References of "Buldgen, Gaël"
     in
Bookmark and Share    
Full Text
Peer Reviewed
See detailSeismic inversion of the solar entropy. A case for improving the standard solar model
Buldgen, Gaël ULiege; Salmon, Sébastien ULiege; Noels-Grötsch, Arlette ULiege et al

in Astronomy and Astrophysics (2017), 607

Context. The Sun is the most constrained and well-studied of all stars. As a consequence, the physical ingredients entering solar models are used as a reference to study all other stars observed in the ... [more ▼]

Context. The Sun is the most constrained and well-studied of all stars. As a consequence, the physical ingredients entering solar models are used as a reference to study all other stars observed in the Universe. However, our understanding of the solar structure is still imperfect, as illustrated by the current debate on the heavy element abundances in the Sun. <BR /> Aims: We provide additional information on the solar structure by carrying out structural inversions of a new physical quantity, a proxy of the entropy of the solar plasma whose properties are very sensitive to the temperature gradient below the convective zone. <BR /> Methods: We use new structural kernels to carry out direct inversions of an entropy proxy of the solar plasma and compare the solar structure to various standard solar models built using various opacity tables and chemical abundances. We also link our results to classical tests commonly found in the literature. <BR /> Results: Our analysis allows us to probe more efficiently the uncertain regions of the solar models, just below the convective zone, paving the way for new in-depth analyses of the Sun taking into account additional physical uncertainties of solar models beyond the specific question of chemical abundances. [less ▲]

Detailed reference viewed: 10 (1 ULiège)
Full Text
Peer Reviewed
See detailDetermining the metallicity of the solar envelope using seismic inversion techniques
Buldgen, Gaël ULiege; Salmon, Sébastien ULiege; Noels-Grötsch, Arlette ULiege et al

in Monthly Notices of the Royal Astronomical Society (2017), 472(1), 751-764

The solar metallicity issue is a long-lasting problem of astrophysics, impacting multi- ple fields and still subject to debate and uncertainties. While spectroscopy has mostly been used to determine the ... [more ▼]

The solar metallicity issue is a long-lasting problem of astrophysics, impacting multi- ple fields and still subject to debate and uncertainties. While spectroscopy has mostly been used to determine the solar heavy elements abundance, helioseismologists at- tempted providing a seismic determination of the metallicity in the solar convective enveloppe. However, the puzzle remains since two independent groups prodived two radically different values for this crucial astrophysical parameter. We aim at provid- ing an independent seismic measurement of the solar metallicity in the convective enveloppe. Our main goal is to help provide new information to break the current stalemate amongst seismic determinations of the solar heavy element abundance. We start by presenting the kernels, the inversion technique and the target function of the inversion we have developed. We then test our approach in multiple hare-and-hounds exercises to assess its reliability and accuracy. We then apply our technique to solar data using calibrated solar models and determine an interval of seismic measurements for the solar metallicity. We show that our inversion can indeed be used to estimate the solar metallicity thanks to our hare-and-hounds exercises. However, we also show that further dependencies in the physical ingredients of solar models lead to a low accuracy. Nevertheless, using various physical ingredients for our solar models, we determine metallicity values between 0.008 and 0.014. [less ▲]

Detailed reference viewed: 20 (3 ULiège)
Full Text
Peer Reviewed
See detailInversions of the Ledoux discriminant: a closer look at the tachocline
Buldgen, Gaël ULiege; Salmon, Sébastien ULiege; Godart, Mélanie ULiege et al

in Monthly Notices of the Royal Astronomical Society : Letters (2017), 472(1), 70-74

Modelling the base of the solar convective envelope is a tedious problem. Since the first rotation inversions, solar modellers are confronted with the fact that a region of very limited extent has an ... [more ▼]

Modelling the base of the solar convective envelope is a tedious problem. Since the first rotation inversions, solar modellers are confronted with the fact that a region of very limited extent has an enormous physical impact on the Sun. Indeed, it is the transition region from differential to solid body rotation, the tachocline, which furthermore is influenced by turbulence and is also supposed to be the seat of the solar magnetic dynamo. Moreover, solar models show significant disagreement with the sound speed profile in this region. In this paper, we show how helioseismology can provide further constraints on this region by carrying out an inversion of the Ledoux discriminant. We compare these inversions for Standard Solar Models built using various opacity tables and chemical abundances and discuss the origins of the discrepancies between Solar Models and the Sun. [less ▲]

Detailed reference viewed: 23 (7 ULiège)
Full Text
See detailDevelopment of inversion techniques in Asteroseismology
Buldgen, Gaël ULiege

Doctoral thesis (2017)

Detailed reference viewed: 34 (5 ULiège)
Full Text
Peer Reviewed
See detailIn-depth study of 16CygB using inversion techniques
Buldgen, Gaël ULiege; Salmon, Sébastien ULiege; Reese, D. R. et al

in Astronomy and Astrophysics (2016), 596

Context. The 16Cyg binary system hosts the solar-like Kepler targets with the most stringent observational constraints. Indeed, we benefit from very high quality oscillation spectra, as well as ... [more ▼]

Context. The 16Cyg binary system hosts the solar-like Kepler targets with the most stringent observational constraints. Indeed, we benefit from very high quality oscillation spectra, as well as spectroscopic and interferometric observations. Moreover, this system is particularly interesting since both stars are very similar in mass but the A component is orbited by a red dwarf, whereas the B component is orbited by a Jovian planet and thus could have formed a more complex planetary system. In our previous study, we showed that seismic inversions of integrated quantities could be used to constrain microscopic diffusion in the A component. In this study, we analyse the B component in the light of a more regularised inversion. <BR /> Aims: We wish to analyse independently the B component of the 16Cyg binary system using the inversion of an indicator dedicated to analyse core conditions, denoted t[SUB]u[/SUB]. Using this independent determination, we wish to analyse any differences between both stars due to the potential influence of planetary formation on stellar structure and/or their respective evolution. <BR /> Methods: First, we recall the observational constraints for 16CygB and the method we used to generate reference stellar models of this star. We then describe how we improved the inversion and how this approach could be used for future targets with a sufficient number of observed frequencies. The inversion results were then used to analyse the differences between the A and B components. <BR /> Results: The inversion of the t[SUB]u[/SUB] indicator for 16CygB shows a disagreement with models including microscopic diffusion and sharing the chemical composition previously derived for 16CygA. We show that small changes in chemical composition are insufficient to solve the problem but that extra mixing can account for the differences seen between both stars. We use a parametric approach to analyse the impact of extra mixing in the form of turbulent diffusion on the behaviour of the t[SUB]u[/SUB] values. We conclude on the necessity of further investigations using models with a physically motivated implementation of extra mixing processes including additional constraints to further improve the accuracy with which the fundamental parameters of this system are determined. [less ▲]

Detailed reference viewed: 24 (0 ULiège)
Full Text
Peer Reviewed
See detailConstraints on the structure of 16 Cygni A and 16 Cygni B using inversion techniques
Buldgen, Gaël ULiege; Reese, Daniel; Dupret, Marc-Antoine ULiege

in Astronomy and Astrophysics (2016)

Abstract Constraining mixing processes and chemical composition is a central problem in stellar physics as their impact on stellar age determinations leads to biases in our studies of stellar evolution ... [more ▼]

Abstract Constraining mixing processes and chemical composition is a central problem in stellar physics as their impact on stellar age determinations leads to biases in our studies of stellar evolution, galactic history and exoplanetary systems. In two previous papers, we showed how seismic inversion techniques could offer strong constraints on such processes by pointing out weaknesses in theoretical models. We now apply our technique to the solar analogues 16CygA and 16CygB, being amongst the best targets in the Kepler field to test the diagnostic potential of seismic inversions. The combination of various seismic indicators helps to provide more constrained and accurate fundamendal parameters for these stars. We use the latest seismic, spectroscopic and interferometric observational constraints in the litterature for this system to determine reference models independently for both stars. We carry out seismic inversions of the acoustic radius, the mean density and a core conditions indicator. We note that a degeneracy exists for the reference models. Namely, changing the diffusion coefficient or the chemical composition within the observational values leads to 5% changes in mass, 3% changes in radius and up to 8% changes in age. We use acoustic radius and mean density inversions to improve our reference models then carry out inversions for a core conditions indicator. Thanks to its sensitivity to microscopic diffusion and chemical composition mismatches, we are able to reduce the mass dispersion to 2%, namely [0.96, 1.0] M_sun, the radius dispersion to 1%, namely [1.188, 1.200] R_sun and the age dispersion to 3%, namely [7.0, 7.4] Gy, for 16CygA. For 16CygB, we can check the consistency of the models but not reduce independently the age dispersion. Nonetheless, assuming consistency with the age of 16CygA helps to further constrain its mass and radius. [less ▲]

Detailed reference viewed: 20 (3 ULiège)
Full Text
See detailStellar acoustic radii and ages from seismic inversion techniques
Buldgen, Gaël ULiege; Dupret, Marc-Antoine ULiege; Reese, Daniel Roy et al

in The Space Photometry Revolution CoRoT Symposium 3, Kepler KASC-7 joint meeting (2015, September)

Determining stellar characteristics such as the radius, mass or age is crucial and asteroseismology is currently the most promising tool to provide these results accurately. We extend the SOLA inversion ... [more ▼]

Determining stellar characteristics such as the radius, mass or age is crucial and asteroseismology is currently the most promising tool to provide these results accurately. We extend the SOLA inversion technique to new global characteristics in addition to the mean density (see Reese et al. 2012) and apply our methodology to the acoustic radius and an age indicator based on the sound speed derivative. The results from SOLA inversions are compared with estimates based on the small and large frequency separations for several test cases.We show that SOLA inversions yield more accurate results than other techniques which are more sensitive to surface effects. [less ▲]

Detailed reference viewed: 15 (2 ULiège)
Full Text
Peer Reviewed
See detailUsing seismic inversions to obtain an indicator of internal mixing processes in main-sequence solar-like stars
Buldgen, Gaël ULiege; Dupret, Marc-Antoine ULiege; Reese, Daniel Roy

in Astronomy and Astrophysics (2015)

Context. Currently, seismic modelling is one of the best ways of building accurate stellar models, thereby providing accurate ages. However, current methods are affected by simplifying assumptions ... [more ▼]

Context. Currently, seismic modelling is one of the best ways of building accurate stellar models, thereby providing accurate ages. However, current methods are affected by simplifying assumptions concerning stellar mixing processes. In this context, providing new structural indicators that are less model-dependent and more sensitive to mixing processes is crucial. Aims. We wish to build a new indicator for core conditions (i.e. mixing processes and evolutionary stage) on the main sequence. This indicator tu should be more sensitive to structural differences and applicable to older stars than the indicator t presented in a previous paper. We also wish to analyse the importance of the number and type of modes for the inversion, as well as the impact of various constraints and levels of accuracy in the forward modelling process that is used to obtain reference models for the inversion. Methods. First, we present a method of obtaining new structural kernels in the context of asteroseismology. We then use these new kernels to build a new indicator of central conditions in stars, denoted tu, and test it for various effects including atomic diffusion, various initial helium abundances, and various metallicities, following the seismic inversion method presented in our previous paper. We then study the indicator’s accuracy for seven different pulsation spectra including those of 16CygA and 16CygB and analyse how it depends on the reference model by using different constraints and levels of accuracy for its selection Results. We observe that the inversion of the new indicator tu using the SOLA method provides a good diagnostic for additional mixing processes in central regions of stars. Its sensitivity allows us to test for diffusive processes and chemical composition mismatch. We also observe that using modes of degree 3 can improve the accuracy of the results, as well as using modes of low radial [less ▲]

Detailed reference viewed: 23 (5 ULiège)
Full Text
Peer Reviewed
See detailStellar acoustic radii, mean densities, and ages from seismic inversion techniques
Buldgen, Gaël ULiege; Dupret, Marc-Antoine ULiege; Reese, Daniel et al

in Astronomy and Astrophysics (2015), 574

Context. Determining stellar characteristics such as the radius, mass or age is crucial when studying stellar evolution or exoplanetary systems, or when characterising stellar populations in the Galaxy ... [more ▼]

Context. Determining stellar characteristics such as the radius, mass or age is crucial when studying stellar evolution or exoplanetary systems, or when characterising stellar populations in the Galaxy. Asteroseismology is the golden path to accurately obtain these characteristics. In this context, a key question is how to make these methods less model-dependent. Aims: Building on the previous work of Daniel Reese, we wish to extend the Substractive Optimally Localized Averages (SOLA) inversion technique to new stellar global characteristics beyond the mean density. The goal is to provide a general framework in which to estimate these characteristics as accurately as possible in low-mass main-sequence stars. Methods: First, we describe our framework and discuss the reliability of the inversion technique and possible sources of error. We then apply this methodology to the acoustic radius, an age indicator based on the sound speed derivative and the mean density, and compare it to estimates based on the average large and small frequency separations. These inversions are carried out for several test cases including various metallicities, different mixing-lengths, non-adiabatic effects, and turbulent pressure. Results: We observe that the SOLA method yields accurate results in all test cases whereas results based on the large and small frequency separations are less accurate and more sensitive to surface effects and structural differences in the models. If we include the surface corrections of Kjeldsen et al. (2008, ApJ, 683, L175), we obtain results of comparable accuracy for the mean density. Overall, the mean density and acoustic radius inversions are more robust than the inversions for the age indicator. Moreover, the current approach is limited to relatively young stars with radiative cores. Increasing the number of observed frequencies improves the reliability and accuracy of the method. [less ▲]

Detailed reference viewed: 13 (3 ULiège)
Full Text
Peer Reviewed
See detailConstraining mixing processes in 16CygA using Kepler data and seismic inversion techniques
Buldgen, Gaël ULiege; Dupret, Marc-Antoine ULiege; Reese, Daniel Roy

Conference (2015)

Constraining additional mixing processes is a central problem in stellar physics. Indeed, their impact on determined stellar ages is non-negligible and thus strongly affects our studies of stellar ... [more ▼]

Constraining additional mixing processes is a central problem in stellar physics. Indeed, their impact on determined stellar ages is non-negligible and thus strongly affects our studies of stellar evolution, galactic history, and exoplanetary systems. However, the quality of the Kepler data allows us to use new seismic tools to constrain these processes. In this talk, we will show a particularly efficient method for constraining chemical mixing in stellar interiors using custom-made structural integrated quantities. These quantities are designed to probe particular regions of the stellar interior and are estimated via the SOLA inversion method (Pijpers and Thompson 1994). They help us determine the values of parameters describing extra mixing processes. Inversions of such quantities have been originally described for the mean density in Reese et al. (2012) and have been extended to the acoustic radius and a first indicator of core conditions in Buldgen et al. (2015). A more efficient indicator for core conditions has now been derived and successfully tested using test cases similar to the 16Cyg binary system (Buldgen et al. in prep.). In this talk, we will show how our technique applies to the system 16Cyg and constrains additional mixing processes using the above structural indicators. Additional indicators and further studies will lead to seismically constrained chemical profiles for stars observed by Kepler, thereby helping us to disentangle the problem of additional mixing processes and ultimately to provide better stellar ages. [less ▲]

Detailed reference viewed: 16 (1 ULiège)
Full Text
See detailStellar acoustic radii and ages from seismic inversion techniques
Buldgen, Gaël ULiege; Dupret, Marc-Antoine ULiege; Reese, Daniel Roy et al

Poster (2014)

Context: Determining stellar characteristics such as the radius, mass or age is crucial for the study of stellar evolution, exoplanetary systems or the characterisation of stellar populations in the ... [more ▼]

Context: Determining stellar characteristics such as the radius, mass or age is crucial for the study of stellar evolution, exoplanetary systems or the characterisation of stellar populations in the Galaxy. Asteroseismology is currently the most promising tool to accurately determine these characteristics. However, a key question is how to reduce the model dependence of asteroseismic methods. Method: We extend the SOLA inversion technique to new global characteristics in addition to the mean density (see Reese et al. 2012). We apply our methodology to the acoustic radius and an age indicator based on the sound speed derivative. The results from SOLA inversions are compared with estimates based on the small and large frequency separations for several test cases, including differing mixing-lengths, and the presence or absence of non-adiabatic effects or turbulent pressure. Results: We show that SOLA inversions yield accurate results in all test cases, unlike the other techniques which are more sensitive to surface effects. We observe that the acoustic radius and mean density inversions are more robust than the age indicator inversions, which are limited to relatively young stars with radiative cores. [less ▲]

Detailed reference viewed: 14 (0 ULiège)
Full Text
Peer Reviewed
See detailAsteroseismic inversions in the context of Plato
Buldgen, Gaël ULiege; Dupret, Marc-Antoine ULiege; Reese, Daniel Roy

Conference (2014)

The determination of stellar characteristics such as the mass, the age or the radius, is crucial when studying both stellar evolution and exoplanetary systems. In the context of the Plato mission, the ... [more ▼]

The determination of stellar characteristics such as the mass, the age or the radius, is crucial when studying both stellar evolution and exoplanetary systems. In the context of the Plato mission, the high quality data allows us to reach a new level of accuracy and model-independence for stellar characterization by using inversion techniques. In this study, we will use the SOLA method (F. Pijpers and M. J. Thompson 1994), known for its successes in helioseismology, and show how it can offer us new insights into the structure of observed stars. We will present results for the mean density, building on the approach of Reese et al. 2012, and other custom-made structural characteristics constraining the outer layers of stars and the chemical content of their cores. We will show that inversions can determine these characteristics within 0.5%, a significant improvement over the capabilities of the classical forward modelling process. Consequently, such inversions lead to better stellar models and help us obtain more accurate estimates of fundamental parameters such as the age, mass or radius. [less ▲]

Detailed reference viewed: 12 (1 ULiège)