6 research outputs found
The diameter of the CoRoT target HD 49933. Combining the 3D limb darkening, asteroseismology, and interferometry
Context. The interpretation of stellar pulsations in terms of internal
structure depends on the knowledge of the fundamental stellar parameters.
Long-base interferometers permit us to determine very accurate stellar radii,
which are independent constraints for stellar models that help us to locate the
star in the HR diagram. Aims: Using a direct interferometric determination of
the angular diameter and advanced three-dimensional (3D) modeling, we derive
the radius of the CoRoT target HD 49933 and reduce the global stellar parameter
space compatible with seismic data. Methods: The VEGA/CHARA
spectro-interferometer is used to measure the angular diameter of the star. A
3D radiative hydrodynamical simulation of the surface is performed to compute
the limb darkening and derive a reliable diameter from visibility curves. The
other fundamental stellar parameters (mass, age, and Teff) are found by fitting
the large and small p-mode frequency separations using a stellar evolution
model that includes microscopic diffusion. Results: We obtain a limb-darkened
angular diameter of {\theta}LD = 0.445 \pm 0.012 mas. With the Hipparcos
parallax, we obtain a radius of R = 1.42 \pm 0.04 Rsun. The corresponding
stellar evolution model that fits both large and small frequency separations
has a mass of 1.20 \pm 0.08 Msun and an age of 2.7 Gy. The atmospheric
parameters are Teff = 6640 \pm 100 K, log g = 4.21 \pm 0.14, and [Fe/H] =
-0.38.Comment: 4 pages, 4 figure
Time Series and Spectral Analysis in Asteroseismology
A major breakthrough in stellar astrophysics occurred a decade ago when a number of space photometry telescopes were launched and began operations. In particular, the NASA space telescope Kepler was constructed with the goal of finding Earth-like planets around other stars in our galaxy. The technique involved observing the same field of stars, searching for dips in the stellar light curves caused by transits of exoplanets. For four years, the Kepler mission observed almost 200,000 stars with a wide variety of spectral types and evolutionary states. The light curves are also ideal for asteroseismology, the study of stellar oscillations. Fitting the frequencies of these oscillations to stellar models returns accurate fundamental properties including mass, luminosity, radius, and age of the observed star. The goal of this thesis is to use a range of asteroseismic data analysis techniques to improve the understanding of the physical properties of various classes of oscillating stars. This thesis is split into four main chapters. Firstly, I follow the adiabatic frequency pattern of the most evolved solar-like oscillators and observe a departure to the well known asymptotic relation. Secondly, I compare Kepler data and stellar models of main-sequence solar-like oscillators to characterise the frequency discrepancy, known as the surface correction. Thirdly, I devise a technique to use the centroid of blended radial-quadrupole modes to accurately determine fundamental stellar parameters in F-type stars. Finally, I investigate a method to detect stellar companions by measuring the modulation of light arrival time using stable oscillation modes, and attempt to apply it to stars of different spectral types
Interpretation of asteroseismic data: Proceedings of the Wroclaw HELAS Workshop 'Interpretation of Asteroseismic Data' Wroclaw/Poland, 2008
Validation and Application of The Stellar Abundances and atmospheric Parameter Pipeline to derive fundamental parameters of stars in the era of large-scale stellar surveys
SAPP is a pipeline designed to determine accurate parameters of stars in large surveys like Gaia-ESO and Gaia. It combines various observations, including spectra, photometry, astrometry, and asteroseismic data, using Bayesian inference. Validated with benchmark stars, SAPP breaks degeneracies between parameters, yielding precise results. For effective temperature, the typical error is about 100 K, with spectroscopic models dominating uncertainty. Log(g) uncertainty depends on observables, ranging from 0.03 dex to 0.06 dex. Metallicities are recovered with a precision of 0.03 dex for PLATO targets, improved by seismic priors. SAPP also employs an iterative scheme using nu_max = f(Teff, log g) relation, yielding robust results with small differences in temperature and metallicity. It provides fundamental parameters accurate within 1%, meeting PLATO’s goals and enabling exploration of the Galactic structure, including Radial Migration and Age Metallicity Relation. SAPP is used to investigate the alpha-poor and alpha-rich populations in the Galactic disc using Gaia-ESO spectra, Gaia EDR3 astrometry, and photometry. Non-Local Thermodynamic Equilibrium models determine parameters and abundances. A cold metal-poor alpha-poor disc is found in local distributions, suggesting co-evolution of the thick and thin disc. These distributions show well-defined trends in age and kinematic space (Vφ ). SAPP’s accurate age and abundance estimations contribute to understanding Galactic characteristics such as Radial Gradient Measurements
Caractérisation interférométrique de la relation brillance de surface-couleur des binaires à éclipse et étalonnage des échelles de distance dans l'univers
Measuring distances separating our own Galaxy from nearby ones revolutionized our understanding of the distance scale and provided the evidence for the expansion of the universe. The distances to the Small and Large Magellanic Clouds are critical steps of the cosmic distance ladder, and they have been determined using numerous independent methods (as, RR Lyrae stars, Cepheids and "red clump" stars). The aim of my thesis work is to improve our understanding of the Surface Brightness-Color relation (SBC) using optical interferometry. For this, we use the interferometer VEGA on CHARA. This instrument operates in the visible and benefits from the baselines of the CHARA interferometer. It has a spatial resolution of 0.3 mas, which makes it an ideal tool to determine diameters of stars. At first I determined the diameter of eight OBA-type stars with an average accuracy of 1.5%. Then I combined these diameters with others collected from the literature, to determine a new SBC relation for this type of stars. In a second step, a theoretical study of the impact of the rotation on the SBC relation was made to understand the physical effects affecting the accuracy of this relation and suppress the currently existing dispersion in order to further improve the accuracy of extragalactic distances.La mesure des distances aux galaxies proches de notre Voie Lactée a révolutionné notre compréhension de l'échelle de distance et a fourni la preuve de l'expansion de l'univers. Notamment les distances aux Petit et Grand Nuages de Magellan sont deux échelons essentiels de l'échelle des distances cosmiques. De nombreuses méthodes indépendantes (comme celle des RR Lyrae, des Céphéides ou des étoiles Red clump) ont été utilisées pour déterminer ces distances. Le but de mon travail de thèse est d'améliorer notre compréhension de la relation BSC grâce à l'interférométrie optique. Pour cela, j'ai utilisé l'instrument VEGA installé sur l'interféromètre CHARA. Cet instrument fonctionne dans le visible et bénéficie de la plus longue base du monde. VEGA a une résolution spatiale de 0.3 mas, ce qui en fait un outil idéal pour une détermination précise des diamètres des étoiles. Dans un premier temps j'ai déterminé le diamètre de huit étoiles de type OBA avec une précision moyenne de 1.5%. Ensuite j'ai combiné ces diamètres avec d'autres mesures collectées dans la littérature pour ainsi donner une nouvelle relation BSC pour ce type d'étoiles. Dans un second temps, une étude théorique de l'impact de rotation sur la relation BSC a été faite pour comprendre les effets physiques influant sur la précision de cette relation de manière à compenser la dispersion existant actuellement et ce dans le but d'améliorer encore la précision sur les distances extragalactiques