Formation of globular clusters (GCs), the Galactic bulge, or galaxy bulges in general is an important unsolved problem in Galactic astronomy. Homogeneous infrared observations of large samples of stars belonging to GCs and the Galactic bulge field are one of the best ways to study these problems. We report the discovery by APOGEE (Apache Point Observatory Galactic Evolution Experiment) of a population of field stars in the inner Galaxy with abundances of N, C, and Al that are typically found in GC stars. The newly discovered stars have high [N/Fe], which is correlated with [Al/Fe] and anticorrelated with [C/Fe]. They are homogeneously distributed across, and kinematically indistinguishable from, other field stars within the same volume. Their metallicity distribution is seemingly unimodal, peaking at [Fe/H] similar to -1, thus being in disagreement with that of the Galactic GC system. Our results can be understood in terms of different scenarios. N-rich stars could be former members of dissolved GCs, in which case the mass in destroyed GCs exceeds that of the surviving GC system by a factor of similar to 8. In that scenario, the total mass contained in so-called 'first-generation' stars cannot be larger than that in 'second-generation' stars by more than a factor of similar to 9 and was certainly smaller. Conversely, our results may imply the absence of a mandatory genetic link between 'second-generation' stars and GCs. Last, but not least, N-rich stars could be the oldest stars in the Galaxy, the by-products of chemical enrichment by the first stellar generations formed in the heart of the Galaxy.

Magnetic fields play a crucial roll in the stellar activity and evolutionof stars. Despite much research there is much that we do notunderstand. Among Ap stars, empirical evidence has suggested a minimumthreshold for the dipolar magnetic field strength of Bp ≈ 300G.This thesis studies the magnetic field of the Ap star EP UMa usingthe oblique rotator model, which is modeling the star’s magnetic fieldas a dipole. The magnetic field was calculated through the StokesV- and I-spectrum emitted by the star. In total 16 observations ofthe Stokes V and I spectrum were used, collected from the spectropolarimeterNARVAL. These spectra were then analysed using the leastsquares deconvolution method, creating average Stokes V and I profiles,through which the magnetic field were calculated. The result ofthis study indicates a magnetic field of EP UMa with polar strengthof 74G ≤ Bp ≤ 196G, which is well below the suggested minimumthreshold.

Within this past decade, advances in spectropolarimetric analyses allowed mapping surface characteristics of nearby non-resolved stars with unique characteristics. In this study, we attempt mapping of the magnetic field structure and surface distribution of elements for such a star, the magnetic Ap star phi Dra. Using high-precision photometric data from TESS satellite, we improve its rotation period. Spectrum synthesis calculations allow compilation of a list of lines present in its spectrum. The resulting synthetic spectrum and observed NARVAL spectra are used to re-estimate element abundances. Least-squares deconvolved (LSD) intensity and circular polarisation profiles are then computed from a sample of 1260 metal lines. To determine which element(s) are most suitable for in-depth analysis, variability of LSD profiles is studied qualitatively for Fe, Cr and Si. The longitudinal magnetic field of phi Dra is calculated from LSD circular polarisation profiles. Stellar magnetic field maps and distributions of Fe concentration are derived through Zeeman Doppler Imaging (ZDI). The resulting maps of this study show five areas of high Fe concentrations, in the Northern stellar hemisphere. The magnetic field topology of phi Dra resulting from our analysis is that of an offset dipole with small quadrupole contributions. Our abundance and magnetic maps suggest correlation between high concentrations of Fe and high magnetic field strength. The field is primarily radial in 4 out of 5 such regions, contrary to theoretical expectations.

J'ai effectué une étude des éléments C, N, et O dans les atmosphères d'étoiles chimiquement particulières de la séquence principale, où ils sont sous-abondants et répartis de manière inhomogène. J'ai recensé les processus physiques qu'il faut inclure dans les calculs d'accélération radiative et montré leur importance relative. Des améliorations majeures par rapport aux approximations usuelles ont été obtenues grâce à l'utilisation systématique des données atomiques du projet OPACITY. Elles permettent de calculer précisément la dépendance en fréquence des opacités, et d'améliorer l'évaluation des largeurs de raies. Les contributions des raies et de la photoionisation sont calculées pour chaque ion et l'accélération totale sur un élément donné est obtenue grâce à un modèle prenant en compte les réactions rapides entre ions. Les accélérations radiatives calculées pour le carbone, l'azote, et l'oxygène, les poussant vers le haut, apparaissent inférieures à la gravité dans tous les modèles d'enveloppe considérés (étoiles de type A à F), pour une large gamme de paramètres, expliquant leurs déficits marquées à la surface de la plupart des étoiles chimiquement particulières. Des tables donnant d'une part l'opacité de ces éléments, d'autre part leur accélération radiative sur une grille contenant de nombreuses conditions de plasma permettent d'effectuer des calculs d'évolution stellaire prenant en compte tous les aspects de la diffusion des éléments C, N, et O, les plus abondants après H et He. <br /> <br />Je me suis ensuite intéressé aux étoiles de type lambda Bootis, un petit sous-groupe singulier d'étoiles chimiquement particulières, dont les anomalies d'abondance ne sont pas expliquées par le modèle de la diffusion radiative. Il s'agirait plutôt d'étoiles jeunes, encore entourées des restes du disque à partir duquel elles se sont formées, et dont elles accrèteraient un gaz appauvri en éléments lourds, ceux-ci s'étant condensés en grains. Afin de vérifier cette hypothèse, nous avons recherché la signature de matière circumstellaire dans le spectre de ces étoiles. Peu d'étoiles de notre échantillon montrent un tel indice et nos résultats suggèrent une anti-corrélation entre la présence de gaz ou de poussières, pouvant caractériser deux états différents dans l'évolution du disque protostellaire. Au cours de cette étude, nous avons découvert par hasard le premier cas de pulsations non radiales dans une étoile de type lambda Bootis, puis montré qu'elles sont communes dans ce groupe. L'identification des modes de pulsation permet de remonter à la structure interne de ces étoiles et à leur état d'évolution, permettant ainsi de tester le modèle d'accrétion. <br /> <br />Mon étude des environnements circumstellaires des étoiles lambda Bootis m'a conduit à m'intéresser aux disques protoplanétaires. Jusqu'à récemment, nous n'avions observé qu'un seul système solaire (le nôtre) dans lequel nous pouvions tester notre compréhension du processus de formation de planètes. Maintenant, plus d'une centaine de planètes ont été découvertes autour d'autres étoiles et les contraintes sur les modèles théoriques sont devenues très serrées. Nous savons que, dans la nébuleuse solaire, les particules de poussière de la taille du micron se sont agglomérées pour former des planètes, objets 10^13 à 10^14 fois plus grands. Bien qu'il y ait beaucoup de travail réalisé sur les dernières étapes de cette formation, et sur la migration de planètes déjà formées, peu de travail a été fait pour développer des modèles hydrodynamiques décrivant l'interaction du gaz et de la poussière dans les disques proto-planétaires. Nous développons un code hydrodynamique SPH permettant de modéliser cette interaction, principalement par la force de friction, entre deux phases: du gaz et des grains de poussière d'une taille donnée. Nous obtenons ainsi la répartition spatiale des grains dans le disque en fonction de leur taille. Ce travail correspond à la thèse de Laure Barrière-Fouchet, qui se termine en 2005. Nous projetons ensuite d'ajouter les mécanismes de coagulation, croissance, et évaporation des grains de poussière en modélisant plusieurs phases pour différentes tailles de grains et la variation du nombre de particules dans chaque phase qui en résulte. Ceci permettra de caractériser les zones du disque les plus favorables à la formation de planétésimaux. Ensuite, il s'agira d'explorer plus profondément les mécanismes de formation de planètes. En effet, si l'on arrive assez bien à faire croître les grains microscopiques jusqu'à une taille de l'ordre du centimètre, les collisions entre ces gros grains les refragmentent et empêchent de dépasser cette taille. Plusieurs solutions sont à envisager pour permettre de passer ce cap: diminution des vitesses de collisions dans les régions plus denses, rôle de la turbulence, etc... <br /> <br />Un peu à part de mes travaux précédents, avec mes collègues de l'ESO, j'ai observé et pris le premier spectre de la contrepartie optique du sursaut gamma GRB980425, qui s'est avéré être une supernova très particulière: SN1998bw. Son spectre en évolution rapide ne permettait pas de classer cette supernova, la première à être associée à un sursaut gamma, dans les types connus. Notre équipe a suivi régulièrement l'évolution de sa courbe de lumière et de son spectre, la somme de données recueillie ayant conduit à un modèle d'hypernova.

We present new radio/millimeter measurements of the hot magnetic star HR 5907 obtained with the VLA and ALMA interferometers. We find that HR 5907 is the most radio luminous early type star in the cm–mm band among those presently known. Its multi-wavelength radio light curves are strongly variable with an amplitude that increases with radio frequency. The radio emission can be explained by the populations of the non-thermal electrons accelerated in the current sheets on the outer border of the magnetosphere of this fast-rotating magnetic star. We classify HR 5907 as another member of the growing class of strongly magnetic fast-rotating hot stars where the gyro-synchrotron emission mechanism efficiently operates in their magnetospheres. The new radio observations of HR 5907 are combined with archival X-ray data to study the physical condition of its magnetosphere. The X-ray spectra of HR 5907 show tentative evidence for the presence of non-thermal spectral component. We suggest that non-thermal X-rays originate a stellar X-ray aurora due to streams of non-thermal electrons impacting on the stellar surface. Taking advantage of the relation between the spectral indices of the X-ray power-law spectrum and the non-thermal electron energy distributions, we perform 3-D modelling of the radio emission for HR 5907. The wavelength-dependent radio light curves probe magnetospheric layers at different heights above the stellar surface. A detailed comparison between simulated and observed radio light curves leads us to conclude that the stellar magnetic field of HR 5907 is likely non-dipolar, providing further indirect evidence of the complex magnetic field topology of HR 5907.

In this paper we investigate the multiwavelengths properties of the magnetic early B-type star HR7355. We present its radio light curves at several frequencies, taken with the Jansky Very Large Array, and X-ray spectra, taken with the XMM X-ray telescope. Modeling of the radio light curves for the Stokes I and V provides a quantitative analysis of the HR7355 magnetosphere. A comparison between HR7355 and a similar analysis for the Ap star CUVir, allows us to study how the different physical parameters of the two stars affect the structure of the respective magnetospheres where the non-thermal electrons originate. Our model includes a cold thermal plasma component that accumulates at high magnetic latitudes that influences the radio regime, but does not give rise to X-ray emission. Instead, the thermal X-ray emission arises from shocks generated by wind stream collisions close to the magnetic equatorial plane. The analysis of the X-ray spectrum of HR7355 also suggests the presence of a non-thermal radiation. Comparison between the spectral index of the power-law X-ray energy distribution with the non-thermal electron energy distribution indicates that the non-thermal X-ray component could be the auroral signature of the non-thermal electrons that impact the stellar surface, the same non-thermal electrons that are responsible for the observed radio emission. On the basis of our analysis, we suggest a novel model that simultaneously explains the X-ray and the radio features of HR7355 and is likely relevant for magnetospheres of other magnetic early type stars.

Large spectroscopic surveys are trailblazing endeavours in the study of stellar archaeology and near eld cosmology. Access to homogeneous databases of thousands of stellar spectra allow for a detailed and statistically satisfying look into the chemical abundance distribution of our Galaxy and its surrounding satellites, ultimately working towards a better understanding of galactic chemical evolution. This thesis presents the work of three new studies at the current frontier of stellar archaeology. Through the rst look at carbon-enhanced metal-poor (CEMP) stars using H-band spectra, six new CEMP stars and another seven likely candidates were found within the APOGEE database following Data Release 12. These stars have chemical compositions typical of metal-poor halo stars, however the alpha-abundances of two stars indicate possible origins in an accreted dwarf galaxy. A lack of heavy element spectral lines impedes further sub-classi cation of these CEMP stars, however, based on radial velocity scatter, we predict most are not CEMP-s stars which are typically found in binary systems. This preliminary investigation warrants optical observations to con rm the stellar parameters and low metallicities of these stars, to determine the heavy-element abundance ratios and improve the precision in the derived abundances, and to examine their CEMP sub-classi cations. Additionally, the rst results for the spectroscopic follow up to the Pristine survey are presented. Using a sample of 149 stars, a success rate of 70% for finding stars with [Fe/H]<-2.5 and 22% for finding stars with [Fe/H]<-3.0 is reported, significantly higher than other surveys that typically report success rates of 3-4% for recovering stars with [Fe/H]<-3.0. Finally, the new spectral analysis tool StarNet is introduced. A deep neural network architecture is used to examine both synthetic stellar spectra and SDSS-III APOGEE spectral data and can produce the stellar parameters of temperature, gravity, and metallicity with similar or better precision as the APOGEE pipeline values when trained directly with the APOGEE spectra. StarNet is capable of being trained on synthetic data as well, and is able to reproduce the stellar parameters for both synthetic and APOGEE spectra, including low signal-to-noise spectra, with similar precision to training on the APOGEE spectra itself. The residuals between StarNet predictions and APOGEE DR13 parameters are similar to or better than the di erences between the APOGEE DR13 results and optical high resolution spectral analyses for a subset of benchmark stars. While developed using the APOGEE spectral database (real spectra and corresponding ASSET synthetic data with similar normalization functions), StarNet should be applicable to other large spectroscopic surveys like Pristine. / Graduate

No description available.

We review the recent discoveries of magnetic fields in different types of massive stars and briefly discuss strategies for spectropolarimetric observations to be carried out in the future.

Context. KQ Vel is a peculiar A0p star with a strong surface magnetic field of about 7.5 kG. It has a slow rotational period of nearly 8 years. Bailey et al. (AandA, 575, A115) detected a binary companion of uncertain nature and suggested that it might be a neutron star or a black hole. Aims. We analyze X-ray data obtained by the Chandra telescope to ascertain information about the stellar magnetic field and/or interaction between the star and its companion. Methods. We confirm previous X-ray detections of KQ Vel with a relatively high X-ray luminosity of 2 × 1030 erg s-1. The X-ray spectra suggest the presence of hot gas at > 20 MK and, possibly, of a nonthermal component. The X-ray light curves are variable, but data with better quality are needed to determine a periodicity, if any. Results. We interpret the X-ray spectra as a combination of two components: the nonthermal emission arising from the aurora on the A0p star, and the hot thermal plasma filling the extended shell that surrounds the "propelling"neutron star. Conclusions. We explore various alternatives, but a hybrid model involving the stellar magnetosphere along with a hot shell around the propelling neutron star seems most plausible. We speculate that KQ Vel was originally a triple system and that the Ap star is a merger product. We conclude that KQ Vel is an intermediate-mass binary consisting of a strongly magnetic main-sequence star and a neutron star.