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Context. The study of star formation is extremely challenging, due to the lack of complete and clean samples of young nearby clusters and star-forming regions. The recent Gaia DR2 catalogue complemented with the deep ground-based COSMIC DANCe catalogue offers a new database of unprecedented accuracy to revisit the membership of clusters and star-forming regions. The 30 Myr open cluster IC 4665 is one of the few well-known clusters of this age and it is an excellent target where evolutionary models can be tested and planetary formation studied. Aims. We provide a comprehensive membership analysis of IC 4665 and study the following properties: empirical isochrones, distance, magnitude distribution, present-day system mass function, and spatial distribution. Methods. We used the Gaia DR2 catalogue together with the DANCe catalogue to look for members via a probabilistic model of the distribution of the observable quantities in both the cluster and background populations. Results. We obtained a final list of 819 candidate members that cover a 12.4 magnitude range (7 < J < 19.4). We find that 50% are new candidates, and we estimate a conservative contamination rate of 20%. This unique sample of members allows us to obtain a present-day system mass function in the range of 0.02–6 M⊙, which reveals a number of details not seen in previous studies. In addition, we find that a spherically symmetric spatial distribution is favoured by our final list of members for this young open cluster. Conclusions. Our membership analysis represents a significant increase in the quantity and quality (low contamination) with respect to previous studies. It offers an excellent opportunity to revisit other fundamental parameters such as age.

International audience; Evolved Open Clusters (OCs) are excellent tracers of the formation and evolution of the galaxy, as well as an ideal laboratory to test theories of star formation and evolution. In particular, nearby OCs are commonly used as benchmark objects to assess the determination of physical properties of field stars. We have designed a project to perform an in-depth study of the physical properties of a sample of benchmark evolved clusters inside a radius of 500 pc around the Sun. We aim to determine shape, radii, extinction, galactic velocity, age and chemical composition, using recent data from Gaia DR2, and complementary ground-based high resolution spectroscopic data. Here we present the first results of this project after the release of Gaia DR2 concerning the Hyades, the most nearby open cluster. We do a membership selection around a large region from the cluster center. And we perform a spectroscopic analysis of the cluster using HARPS spectra of main sequence stars and red giants. We obtain chemical abundances for 11 species with a typical precision of 0.01 dex.

The evolution of lithium abundance over a star's lifetime is indicative of transport processes operating in the stellar interior. We revisit the relationship between lithium content and rotation rate previously reported for cool dwarfs in the Pleiades cluster. We derive new LiI 670.8 nm equivalent width measurements from high-resolution spectra obtained for low-mass Pleiades members. We combine these new measurements with previously published ones, and use the Kepler/K2 rotational periods recently derived for Pleiades cool dwarfs to investigate the lithium-rotation connection in this 125 Myr-old cluster. The new data confirm the correlation between lithium equivalent width and stellar spin rate for a sample of 51 early K-type members of the cluster, where fast rotating stars are systematically lithium-rich compared to slowly rotating ones. The correlation is valid for all stars over the (J-Ks) color range 0.50-0.70 mag, corresponding to a mass range from about 0.75 to 0.90 solar mass, and may extend down to lower masses. We argue that the dispersion in lithium equivalent widths observed for cool dwarfs in the Pleiades cluster reflects an intrinsic scatter in lithium abundances, and suggest that the physical origin of the lithium dispersion pattern is to be found in the pre-main sequence rotational history of solar-type stars., 10 pages, 8 figures, accepted for publication in Astronomy & Astrophysics

The search for minor bodies in the solar system promises insights into its formation history. Wide imaging surveys offer the opportunity to serendipitously discover and identify these traces of planetary formation and evolution. We aim to present a method to acquire position, photometry, and proper motion measurements of solar system objects in surveys using dithered image sequences. The application of this method on the Kilo-Degree Survey is demonstrated. Optical images of 346 square degree fields of the sky are searched in up to four filters using the AstrOmatic software suite to reduce the pixel to catalog data. The solar system objects within the acquired sources are selected based on a set of criteria depending on their number of observation, motion, and size. The Virtual Observatory SkyBoT tool is used to identify known objects. We observed 20,221 SSO candidates, with an estimated false-positive content of less than 0.05%. Of these SSO candidates, 53.4% are identified by SkyBoT. KiDS can detect previously unknown SSOs because of its depth and coverage at high ecliptic latitude, including parts of the Southern Hemisphere. Thus we expect the large fraction of the 46.6% of unidentified objects to be truly new SSOs. Our method is applicable to a variety of dithered surveys such as DES, LSST, and Euclid. It offers a quick and easy-to-implement search for solar system objects. SkyBoT can then be used to estimate the completeness of the recovered sample., Comment: Final version to be published in A&A. Supplementary data will be published at CDS. 11 pages, 15 figures

We aim at performing a kinematic census of young stellar objects (YSOs) in the Rho Ophiuchi F core and partially in the E core of the L1688 dark cloud. We run a proper motion program at the ESO New Technology Telescope (NTT) with the Son of ISAAC (SOFI) instrument over nine years in the near-infrared. We complemented these observations with various public image databases to enlarge the time base of observations and the field of investigation to 0.5 deg X 0.5 deg. We derived positions and proper motions for 2213 objects. From these, 607 proper motions were derived from SOFI observations with a ~1.8 mas/yr accuracy while the remaining objects were measured only from auxiliary data with a mean precision of about ~3 mas/yr. We performed a kinematic analysis of the most accurate proper motions derived in this work, which allowed us to separate cluster members from field stars and to derive the mean properties of the cluster. From the kinematic analysis we derived a list of 68 members and 14 candidate members, comprising 26 new objects with a high membership probability. These new members are generally fainter than the known ones. We measured a mean proper motion of (mu_RA*, mu_DEC)=(-8.2, -24.3)+/-0.8 mas/yr for the L1688 dark cloud. A supervised classification was applied to photometric data of members to allocate a spectral energy distribution (SED) classification to the unclassified members.} We kinematically confirmed that the 56 members that were known from previous studies of the Rho Ophiuchi F cluster and that were also part of our survey are members of the cluster, and we added 26 new members. We defined the evolutionary status of the unclassified members of the cluster. We showed that a large part (23) of these new members are probably brown dwarfs, which multiplies the number of known substellar objects in the cluster by a factor of 3.3., Comment: 16 pages, 13 figures, 3 tables in electronic form

International audience; The COSMIC-DANCE project is an observational program aiming at understanding the origin and evolution of ultracool objects by measuring the mass function and internal dynamics of young nearby associations down to the fragmentation limit. The least massive members of young nearby associations are identified using modern statistical methods in a multi-dimensional space made of optical and infrared luminosities and colors and proper motions. The photometry and astrometry are obtained by combining ground and in some case space based archival observations with new observations, covering between one and two decades.

Context. Observations of nearby star-forming regions with the Herschel Space Observatory complement our view of the protoplanetary disks in Ophiuchus with information about the outer disks. Aims. The main goal of this project is to provide new far-infrared fluxes for the known disks in the core region of Ophiuchus and to identify potential transitional disks using data from Herschel. Methods. We obtained PACS and SPIRE photometry of previously spectroscopically confirmed young stellar objects (YSO) in the region and analysed their spectral energy distributions. Results. From an initial sample of 261 objects with spectral types in Ophiuchus, we detect 49 disks in at least one Herschel band. We provide new far-infrared fluxes for these objects. One of them is clearly a new transitional disk candidate. Conclusions. The data from Herschel Space Observatory provides fluxes that complement previous infrared data and that we use to identify a new transitional disk candidate., 21 pages, with 5 figures. Accepted in Astronomy & Astrophysics

From a set of adaptive optics (AO) observations collected with the W.M. Keck telescope between August and September 2009, we derived the orbital parameters of the most recently discovered satellites of the large C-type asteroid (93) Minerva. The satellites of Minerva, which are approximately 3 and 4 km in diameter, orbit very close to the primary $\sim$5 & $\sim$8 $\times$ Rp and $\sim$1% & $\sim$2% $\times$ RHill) in a circular manner, sharing common characteristics with most of the triple asteroid systems in the main-belt. Combining these AO observations with lightcurve data collected since 1980 and two stellar occultations in 2010 & 2011, we removed the ambiguity of the pole solution of Minerva's primary and showed that it has an almost regular shape with an equivalent diameter Deq = 154 $\pm$ 6 km in agreement with IRAS observations. The surprisingly high bulk density of 1.75 $\pm$ 0.30 g/cm$\^3$ for this C-type asteroid, suggests that this taxonomic class is composed of asteroids with different compositions, For instance, Minerva could be made of the same material as dry CR, CO, and CV meteorites. We discuss possible scenarios on the origin of the system and conclude that future observations may shine light on the nature and composition of this fifth known triple main-belt asteroid., 35 pages, 9 figures, 5 tables

The Z CMa binary is understood to undergo both FU Orionis (FUOR) and EX Orionis (EXOR) type outbursts. While the SE component has been spectro- scopically identified as an FUOR, the NW component, a Herbig Be star, is the source of the EXOR outbursts. The system has been identified as the source of a large outflow, however, previous studies have failed to identify the driver. Here we present adaptive optics (AO) assisted [FeII] spectro-images which reveal for the first time the presence of two jets. Observations made using OSIRIS at the Keck Observatory show the Herbig Be star to be the source of the parsec-scale outflow, which within 2'' of the source shows signs of wiggling and the FUOR to '' be driving a ~ 0.4 jet. The wiggling of the Herbig Be star's jet is evidence for an additional companion which could in fact be generating the EXOR outbursts, the last of which began in 2008 (Grankin & Artemenko 2009). Indeed the dy- namical scale of the wiggling corresponds to a time-scale of 4-8 years which is in agreement with the time-scale of these outbursts. The spectro-images also show a bow-shock shaped feature and possible associated knots. The origin of this structure is as of yet unclear. Finally interesting low velocity structure is also observed. One possibility is that it originates in a wide-angle outflow launched from a circumbinary disk., accepted by ApJ letters

Par-Lup3-4 is a very low-mass star (spectral type M5) in the Lupus III star-forming region. The object is underluminous by ~4 mag when compared to objects of similar mass in the same association. To better understand the origin of its underluminosity, we have analyzed high angular resolution near-IR imaging data and mid-IR spectroscopy. We have also compared the SED of the target (from the optical to the sub-millimeter regime) to a grid of radiative transfer models of circumstellar disks. The diffraction-limited infrared observations do not show obvious extended emission, allowing us to put an upper limit to the disk outer radius of ~20AU. The lack of extended emission, together with the non detection of a strong 9.8 microns silicate in absorption indicates that Par-Lup3-4 is probably in a Class II (rather than Class I) evolutionary stage. The SED of Par-Lup3-4 resembles that of objects with edge-on disks seen in scattered light, that is, a double peaked-SED and a dip at ~10 microns. We can fit the whole SED with a single disk model with an inclination of 81+/-6 degrees which provides a natural explanation for the under-luminosity of the target. Our analysis allows to put constraints on the disk inner radius, Rin < 0.05 AU, which is very close to the dust sublimation radius, and the maximum size of the dust grains, a_max > 10 microns, which indicates that dust processing has already taken place in Par-Lup3-4., 9 pages, 6 figures, accepted for publication in A&A

HIP96515A is a double-lined spectroscopic binary with a visual companion (HIP96515B) at 8.6 arcsec. It is included in the SACY catalog as a potential young star and classified as an eclipsing binary in the ASAS Catalog. We have analyzed spectroscopic and photometric observations of the triple system. The high-resolution optical spectrum of HIP96515A has been used to derive a mass ratio, M_2/M_1, close to 0.9, with the SB2 components showing spectral types of M1 and M2. The ASAS and Hipparcos light-curves of HIP96515A show periodic variations with P=2.3456 days, confirming that HIP96515A is an eclipsing binary with preliminary parameters of i=89, M_Aa=0.59+-0.03 Msun and M_Ab=0.54+-0.03 Msun, for the primary and secondary, respectively, at an estimated distance of 42+-3 pc. This is a new eclipsing binary with component masses below 0.6 Msun. Multi-epoch observations of HIP 96515 A&B show that the system is a common proper motion pair. The optical spectrum of HIP 96515B is consistent with a pure helium atmosphere (DB) white dwarf. We estimate a total age (main-sequence lifetime plus cooling age) of 400 Myr for the white dwarf. If HIP 96515 A&B are coeval, and assuming a common age of 400 Myr, the comparison of the masses of the eclipsing binary members with evolutionary tracks shows that they are underestimated by ~15% and ~10%, for the primary and secondary, respectively., Accepted for publication in A&A, 9 pages, 9 figures

In order to improve our understanding of substellar formation, we have performed a compositional and structural study of a brown dwarf disk. We present the result of photometric, spectroscopic and imaging observations of 2MASS J04442713+2512164, a young brown dwarf (M7.25) member of the Taurus association. Our dataset, combined with results from the literature, provides a complete coverage of the spectral energy distribution from the optical to the millimeter including the first photometric measurement of a brown dwarf disk at 3.7mm, and allows us to perform a detailed analysis of the disk properties. The target was known to have a disk. High resolution optical spectroscopy shows that it is intensely accreting, and powers a jet and an outflow. The disk structure is similar to that observed for more massive TTauri stars. Spectral decomposition models of Spitzer/IRS spectra suggest that the mid-infrared emission from the optically thin disk layers is dominated by grains with intermediate sizes (1.5micron). Crystalline silicates are significantly more abondant in the outer part and/or deeper layers of the disk, implying very efficient mixing and/or additional annealing processes. Sub-millimeter and millimeter data indicate that most of the disk mass is in large grains (>1mm), 17 pages, 10 figures, 7 tables, accepted for A&A

We present the results of a high-resolution imaging survey for brown dwarf binaries in the Pleiades open cluster. The observations were carried out with the Advance Camera for Surveys onboard the Hubble Space Telescope. Our sample consists of 15 bona-fide brown dwarfs. We confirm 2 binaries and detect their orbital motion, but we did not resolve any new binary candidates in the separation range between 5.4AU and 1700AU and masses in the range 0.035--0.065~Msun. Together with the results of our previous study (Martin et al., 2003), we can derive a visual binary frequency of 13.3$^{+13.7}_{-4.3}$\% for separations greater than 7~AU masses between 0.055--0.065~M$_{\sun}$ and mass ratios between 0.45--0.9$

The star formation process usually proceeds with protoplanetary disks. These disks contain a mixture of gas, accounting for 99 % of the disk mass, and of solid particles called dust grains (1 % of the disk mass). These grains, initially at sub-micro metric sizes, gradually coagulate, grow, and potentially allow for the formation of planets about the star.The study of the dust and molecular composition of young disks is fundamental to constraint the physical and chemical initial conditions of planetary formation and the origins of the chemical composition of the planetary cores.The goal of this thesis was to build state-of-the-art models of typical young disks consisting of gas and of a population of grains of multiple sizes, then, in a new approach, to test with the use of numerical simulations the implication of the size and temperature distributions on the chemical evolution of disks.To achieve this, we have coupled the 3D Monte-Carlo radiative transfer code POLARIS to the time-dependent gas-grain code NAUTILUS. The radiative transfer code allowed us to finely compute the grain temperature as a function of the size and location as well as the UV flux within the disk. The gas-grain code was able to simulate the evolution of the chemical abundances in our disk models. Moreover, the computation of the UV flux by POLARIS coupled to a set of molecular cross-sections extracted from a comprehensive database allowed us to compute as a function of the frequency the rates of molecular photoabsorption, photodissociation, and photoionization., La formation des étoiles s’accompagne généralement de la formation d'undisque où peuvent se former les planètes. Ces disques protoplanétaires contiennent un mélange de 99 % de gaz et de 1 % de particules solides appelées grains de poussière. Initialement de taille sub-micrométrique, ces grains vont progressivement s’agglomérer, grossir, et potentiellement permettre la formation de planètes autour de l’étoile.L’étude de la composition en molécules et en grains des disques jeunes est fondamentale pour contraindre les conditions physico-chimiques initiales de la formation planétaire et l’origine de la composition chimique des planètes.L’objectif de la thèse a été de construire des modèles sophistiqués de disques jeunes typiques constitués de gaz et d’une population de grains de différentes tailles puis, de manière inédite, de tester par simulations numériques l’implication de cette distribution en taille et en température sur l’évolution chimique.Pour ce faire, nous avons couplé le code de transfert radiatif 3D Monte-Carlo POLARIS au code de simulation gaz-grain dépendant du temps NAUTILUS. Le code de transfert radiatif nous a permis de calculer finement la température des grains en fonction de leur taille et de leur position ainsi que le flux UV au sein du disque. Le code gaz-grain, quant à lui, a ensuite pu simuler l’évolution des abondances chimiques dans nos modèles de disques. De plus, le calcul du flux UV effectué par POLARIS couplé à l’utilisation de section efficaces moléculaires provenant de bases de données a permis le calcul en fonction de la fréquence des taux de photoabsorption, de photodissociation et de photoionisation des molécules.

Extracting reliable source catalogs from images is crucial for a broad range of astronomical research topics.However, the efficiency of current source detection methods becomes severely limited in crowded fields, or when images are contaminated by optical, electronic and environmental defects.Performance in terms of reliability and completeness is now often insufficient with regard to the scientific requirements of large imaging surveys.In this thesis, we develop new methods to produce more robust and reliable source catalogs.We leverage recent advances in deep supervised learning to design generic and reliable models based on convolutional neural networks (CNNs).We present MaxiMask and MaxiTrack, two convolutional neural networks that we trained to automatically identify 13 different types of image defects in astronomical exposures.We also introduce a prototype of a multi-scale CNN-based source detector robust to image defects, which we show to significantly outperform existing algorithms.We discuss the current limitations and potential improvements of our approach in the scope of forthcoming large scale surveys such as Euclid., L'extraction de catalogues de sources fiables à partir des images est cruciale pour un large éventail de recherches en astronomie.Cependant, l'efficacité des méthodes de détection de source actuelles est sérieusement limitée dans les champs encombrés, ou lorsque les images sont contaminées par des défauts optiques, électroniques et environnementaux.Les performances en termes de fiabilité et de complétude sont aujourd'hui souvent insuffisantes au regard des exigences scientifiques des grands relevés d'imagerie.Dans cette thèse, nous développons de nouvelles méthodes pour produire des catalogues sources plus robustes et fiables.Nous tirons parti des progrès récents en apprentissage supervisé profond pour concevoir des modèles génériques et fiables basés sur des réseaux de neurones à convolutions (CNNs).Nous présentons MaxiMask et MaxiTrack, deux réseaux de neurones à convolutions que nous avons entrainés pour identifier automatiquement 13 types différents de défauts d'image dans des expositions astronomiques.Nous présentons également un prototype de détecteur de sources multi-échelle et robuste vis-à-vis des défauts d'image, dont nous montrons qu'il surpasse largement les algorithmes existants en terme de performances.Nous discutons des limites actuelles et des améliorations potentielles de notre approche dans le cadre des prochains grands relevés tels que Euclid.

Understanding how stars form is one of the fundamental questions which astronomy aims to answer. Currently, it is well accepted that the majority of stars form in groups and that their predominant mechanism of formation is the core-collapse. However, several mechanisms have been suggested to explain the formation of substellar objects, and their contribution is still under debate. The main goal of this thesis is to determine the initial mass function, the mass distribution of stars at birth time, in different associations and star-forming regions. The mass function constitutes a fundamental observational parameter to constrain stellar and substellar formation theories since different formation mechanisms predict different fraction of stellar and substellar objects. We used the Gaia Data Release 2 catalogue together with ground-based observations from the COSMIC-DANCe project to look for high probability members via a probabilistic model of the distribution of the observable quantities in both the cluster and background populations. We applied this method to the 30 Myr open cluster IC 4665 and the 1 - 10 Myr star-forming region Upper Scorpius (USC) and r Ophiuchi (r Oph). We found very rich populations of substellar objects which largely exceed the numbers predicted by core-collapse models. In USC, where our sensitivity is best, we found a large number of free-floating planets and we suggest that ejection from planetary systems must have a similar contribution than core-collapse in their formation. The age is a fundamental parameter to study the formation and evolution of stars and is essential to accurately convert luminosities to masses. For that, we also presented a strategy to study the dynamical traceback age of young local associations through an orbital traceback analysis. We applied this method to determine the age of the b Pictoris moving group and in the future, we plan to apply it to other regions such as USC. The members we identified with the membership analysis are excellent targets for follow-up studies such as a search for discs, exoplanets, characterisation of brown dwarfs and free-floating planets. I this thesis, we presented a search for discs hosted by members of IC 4665 and we found six excellent candidates to be imaged with ALMA or the JWST. The tools we developed, are ready to be used in other regions such as USC and r Oph, where we expect to find a larger number of disc-host stars., Comprendre comment se forment les étoiles est l’une des questions fondamentales auxquelles l’astronomie entend répondre. Malheureusement, nous ne pouvons pas étudier la formation stellaire en temps réel et différentes méthodes indirectes doivent être utilisées pour faire la lumière sur ce sujet. L’objectif principal de cette thèse est de déterminer la fonction de masse initiale, la distribution de masse des étoiles à leur naissance, dans différentes associations et régions de formation d’étoiles. La fonction de masse est le produit direct de la formation stellaire et constitue donc un paramètre d’observation fondamental pour contraindre les théories de formation stellaire et sous-estellaire. Nous nous sommes concentrés sur l’amas ouvert de 30 Ma IC 4665 et la région de formation d’étoiles de 1 - 10 Ma de Upper Scorpius (USC) et r Ophiuchi (r Oph). Nous avons combiné l’astrométrie et la photométrie de Gaia Data Release 2 avec nos observations au sol pour préparer un catalogue profond et étendu de chaque région. Ensuite, nous avons calculé les probabilités d’appartenance en utilisanttoute l’astrométrie et la photométrie disponibles et identifié les membres à haute probabilité. Nous avons utilisé la liste finale des membres pour estimer la distribution de magnitude, et les fonctions de luminosité et masse de ces associations. Alors que la première a l’avantage d’être indépendante des modèles d’évolution, tandis que les fonctions de luminosité et de masse peuvent être utilisées pour contraindre les mécanismes de formation d’étoiles. L’étude d’IC 4556 nous a permis d’identifier des objets sous-stellaires, sans pour autant pouvoir fournir un recensement complet dans ce domaine de masse. Dans USC et r Oph, nous avons identifié une population très riche d’objets sous-stellaires, significativement plus nombreux que les prédictions des modèles de formation par effondrement de coeurs moléculaires, suggérant que la formation de naines brunes et d’objets de masses planétaires isolés par des phénomènes d’éjection dans des systèmes planétaires a une contribution importante et du même ordre que l’effondrement des coeurs moléculaires à la population finale d’objets dans un amas. L’âge est un paramètre fondamental pour étudier la formation et l’évolution des étoiles pour plusieurs raisons: premièrement puisqu’il établit une échelle de temps sur laquelle placer les observations. Deuxièmement car il est essentiel pour convertir les luminosités en masses, avec l’aide de modèles d’évolution stellaire. Les incertitudes sur l’age de USC et r Oph se traduisant en erreurs importantes dans notre estimation de la fonction de masse, j’ai développé une stratégie d’étude de "l’âge dynamique" au moyen d’une analyse orbitale de traçage des mouvements des membres d’associations jeunes. J’ai ainsi mis au point une stratégie incluant i) les observations et la recherche de données dans les archives publiques, ii) la réduction et l’analyse des spectres échelles obtenus; iii) et l’analyse dynamique, pour déterminer l’âge d’une association. La méthodologie, développée avec l’association b Pictoris (b Pic), est prête à être appliquée à d’autres régions et en particulier à USC et r Oph. Les membres que nous avons identifiés sont par ailleurs d’excellentes cibles pour des études complémentaires telles que la recherche de disques (produit également fondamental de la formation stellaire), d’exoplanètes, de système multiples, mais aussi pour la caractérisation des atmosphères et propriétés physiques des naines brunes et des planètes errantes. [...]