OTS44 is one of only four free-floating planets known to have a disk. We have previously shown that it is the coolest and least massive known free-floating planet (similar to 12 M-Jup) with a substantial disk that is actively accreting. We have obtained Band 6 (233 GHz) ALMA continuum data of this very young disk-bearing object. The data show a clear unresolved detection of the source. We obtained disk-mass estimates via empirical correlations derived for young, higher-mass, central (substellar) objects. The range of values obtained are between 0.07 and 0.63 M-circle plus (dust masses). We compare the properties of this unique disk with those recently reported around higher-mass (brown dwarfs) young objects in order to infer constraints on its mechanism of formation. While extreme assumptions on dust temperature yield disk-mass values that could slightly diverge from the general trends found for more massive brown dwarfs, a range of sensible values provide disk masses compatible with a unique scaling relation between M-dust and M* through the substellar domain down to planetary masses.

The first transiting planetesimal orbiting a white dwarf was recently detected in K2 data of WD 1145+017 and has been followed up intensively. The multiple, long and variable transits suggest the transiting objects are dust clouds, probably produced by a disintegrating asteroid. In addition, the system contains circumstellar gas, evident by broad absorption lines, mostly in the u' band, and a dust disc, indicated by an infrared excess. Here we present the first detection of a change in colour of WD 1145+017 during transits, using simultaneous multiband fast-photometry ULTRACAM measurements over the u'g'r'i' bands. The observations reveal what appears to be 'bluing' during transits; transits are deeper in the redder bands, with a u' - r' colour difference of up to similar to-0.05 mag. We explore various possible explanations for the bluing, including limb darkening or peculiar dust properties. 'Spectral' photometry obtained by integrating over bandpasses in the spectroscopic data in and out of transit, compared to the photometric data, shows that the observed colour difference is most likely the result of reduced circumstellar absorption in the spectrum during transits. This indicates that the transiting objects and the gas share the same line of sight and that the gas covers the white dwarf only partially, as would be expected if the gas, the transiting debris and the dust emitting the infrared excess are part of the same general disc structure (although possibly at different radii). In addition, we present the results of a week-long monitoring campaign of the system using a global network of telescopes.

We present the discovery of a brown dwarf companion to the debris disk host star HR 2562. This object, discovered with the Gemini Planet Imager (GPI), has a projected separation of 20.3 +/- 0.3 au (0".618 +/- 0".004) from the star. With the high astrometric precision afforded by GPI, we have confirmed, to more than 5 sigma, the common proper motion of HR 2562B with the star, with only a month-long time baseline between observations. Spectral data in the J-, H-, and K-bands show a morphological similarity to L/T transition objects. We assign a spectral type of L7 +/- 3 to HR 2562B. and derive a luminosity of log(L-bol/L-circle dot) = -4.62 +/- 0.12, corresponding to a mass of 30 +/- 15 M-Jup from evolutionary models at an estimated age of the system of 300-900 Myr. Although the uncertainty in the age of the host star is significant, the spectra and photometry exhibit several indications of youth for HR 2562B. The source has a position angle that is consistent with an orbit in the same plane as the debris disk recently resolved with Herschel. Additionally, it appears to be interior to the debris disk. Though the extent of the inner hole is currently too uncertain to place limits on the mass of HR 2562B, future observations of the disk with higher spatial resolution may be able to provide mass constraints. This is the first brown-dwarf-mass object found to reside in the inner hole of a debris disk, offering the opportunity to search for evidence of formation above the deuterium burning limit in a circumstellar disk.

Estrelas anãs brancas são o último estágio evolutivo observável de mais de 95% das estrelas e também um resultado comum na evolução de estrelas binárias. O estudo de anãs brancas é, portanto, uma ferramenta poderosa na compreensão da evolução de estrelas simples e binárias, da função de massa inicial local, e da perda de massa após a sequência principal, levando-nos a uma melhor compreensão do histórico de formação e evolução estelar de diferentes populações. Para que esses estudos sejam possíveis, é necessária uma amostra grande e preferencialmente completa de anãs brancas, cobrindo todo o intervalo de parâmetros físicos. A maneira mais simples de obter isso é utilizando dados de grandes projetos de mapeamento. O Sloan Digital Sky Survey já permitiu o aumento do número de anãs brancas conhecidas em cinco vezes até o data release 10. Neste trabalho, estendemos a busca por anãs brancas aos novos objetos no data release 12, descobrindo 3 157 novas anãs brancas e 1 349 novas subanãs. Pela primeira vez, estendemos essa busca para log g < 6.5, correspondente a M < 0.3 M⊙. Anãs brancas abaixo desse limite de massa não podem formar-se em um tempo de Hubble; contudo, se a estrela é parte de um sistema de binárias próximas, a perda de massa pode ser tão intensa que a anã branca resultante tem massa inferior ao limite por evolução simples. Esses objetos são chamados anãs brancas de massa extremamente-baixa (ELMs, do inglês extremely-low mass white dwarfs) Elas têm Teff < 20 000 K e 5.0 . log g . 6.5 e espectros muito similares a estrelas A de sequência principal. Menos de cem são conhecidas, e a maioria dos objetos foi descoberta tendo em conta um critério de seleção tendencioso, que excluiu ELMs frias (Teff < 9 000 K) e com mais baixa massa (M . 0.15 M⊙), tornando difícil verificar modelos e compreender as propriedades das ELMs como classe. Nós identificamos milhares de objetos cujas propriedades físicas, temperatura efetiva e log g, estão no intervalo de produtos de evolução binária, como as ELMs. Nós os chamamos de sdAs, referindo-nos ao seu log g que as coloca abaixo da sequência principal e seu espectro dominado por hidrogênio. As sdAs parecem conter populações estelares sobrepostas, e encontramos que 7% são mais compatíveis com (pre-)ELMs do que com objetos de sequência principal. Obtivemos espectroscopia resolvida temporalmente para 26 objetos e pudemos confirmar que 15 estão em binárias. Um objeto faz parte de um sistema eclipsante, enquanto outro é pulsante — o oitavo membro da classe de ELMs pulsantes. Outros seis objetos também apresentaram pulsações em nossas observações, cinco desses estão na vizinhança da faixa de instabilidade das ELMs. Com estes resultados, aumentamos a população de ELMs por um fator de 20%, elevando a fração de ELMs de 4 para 20%, um valor que é consistente com as previsões de modelos evolucionários. / White dwarf stars are the final observable evolutionary state of over 95% of stars and also a common outcome in binary evolution. Therefore, studying white dwarfs is a powerful tool to understand both single and binary stellar evolution, local initial mass function, and post-main sequence mass loss, leading us to a better comprehension of the history of stellar formation and evolution of different stellar populations. In order to make this type of studies possible, a large and preferably complete sample of white dwarf stars, covering the whole range of physical parameters, is required. The simplest way to achieve that is to take advantage of data provided by large surveys. The Sloan Digital Sky Survey has already allowed the increase of the number of known white dwarf stars fivefold up to its data release 10. In this work, we extended the search for white dwarfs to the new objects in the data release 12, discovering 3 157 new white dwarfs and 1 349 new subdwarfs. For the first time, we have extended this search to log g < 6.5, corresponding to M < 0.3 M⊙. White dwarfs below this mass limit cannot be formed through single evolution within a Hubble time; however, if the star is part of a close binary system, the mass loss of the system may be so intense that the resulting white dwarf has mass below the single evolution limit. These objects are known as extremely-low mass white dwarfs (ELMs) They show Teff < 20 000 K and 5.0 . log g . 6.5 and spectra very similar to main sequence A stars. Less than a hundred of them are known, and most objects were discovered relying on biased selection criteria, that excluded cool (Teff < 9 000 K), lowermass (M . 0.15 M⊙) ELMs, making it difficult to validate the models and comprehend the properties of the ELMs as a class. We have identified thousands of objects whose physical properties, effective temperature and surface gravity, place them in the range of by-products of binary interaction such as the ELMs. We have called them sdAs, referring to their sub-main sequence log g and hydrogen dominated spectra. They seem to be composed of overlapping stellar populations, and we found that at least 7% are more likely ELMs or their precursors, the pre-ELMs, than main sequence stars. Obtaining time-resolved spectroscopy for 26 objects, we could confirm 15 to be in close binaries. One of them is also an eclipsing system, while another is a pulsator — the eighth member of the pulsating ELM class. Other six new pulsators were found as part of our follow-up, five of them in the vicinity of the ELM instability strip. With these results, we increase the population of ELMs by 20%, raising the fraction of cool ELMs from 4 to 20%, which is consistent with the predictions from the evolutionary models.

No description available.

Brown dwarfs are substellar objects that fall in-between the smallest stars and largest planets in size and temperature. Due to their relatively cool temperatures, the atmospheres of these 'failed stars' have been shown to exhibit interesting properties such as iron, silicate, and salt clouds. Theoretical atmospheric models based on known physics and chemistry can be used as tools to interpret and understand our observations of brown dwarfs. I have fit archival and new infrared spectra of over 300 brown dwarfs with atmospheric models. Using the parameters of the best-fit models as estimates for the physical properties of the brown dwarfs in my sample, I have performed a survey of how brown dwarfs evolve with spectral type and temperature. I present my fit results and observed trends. I use these fit results to note where current atmospheric models are able to well-replicate the data and where the models and data conflict.

M dwarfs have been the subject of renewed interest as potential habitable planet hosts and have increasingly become the targets of planet detection surveys. Currently, however, the number of detections of transiting M dwarf planets remain low. The characterisation of M dwarf activity is an important consideration for such surveys, and provides constraints on the modelling of magnetically active low mass stars. Currently the spottedness of M dwarfs is not well understood owing to their intrinsic faintness and the lack of diagnostics for assessing starspot morphologies and distributions. The WFCAM Transit Survey (WTS) contains long term observations of M dwarfs in the near infra-red and presents an opportunity to study the long term variability of M dwarfs. The M dwarfs in the WTS are identified by use of colour-spectral type relations, and the periodically variable M dwarfs in the sample are detected using a Lomb-Scargle periodogram analysis. A total of 72 periodically variable M dwarfs are found with periods ranging from 0.16 to 90.33 days. The relations between the spectral subtypes, amplitudes and periods are studied and comparisons to earlier works studying M dwarf rotation are made. A number of examples of significant spot evolution are found, which exhibit complex light curve morphologies that vary in form and amplitude over periods of months to years. This provides an indication as to the nature of the spottedness of these stars. Simulations are performed to probe the connection between spot coverage, temperature and light curve amplitude. Using the results from these simulations, the spot coverage fractions of the WTS M dwarfs are estimated and they are found to be heavily spotted. Dynamic models with spots evolving at various average rates are used to explore how spot evolution can drive increased dispersion in the light curves, and to what extent this affects the detectability of periodicity by the method used. It is found that spot evolution can invoke significant noise in an M dwarf light curve, and in combination with photon noise, can in some instances inhibit the detection of a period. In reflection of the results, the relation between the light curve dispersion and spot coverage of the WTS M dwarfs is considered and it is found that more heavily spotted M dwarfs have intrinsically noisier light curves. The morphologies of the light curves produced by the simulations, and the manner in which they evolve, are qualitatively similar to the real M dwarfs in the WTS sample and indicate how models extrapolated from sunspot distributions can explain behaviour seen in active M dwarf light curves.

HD 11112 is an old, Sun-like star that has a long-term radial velocity (RV) trend indicative of a massive companion on a wide orbit. Here we present direct images of the source responsible for the trend using the Magellan Adaptive Optics system. We detect the object (HD 11112B) at a separation of 2 2 (100 au) at multiple wavelengths spanning 0.6-4 mu m. and show that it is most likely a gravitationally bound cool white dwarf. Modeling its spectral energy distribution suggests that its mass is 0.9-1.1M(circle dot), which corresponds to very high eccentricity, near edge-on orbits from a. Markov chain Monte Carlo analysis of the RV and imaging data together. The total age of the white dwarf is > 2 sigma, which is discrepant with that of the primary star under most assumptions. The problem can be resolved if the white dwarf progenitor was initially a double white dwarf binary that then merged into the observed high-mass white dwarf. HD 11112B is a unique and intriguing benchmark object that can be used to calibrate atmospheric and evolutionary models of cool white dwarfs and should thus continue to be monitored by RV and direct imaging over the coming years.

Les naines brunes sont des objets de masse intermédiaire entre celle nécessaire pour former une étoile et celle d'une planète. Les naines brunes sont classées, des plus chaudes aux plus froides, en types spectraux L, T et Y, caractérisés par une couleur J-K moyenne qui varie de 1.2 à 1.8 pour les étoiles de type L0 à L8, et de 1.8 à -0.5 pour les étoiles de type L8 à T8. Par ailleurs, la couleur J-K de certains types spectraux présente une dispersion de l'ordre d'une magnitude. Ce travail tente de faire la lumière sur la nature de cette grande dispersion, présente dans la couleur J-K des naines brunes de type L2. Les observations ont été réalisées avec la caméra infrarouge CPAPIR à l'Observatoire du Mont Mégantic. Nous avons ciblé un total de 22 naines brunes qui ont été observées en K, et 12 parmi celles-ci ont aussi été observées en J. Chacune des naines brunes a été calibrée à l'aide d'une étoile standard, ce qui rend nos résultats indépendants des données 2MASS. Nous observons une corrélation entre les couleurs J-K de nos données et de celles de 2MASS. Cela montre que la grande dispersion en J-K de nos données et des données 2MASS est due aux propriétés physiques des naines brunes et non à des erreurs observationnelles. L'examen des facteurs qui pourraient être responsables de cette grande dispersion, soit la classification spectrale, la métallicité, la gravité de surface, une binarité non résolue, la présence de nuages de condensats et la rotation, montre que la gravité de surface serait le facteur le plus susceptible d'être responsable de la grande dispersion des valeurs de J-K. / Brown dwarfs are objects with a mass intermediate between that required to form a star and that of a planet. Brown dwarfs are classified, from higher to lower temperature, under spectral types L, T and Y, caracterized by a J-K average color that varies from 1.2 to 1.8 for types L0 to L8, and from 1.8 to -0.5 for types L8 to T8. Furthermore, the J-K color of some spectral types presents more than a magnitude of dispersion. This study attempts to explain the large dispersion of the J-K color of the type L2 brown dwarfs. Observations were made with the infrared camera CPAPIR at the Observatoire du Mont Mégantic. We targeted a total of 22 brown dwarfs that were observed in the K band, and 12 among them were also observed in the J band. Each brown dwarf was calibrated with a standard star, which makes our data independent from those of 2MASS. We observe a correlation between the J-K colors obtained from our data and those from 2MASS. This shows that the large J-K dispersion in the data is due to brown dwarf physical properties and not to observational errors. Consideration of the factors that could be responsible for this large dispersion, namely the spectral classification, the metallicity, the surface gravity, an unresolved binarity, the presence of clouds and rotation, shows that surface gravity is the factor most likely to explain the large J-K color dispersion.

L'outil développé dans le cadre de cette thèse est disponible à l'adresse suivante: www.astro.umontreal.ca/~malo/banyan.php / Près de 70% des étoiles de la Galaxie ont une masse inférieure à ~0.8 Msun. Cependant, étant donné que ces étoiles sont plus difficilement observables en raison de leur plus faible luminosité, cette statistique ne reflète pas le recensement actuel de la population d'étoiles de faible masse dans le voisinage solaire, ni dans les groupes cinématiques d'étoiles jeunes. Cette population a une grande importance pour contraindre la forme de la fonction de masse Galactique, et aussi pour contraindre les modèles évolutifs. Les étoiles de faible masse sont aussi d'excellentes cibles pour la recherche d'exoplanètes avec des techniques variées (imagerie directe, vitesse radiale, transit). La caractérisation des exoplanètes autour de ces étoiles est tributaire des connaissances fondamentales sur celles-ci, c'est-à-dire de leur luminosité bolométrique, température effective, rayon et âge. Dans la présente thèse, dont le but est d'identifier et caractériser les étoiles de faible masse, une méthode statistique a été développée afin d'établir quantitativement l'appartenance d'une étoile à un groupe en dérivant une probabilité d'association. Cette méthode combine l'inférence Bayesienne et des modèles empiriques de plusieurs observables, dont la luminosité, vitesse spatiale et position galactique, de membres confirmés de 7 groupes d'étoiles jeunes (8-120 Mans) ainsi que d'étoiles vieilles du champ. Les étoiles ayant une probabilité d'association minimale de 90% sont considérées comme des candidates. L'analyse développée prédit aussi la vitesse radiale et la distance trigonométrique qu'une étoile aurait dans une association donnée. L'analyse a montré, pour les 177 membres confirmés, un excellent accord entre les paramètres prédits et observés, soit de 1.9 km/s et 10% respectivement, pour la vitesse radiale et la parallaxe. La mesure de ces paramètres pour les candidates est donc une bonne manière de confirmer leur appartenance à l'association. Cette méthode robuste a été appliquée sur un échantillon de 758 étoiles montrant des signes de jeunesse (émission H$\alpha$ et rayons X). L'analyse a permis d'identifier 214 candidates hautement probables, et le suivi spectroscopique de ces étoiles a permis, jusqu'à présent, de confirmer la justesse de la prédiction en vitesse radiale pour 130 étoiles. Ces observations spectroscopiques ont aussi permis de mesurer leur vitesse de rotation, qui s'est avérée élevée comparativement aux étoiles vieilles du champs. La mesure de la distance trigonométrique était aussi en accord avec la prédiction pour 18 candidates jeunes. Grâce aux membres dont l'appartenance à un groupe jeune a été confirmée, un modèle empirique de la luminosité en rayon X des étoiles a pu être établi. Cette luminosité s'est avérée significativement plus élevée (environ 4 fois plus) pour les étoiles des groupes les plus jeunes (~8-12 Mans) que pour celles des groupes plus vieux (~120 Mans). Cet observable constitue donc un bon indicateur d'âge. La comparaison des spectres de 59 candidates à des modèles d'atmosphère a permis de déterminer trois paramètres fondamentaux: la luminosité bolométrique, la température effective et le rayon. Globalement, les candidates jeunes ont une luminosité plus élevée et un rayon plus grand que les étoiles vieilles. De récents modèles évolutifs incluant le traitement d'une dynamo de type rotationnel et générant un champ magnétique de surface de 1 à 2.5 kGauss ont été utilisés pour déterminer l'âge isochronal de ces étoiles. Les âges ainsi déterminés pour les étoiles de l'association \beta Pictoris en utilisant des étoiles de types spectraux différents sont davantage cohérents (types K5V-M0V: 24 Mans, types M1V-M4V: 14 Mans) et sont aussi cohérents avec l'âge déterminé indépendamment pour le groupe en utilisant l'abondance du lithium des membres de faible masse (26 Mans). / About 70% of the stars in the Galaxy have a mass inferior than ~0.8 Msun. However, this statistic does not reflect the current census population of low mass in the solar neighborhood and in young kinematic groups, since their low luminosity make their observation more difficult. This population is of great interest to check the validity of the Galactic mass function, and also to constraint evolutionary models. The low-mass stars are also excellent targets for the search for exoplanets using various techniques (direct imaging, radial velocity, transit). The characterization of the exoplanets orbiting these stars depends mostly on our basic knowledge of the host star, that is their bolometric luminosity, effective temperature, radius and age. The present thesis aim to identify and characterize low-mass stars. Toward that end, a statistical method has been developed to determine quantitatively the membership probability of a star to a young kinematic group. This method combines the Bayesian inference and empirical models of several observables such as the brightness, Galactic space velocity and position of bona fide members of 7 young stars groups (8-120 Mans), as well as old field stars. Stars with a membership probability greater than 90% are considered candidate members. The analysis also predicts the radial velocity and distance that a star would have if it was an actual member. For the 177 previously-known members, an excellent agreement was found between the predicted and observed parameters (1.9 km/s and 10% for the radial velocity and parallax, respectively). Measuring these observables for the candidates stars is thus a good way to confirm their membership. This robust method was applied to a sample of 758 stars which showed signs of youth (H$\alpha$ and X-ray emission). It allowed to identify 214 highly probable candidates. The spectroscopic follow-up yields a radial velocity in agreement with predictions for 130 stars. These spectroscopic observations also allowed to measure their projected rotational velocity, which turned out to be higher than that of the old population of stars. Trigonometric distance measurements were also obtained and were coherent with predictions for 18 young candidates. Using the confirmed members, a new empirical model of the X-ray luminosity was developed. The X-ray luminosity was found to be about 4 times higher for stars around ~8-12Myr than for older, ~120Myr stars, thus, this observable is a good age indicator in this range. Comparing the spectra of 59 young candidate members to atmosphere models allowed to determine three basic parameters: the bolometric luminosity, the effective temperature and the radius. Overall, these candidates are more luminous and have a greater radius than old stars. Recent evolutionary models that include the rotational dynamo-type treatment and produce magnetic field strength of 1 to 2.5 kGauss were used to derive an isochronal age for each star. The ages determined for \beta Pictoris moving group members using stars of different spectral types are coherent with one another (types K5V-M0V: 24 Mans, types M1V-M4V: 14 Mans) and are also coherent with age determined independently using lithium abundance of the low-mass members (26 Mans).