be driven. The origin of the turbulence remains unknown but the presence of pulsations gives tight constraints on the underlying turbulence model. [less ▲]

Detailed reference viewed: 4 (0 ULg)
Full Text
Peer Reviewed
See detailQuantitative estimates of the constraints on solar-like models imposed by observables
Ozel, Nesibe; Dupret, Marc-Antoine ULg; Baglin, A.

in Astronomy and Astrophysics (2011), 532

Context. Seismic parameters such as the large Δ[SUB]0[/SUB] and small δ[SUB]02[/SUB] frequency separations are now being measured in a very large number of stars and begin to be used to test the physics ... [more ▼]

Context. Seismic parameters such as the large Δ[SUB]0[/SUB] and small δ[SUB]02[/SUB] frequency separations are now being measured in a very large number of stars and begin to be used to test the physics of stellar models. <BR /> Aims: We estimate the influence of different observed quantities (oscillation frequencies, interferometry, etc.) and the impact of their accuracy in constraining stellar model parameters. <BR /> Methods: To relate the errors in observed quantities to the precision of the theoretical model parameters, we analyse the behaviour of the χ[SUP]2[/SUP] fitting function around its minimum using the singular value decomposition (SVD) formalism. A new indicator called "weighting" quantifies the relative importance of observational constraints on the determination of each physical parameter individually. These tools are applied to a grid of evolutionary sequences for solar-like stellar models with varying age and mass, and to a real case: HD 49933 - a typical case for which seismic observations are available from space using CoRoT. <BR /> Results: The mass ℳ is always the best determined parameter. The new indicator "weighting" allows us to rank the importance of the different constraints: the mean large separation Δ[SUB]0[/SUB], the radius R/R[SUB]&sun;[/SUB], the mean small separation δ[SUB]02[/SUB], the luminosity L/L[SUB]&sun;[/SUB], the effective temperature T[SUB]eff[/SUB]. If the metallicity and age parameters are known, for example in an open cluster, using either individual or mean frequency separations yields the same uncertainties for masses less than 1.1 M[SUB]&sun;[/SUB]. For HD 49933 the combination of ℳ and Y[SUB]0[/SUB]: ℳ[SUP]2[/SUP]Y[SUB]0[/SUB] is well determined because of their correlation. However, they are poorly constrained individually. The frequency difference δ[SUB]01[/SUB], if known with an error of about 0.3%, can determine the size of the convective core overshooting with about 3% accuracy. Appendices A and B are available in electronic form at <A href="http://www.aanda.org">http://www.aanda.org</A> [less ▲]

Detailed reference viewed: 16 (2 ULg)
Full Text
Peer Reviewed
See detailMixed modes in red-giant stars observed with CoRoT
Mosser, B.; Barban, C.; Montalban Iglesias, Josefa ULg et al

in Astronomy and Astrophysics (2011), 532

Context. The CoRoT mission has provided thousands of red-giant light curves. The analysis of their solar-like oscillations allows us to characterize their stellar properties. <BR /> Aims: Up to now, the ... [more ▼]

Context. The CoRoT mission has provided thousands of red-giant light curves. The analysis of their solar-like oscillations allows us to characterize their stellar properties. <BR /> Aims: Up to now, the global seismic parameters of the pressure modes have been unable to distinguish red-clump giants from members of the red-giant branch. As recently done with Kepler red giants, we intend to analyze and use the so-called mixed modes to determine the evolutionary status of the red giants observed with CoRoT. We also aim at deriving different seismic characteristics depending on evolution. <BR /> Methods: The complete identification of the pressure eigenmodes provided by the red-giant universal oscillation pattern allows us to aim at the mixed modes surrounding the ℓ = 1 expected eigenfrequencies. A dedicated method based on the envelope autocorrelation function is proposed to analyze their period separation. <BR /> Results: We have identified the mixed-mode signature separation thanks to their pattern that is compatible with the asymptotic law of gravity modes. We have shown that, independent of any modeling, the g-mode spacings help to distinguish the evolutionary status of a red-giant star. We then report the different seismic and fundamental properties of the stars, depending on their evolutionary status. In particular, we show that high-mass stars of the secondary clump present very specific seismic properties. We emphasize that stars belonging to the clump were affected by significant mass loss. We also note significant population and/or evolution differences in the different fields observed by CoRoT. The CoRoT space mission, launched 2006 December 27, was developed and is operated by the CNES, with participation of the Science Programs of ESA, ESAŠs RSSD, Austria, Belgium, Brazil, Germany, and Spain.Apeendix A is available in electronic form at <A href="http://www.aanda.org">http://www.aanda.org</A> [less ▲]

Detailed reference viewed: 17 (0 ULg)
Full Text
See detailSeismic modelling of OB stars
Dupret, Marc-Antoine ULg; Godart, Mélanie ULg; Belkacem, Kevin ULg et al

in Proceedings of the International Astronomical Union (2011), 272

A review of the ability of asteroseismology to probe the internal physics of OB stars is presented. The main constraints that can be obtained from the frequency spectrum in p- and g-modes pulsators are ... [more ▼]

A review of the ability of asteroseismology to probe the internal physics of OB stars is presented. The main constraints that can be obtained from the frequency spectrum in p- and g-modes pulsators are discussed. Next, we consider energetic aspects of the pulsations in OB stars and show how such study also allows to constrain their internal physics. The cases of p-mixed modes (β Cep stars), g-modes (SPB stars), strange modes and stochastically excited modes are considered. [less ▲]

Detailed reference viewed: 5 (1 ULg)
Full Text
See detailPulsations in massive stars: effect of the atmosphere on the strange mode pulsations
Godart, Mélanie ULg; Dupret, Marc-Antoine ULg; Noels-Grötsch, Arlette ULg et al

in Proceedings of the International Astronomical Union (2011), 272

Recent space observations with CoRoT and ground-based spectroscopy have shown the presence of different types of pulsations in OB stars. These oscillations could be due to acoustic and gravity modes ... [more ▼]

Recent space observations with CoRoT and ground-based spectroscopy have shown the presence of different types of pulsations in OB stars. These oscillations could be due to acoustic and gravity modes, solar-like oscillations or even other pulsations of large growth rates. We present a first attempt at interpreting the latter as strange modes. [less ▲]

Detailed reference viewed: 11 (4 ULg)
Full Text
Peer Reviewed
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 ▲]

Detailed reference viewed: 11 (6 ULg)
Full Text
Peer Reviewed
See detailThe underlying physical meaning of the νmax - νc relation
Belkacem, K.; Goupil, M. J.; Dupret, Marc-Antoine ULg et al

in Astronomy and Astrophysics (2011), 530

Asteroseismology of stars that exhibit solar-like oscillations are enjoying a growing interest with the wealth of observational results obtained with the CoRoT and Kepler missions. In this framework ... [more ▼]

Asteroseismology of stars that exhibit solar-like oscillations are enjoying a growing interest with the wealth of observational results obtained with the CoRoT and Kepler missions. In this framework, scaling laws between asteroseismic quantities and stellar parameters are becoming essential tools to study a rich variety of stars. However, the physical underlying mechanisms of those scaling laws are still poorly known. Our objective is to provide a theoretical basis for the scaling between the frequency of the maximum in the power spectrum (ν[SUB]max[/SUB]) of solar-like oscillations and the cut-off frequency (ν[SUB]c[/SUB]). Using the SoHO GOLF observations together with theoretical considerations, we first confirm that the maximum of the height in oscillation power spectrum is determined by the so-called plateau of the damping rates. The physical origin of the plateau can be traced to the destabilizing effect of the Lagrangian perturbation of entropy in the upper-most layers, which becomes important when the modal period and the local thermal relaxation time-scale are comparable. Based on this analysis, we then find a linear relation between ν[SUB]max[/SUB] and ν[SUB]c[/SUB], with a coefficient that depends on the ratio of the Mach number of the exciting turbulence to the third power to the mixing-length parameter. [less ▲]

Detailed reference viewed: 10 (2 ULg)
Full Text
Peer Reviewed
See detailKepler Detected Gravity-Mode Period Spacings in a Red Giant Star
Beck, P. G.; Bedding, T. R.; Mosser, B. et al

in Science (2011), 332

Stellar interiors are inaccessible through direct observations. For this reason, helioseismologists made use of the Sun’s acoustic oscillation modes to tune models of its structure. The quest to detect ... [more ▼]

Stellar interiors are inaccessible through direct observations. For this reason, helioseismologists made use of the Sun’s acoustic oscillation modes to tune models of its structure. The quest to detect modes that probe the solar core has been ongoing for decades. We report the detection of mixed modes penetrating all the way to the core of an evolved star from 320 days of observations with the Kepler satellite. The period spacings of these mixed modes are directly dependent on the density gradient between the core region and the convective envelope. [less ▲]

Detailed reference viewed: 17 (1 ULg)
Full Text
Peer Reviewed
See detailGravity modes as a way to distinguish between hydrogen- and helium-burning red giant stars
Bedding, Timothy R; Mosser, Benoit; Huber, Daniel et al

in Nature (2011), 471

Red giants are evolved stars that have exhausted the supply of hydrogen in their cores and instead burn hydrogen in a surrounding shell. Once a red giant is sufficiently evolved, the helium in the core ... [more ▼]

Red giants are evolved stars that have exhausted the supply of hydrogen in their cores and instead burn hydrogen in a surrounding shell. Once a red giant is sufficiently evolved, the helium in the core also undergoes fusion. Outstanding issues in our understanding of red giants include uncertainties in the amount of mass lost at the surface before helium ignition and the amount of internal mixing from rotation and other processes. Progress is hampered by our inability to distinguish between red giants burning helium in the core and those still only burning hydrogen in a shell. Asteroseismology offers a way forward, being a powerful tool for probing the internal structures of stars using their natural oscillation frequencies. Here we report observations of gravity-mode period spacings in red giants that permit a distinction between evolutionary stages to be made. We use high-precision photometry obtained by the Kepler spacecraft over more than a year to measure oscillations in several hundred red giants. We find many stars whose dipole modes show sequences with approximately regular period spacings. These stars fall into two clear groups, allowing us to distinguish unambiguously between hydrogen-shell-burning stars (period spacing mostly ~50seconds) and those that are also burning helium (period spacing ~100 to 300 seconds). [less ▲]

Detailed reference viewed: 18 (4 ULg)