References of "Théado, S"
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See detailEstimating the p-mode frequencies of the solar twin 18 Scorpii
Bazot, M.; Campante, T.L.; Chaplin, W.J. et al

in Astronomy and Astrophysics (2012), 544

Solar twins have been a focus of attention for more than a decade, because their structure is extremely close to that of the Sun. Today, thanks to high-precision spectrometers, it is possible to use ... [more ▼]

Solar twins have been a focus of attention for more than a decade, because their structure is extremely close to that of the Sun. Today, thanks to high-precision spectrometers, it is possible to use asteroseismology to probe their interiors. Our goal is to use time series obtained from the HARPS spectrometer to extract the oscillation frequencies of 18 Sco, the brightest solar twin. We used the tools of spectral analysis to estimate these quantities. We estimate 52 frequencies using an MCMC algorithm. After examination of their probability densities and comparison with results from direct MAP optimization, we obtain a minimal set of 21 reliable modes. The identification of each pulsation mode is straightforwardly accomplished by comparing to the well-established solar pulsation modes. We also derived some basic seismic indicators using these values. These results offer a good basis to start a detailed seismic analysis of 18 Sco using stellar models. [less ▲]

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See detailLong-term magnetic field monitoring of the Sun-like star \xi Bootis A
Morgenthaler, A.; Petit, P.; Saar, S. et al

in Astronomy and Astrophysics (2012), 540

Aims: We aim to investigate the long-term temporal evolution of the magnetic field of the solar-type star ξ Bootis A, both from direct magnetic field measurements and from the simultaneous estimate of ... [more ▼]

Aims: We aim to investigate the long-term temporal evolution of the magnetic field of the solar-type star ξ Bootis A, both from direct magnetic field measurements and from the simultaneous estimate of indirect activity indicators. Methods: We obtained seven epochs of high-resolution, circularly-polarized spectra from the NARVAL spectropolarimeter between 2007 and 2011, for a total of 76 spectra. Using approximately 6100 photospheric spectral lines covering the visible domain, we employed a cross-correlation procedure to compute a mean polarized line profile from each spectrum. The large-scale photospheric magnetic field of the star was then modelled by means of Zeeman-Doppler Imaging, allowing us to follow the year-to-year evolution of the reconstructed magnetic topology. Simultaneously, we monitored the width of several magnetically sensitive spectral lines, the radial velocity, the line asymmetry of intensity line profiles, and the chromospheric emission in the cores of the Ca II H and Hα lines. Results: During the highest observed activity states, in 2007 and 2011, the large-scale field of ξ Bootis A is almost completely axisymmetric and is dominated by its toroidal component. The toroidal component persists with a constant polarity, containing a significant fraction of the magnetic energy of the large-scale surface field through all observing epochs. The magnetic topologies reconstructed for these activity maxima are very similar, suggesting a form of short cyclicity in the large-scale field distribution. The mean unsigned large-scale magnetic flux derived from the magnetic maps varies by a factor of about 2 between the lowest and highest observed magnetic states. The chromospheric flux is less affected and varies by a factor of 1.2. Correlated temporal evolution, due to both rotational modulation and seasonal variability, is observed between the Ca II emission, the Hα emission and the width of magnetically sensitive lines. The rotational dependence of polarimetric magnetic measurements displays a weak correlation with other activity proxies, presumably due to the different spatial scales and centre-to-limb darkening associated with polarimetric signatures, as compared to non-polarized activity indicators. Better agreement is observed on the longer term. When measurable, the differential rotation reveals a strong latitudinal shear in excess of 0.2 rad d-1. [less ▲]

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See detailLong-term magnetic field monitoring of the sun-like star ξ Bootis A
Morgenthaler, A.; Petit, P.; Aurière, M. et al

in Boissier, S.; Heydari-Malayeri, M.; Samadi, R. (Eds.) et al SF2A-2010: Proceedings of the Annual meeting of the French Society of Astronomy and Astrophysics (2010, December 01)

Phase-resolved observations of the solar-type star ξ Bootis A were obtained using the Narval spectropolarimeter at Telescope Bernard Lyot (Pic du Midi, France) during years 2007, 2008, 2009 and 2010. The ... [more ▼]

Phase-resolved observations of the solar-type star ξ Bootis A were obtained using the Narval spectropolarimeter at Telescope Bernard Lyot (Pic du Midi, France) during years 2007, 2008, 2009 and 2010. The data sets enable us to study both the rotational and the long-term evolution of various activity tracers. Here, we focus on the large-scale photospheric magnetic field (reconstructed by Zeeman-Doppler Imaging), the Zeeman broadening of the FeI 846.84 nm magnetic line, and the chromospheric CaII H and Hα emission. [less ▲]

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See detailExploring the magnetic topologies of cool stars
Morin, J.; Donati, J *-F; Petit, P. et al

in IAU Symposium Proceedings (2010, September 01)

Magnetic fields of cool stars can be directly investigated through the study of the Zeeman effect on photospheric spectral lines using several approaches. With spectroscopic measurement in unpolarised ... [more ▼]

Magnetic fields of cool stars can be directly investigated through the study of the Zeeman effect on photospheric spectral lines using several approaches. With spectroscopic measurement in unpolarised light, the total magnetic flux averaged over the stellar disc can be derived but very little information on the field geometry is available. Spectropolarimetry provides a complementary information on the large-scale component of the magnetic topology. With Zeeman-Doppler Imaging (ZDI), this information can be retrieved to produce a map of the vector magnetic field at the surface of the star, and in particular to assess the relative importance of the poloidal and toroidal components as well as the degree of axisymmetry of the field distribution. The development of high-performance spectropolarimeters associated with multi-lines techniques and ZDI allows us to explore magnetic topologies throughout the Hertzsprung-Russel diagram, on stars spanning a wide range of mass, age and rotation period. These observations bring novel constraints on magnetic field generation by dynamo effect in cool stars. In particular, the study of solar twins brings new insight on the impact of rotation on the solar dynamo, whereas the detection of strong and stable dipolar magnetic fields on fully convective stars questions the precise role of the tachocline in this process. [less ▲]

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See detailThe driving mechanism of roAp stars : effects of local metallicity enhancement
Théado, S.; Dupret, Marc-Antoine ULg; Noels-Grötsch, Arlette ULg

in Communications in Asteroseismology (2009), 158

We have investigated the influence of a local metallicity enhancement on the excitation mechanism of roAp star pulsations. Our computations show that such accumulations poorly affect the position of the ... [more ▼]

We have investigated the influence of a local metallicity enhancement on the excitation mechanism of roAp star pulsations. Our computations show that such accumulations poorly affect the position of the theoretical roAp star instability strip although the opacity in the driving region of roAp modes is affected by metal accumulation. [less ▲]

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See detailNew light on the driving mechanism in roAp stars. I. Effects of metallicity
Théado, S.; Dupret, Marc-Antoine ULg; Noels-Grötsch, Arlette ULg et al

in Astronomy and Astrophysics (2009), 493

Context: Observations suggest that a relationship exists between the driving mechanism of roAp star pulsations and the heavy element distribution in these stars. Aims: We attempt to study the effects of ... [more ▼]

Context: Observations suggest that a relationship exists between the driving mechanism of roAp star pulsations and the heavy element distribution in these stars. Aims: We attempt to study the effects of local and global metallicity variations on the excitation mechanism of high order p-modes in A star models. Methods: We developed stellar evolutionary models to describe magnetic A stars with different global metallicity or local metal accumulation profiles. These models were computed with CLES (``Code Liègeois d'évolution stellaire''), and the stability of our models was assessed with the non-adiabatic oscillation code MAD. Results: Our models reproduce the blue edge of the roAp star instability strip, but generate a red edge hotter than the observed one, regardless of metallicity. Surprisingly, we find that an increase in opacity inside the driving region can produce a lower amount of driving, which we refer to as the ``inverse kappa-mechanism''. [less ▲]

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See detailThe driving mechanism of roAp stars : effects of global metallicity
Theado, S.; Dupret, Marc-Antoine ULg; Noels-Grötsch, Arlette ULg

in Communications in Asteroseismology (2008), 157

We have investigated the influence of global metallicity on the excitation mechanism of roAp star pulsations. Our computations show that the opacity in the driving region of the roAp modes is strongly ... [more ▼]

We have investigated the influence of global metallicity on the excitation mechanism of roAp star pulsations. Our computations show that the opacity in the driving region of the roAp modes is strongly sensitive to the metal content but surprisingly the roAp theoretical instability strip is only weakly affected by metallicity changes. [less ▲]

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See detailThe driving mechanism of roAp stars
Dupret, Marc-Antoine ULg; Théado, S.; Noels-Grötsch, Arlette ULg

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

We analyse in detail the driving mechanism of roAp stars and present the theoretical instability strip predicted by our models with solar metallicity. A particular attention is given to the interpretation ... [more ▼]

We analyse in detail the driving mechanism of roAp stars and present the theoretical instability strip predicted by our models with solar metallicity. A particular attention is given to the interpretation of the role played by the different eigenfunctions in the stabilization of the modes at the red edge of the instability strip. The gradient of temperature in the H[SUB]I[/SUB] opacity bump appears to play a major role in this context. We also consider the particular and complex role played by the shape of the eigenfunctions (location of the nodes, ...). [less ▲]

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See detailThe Liège Oscillation code
Scuflaire, Richard ULg; Montalban Iglesias, Josefa ULg; Théado, S. et al

in Astrophysics & Space Science (2008), 316

The Liège Oscillation code can be used as a stand-alone program or as a library of subroutines that the user calls from a Fortran main program of his own to compute radial and nonradial adiabatic ... [more ▼]

The Liège Oscillation code can be used as a stand-alone program or as a library of subroutines that the user calls from a Fortran main program of his own to compute radial and nonradial adiabatic oscillations of stellar models. We describe the variables and the equations used by the program and the methods used to solve them. A brief account is given of the use and the output of the program. [less ▲]

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See detailEffects of diffusion in β Cephei stars
Bourge, P*-O; Théado, S.; Thoul, Anne ULg

in Communications in Asteroseismology (2007), 150

We investigate the effects of the radiatively-driven diffusion of Fe, C, N and O in β Cephei stellar models.

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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 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 detailEnergetic effects in classical pulsators
Dupret, Marc-Antoine ULg; Grigahcène, A.; Théado, S. et al

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

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See detailRadiative forces and pulsation in beta Cephei stars
Bourge, P*-O; Alecian, G.; Thoul, Anne ULg et al

in Communications in Asteroseismology (2006), 147

Recently radiatively-driven diffusion in beta Cephei stars has been suggested as a possible explanation to account for excitation of the observed oscillation modes of the beta Cephei stars nu Eridani and ... [more ▼]

Recently radiatively-driven diffusion in beta Cephei stars has been suggested as a possible explanation to account for excitation of the observed oscillation modes of the beta Cephei stars nu Eridani and 12 Lacertae. Preliminary results (Bourge and Alecian 2006) show that microscopic diffusion of iron could indeed occur in a region situated just above the opacity bump of the iron group elements. This diffusion has little influence on the position of the star in the HR diagram and on the spectrum of low radial order eigenfrequencies. It does, however, increase the number of excited frequencies. We show here that the ratio between the number of p-modes to the number of g-modes could be used as a signature of this diffusion process. [less ▲]

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See detailThe new solar abundances - Part II: the crisis and possible solutions
Montalban Iglesias, Josefa ULg; Miglio, Andrea ULg; Theado, S. et al

in Communications in Asteroseismology (2006), 147

Recent 3D, NLTE analysis of the solar spectrum (Asplund et al. 2006, Part I; Asplund et al. 2005) have led to a significant reduction of the CNO and Ne abundances leading to a (Z/X) 30% smaller than the ... [more ▼]

Recent 3D, NLTE analysis of the solar spectrum (Asplund et al. 2006, Part I; Asplund et al. 2005) have led to a significant reduction of the CNO and Ne abundances leading to a (Z/X) 30% smaller than the previously recommended value. The corresponding decrease in opacity increases dramatically the discrepancies between the sound-speed derived from helioseismology and our new standard solar models (SSM). We present in this paper some numerical experiments trying to reduce this discrepancy. [less ▲]

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