En 1972, Skumanich découvre une relation empirique unique entre la période de rotation de surface des étoiles G et leur âge sur la séquence principale. Cette découverte ouvrit alors une nouvelle voie pour la datation stellaire : la gyrochronologie. Dès lors, bon nombre d'auteurs, entre la fin des années 80 et 90, se sont intéressés à l'évolution de la vitesse de rotation de surface des étoiles de faible masse ($M_*$ = 0.4 $M_{odot}$- 1.1 $M_{odot}$). Les premiers modèles phénoménologies sur le sujet été nés.L'évolution de la vitesse de rotation de ces étoiles commence à être raisonnablement bien reproduite par la classe de modèle paramétrique que je présente dans cette thèse. Par manque de descriptions théoriques satisfaisantes, seuls les effets globaux des mécanismes physiques impliqués sont ici décris. Le principal enjeu est d'étudier le cadre et la façon dont le moment cinétique stellaire est impacté par ces processus tout en contraignant leurs principales caractéristiques.Au cours de ma thèse, j'ai modélisé les trajets rotationnels des enveloppes externes et médianes des distributions de période de rotation de 18 amas stellaire entre 1 Myr et 1 Gyr. Ceci m'a permis d'analyser la dépendance temporelle des mécanismes physiques impliqués dans l'évolution du moment cinétique des étoiles de type solaire. Les résultats que j'ai obtenus montrent que l'évolution de la rotation différentielle interne impact fortement la convergence rotationnelle (relation empirique de Skumanich), l'évolution de l'abondance de surface en lithium, et les intensités du champ magnétique généré par effet dynamo. En plus de reproduire ces enveloppes externes, le modèle que j'ai développé fournit des contraintes sur les mécanismes de redistribution interne du moment cinétique et sur les durées de vie des disques circumstellaires, supposés responsables de la régulation rotationnelle observée durant les quelques premiers millions d'années de la pré-séquence principale. L'extension du modèle aux étoiles moins massives (0.5 et 0.8 $M_{odot}$) que j'ai réalisé, a également fournis la dépendance en masse de ces différents processus physiques.Cette étape à notamment ajoutée de fortes contraintes sur les temps caractéristiques associés au transport de moment cinétique entre le coeur et l'enveloppe, sur l'efficacité du freinage magnétique vraisemblablement reliée à un changement de topologie des étoiles de type solaire vers celles de 0.5 $M_{odot}$, et sur l'histoire rotationnelle, interne comme de surface, des étoiles entre 1 Myr à 1 Gyr. / In 1972, Skumanich discovers a unique empirical relationship between the rotation period of the surface of G star and their age on the main sequence. This discovery then opened a new path for stellar dating: the gyrochronology. Therefore, many authors in the late 80's and the begenning 90's, were interested in the evolution of the surface angular velocity of low-mass stars ($M_*$ = 0.4 $M_{odot}$- 1.1 $M_{odot}$). The first phenomenological models on the subject were born.The angular velocity evolution of these stars begins to be reasonably well reproduced by the class of parametrical model that I present in this thesis. Because of the lack of adequate theoretical descriptions, only the overall effects of the physical mechanisms involved are described here. The main issue is to study the framework and how the stellar angular momentum is affected by these processes and to constrain their main characteristics.Over the course of my thesis, I modelled the rotational tracks of external and median envelopes and median of rotation period distributions of 18 stellar clusters between 1 Myr and 1 Gyr. This allowed me to analyse the time dependence of the physical mechanisms involved in the angular momentum evolution of solar-type stars. The results I obtained show that the evolution of the internal differential rotation significantly impact the rotational convergence (empirical Skumanich's relationship), the evolution of the surface lithium abundance, and the intensity of the magnetic field generated by dynamo effect. In addition to the reproduction of these external envelopes, the model I developed provides constraints on the mechanisms of internal redistribution of angular momentum and the lifetimes of circumstellar disks, that are held responsible for the rotational regulation observed during the first few million years of pre-main sequence. The extension of the model to less massive stars (0.5 et 0.8 $M_{odot}$) that I performed also provided the mass dependence of these physical processes. Most specifically, this step added strong constraints on the characteristic time associated to the transport of angular momentum between the core and the envelope, on the efficiency of magnetic braking likely related to a change of topology from solar-type stars to those of 0.5 $M_{odot}$, and on the internal and external rotational history of stars from 1 Myr to 1 Gyr.

The study of magnetic fields on low-mass stars is important due to their ubiquity. They are responsible for phenomena spanning a wide range of spatial and temporal scales. Over the last two decades, the Zeeman-Doppler imaging (ZDI) technique has been used to study the topologies of stellar magnetic fields. A great deal has been learnt about how the magnetic characteristics of cool dwarfs vary as a function of parameters such as mass, rotation or age. In this thesis, I assemble a sample of stars with Zeeman-Doppler maps. I study their poloidal and toroidal components as a function of fundamental parameters and also in relation to activity cycles. I find that the relationship between poloidal and toroidal fields is different for stars above and below the fully convective boundary, in line with previous ZDI studies. I also find that the fields of strongly toroidal stars must be generated axisymmetrically. With regards to activity cycles, I find that so called “inactive branch" stars appear to remain poloidal throughout their activity cycle while so called “active branch" stars appear to be able to generate strong toroidal fields. Magnetic activity can also interact with exoplanets that may be orbiting a star. In this thesis, I consider two such interactions. The first is the compression of planetary magnetospheres by stellar winds. Sufficiently powerful winds can strip a planet of its atmosphere and render it uninhabitable. However magnetospheric shielding can provide some protection. I show that planets around 0.6 M⊙ - 0.8 M⊙ stars are the most likely to be able to protect their atmospheres. The second interaction I consider is exoplanetary radio emission. I present a wind model and show that exoplanetary radio emissions will depend strongly on the structure of the magnetic field structure of the central star.

We image 104 newly identified low-mass (mostly M-dwarf) pre-main sequence (PMS) members of nearby young moving groups (YMGs) with Magellan Adaptive Optics (MagAO) and identify 27 stellar binaries with instantaneous projected separation as small as 40 mas. Fifteen were previously unknown. The total number of multiple systems in this sample including spectroscopic and visual binaries from the literature is 36, giving a raw stellar multiplicity rate of at least 35(-4)(+5)% for this population. In the separation range of roughly 1-300 au in which infrared AO imaging is most sensitive, the raw multiplicity rate is at least 24(-4)(+5)% for binaries resolved by the MagAO infrared camera (Clio). The M-star subsample of 87 stars yields a raw multiplicity of at least 30(-4)(+5)% over all separations, 21(-4)(+5)% for secondary companions resolved by Clio from 1 to 300 au (23(-4)(+5)% for all known binaries in this separation range). A combined analysis with binaries discovered by the Search for Associations Containing Young stars shows that stellar multiplicity fraction as a function of mass over the range of 0.2 to 1.2M(circle dot) appears to be linearly flat, in contrast to the field, where multiplicity increases with mass. After bias corrections are applied, the multiplicity of low-mass YMG members (0.2-0.6M(circle dot)) is in excess of the field. The overall multiplicity fraction is also consistent with being constant in age and across YMGs, which suggests that multiplicity rates for this mass range are largely set by 10 Myr without appreciable evolution thereafter.

In this paper, we investigate the relation between disk mass and mass accretion rate to constrain the mechanism of angular momentum transport in protoplanetary disks. We find a correlation between dust disk mass and mass accretion rate in Chamaeleon I with a slope that is close to linear, similar to the one recently identified in Lupus. We investigate the effect of stellar mass and find that the intrinsic scatter around the best-fit M-dust-M star and M-acc-M star relations is uncorrelated. We simulate synthetic observations of an ensemble of evolving disks using a Monte Carlo approach and find that disks with a constant alpha viscosity can fit the observed relations between dust mass, mass accretion rate, and stellar mass but overpredict the strength of the correlation between disk mass and mass accretion rate when using standard initial conditions. We find two possible solutions. In the first one, the observed scatter in M-dust and M-acc is not primordial, but arises from additional physical processes or uncertainties in estimating the disk gas mass. Most likely grain growth and radial drift affect the observable dust mass, while variability on large timescales affects the mass accretion rates. In the second scenario, the observed scatter is primordial, but disks have not evolved substantially at the age of Lupus and Chamaeleon I owing to a low viscosity or a large initial disk radius. More accurate estimates of the disk mass and gas disk sizes in a large sample of protoplanetary disks, through either direct observations of the gas or spatially resolved multiwavelength observations of the dust with ALMA, are needed to discriminate between both scenarios or to constrain alternative angular momentum transport mechanisms such as MHD disk winds.

The K2 mission is targeting large numbers of nearby (d < 100 pc) GKM dwarfs selected from the SUPERBLINK proper motion survey (mu > 40 mas yr(-1), V < 20). Additionally, the mission is targeting low-mass, high proper motion stars associated with the local (d < 500 pc) Galactic halo population also selected from SUPERBLINK. K2 campaigns 0 through 8 monitored a total of 26,518 of these cool main-sequence stars. We used the auto-correlation function to search for fast rotators by identifying short-period photometric modulations in the K2 light curves. We identified 481 candidate fast rotators with rotation periods < 4 days that show light-curve modulations consistent with starspots. Their kinematics show low average transverse velocities, suggesting that they are part of the young disk population. A subset (13) of the fast rotators is found among those targets with colors and kinematics consistent with the local Galactic halo population and may represent stars spun up by tidal interactions in close binary systems. We further demonstrate that the M dwarf fast rotators selected from the K2 light curves are significantly more likely to have UV excess and discuss the potential of the K2 mission to identify new nearby young GKM dwarfs on the basis of their fast rotation rates. Finally, we discuss the possible use of local halo stars as fiducial, non-variable sources in the Kepler fields.

The aim of this thesis is to use a cross-match between WISE, 2MASS and SDSS to identify a large sample of M dwarfs. Through the careful characterisation and quality control of these M dwarfs I aim to identify rare systems (i.e. unresolved UCD companions, young M dwarfs, late M dwarfs and M dwarfs with common proper motion companions). Locating ultracool companions to M dwarfs is important for constraining low-mass formation models, the measurement of substellar dynamical masses and radii, and for testing ultracool evolutionary models. This is done by using an optimised method for identifying M dwarfs which may have unresolved ultracool companions. To do this I construct a catalogue of 440 694 M dwarf candidates, from WISE, 2MASS and SDSS, based on optical- and near-infrared colours and reduced proper motion. With strict reddening, photometric and quality constraints I isolate a sub-sample of 36 898 M dwarfs and search for possible mid-infrared M dwarf + ultracool dwarf candidates by comparing M dwarfs which have similar optical/near-infrared colours (chosen for their sensitivity to effective temperature and metallicity). I present 1 082 M dwarf + ultracool dwarf candidates for follow-up. Using simulated ultracool dwarf companions to M dwarfs, I estimate that the occurrence of unresolved ultracool companions amongst my M dwarf + ultracool dwarf candidates should be at least four times the average for my full M dwarf catalogue. I discuss yields of candidates based on my simulations. The possible contamination and bias from misidentified M dwarfs is then discussed, from chance alignments with other M dwarfs and UCDs, from chance alignments with giant stars, from chance alignments with galaxies, and from blended systems (via visual inspection). I then use optical spectra from LAMOST to spectral type a subset of my M dwarf + ultracool dwarf candidates. These candidates need confirming as true M dwarf + ultracool dwarf systems thus I present a new method I developed to use low resolution near-infrared spectra which relies on two colour similar objects (one an excess candidate, one not) having very similar spectra. A spectral difference of these two colour similar objects should leave the signature of a UCD in the residual of their differences, which I look for using the difference in two spectral bands designed to identify UCD spectral features. I then present the methods used to identify other rare systems from my full M dwarf catalogue. Young M dwarfs were identified by measuring equivalent widths of Hα from the LAMOST spectra, and by measuring rotation periods from Kepler 2 light curves. I identify late M dwarfs photometrically (using reduced proper motion and colour cuts) and spectroscopically (using the LAMOST spectra with spectral indices from the literature). Also I present common proper motion analysis aimed at finding Tycho-2 primaries for my M dwarfs and look for physically separated M dwarf + M dwarf pairs (internally within my full M dwarf catalogue).

Context. One of the key questions of the star formation problem is whether brown dwarfs (BDs) form in the manner of stars directly from the gravitational collapse of a molecular cloud core (star-like) or whether BDs and some very low-mass stars (VLMSs) constitute a separate population that forms alongside stars comparable to the population of planets, for example through circumstellar disk (peripheral) fragmentation. Aims. For young stars in Taurus-Auriga the binary fraction has been shown to be large with little dependence on primary mass above approximate to 0.2 M-circle dot, while for BDs the binary fraction is < 10%. Here we investigate a case in which BDs in Taurus formed dominantly, but not exclusively, through peripheral fragmentation, which naturally results in small binary fractions. The decline of the binary frequency in the transition region between star-like formation and peripheral formation is modelled. Methods. We employed a dynamical population synthesis model in which stellar binary formation is universal with a large binary fraction close to unity. Peripheral objects form separately in circumstellar disks with a distinctive initial mass function (IMF), their own orbital parameter distributions for binaries, and small binary fractions, according to observations and expectations from smoothed particle hydrodynamics (SPH) and grid-based computations. A small amount of dynamical processing of the stellar component was accounted for as appropriate for the low-density Taurus-Auriga embedded clusters. Results. The binary fraction declines strongly in the transition region between star-like and peripheral formation, exhibiting characteristic features. The location of these features and the steepness of this trend depend on the mass limits for star-like and peripheral formation. Such a trend might be unique to low density regions, such as Taurus, which host binary populations that are largely unprocessed dynamically in which the binary fraction is large for stars down to M-dwarfs and small for BDs. Conclusions. The existence of a strong decline in the binary fraction - primary mass diagram will become verifiable in future surveys on BD and VLMS binarity in the Taurus-Auriga star-forming region. The binary fraction -primary mass diagram is a diagnostic of the (non-)continuity of star formation along the mass scale, the separateness of the stellar and BD populations, and the dominant formation channel for BDs and BD binaries in regions of low stellar density hosting dynamically unprocessed populations.

Les étoiles de petite masse (naines M) forment la composante stellaire dominante de notre galaxie et contribuent à la majeure partie de la matière baryonique dans la galaxie. Notre compréhension de la galaxie repose donc sur la connaissance de cette composante peu lumineuse. Par ailleurs, un nombre grandissant d’exoplanètes sont découverte autour de naines M, y compris des super-terres. L’étude des naines M a ainsi des implications importantes sur la physique stellaire, permettant de comprendre les processus en jeu dans ces atmosphères froides. Ces astres restent cependant peu connus du fait de leur faible luminosité intrinsèque. La description de ces étoiles nécessite une validation empirique fort, notamment en ce qui concerne les effets de l’abondance chimique sur la physique des atmosphères froides.Le but de cette thèse est de déterminer les paramètres fondamentaux des naines M et de tester les modèles d’atmosphères d’étoiles froides.Dans la première partie nous comparons des spectres calibrés dans le visible et l’infrarouge des composantes de système triple LHS 1070 avec des spectres synthétiques. Cette étude permet d’améliorer notre compréhension de la formation de la poussière dans les atmosphères très froides. Ce travail est étendu à un échantillon de naines M et permet de déterminer l’échelle de température des naines M. La seconde partie présente l’analyse des spectres à haute résolution de 21 naines M de faible métallicité (sous naines). Nous analysons en détail les signature moléculaires et atomiques dans le spectre. La comparaison avec des spectres synthétiques permet de mesurer les paramètres stellaires des étoiles et de déterminer avec précision leur métallicité. Ce travail doit initier des recherches futures. En particulier nous voulons étendre cette étude dans le domaine proche-infrarouge pour s’assurer que les résultats obtenus sont cohérents du domaine optique jusqu’à l’infrarouge proche. / 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.[...]

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.

A great deal of work has been conducted on in-tube condensation in horizontal and vertical smooth tubes. The available literature points to mechanisms governing two-phase condensation heat transfer coefficients and pressure drops, which are directly linked to the local flow pattern for both horizontal and inclined configurations. However, the work has been limited to flow pattern observations, heat transfer, pressure drops and void fractions for both horizontal and inclined tubes at high mass fluxes. No work has been conducted on the analysis of the observed flow patterns and the effect of temperature difference between the average wall temperature and average saturation temperature for different inclination angles at mass fluxes of 100 kg/m2.s and below. The purpose of this study is to carry out a qualitative analysis of flow patterns, and show the effect of temperature difference on the heat transfer coefficient for inclination angles from +90° (upward flow) to -90° (downward flow) at mass fluxes below 100 kg/m2.s. An experimental set-up provided the measurements for the two-phase condensation of R-143a in a smooth tube with an inside diameter of 8.38 mm and a length of 1.5 m. The mass fluxes were 25 kg/m2.s to 100 kg/m2.s, the saturation temperature was 40 °C and the mean qualities were 0.1 to 0.9. A high-speed camera was used to visually analyse and determine the flow patterns for both the inlet and the outlet of the test section. Through the results, eight flow patterns were observed: stratified-wavy, stratified, wavy, wavy-churn, intermittent, churn, annular and wavy-annular. The maximum heat transfer was observed for downward flow between inclination angles of -15° and -30°. The Thome-Hajal flow pattern map correctly predicted horizontal flow patterns, but failed to predict most of the inclined flow patterns. Various flow pattern transitions were identified and proposed for all the investigated inclination angles in this study. Finally, the heat transfer coefficient was found to be dependent on quality, mass flux, temperature difference and inclination angle. / Dissertation (MSc)--University of Pretoria, 2016. / Mechanical and Aeronautical Engineering / MSc / Unrestricted