Search

The hunt for compact objects is on. Rarely seen massive binaries with a compact object are a crucial phase in the evolution towards compact object mergers. In Gaia data release 3 (DR3), the first Gaia astrometric orbital solutions for binary sources will become available. We investigate how many black holes (BH) with massive main-sequence dwarf companions (OB+BH binaries) are expected to be detected as binaries in DR3 and at the end of the nominal 5-yr mission (DR4). We estimate the fraction of identifiable OB+BH binaries and discuss the distributions of the masses of both components and the orbital periods. We study the impact of different BH-formation scenarios. Using tailored models for the massive star population, which assume a direct collapse and no kick upon BH formation (the fiducial case), we estimate the fraction of OB+BH systems that Gaia will detect as binaries. A distance distribution according to that of the second Alma Luminous Star catalogue (ALSII) is assumed. We investigate how many of the systems detected as binaries are identifiable as OB+BH binaries, using a method based on astrometric data. In the fiducial case we conservatively estimate that 77% of the OB+BH binaries in ALSII will be detected as binaries in DR3, of which 89% are identifiable as OB+BH binaries. This leads to a total of around 190 OB+BH binaries, a 20-fold increase in the known sample of OB+BH binaries, covering an uncharted parameter space of long-period binaries. The size and properties of the identifiable OB+BH population will contain crucial observational constraints to improve our understanding of BH formation. In DR4, the detected fraction will increase to 85%, of which 82% will be identifiable. Hence, an additional ~5 systems could be identified, which are expected to have either very short or long periods. The fractions become smaller for different BH-formation scenarios. (truncated), Comment: 22 pages, 18 figures, 2 tables, accepted for publication in A&A

Full list of authors: Blomme, R.; Daflon, S.; Gebran, M.; Herrero, A.; Lobel, A.; Mahy, L.; Martins, F.; Morel, T.; Berlanas, S. R.; Blazère, A.; Frémat, Y.; Gosset, E.; Maíz Apellániz, J.; Santos, W.; Semaan, T.; Simón-Díaz, S.; Volpi, D.; Holgado, G.; Jiménez-Esteban, F.; Nieva, M. F.; Przybilla, N.; Gilmore, G.; Randich, S.; Negueruela, I.; Prusti, T.; Vallenari, A.; Alfaro, E. J.; Bensby, T.; Bragaglia, A.; Flaccomio, E.; Francois, P.; Korn, A. J.; Lanzafame, A.; Pancino, E.; Smiljanic, R.; Bergemann, M.; Carraro, G.; Franciosini, E.; Gonneau, A.; Heiter, U.; Hourihane, A.; Jofré, P.; Magrini, L.; Morbidelli, L.; Sacco, G. G.; Worley, C. C.; Zaggia, S., Context. The Gaia-ESO Survey (GES) is a large public spectroscopic survey that has collected, over a period of six years, spectra of ~105 stars. This survey provides not only the reduced spectra, but also the stellar parameters and abundances resulting from the analysis of the spectra. Aims. The GES dataflow is organised in 19 working groups. Working group 13 (WG13) is responsible for the spectral analysis of the hottest stars (O, B, and A type, with a formal cutoff of Teff > 7000 K) that were observed as part of GES. We present the procedures and techniques that have been applied to the reduced spectra in order to determine the stellar parameters and abundances of these stars. Methods. The procedure used was similar to that of other working groups in GES. A number of groups (called Nodes) each independently analyse the spectra via state-of-the-art techniques and codes. Specific for the analysis in WG13 was the large temperature range covered (Teff ≈ 7000–50 000 K), requiring the use of different analysis codes. Most Nodes could therefore only handle part of the data. Quality checks were applied to the results of these Nodes by comparing them to benchmark stars, and by comparing them to one another. For each star the Node values were then homogenised into a single result: the recommended parameters and abundances. Results. Eight Nodes each analysed part of the data. In total 17 693 spectra of 6462 stars were analysed, most of them in 37 open star clusters. The homogenisation led to stellar parameters for 5584 stars. Abundances were determined for a more limited number of stars. The elements studied are He, C, N, O, Ne, Mg, Al, Si, and Sc. Abundances for at least one of these elements were determined for 292 stars. Conclusions. The hot-star data analysed here, as well as the GES data in general, will be of considerable use in future studies of stellar evolution and open clusters. © ESO 2022., This paper is based on data products from observations made with ESO Telescopes at the La Silla Paranal Observatory under programmes ID 188.B-3002, 193.B-0936, and 197.B-1074. These data products have been processed by the Cambridge Astronomy Survey Unit (CASU) at the Institute of Astronomy, University of Cambridge, and by the FLAMES/UVES reduction team at INAF/Osservatorio Astrofisico di Arcetri. This work was partly supported by the European Union FP7 programme through ERC grant number 320360 and by the Leverhulme Trust through grant RPG-2012-541. We acknowledge the support from INAF and Ministero dell’ Istruzione, dell’ Université’ e della Ricerca (MIUR) in the form of the grant ‘Premiale VLT 2012’. The results presented here benefitted from discussions held during the Gaia-ESO workshops and conferences supported by the ESF (European Science Foundation) through the GREAT Research Network Programme. We are grateful to E. Bertone for making their high-resolution models available, as well as to R. Sordo for making the high-resolution Munari models available. This research has made use of the WEBDA database, originally developed by J.-C. Mermilliod, and now operated at the Department of Theoretical Physics and Astrophysics of the Masaryk University. This research has also made use of the SIMBAD database, operated at CDS, Strasbourg, France. A.H. acknowledges support by the Spanish Ministerio de Ciencia e Innovación through grants PGC2018-091, 3741-B-C22 and CEX-2019-000920-S. The work of S.R.B, I.N., and S.S.D. is partially supported by the Spanish Government Ministerio de Economía y Competitivad (MINECO/FEDER) under grants PGC2018-093741-B-C21/C22 (MICIU/AEI/FEDER, UE). A.L. acknowledges that his research has been funded in part by the Belgian Federal Science Policy Office under contract No. BR/143/A2/BRASS. The Liège Node is grateful to Belgian F.R.S.-FNRS for various supports. They are also indebted for an ESA/PRODEX Belspo contract related to the Gaia Data Processing and Analysis Consortium and for support through an ARC grant for Concerted Research Actions financed by the Federation Wallonie-Brussels. J.M.A. acknowledges support from the Spanish Government Ministerio de Ciencia through grants AYA2016-75 931-C2-2-P and PGC2018-095 049-B-C22. W.S. acknowledges CAPES for a Ph.D. Fellowship. G.H. acknowledges that this research has been partially funded by the Canarian Agency for Economy, Knowledge, and Employment and the European Regional Development Fund (ERDF/EU), under grant with reference ProID2020010016. F.J.E. acknowledges financial support from the Spanish MINECO/FEDER through the grant AYA2017-84089 and MDM-2017-0737 at Centro de Astrobiología (CSIC-INTA), Unidad de Excelencia María de Maeztu, and from the European Union’s Horizon 2020 research and innovation programme under Grant Agreement No. 824064 through the ESCAPE - The European Science Cluster of Astronomy & Particle Physics ESFRI Research Infrastructures project. E.J.A acknowledges funding from the State Agency for Research of the Spanish MCIU through the ‘Center of Excellence Severo Ochoa’ award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709) and from MCIU grant PGC2018-095049-B-C21. T.B. acknowledges financial support by grant No. 2018-04857 from the Swedish Research Council. A.J.K. and U.H. acknowledge support from the Swedish National Space Agency (SNSA). R.S. acknowledges support from the National Science Centre (2014/15/B/ST9/03981).

The Tillinghast Reflector Echelle Spectrograph (TRES) is a fiber-fed optical spectrograph with a resolution of 44,000. TRES is a workhorse for recon spectroscopy as part of the TESS Follow-up Observing Program (TFOP). To date, over 2500 spectra of over 1000 TOIs have been obtained with TRES. In addition, over 200 TOIs have been observed with the Fiber-fed Echelle Spectrograph (FIES). These spectra are carefully examined to search for possible false positives such as double-lined spectra or targets that show large velocity variation indicating a stellar companion. The spectra are also used to derive stellar parameters using the Stellar Parameter Classification (SPC) tool. Extracted spectra along with quick-look stellar classification plots from the TRES pipeline are uploaded to ExoFOP-TESS weekly to encourage community collaboration. A full SPC analysis of all TRES and FIES spectra is being run to provide effective temperature, surface gravity, projected rotational velocity and metallicity. A quality flag indicator will also be applied to the classifications based on the quality of the data and the stellar type/parameters. These parameters will be uploaded to ExoFOP-TESS once the analysis is complete for community use., {"references":["Buchhave L. A., et al. 2010, ApJ. Vol. 720. Iss. 2. pp. 1118. arXiv:1005.2009","Buchhave L. A., et al. 2012, Nature, Vol. 486, Iss. 7403, pp. 375. arXiv:10.1038/nature11121","Kurucz, R. L., 1992, Model Atmospheres for Population Synthesis bibcode:https://ui.adsabs.harvard.edu/abs/1992IAUS..149..225K/abstract"]}

Isochrone fitting can be used for estimating stellar parameters which are not directly observable. Theoretical isochrones create a grid in the Hertzsprung- Russell diagram, in which it is possible to fit incomplete data and obtain missing parameters. The Stellar Isochrone Fitting Tool (StIFT) is a desktop Java application that computes stellar mass, radius, age, and evolutionary phase by repetitive linear interpolation of the loaded models. Monte Carlo simulation then works with uncertainties of the parameters and estimates uncertainty of the result parameters. The tool allows visualising and changing the input grids, in which the interpolation is performed, thus can be used for processing various ranges of data (for example, photometric grids). As a firs case study, the tool was used to estimate astrophysical parameters of a sample of magnetic chemically peculiar stars known as Alpha2 Canum Venaticorum (ACV) variables observed by the Solar Mass Ejection Imager (SMEI). The results for these variables comprise more than a hundred estimated stellar mass and evolutionary phase parameters which were analysed in a statistical way.

In young pre-main sequence clusters, observations typically reveal a large luminosity spread between cluster members, which may be the result of observational errors or evolutionary processes. In this work, we attempt to constrain observational uncertainties through improved membership, extinction, empirical colors, and independent Teff information. The recent success of magnetic and spotted models in explaining stellar parameter discrepancies for eclipsing binaries and pre-main sequence stars motivates a comparison with classical models in a young cluster. We apply our open cluster-calibrated, spectral energy distribution fitting technique to pre-MS stars on a sample of selected literature and APOGEE Net Teff's. We develop a Gaussian Mixture Model for determining cluster membership with Gaia DR2 data, and apply a highly selective cut in parallax and proper motion space. After obtaining stellar luminosity and temperature for our members, we provide isochronal ages with classical stellar models as well as spotted and magnetic models. Our luminosities imply a significant luminosity spread even with star-by-star extinction. We suggest that the ages of Class II stars in our cluster appear systematically younger than the Class III stars after accounting for star-by-star extinction. We find an age of Lambda Orionis of 3.9±0.2 Myr; with empirical intrinsic colors, classical models may underestimate stellar ages by as much as 50% at these early ages, if accounting for low mass stars. With multiple families of isochrones, we suggest that the inconsistency in isochrone ages between stars of high and low mass with classical models may be improved through the introduction of spots or magnetic effects in stellar evolution., Supported by NASA grant 80NSSC19K0597

Elemental abundances of the most metal-poor stars reflect the conditions in the early Galaxy and the properties of the first stars. We present a spectroscopic follow-up of two ultra metal-poor stars ([Fe/H], Comment: 17 pages, 11 figures, accepted for publication in MNRAS

We present an optical analysis of 55 members of R136, the central cluster in the Tarantula Nebula of the Large Magellanic Cloud. Our sample was observed with STIS aboard the Hubble Space Telescope, is complete down to about 40\,$M_{\odot}$, and includes 7 very massive stars with masses over 100\,$M_{\odot}$. We performed a spectroscopic analysis to derive their physical properties. Using evolutionary models we find that the initial mass function (IMF) of massive stars in R136 is suggestive of being top-heavy with a power-law exponent $\gamma \approx 2 \pm 0.3$, but steeper exponents cannot be excluded. The age of R136 lies between 1 and 2\,Myr with a median age of around 1.6\,Myr. Stars more luminous than $\log L/L_{\odot} = 6.3$ are helium enriched and their evolution is dominated by mass loss, but rotational mixing or some other form of mixing could be still required to explain the helium composition at the surface. Stars more massive than 40\,$M_{\odot}$ have larger spectroscopic than evolutionary masses. The slope of the wind-luminosity relation assuming unclumped stellar winds is $2.41\pm0.13$ which is steeper than usually obtained ($\sim 1.8$). The ionising ($\log Q_0\,[{\rm ph/s}] = 51.4$) and mechanical ($\log L_{\rm SW}\,[{\rm erg/s}] = 39.1$) output of R136 is dominated by the most massive stars ($>100\,M_{\odot}$). R136 contributes around a quarter of the ionising flux and around a fifth of the mechanical feedback to the overall budget of the Tarantula Nebula. For a census of massive stars of the Tarantula Nebula region we combined our results with the VLT-FLAMES Tarantula Survey plus other spectroscopic studies. We observe a lack of evolved Wolf-Rayet stars and luminous blue and red supergiants., Comment: Accepted for publication in MNRAS, 21 pages, 14 figures, 2 tables plus 33 pages of supplementary material

Stellar spectral analysis package. FASMA delivers the atmospheric stellar parameters (effective temperature, surface gravity, metallicity, microturbulence, macroturbulence, and rotational velocity) based on the spectral synthesis technique. More information: https://github.com/MariaTsantaki/FASMA-synthesis, {"references":["Tsantaki et al. 2018, https://doi.org/10.1093/mnras/stx2564"]}

Asteroseismology is a powerful tool for probing the internal structures of stars by using their natural pulsation frequencies. It relies on identifying sequences of pulsation modes that can be compared with theoretical models, which has been done successfully for many classes of pulsators, including low-mass solar-type stars, red giants, high-mass stars and white dwarfs. However, a large group of pulsating stars of intermediate mass--the so-called delta Scuti stars--have rich pulsation spectra for which systematic mode identification has not hitherto been possible. This arises because only a seemingly random subset of possible modes are excited, and because rapid rotation tends to spoil the regular patterns. Here we report the detection of remarkably regular sequences of high-frequency pulsation modes in 60 intermediate-mass main-sequence stars, allowing definitive mode identification. Some of these stars have space motions that indicate they are members of known associations of young stars, and modelling of their pulsation spectra confirms that these stars are indeed young., published in Nature https://www.nature.com/articles/s41586-020-2226-8

Pulsating stars are used to determine distances in the Universe owing to the relation between their pulsation period and absolute magnitude. The calibration of this period-luminosity relation is performed through the determination of independant distances, in particular with the parallax of pulsation method. However, the accuracy of this method is still mainly limited by the projection factor, a parameter used to convert radial velocities measured from spectroscopy into the true pulsating velocity of the star. The objective of this thesis is to use the SPIPS (Spectro-Photo-Interferometry of Pulsating Stars) algorithm to precisely determine this p-factor. Based on the parallax of pulsation approach, this code allows to obtain a better precision on the fundamental parameters of a pulsating star, thanks to a simultaneous modeling of the observables (spectroscopy, photometry and interferometry). Using this method while fixing an independent distance estimate allowed me to retrieve the value of the p-factor. In view of a large scale use of the SPIPS code, I first did an important work of optimization and data collection. These catalogs as well as various observations I made throughout these three years (with among others the PIONIER instrument of the VLTI) allowed me to create a complete and heterogeneous database of more than 300 stars. I then apply the SPIPS algorithm to a large sample of stars using the parallaxes from the second data release of the Gaia satellite. Using these parallaxes, I was first able to accurately calculate the p-factor of some RR Lyrae type variables. Applied to a larger sample of Cepheids, the SPIPS code allowed me to determine a linear relation between the p-factor and the pulsation period, resulting in a new calibration of the period-luminosity relation. These promising results of the SPIPS code suggest that the future releases of the Gaia satellite will unveil the nature of the p-factor, bringing the parallax of pulsation method to the forefront of in the calibration of the distance scale and the determination of the Hubble constant.; Les étoiles pulsantes sont utilisées pour estimer les distances dans l’Univers, au moyen de la relation liant leur période de pulsation à leur magnitude absolue. La calibration de cette relation période-luminosité est réalisée via la détermination de distances indépendantes, notamment par la méthode de la parallaxe de pulsation. Cependant, la précision de cette méthode est aujourd’hui encore principalement limitée par le facteur de projection (p-facteur), un paramètre permettant de remonter à la vitesse pulsante de l’étoile à partir des vitesses radiales mesurées par spectroscopie. Mon travail de thèse a consisté à utiliser le code SPIPS (Spectro-Photo-Interferometry of Pulsating Stars) afin de parvenir à une détermination précise du p-facteur. Reprenant le principe de la parallaxe de pulsation, ce code permet d’obtenir une meilleure précision statistique des paramètres fondamentaux d’une étoile pulsante, grâce à une modélisation simultanée des observables spectroscopique, photométrique et interférométrique. L’utilisation de cette méthode en fixant une distance mesurée indépendamment permet alors de remonter à la valeur du p-facteur. En vue d’une utilisation à grande échelle du code SPIPS, j’ai effectué dans un premier temps un important travail d’optimisation et de collecte de données de la littérature. Ces catalogues ainsi que diverses observations réalisées tout au long de ces trois années (avec entre autres l’instrument PIONIER du VLTI) m’ont permis de construire une base de données complète et homogène de plus de 300 étoiles. L’objectif de ma thèse fût alors d’appliquer le code SPIPS à un large échantillon d’étoiles, en utilisant les parallaxes du second catalogue du satellite Gaia. Au moyen de ces parallaxes, j’ai tout d’abord pu calculer avec précision le p-facteur de quelques variables de type RR Lyrae. Appliqué dans un second temps à un plus grand échantillon de Céphéides, le code SPIPS m’a permis de déterminer une relation linéaire entre le p-facteur et la période de pulsation, aboutissant à un nouvel étalonnage de la relation période-luminosité. Ces résultats prometteurs du code SPIPS laissent envisager que les prochaines publications de données du satellite Gaia permettront de lever le voile sur la nature du p-facteur, remettant ainsi au premier plan la méthode de la parallaxe de pulsation dans l’étalonnage de l’échelle des distances et la détermination de la constante de Hubble.

Pair-instability and pulsational pair-instability supernovae (PPISN) have not been unambiguously observed so far. They are, however, promising candidates for the progenitors of the heaviest binary black hole (BBH) mergers detected. If these BBHs are the product of binary evolution, then PPISNe could occur in very close binaries. Motivated by this, we discuss the implications of a PPISN happening with a close binary companion, and what impact these events have on the formation of merging BBHs through binary evolution. For this, we have computed a set of models of metal-poor ($Z_\odot/10$) single helium stars using the \texttt{MESA} software instrument. For PPISN progenitors with pre-pulse masses $>50M_\odot$ we find that, after a pulse, heat deposited throughout the layers of the star that remain bound cause it to expand to more than $100R_\odot$ for periods of $10^2-10^4\;$~yrs depending on the mass of the progenitor. This results in long-lived phases of Roche-lobe overflow or even common-envelope events if there is a close binary companion, leading to additional electromagnetic transients associated to PPISN eruptions. If we ignore the effect of these interactions, we find that mass loss from PPISNe reduces the final black hole spin by $\sim 30\%$, induces eccentricities below the threshold of detectability of the LISA observatory, and can produce a double-peaked distribution of measured chirp masses in BBH mergers observed by ground-based detectors., 22 pages, 17 figures, including 4 pages of appendices

We develop a novel method based on machine learning principles to achieve optimal initiation of CPU-intensive computations for forward asteroseismic modeling in a multi-D parameter space. A deep neural network is trained on a precomputed asteroseismology grid containing about 62 million coherent oscillation-mode frequencies derived from stellar evolution models. These models are representative of the core-hydrogen burning stage of intermediate-mass and high-mass stars. The evolution models constitute a 6D parameter space and their predicted low-degree pressure- and gravity-mode oscillations are scanned, using a genetic algorithm. A software pipeline is created to find the best fitting stellar parameters for a given set of observed oscillation frequencies. The proposed method finds the optimal regions in the 6D parameters space in less than a minute, hence providing the optimal starting point for further and more detailed forward asteroseismic modeling in a high-dimensional context. We test and apply the method to seven pulsating stars that were previously modeled asteroseismically by classical grid-based forward modeling based on a $\chi^2$ statistic and obtain good agreement with past results. Our deep learning methodology opens up the application of asteroseismic modeling in +6D parameter space for thousands of stars pulsating in coherent modes with long lifetimes observed by the $Kepler$ space telescope and to be discovered with the TESS and PLATO space missions, while applications so far were done star-by-star for only a handful of cases. Our method is open source and can be used by anyone freely., Comment: Accepted for publication in PASP Speciale Volume on Machine Learning

We analysed solar-like oscillations in 1523 $\textit{Kepler}$ red giants which have previously been misclassified as subgiants, with predicted $\nu_{\rm max}$ values (based on the Kepler Input Catalogue) between 280$\mu$Hz to 700$\mu$Hz. We report the discovery of 626 new oscillating red giants in our sample, in addition to 897 oscillators that were previously characterized by Hekker et al. (2011) from one quarter of $\textit{Kepler}$ data. Our sample increases the known number of oscillating low-luminosity red giants by $26\%$ (up to $\sim$ 1900 stars). About three quarters of our sample are classified as ascending red-giant-branch stars, while the remainder are red-clump stars. A novel scheme was applied to determine $\Delta \nu$ for 108 stars with $\nu_{\rm max}$ close to the Nyquist frequency (240$\mu$Hz < $\nu_{\rm max}$ < 320$\mu$Hz). Additionally, we identified 47 stars oscillating in the super-Nyquist frequency regime, up to 387$\mu$Hz, using long-cadence light curves. We show that the misclassifications are most likely due to large uncertainties in KIC surface gravities, and do not result from the absence of broadband colors or from different physical properties such as reddening, spatial distribution, mass or metallicity. The sample will be valuable to study oscillations in low-luminosity red giants and to characterize planet candidates around those stars.

In this paper we present the most promising science cases for a new generation visible instrument on the VLTI and the conceptual idea for the instrumental configuration. We also present a statistical study of the potential targets that may be accessible for the different classes of objects and for the required spectral resolutions., Experimental Astronomy, Springer Link, In press

The definition and optimisation studies for the Gaia satellite spectrograph, the Radial Velocity Spectrometer (RVS), converged in late 2002 with the adoption of the instrument baseline. This paper reviews the characteristics of the selected configuration and presents its expected performance. The RVS is a 2.0 by 1.6 degree integral field spectrograph, dispersing the light of all sources entering its field of view with a resolving power R=11 500 over the wavelength range [848, 874] nm. The RVS will continuously and repeatedly scan the sky during the 5 years of the Gaia mission. On average, each source will be observed 102 times over this period. The RVS will collect the spectra of about 100-150 million stars up to magnitude V~17-18. At the end of the mission, the RVS will provide radial velocities with precisions of ~2 km/s at V=15 and \~15-20 km/s at V=17, for a solar metallicity G5 dwarf. The RVS will also provide rotational velocities, with precisions (at the end of the mission) for late type stars of sigma_vsini ~5 km/s at V~15 as well as atmospheric parameters up to V~14-15. The individual abundances of elements such as Silicon and Magnesium, vital for the understanding of Galactic evolution, will be obtained up to V~12-13. Finally, the presence of the 862.0 nm Diffuse Interstellar Band (DIB) in the RVS wavelength range will make it possible to derive the three dimensional structure of the interstellar reddening., 17 pages, 9 figures, accepted for publication in MNRAS. Fig. 1,2,4,5, 6 in degraded resolution; available in full resolution at http://blackwell-synergy.com/links/doi/10.1111/j.1365-2966.2004.08282.x/pdf

National audience; We propose to create a high resolution spectral library in the optical and infrared range with PHOENIX model atmospheres code that can compute molecular lines in order to estimate stellar parameters. The chosen grid of stellar parameters will be as follows: δT eff =25K, δlogg = 0.05, δ[M/H] = 0.05, ranging between: T eff = [2500K, 4000K], logg = [4.0, −5.5], [M/H] = [−1, −1] but first, we need to calibrate on F to K stars. We present here the preliminary tests and calibration on the Sun and 18sco. The method yield respectively T eff = 5770 ± 3K, logg = 4.458 ± 0.006 and T eff = 5763 ± 3.5K, logg = 4.451 ± 0.007

We present a set of 87 RAVE stars with detected solar like oscillations, observed during Campaign 1 of the K2 mission (RAVE K2-C1 sample). This dataset provides a useful benchmark for testing the gravities provided in RAVE Data Release 4 (DR4), and is key for the calibration of the RAVE Data Release 5 (DR5). In the present work, we use two different pipelines, GAUFRE (Valentini et al. 2013) and Sp_Ace (Boeche & Grebel 2016), to determine atmospheric parameters and abundances by fixing log(g) to the seismic one. Our strategy ensures highly consistent values among all stellar parameters, leading to more accurate chemical abundances. A comparison of the chemical abundances obtained here with and without the use of seismic log(g) information has shown that an underestimated (overestimated) gravity leads to an underestimated (overestimated) elemental abundance (e.g. [Mg/H] is underestimated by ~0.25 dex when the gravity is underestimated by 0.5 dex). We then perform a comparison between the seismic gravities and the spectroscopic gravities presented in the RAVE DR4 catalogue, extracting a calibration for log(g) of RAVE giants in the colour interval 0.50, Comment: 20 pages, 27 figures, accepted

Aims. HD 96446 is a magnetic B2p He-strong star that has been reported to be a β Cep pulsator. We present a detailed spectroscopic analysis of this object based on an intensive observational data set obtained in a multisite campaign with the spectrographs CORALIE, FEROS, and HARPS (La Silla); UVES (Paranal); HERCULES (Mt. John Observatory); and GIRAFFE (SAAO). Methods. Radial velocities were measured by cross-correlations and analysed to detect periodic variations. On the other hand, the mean spectrum was fit with spectral synthesis to derive atmospheric parameters and chemical abundances. Results. From the analysis of radial velocities, HD 96446 was found to be a spectroscopic binary with a period of 799 days. The stellar companion, which contributes only ∼5% of the total flux, is an A0-Type star. A frequency analysis of the radial velocities allowed us to detect two pulsational modes with periods 2.23 h and 2.66 h. The main mode is most probably a low-inclination, dipole mode (l, m) = (1, 0), and the second pulsation mode corresponds to (l, m) = (2, 2) or to a pole-on (l, m) = (3, 2) configuration. In addition to radial velocities, the main pulsation mode is evidenced through small variations in the spectral morphology (temperature variations) and the light flux. The rotation period of 23.4 d, was detected through the variation in line intensities. Chemical abundances are unevenly distributed over the stellar surface, with helium concentrated at the negative magnetic pole and most metals strengthened at lower latitudes. The mean chemical abundance of helium is strongly abnormal, reaching a value of 0.60 (number fraction). Fil: Gonzalez, Jorge Federico. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Instituto de Ciencias Astronómicas, de la Tierra y del Espacio. Universidad Nacional de San Juan. Instituto de Ciencias Astronómicas, de la Tierra y del Espacio; Argentina Fil: Briquet, M.. Leibniz Institute For Astrophysics Potsdam; Alemania. Université de Liège; Bélgica Fil: Przybilla, N.. Universidad de Innsbruck; Austria Fil: Nieva, M.-F.. Universidad de Innsbruck; Austria Fil: De Cat, P.. Royal Observatory Of Belgium; Bélgica Fil: Saesen, S.. Universidad de Ginebra; Suiza Fil: Hubrig, S.. Leibniz Institute For Astrophysics Potsdam; Alemania Fil: Thoul, A.. Université de Liège; Bélgica Fil: Pápics, P.I.. Instituut voor Sterrenkunde; Bélgica Fil: Palaversa, L.. Institute Of Astronomy; Austria. Universidad de Ginebra; Suiza Fil: Naef, D.. Universidad de Ginebra; Suiza Fil: Neveu-Van Malle, M.. Universidad de Ginebra; Suiza Fil: Järvinen, S.. Leibniz-Institut für Astrophysik Potsdam; Alemania Fil: Pollard, K.R.. University of Canterbury; Nueva Zelanda Fil: Kilmartin, P.. University of Canterbury; Nueva Zelanda Fil: Mowlavi, N.. Universidad de Ginebra; Suiza Fil: Butler, K.. Ludwig Maximilians Universitat; Alemania

In preparation for future, large-scale, multi-object, high-resolution spectroscopic surveys of the Galaxy, we present a series of tests of the precision in radial velocity and chemical abundances that any such project can achieve at a 4 m class telescope. We briefly discuss a number of science cases that aim at studying the chemo-dynamical history of the major Galactic components (bulge, thin and thick disks, and halo) - either as a follow-up to the Gaia mission or on their own merits. Based on a large grid of synthetic spectra that cover the full range in stellar parameters of typical survey targets, we devise an optimal wavelength range and argue for a moderately high-resolution spectrograph. As a result, the kinematic precision is not limited by any of these factors, but will practically only suffer from systematic effects, easily reaching uncertainties

Thesis is devoted to the research of mixing processes in low-mass metal-rich stars. A classical stellar evolution model is unable to explain the observed abundances of specific elements that are affected by internal mixing processes. Recently developed models aim to explain extra-mixing scenarios in the low-mass stars, however, the amount of observational data is still not large enough to test these models. Using high-resolution spectra we analysed 109 stars in ten open clusters with turn-off masses between 1.1 and 5.6 solar masses. We determined carbon, nitrogen and oxygen abundances for all stars in our sample as well as sodium and ten other chemical elements for some stars. Abundances of elements that are affected by mixing processes were compared with classical 1st dredge-up and two extra-mixing models. The first one includes material mixing induced by thermohaline instability. The second model combines both thermohaline- and rotation-induced mixing. Both models of extra-mixing are quite similar in the range up to 3 solar masses for the carbon isotope ratios and they describe observational data well enough. However, for the mass range over 3 solar masses the data of carbon isotopic ratios disagree with the extra-mixing model that includes both rotational and thermohaline mixing. However, the carbon-to-nitrogen abundance ratios act vice versa. Thus, a universal model of carbon isotopic and carbon-to-nitrogen rations has yet to be developed.

Thesis is devoted to the research of mixing processes in low-mass metal-rich stars. A classical stellar evolution model is unable to explain the observed abundances of specific elements that are affected by internal mixing processes. Recently developed models aim to explain extra-mixing scenarios in the low-mass stars, however, the amount of observational data is still not large enough to test these models. Using high-resolution spectra we analysed 109 stars in ten open clusters with turn-off masses between 1.1 and 5.6 solar masses. We determined carbon, nitrogen and oxygen abundances for all stars in our sample as well as sodium and ten other chemical elements for some stars. Abundances of elements that are affected by mixing processes were compared with classical 1st dredge-up and two extra-mixing models. The first one includes material mixing induced by thermohaline instability. The second model combines both thermohaline- and rotation-induced mixing. Both models of extra-mixing are quite similar in the range up to 3 solar masses for the carbon isotope ratios and they describe observational data well enough. However, for the mass range over 3 solar masses the data of carbon isotopic ratios disagree with the extra-mixing model that includes both rotational and thermohaline mixing. However, the carbon-to-nitrogen abundance ratios act vice versa. Thus, a universal model of carbon isotopic and carbon-to-nitrogen rations has yet to be developed.

TheImmersion Grating Infrared Spectrometer (IGRINS) is a revolutionary instrument that exploits broad spectral coverage at high-resolution in the near-infrared. There are no moving parts in IGRINS and its high-throughput white-pupil design maximizes sensitivity. IGRINS on the 2.7 meter Harlan J. Smith Telescope at McDonald Observatory is nearly as sensitive as CRIRES at the 8 meter Very Large Telescope. However, IGRINS at R=45,000 has more than 30 times the spectral grasp of CRIRES. The use of an immersion grating facilitates a compact cryostat while providing simultaneous H and K band observations with complete wavelength coverage from 1.45 - 2.45 microns. Here we discuss details of instrument performance and summarize the application of IGRINS to stellar characterization, star formation in regions like Taurus and Ophiuchus, the interstellar medium, and photodissociation regions. IGRINS has the largest spectral grasp of any high-resolution, near-infrared spectrograph, allowing us to study star formation and evolution in unprecedented detail. With its fixed format and high sensitivity, IGRINS is a great survey instrument for star clusters, high signal-to-noise (SNR>300) studies of field stars, and for mapping the interstellar medium. As a prototype for GMTNIRS on the Giant Magellan Telescope, IGRINS represents the future of high-resolution spectroscopy. In the future IGRINS will be deployed to numerous facilities and will remain a versatile instrument for the community while producing a rich archive of uniform spectra.

The Wolf-Rayet 98a (WR 98a) system is a prime target for interferometric surveys, since its identification as a "rotating pinwheel nebulae", where infrared images display a spiral dust lane revolving with a 1.4 year periodicity. WR 98a hosts a WC9+OB star, and the presence of dust is puzzling given the extreme luminosities of Wolf-Rayet stars. We present 3D hydrodynamic models for WR 98a, where dust creation and redistribution are self-consistently incorporated. Our grid-adaptive simulations resolve details in the wind collision region at scales below one percent of the orbital separation (~4 AU), while simulating up to 1300 AU. We cover several orbital periods under conditions where the gas component alone behaves adiabatic, or is subject to effective radiative cooling. In the adiabatic case, mixing between stellar winds is effective in a well-defined spiral pattern, where optimal conditions for dust creation are met. When radiative cooling is incorporated, the interaction gets dominated by thermal instabilities along the wind collision region, and dust concentrates in clumps and filaments in a volume-filling fashion, so WR 98a must obey close to adiabatic evolutions to demonstrate the rotating pinwheel structure. We mimic Keck, ALMA or future E-ELT observations and confront photometric long-term monitoring. We predict an asymmetry in the dust distribution between leading and trailing edge of the spiral, show that ALMA and E-ELT would be able to detect fine-structure in the spiral indicative of Kelvin-Helmholtz development, and confirm the variation in photometry due to the orientation. Historic Keck images are reproduced, but their resolution is insufficient to detect the details we predict., Accepted for publication in mnras

This paper presents a summary of four invited and twelve contributed presentations on asymptotic giant branch stars and red supergiants, given over the course of two afternoon splinter sessions at the 19th Cool Stars Workshop. It highlights both recent observations and recent theory, with some emphasis on high spatial resolution, over a wide range of wavelengths. Topics covered include 3D models, convection, binary interactions, mass loss, dust formation and magnetic fields., Comment: To appear in the proceedings of the 19th Cambridge Workshop on Cool Stars, Stellar Systems and the Sun; 19 pages and 20 figures

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.

More than 80% of the stars in the Universe are expected to have initial masses below eight to ten times the mass of our sun. These low mass stars, including our sun, become cool red giants during one of the final evolutionary stages of their life: the Asymptotic Giant Branch (or AGB) phase. AGB stars are among the main producers of carbon and heavy (s-process) elements in the Universe. These elements are synthesized inside the star and mixed to the stellar atmosphere where stellar winds are responsible for the loss of more than 50% of the stellar mass, hence, AGB stars are strong polluters of the interstellar medium. The ejected material can clump together into dusty particles which may serve as ingredients for the birth of new stars and planets. When most of the AGB stellar envelope is lost, the AGB star stops releasing nuclear energy from interior processes and swaps its giant face for a planetary nebulae look, whereafter it fades away as a white dwarf.The dredge-up of carbon and s-process elements into the AGB atmosphere causes an important chemical anomaly among them: initial oxygen-rich stars (M stars) are transformed into carbon-rich stars (C stars). As a consequence, a group of oxygen-rich AGB stars exists which makes the transition between M and C stars. These transition stars are classified as S.Although AGB stars are identified as producers of heavy elements, their nucleosynthesis and mixing processes are weakly constrained due to large uncertainties on their estimated temperature, gravity and chemical composition. Stronger constraints on the atmospheric parameter space, hence interior processes, of AGB stars can be obtained by investigating the atmosphere of S stars. Since they are transition objects on the AGB, they trace the rise of the s-process. S stars are less numerous than C stars, but their optical spectra are brighter making it easier to identify atomic and molecular lines. Therefore, S stars belong to the most interesting objects along the AGB to perform this task.From a practical point of view, the spectra of S stars are extremely difficult to study since they are dominated by different, overlapping molecular bands, and the spectral shape may vary strongly from star to star due to their transition status. Therefore, tailored model atmospheres for S stars are of utmost importance to understand the spectroscopic, and even photometric, changes in terms of variations in the atmospheric parameters. A comparison between the models and observations aims not only at constraining the atmospheric parameter space of S stars, it will also test the reliability of 1D state-of-the-art model atmospheres for such complex stars.From an evolutionary point of view, the S-star family is contaminated with stars who gained their atmospheric enrichment in heavy elements from a companion star. Evidences were found that these binary S stars are not at all located on the AGB, hence, they are labelled as extrinsic S stars while S stars on the AGB are labelled as intrinsic. The difference in evolutionary stages between intrinsic and extrinsic S stars was already found 20 years ago, however, a separation in terms of surface temperature, gravity and chemical composition is not well-established due to the lack of S-star model atmospheres. Such a distinction in atmospheric parameters will facilitate the discovery of these intruders and even help to calibrate stellar evolutionary models of single and binary stars.To achieve these goals, the first step consists in the construction of a grid of model atmospheres for S stars. The grid will be used to quantify the influence of atmospheric parameters on the model structure and emergent flux. These results will be analyzed to derive precise atmospheric parameters of observed S stars, using a set of well-defined photometric and spectroscopic indices. Once the best model atmosphere has been selected for all observed S stars, their atmospheric parameters will be discussed in view of their evolutionary stage. The best-fitting model atmosphere will also be used to derive abundances from spectral syntheses. The abundance profiles are compared with stellar evolution model prediction to constrain nucleosynthesis and mixing processes inside S stars. Derived abundances of unstable elements will be used to estimate, for the first time, the age of AGB stars. Finally, their abundance profile will be discussed as a function of their time spent on the AGB., Doctorat en Sciences, info:eu-repo/semantics/nonPublished

As instruments have become increasingly efficient and accurate, the detection and characterization of exoplanets has become a rapidly expanding field of research. However, the data provided are always dependent on the host stars' parameters. For example, the radial velocity measurements allow the ratio Mp sin(i)/M* to be determined, while the transit method provides Rp/R*. Estimating the stellar radius with sufficient accuracy is thus a very important element to constrain both the stellar parameters and the exoplanets minimum masses. Another phenomenon, the stellar activity, should not be neglected because, as shown by many works on radial velocity, it affects the signal produced by exoplanets. Its knowledge is also essential since it gives information on stellar structure and evolution. Several methods are currently employed to measure stellar parameters : asteroseismology, spectrophotometry and interferometry. I have used the latter one to tackle the problem of the determination of fundamental parameters of host stars using two different approaches. Throughout my thesis I have performed observations with VEGA/CHARA, an interferometer operating in the optical domain and taking advantage of the longest baselines in the world. It allows an angular resolution of 0.3 milliarcseconds to be obtained, coupled with an accuracy of 1 − 3% on diameters. With empirical formulae, I have derived from these diameters the parameters of a sample of 17 stars : linear radius, mass and effective temperature. This sample gathers host stars, but also non-host stars for comparison. Thanks to the accuracy reached on the parameters, those stars could be located on the Hertzsprung-Russell diagram, and the derived minimum masses of exoplanets are in very good agreement with estimates obtained using other methods. I have reinforced this work by modeling stars with a transiting exoplanet or a magnetic spot, in order to quantify their impact on interferometric observables (visibility, phase and closure phase). To do so, I have developed a numerical code including analytical formulae used to probe the ability of VEGA/CHARA to measure these observables, and thus report the improvement to be brought to such an instrument. I also make a complete and more global analysis of parameters able to impact the detection and characterization of spots and exoplanets, like their size or position on the stellar disk, using a fictive interferometer. It turns out that for resolved stars hosting big enough exoplanets or spots, CHARA baselines are long enough to allow their detection. However, the difference between the two signals occurs beyond the 2nd lobe of the visibility function and is mostly perceptible in the closure phase measurements, and thus remains out of reach for VEGA. This thesis provides an important contribution to research performed on host stars because it includes not only the analysis of stellar parameters, but also stellar activity, which allows a better characterization of exoplanets and gives a prospective on future interferometers. At this time, when the characterization of exoplanets is being limited, not only by instruments but also by the accuracy on stellar parameters, the study of host stars and perturbating phenomenon on the measurement of their parameters becomes crucial if we want to make the knowledge on exoplanets progress.; Avec des instruments toujours plus précis et performants, la détection et la caractérisation des exoplanètes sont devenues un domaine de recherche qui ne cesse de croître. Cependant, les données obtenues sont toujours dépendantes des paramètres des étoiles les abritant. Par exemple, les mesures de vitesse radiale permettent de déterminer le rapport Mp sin(i)/M*, tandis que la méthode des transits fournit Rp/R*. L'estimation du rayon stellaire avec une précision suffisante est donc un élément très important pour contraindre à la fois les paramètres de l'étoile et la masse minimale de l'exoplanète. Un autre phénomène, l'activité stellaire, ne doit pas être négligé car, comme l'ont prouvé de nombreux travaux sur les vitesses radiales, elle affecte le signal provoqué par les exoplanètes. Sa connaissance est aussi essentielle puisqu'elle renseigne sur l'évolution et la structure des étoiles. Plusieurs méthodes sont actuellement employées pour mesurer les paramètres stellaires : astérosismologie, spectrophotométrie, interférométrie. J'ai utilisé cette dernière méthode pour aborder le problème de la détermination des paramètres fondamentaux des étoiles hôtes sur deux aspects. J'ai effectué tout au long de ma thèse des observations avec VEGA/CHARA, un interférométre opérant dans le domaine visible et bénéficiant des plus longues bases du monde. Cela permet d'obtenir une résolution angulaire de 0.3 millisecondes d'arc, couplée à des précisions de 1−3% sur les diamètres. En utilisant des formules empiriques, j'en ai déduit les paramètres d'un échantillon de 17 étoiles : rayon linéaire, masse, température effective. Cet échantillon comprend des étoiles hôtes, mais aussi des étoiles sans exoplanète connue à ce jour à titre de comparaison. Grâce aux précisions atteintes sur les paramètres, elles ont pu être placées sur le diagramme de Hertzsprung-Russell, et les masses minimales des exoplanètes qui en sont déduites se sont révélées en très bon accord avec les estimations obtenues avec d'autres méthodes. J'ai complété ce travail par la modélisation d'étoiles abritant une exoplanète en transit ou ayant une tache magnétique afin de quantifier l'impact qu'elles produisent sur les observables interférométriques (visibilités, phases et clôtures de phase). Pour cela, j'ai développé un code numérique incluant des formules analytiques servant à sonder les capacités de VEGA/CHARA à mesurer ces signaux, et ainsi rendre compte des améliorations à apporter sur un tel instrument. J'apporte aussi une analyse complète et plus globale des paramètres pouvant influencer la détection et la caractérisation des taches et exoplanètes, comme leur diamètre ou leur position sur le disque stellaire, en utilisant un interféromètre fictif. Il s'avère que pour des étoiles résolues abritant des exoplanètes ou taches assez grosses, les bases de CHARA sont assez grandes pour permettre leur détection. Par contre, la différence entre les deux signaux s'opère au-delà du 2ème lobe de visibilité et est surtout repérable dans les mesures de clôtures, et demeure donc pour l'instant hors d'atteinte de VEGA. Cette thèse englobe une partie importante du travail effectué généralement sur les étoiles hôtes car elle inclut l'analyse des paramètres stellaires, mais aussi l'activité stellaire, ce qui permet une meilleure caractérisation des exoplanètes et donne des perspectives sur les interféromètres futurs. À l'heure où la précision des instruments permettant de détecter des exoplanètes dépasse la précision disponible sur les paramètres des étoiles hôtes, l'étude des étoiles hôtes et des phénomènes qui perturbent la mesure de leurs paramètres devient cruciale si l'on veut faire progresser la connaissance sur les exoplanètes.

As instruments have become increasingly efficient and accurate, the detection and characterization of exoplanets has become a rapidly expanding field of research. However, the data provided are always dependent on the host stars' parameters. For example, the radial velocity measurements allow the ratio Mp sin(i)/M* to be determined, while the transit method provides Rp/R*. Estimating the stellar radius with sufficient accuracy is thus a very important element to constrain both the stellar parameters and the exoplanets minimum masses. Another phenomenon, the stellar activity, should not be neglected because, as shown by many works on radial velocity, it affects the signal produced by exoplanets. Its knowledge is also essential since it gives information on stellar structure and evolution. Several methods are currently employed to measure stellar parameters : asteroseismology, spectrophotometry and interferometry. I have used the latter one to tackle the problem of the determination of fundamental parameters of host stars using two different approaches. Throughout my thesis I have performed observations with VEGA/CHARA, an interferometer operating in the optical domain and taking advantage of the longest baselines in the world. It allows an angular resolution of 0.3 milliarcseconds to be obtained, coupled with an accuracy of 1 − 3% on diameters. With empirical formulae, I have derived from these diameters the parameters of a sample of 17 stars : linear radius, mass and effective temperature. This sample gathers host stars, but also non-host stars for comparison. Thanks to the accuracy reached on the parameters, those stars could be located on the Hertzsprung-Russell diagram, and the derived minimum masses of exoplanets are in very good agreement with estimates obtained using other methods. I have reinforced this work by modeling stars with a transiting exoplanet or a magnetic spot, in order to quantify their impact on interferometric observables (visibility, phase and closure phase). To do so, I have developed a numerical code including analytical formulae used to probe the ability of VEGA/CHARA to measure these observables, and thus report the improvement to be brought to such an instrument. I also make a complete and more global analysis of parameters able to impact the detection and characterization of spots and exoplanets, like their size or position on the stellar disk, using a fictive interferometer. It turns out that for resolved stars hosting big enough exoplanets or spots, CHARA baselines are long enough to allow their detection. However, the difference between the two signals occurs beyond the 2nd lobe of the visibility function and is mostly perceptible in the closure phase measurements, and thus remains out of reach for VEGA. This thesis provides an important contribution to research performed on host stars because it includes not only the analysis of stellar parameters, but also stellar activity, which allows a better characterization of exoplanets and gives a prospective on future interferometers. At this time, when the characterization of exoplanets is being limited, not only by instruments but also by the accuracy on stellar parameters, the study of host stars and perturbating phenomenon on the measurement of their parameters becomes crucial if we want to make the knowledge on exoplanets progress.; Avec des instruments toujours plus précis et performants, la détection et la caractérisation des exoplanètes sont devenues un domaine de recherche qui ne cesse de croître. Cependant, les données obtenues sont toujours dépendantes des paramètres des étoiles les abritant. Par exemple, les mesures de vitesse radiale permettent de déterminer le rapport Mp sin(i)/M*, tandis que la méthode des transits fournit Rp/R*. L'estimation du rayon stellaire avec une précision suffisante est donc un élément très important pour contraindre à la fois les paramètres de l'étoile et la masse minimale de l'exoplanète. Un autre phénomène, l'activité stellaire, ne doit pas être négligé car, comme l'ont prouvé de nombreux travaux sur les vitesses radiales, elle affecte le signal provoqué par les exoplanètes. Sa connaissance est aussi essentielle puisqu'elle renseigne sur l'évolution et la structure des étoiles. Plusieurs méthodes sont actuellement employées pour mesurer les paramètres stellaires : astérosismologie, spectrophotométrie, interférométrie. J'ai utilisé cette dernière méthode pour aborder le problème de la détermination des paramètres fondamentaux des étoiles hôtes sur deux aspects. J'ai effectué tout au long de ma thèse des observations avec VEGA/CHARA, un interférométre opérant dans le domaine visible et bénéficiant des plus longues bases du monde. Cela permet d'obtenir une résolution angulaire de 0.3 millisecondes d'arc, couplée à des précisions de 1−3% sur les diamètres. En utilisant des formules empiriques, j'en ai déduit les paramètres d'un échantillon de 17 étoiles : rayon linéaire, masse, température effective. Cet échantillon comprend des étoiles hôtes, mais aussi des étoiles sans exoplanète connue à ce jour à titre de comparaison. Grâce aux précisions atteintes sur les paramètres, elles ont pu être placées sur le diagramme de Hertzsprung-Russell, et les masses minimales des exoplanètes qui en sont déduites se sont révélées en très bon accord avec les estimations obtenues avec d'autres méthodes. J'ai complété ce travail par la modélisation d'étoiles abritant une exoplanète en transit ou ayant une tache magnétique afin de quantifier l'impact qu'elles produisent sur les observables interférométriques (visibilités, phases et clôtures de phase). Pour cela, j'ai développé un code numérique incluant des formules analytiques servant à sonder les capacités de VEGA/CHARA à mesurer ces signaux, et ainsi rendre compte des améliorations à apporter sur un tel instrument. J'apporte aussi une analyse complète et plus globale des paramètres pouvant influencer la détection et la caractérisation des taches et exoplanètes, comme leur diamètre ou leur position sur le disque stellaire, en utilisant un interféromètre fictif. Il s'avère que pour des étoiles résolues abritant des exoplanètes ou taches assez grosses, les bases de CHARA sont assez grandes pour permettre leur détection. Par contre, la différence entre les deux signaux s'opère au-delà du 2ème lobe de visibilité et est surtout repérable dans les mesures de clôtures, et demeure donc pour l'instant hors d'atteinte de VEGA. Cette thèse englobe une partie importante du travail effectué généralement sur les étoiles hôtes car elle inclut l'analyse des paramètres stellaires, mais aussi l'activité stellaire, ce qui permet une meilleure caractérisation des exoplanètes et donne des perspectives sur les interféromètres futurs. À l'heure où la précision des instruments permettant de détecter des exoplanètes dépasse la précision disponible sur les paramètres des étoiles hôtes, l'étude des étoiles hôtes et des phénomènes qui perturbent la mesure de leurs paramètres devient cruciale si l'on veut faire progresser la connaissance sur les exoplanètes.