WorldCat Identities

Schultheis, Mathias

Works: 15 works in 21 publications in 2 languages and 28 library holdings
Genres: Conference papers and proceedings 
Roles: Editor, Opponent, Other, Thesis advisor, Author
Publication Timeline
Most widely held works by Mathias Schultheis
Low-mass stars and the transition stars-brown dwarfs by Evry Schatzman School (Conference)( Book )

4 editions published in 2012 in English and held by 11 WorldCat member libraries worldwide

Etude des structures du disque mince Galactique by Chloé Fourtune-Ravard( Book )

3 editions published between 2014 and 2019 in French and held by 3 WorldCat member libraries worldwide

Our Galaxy is to be a barred spiral. Nevertheless, its detailed structure is still unknown and there is ongoing debate on the number and shape of its spiral arms and bar orientation. One of the main tools that has been developped to study our Galaxy is photometry which is the study of stellar light. However, this light is affected by the instellar medium through which it passes between the emission point and the observation point. This effect is called interstellar extinction, and how it affects the light depends on the properties of the environment through which it passes. That is why it is important to have detailed maps of interstellar extinction. Few 3D (position - non cumulative values) extinction maps exist today, and most of them rely on Galactic stellar population models. The goal of my PhD research was to establish new techniques to map the extinction in 3D, independantly of any model. Initialy, I developed an algorithm to find the interstellar extinction in a line of sight, by taking advantage of the properties of Red Clump Stars. I used stars taken from near-infrared observations. In the presence of extinction, Red Clump stars form a tail in colour-magnitude diagrams. If one can detect the position of the tail, one can deduce the distance-extinction relation for this field. By generalising this process to all our data, we build up a 3D map of the insterstellar extinction in the first quadrant. Because of the limitation of the above method, I subsequently used Bayesien deconvolution to develop a method to determine interstellar extinction. The principal benefit of this method is that it is versatile enough that one can also recover the stellar density
Low-mass stars as tracers of the milky way populations : investigating the effects of metallicity in cool atmosphere by Arvind Singh Rajpurohit( Book )

2 editions published in 2013 in English and held by 2 WorldCat member libraries worldwide

Very Low-Mass stars (M dwarfs) are an important source of information for probing the lowmass end of the main sequence, down to the hydrogen burning limit. They are the dominantstellar component of the Galaxy and make up the majority of baryonic matter in the Galaxy.Moreover, an increasing number of M dwarfs are now known to host exoplanets, includingsuper-Earth exoplanets. The determination of accurate fundamental parameters for M dwarfshas therefore relevant implications for both stellar and Galactic astronomy as well as planetology.Despite their large number in the Galaxy, M dwarfs remain elusive objects and themodelling of their photosphere has long remained a challenge (molecular opacities, dust cloudformation). The description of these stars therefore need a strong empirical basis, or validation.In particular, the effect of metallicity on the physics of cool atmospheres are still poorly known,even for early-type M-dwarfs.[...]
Stellar sources in the isogal inner galactic bulge field (l=0°,b= − 1°) by D. K Ojha( )

1 edition published in 2000 in English and held by 2 WorldCat member libraries worldwide

Assembling the puzzle of the Milky Way( Book )

1 edition published in 2012 in English and held by 1 WorldCat member library worldwide

Star formation history of the Galactic bulge from deep HST imaging of low reddening windows( )

1 edition published in 2018 in English and held by 1 WorldCat member library worldwide

Abstract Despite the huge amount of photometric and spectroscopic efforts targeting the Galactic bulge over the past few years, its age distribution remains controversial owing to both the complexity of determining the age of individual stars and the difficult observing conditions. Taking advantage of the recent release of very deep, proper-motion-cleaned colour-magnitude diagrams (CMDs) of four low reddening windows obtained with the Hubble Space Telescope (HST), we used the CMD-fitting technique to calculate the star formation history (SFH) of the bulge at −2° > b > −4° along the minor axis. We find that over 80 per cent of the stars formed before 8 Gyr ago, but that a significant fraction of the super-solar metallicity stars are younger than this age. Considering only the stars that are within reach of the current generation of spectrographs (i.e. $V\lesssim$ 21), we find that 10 per cent of the bulge stars are younger than 5 Gyr, while this fraction rises to 20-25 per cent in the metal-rich peak. The age-metallicity relation is well parametrized by a linear fit, implying an enrichment rate of dZ/dt ∼ 0.005 Gyr. Our metallicity distribution function accurately reproduces that observed by several spectroscopic surveys of Baade's window, with the bulk of stars having metal content in the range [Fe/H]∼−0.7 to ∼0.6, along with a sparse tail to much lower metallicities
AGB stars in the galactic Bulge and DENIS by M Schultheis( Book )

1 edition published in 1998 in English and held by 1 WorldCat member library worldwide

Gaia Data Release 2 observations of solar system objects by F Spoto( )

1 edition published in 2018 in English and held by 1 WorldCat member library worldwide

Context: The Gaia spacecraft of the European Space Agency (ESA) has been securing observations of solar system objects (SSOs) since the beginning of its operations. Data Release 2 (DR2) contains the observations of a selected sample of 14,099 SSOs. These asteroids have been already identified and have been numbered by the Minor Planet Center repository. Positions are provided for each Gaia observation at CCD level. As additional information, complementary to astrometry, the apparent brightness of SSOs in the unfiltered G band is also provided for selected observations. Aims: We explain the processing of SSO data, and describe the criteria we used to select the sample published in Gaia DR2. We then explore the data set to assess its quality. Methods: To exploit the main data product for the solar system in Gaia DR2, which is the epoch astrometry of asteroids, it is necessary to take into account the unusual properties of the uncertainty, as the position information is nearly one-dimensional. When this aspect is handled appropriately, an orbit fit can be obtained with post-fit residuals that are overall consistent with the a-priori error model that was used to define individual values of the astrometric uncertainty. The role of both random and systematic errors is described. The distribution of residuals allowed us to identify possible contaminants in the data set (such as stars). Photometry in the G band was compared to computed values from reference asteroid shapes and to the flux registered at the corresponding epochs by the red and blue photometers (RP and BP). Results: The overall astrometric performance is close to the expectations, with an optimal range of brightness G similar to 12 - 17. In this range, the typical transit-level accuracy is well below 1 mas. For fainter asteroids, the growing photon noise deteriorates the performance. Asteroids brighter than G similar to 12 are affected by a lower performance of the processing of their signals. The dramatic improvement brought by Gaia DR2 astrometry of SSOs is demonstrated by comparisons to the archive data and by preliminary tests on the detection of subtle non-gravitational effects
L'archéologie galactique et son application au centre galactique by Govind Nandakumar( )

1 edition published in 2018 in English and held by 1 WorldCat member library worldwide

L'archéologie galactique consiste à disséquer et analyser les nombreuses composantes de la Voie Lactée afin de mettre en évidence et distinguer les processus physiques qui contribuent à sa formation et son évolution. Ceci est possible grâce à une estimation précise des positions, des vitesses ainsi que des propriétés de l'atmosphère stellaire des étoiles individuelles qui appartiennent aux différents populations stellaires qui composent chacune de ces composantes. De ce fait, ce domaine dépend non seulement d'observations photométriques, astrométriques et spectroscopiques permettant de mesurer en détail les propriétés stellaires mentionnées mais également de modèles théoriques précis afin de les confronter avec les données observationnelles. Au cours de cette thèse, j'ai mené une étude détaillée sur les effets de fonction de sélection sur les abondances métalliques en utilisant des sondages spectroscopiques aux grandes échelles, suivi d'observations spectroscopiques de petites et grandes résolutions sur les parties internes de la Voie Lactée afin de caractériser la nature chimique du bulbe galactique ainsi que le taux de formation stellaire dans la zone centrale moléculaire (CMZ). Avec les présents et futurs grands sondages dédiés à l'archéologie galactique tels que APOGEE, RAVE, LAMOST, GALAH, etc.., il est essentiel de connaître la fonction de sélection spécifique qui est associée à la stratégie de ciblage de chacun de ces sondages. En utilisant des champs communs et des lignes de visée similaires entre APOGEE, LAMOST, GES et RAVE, et tout en considérant des modèles de synthèse de population stellaire, J'ai étudié les effets de fonction de sélection sur la fonction de distribution de la métallicité (MDF) et sur le gradient vertical de métallicité dans le voisinage solaire. Mes résultats indiquent qu'il y a un négligeable effet de fonction de sélection sur la MDF ainsi que sur le gradient vertical de métallicité. Ces résultats suggèrent alors que différents sondages spectroscopiques (de différentes résolutions et de longueurs d'onde) peuvent être combinés dans des études similaires à condition que les métallicité soient placées sur la même échelle. Tandis que de plus en plus d'observations spectroscopiques des régions externes du bulbe de la Voie Lactée révèlent la complexité de sa morphologie, sa cinétique et de sa nature chimique, les études détaillées sur les abondances chimiques de la région interne du bulbe (400-500 pc) font en revanche défaut. Je présenterai alors des spectres de haute résolution dans la bande K d'étoiles géantes K/M issues de cette région obscure et obtenus à partir du spectrographe de haute résolution dans l'infrarouge, CRIRES (R-50,000) situé au VLT. Je discuterai ensuite la MDF et les abondances chimiques détaillées de notre échantillon dans cette région et également la symétrie Nord-Sud dans la MDF le long du petit axe du bulbe. Un enjeu majeur dans les modèles d'évolution chimique est le manque de connaissance vis à vis de l'histoire et du taux de la formation stellaire de la Voie Lactée. La partie centrale de la Voie Lactée (<200 pc), appelée communément la zone centrale moléculaire, possède un grand réservoir de gaz moléculaire avec des indications d'activités de formation stellaire durant les 100 000 dernières années. J'ai utilisé des spectres KMOS (VLT) de petite résolution afin d'identifier et analyser les objects stellaires jeunes et massifs (YSOs) et afin d'estimer le taux de formation stellaire dans la CMZ en utilisant la méthode de contage YSO
A disaggregate model of intercity freight transport demand by V Prins( )

1 edition published in 1987 in English and held by 1 WorldCat member library worldwide

Imagerie à haute résolution des amas R136 et NGC3603 dévoilent la nature de leurs populations stellaires by Zeinab Khorrami( )

1 edition published in 2016 in English and held by 1 WorldCat member library worldwide

Cette thèse a pour objectif de comprendre les différents aspects de l'évolution des amas d'étoiles massives NGC3603 et R136 qui possèdent les étoiles les plus massives connues de l'univers local. L'analyse photométrique des noyaux de R136 et NGC3603 utilisant l'imagerie infrarouge de l'instrument SPHERE sur VLT et son système d'optique adaptative extrême de SPHERE, m'a permis de détecter pour la 1ière fois un grand nombre d'étoiles de faibles masse et luminosité au coeur de ces amas et pour la plupart au voisinage des étoiles les plus lumineuses et massives. La comparaison des données de SPHERE de NGC3603 à celles du HST montre l'absence de ségrégation de masse dans le noyau de cet amas. De plus la pente de la fonction de masse de cette région est la même que celle de la région suivante et similaire aux valeurs de la MF correspondant aux régions extérieures de l'amas connues jusqu'ici. L'amas R136 est partiellement résolu par SPHERE/IRDIS dans l'IR. La majorité de ses étoiles massives ont des compagnons visuels. En prenant compte des mesures spectroscopiques et photométriques et leurs erreurs sur l'extinction et l'âge des membres de l'amas, j'ai estimé une gamme de masse pour chaque étoile identifiée. La MF a été calculée pour différents âges ainsi que les erreurs sur les masses stellaires. J'ai simulé des séries d'images de R136 grâce au code Nbody6, et les ai comparées aux observations du HST/WFPC2. Ces simulations permettent de vérifier l'effet de la binarité initiale des étoiles de l'amas, la ségrégation de masse et l'évolution des étoiles sur l'évolution dynamique propre à R136
Evaluation of the Kigoma rural development project : part I: baseline study( Book )

1 edition published in 1977 in English and held by 1 WorldCat member library worldwide

Indicateurs chimiques d'âge stellaire à l'ère de Gaia by Anastasia Titarenko( )

1 edition published in 2018 in English and held by 1 WorldCat member library worldwide

Stars record the past in their ages, chemical compositions and kinematics. They can provide unprecedented detailed constraints on the early epochs of galaxy formation, back to redshifts greater than two (a look-back time of around 10 billion years). In particular, stellar ages are crucial to the understanding of the Milky Way history and for comparison with galactic evolution models. The advent of the Gaia space mission has opened the path to stellar age estimations for large samples of stars, in particular, based on isochrone fitting methods. In addition, Gaia precise distances allow to develop indirect age estimations based on the stellar population chemical evolution clock. In fact, the chemical abundance patterns imprinted on stellar atmospheres represent the gas conditions at the time of the stars' formation back to redshifts greater than two. The chemical evolution products of different nucleosynthetic channels can therefore provide a time proxy. After calibration, it can be used as an age estimator.This thesis is focussed on the use of a particular chemical clock, the [Y/Mg] abundance. To this purpose, the astrometric Gaia mission data from the first data release was combined with high resolution spectroscopic data from the AMBRE-HARPS catalogue. First of all, the object identification of the AMBRE archival data was improved, thanks to a cross match with the 2MASS catalog, and later the Gaia DR1. In total, 6776 different stars have been identified.Secondly, in order to obtain precise estimations of the [Y/Mg] abundance ratio for galactic disc stars, the automated GAUGUIN tool integrated in the Gaia DPAC APSIS chain, has been optimized and tested. In particular, the abundance estimation capabilities of the APSIS GAUGUIN tool have been improved for irregularly distributed synthetic spectra grids, spanning a large range in stellar atmospheric parameters.Thirdly, the [Y/Mg] abundance ratio has been estimated for about 2000 stars from the AMBRE HARPS spectroscopic data. In addition, the internal and external errors of the abundances were carefully analysed. The studied stars belong mainly to the galactic thin and thick disc, in the metallicity range from --1.0 dex to 0.5 dex.Fourth, thanks to the isochrone fitting age estimations of 342 turn-off stars of the sample, the age sensitivity of the [Y/Mg] ratio has been studied. The analysis reveals a clear correlation between [Y/Mg] and age for thin disk stars of different metallicities, in synergy with previous studies of Solar type stars. In addition, no metallicity dependence with stellar age is detected, allowing to use the [Y/Mg] ratio as a reliable age proxy.Finally, the [Y/Mg] vs. age relation presents a discontinuity between thin and thick disk stars around 9-10 Gyrs. For thick disk stars, the correlation has a different zero point and probably a steeper trend with age, reflecting the different chemical evolution histories of the two disk components
Assembling the puzzle of the Milky Way( )

1 edition published in 2012 in English and held by 0 WorldCat member libraries worldwide

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English (18)

French (3)