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.

We present the final results from a targeted search for brown dwarfs with unusual near-infrared colors. From a positional cross-match of the Sloan Digital Sky Survey (SDSS), 2-Micron All-Sky Survey (2MASS), and Wide-Field Infrared Survey Explorer (WISE) catalogs, we have identified 144 candidate peculiar L and T dwarfs. Spectroscopy confirms that 20 of the objects are peculiar or are candidate binaries. Of the 420 objects in our full sample 9 are young (less than or similar to 200 Myr; 2.1%) and another 8 (1.9%) are unusually red, with no signatures of youth. With a spectroscopic J-K-s color of 2.58 +/- 0.11 mag, one of the new objects, the L6 dwarf 2MASS J03530419 +0418193, is among the reddest field dwarfs currently known and is one of the reddest objects with no signatures of youth known to date. We have also discovered another potentially very-low-gravity object, the L1 dwarf 2MASS J00133470+1109403, and independently identified the young L7 dwarf 2MASS J00440332+0228112, which was first reported by Schneider and collaborators. Our results confirm that signatures of low gravity are no longer discernible in low to moderate resolution spectra of objects older than similar to 200 Myr. The 1.9% of unusually red L dwarfs that do not show other signatures of youth could be slightly older, up to similar to 400 Myr. In this case a red J - K-s color may be more diagnostic of moderate youth than individual spectral features. However, its is also possible that these objects are relatively metal-rich, and thus have enhanced atmospheric dust content.

Past ultraviolet and optical observations of stars hosting close-in Jupiter-mass planets have shown that some of these stars present an anomalously low chromospheric activity, significantly below the basal level. For the hot Jupiter planet host WASP-13, observations have shown that the apparent lack of activity is possibly caused by absorption from the intervening interstellar medium (ISM). Inspired by this result, we study the effect of ISM absorption on activity measurements (S and log R'(HK) indices) for main-sequence late-type stars. To this end, we employ synthetic stellar photospheric spectra combined with varying amounts of chromospheric emission and ISM absorption. We present the effect of ISM absorption on activity measurements by varying several instrumental (spectral resolution), stellar (projected rotational velocity, effective temperature, and chromospheric emission flux), and ISM parameters (relative velocity between stellar and ISM Ca II lines, broadening b-parameter, and Ca II column density). We find that for relative velocities between the stellar and ISM lines smaller than 30-40 km s(-1) and for ISM Ca II column densities log N-CaII greater than or similar to 12, the ISM absorption has a significant influence on activity measurements. Direct measurements and three dimensional maps of the Galactic ISM absorption indicate that an ISM Ca II column density of log N-CaII = 12 is typically reached by a distance of about 100 pc along most sight lines. In particular, for a Sun-like star lying at a distance greater than 100 pc, we expect a depression (bias) in the log R'(HK) value larger than 0.05-0.1 dex, about the same size as the typical measurement and calibration uncertainties on this parameter. This work shows that the bias introduced by ISM absorption must always be considered when measuring activity for stars lying beyond 100 pc. We also consider the effect of multiple ISM absorption components. We discuss the relevance of this result for exoplanet studies and revise the latest results on stellar activity versus planet surface gravity correlation. We finally describe methods with which it would be possible to account for ISM absorption in activity measurements and provide a code to roughly estimate the magnitude of the bias. Correcting for the ISM absorption bias may allow one to identify the origin of the anomaly in the activity measured for some planet-hosting stars.

Model stellar atmospheres are fundamental tools for understanding stellar observations from interferometry, microlensing, eclipsing binaries and planetary transits. However, the calculations also include assumptions, such as the geometry of the model. We use intensity profiles computed for both plane-parallel and spherically symmetric model atmospheres to determine fitting coefficients in the BVRIHK, CoRot and Kepler wavebands for limb darkening using several different fitting laws, for gravity-darkening and for interferometric angular diameter corrections. Comparing predicted variables for each geometry, we find that the spherically symmetric model geometry leads to different predictions for surface gravities log g < 3. In particular, the most commonly used limb-darkening laws produce poor fits to the intensity profiles of spherically symmetric model atmospheres, which indicates the need for more sophisticated laws. Angular diameter corrections for spherically symmetric models range from 0.67 to 1, compared to the much smaller range from 0.95 to 1 for plane-parallel models.

As a preliminary step in deducing Teff and log (g) of Mira variables as a function of phase, a comparison is made between spectra synthesized from LTE stellar atmosphere models and observed spectra. The observed spectra show obvious vanadium oxide (VO) absorption bands. However, the molecular line list used to produce the synthetic spectra does not include the bound-bound VO opacities. The wavenumbers, line oscillator strengths, and lowest energy levels are needed to calculate these opacities. The equations, constants, and experimentally determined factors required to calculate the line oscillator strengths and lowest energy levels from experimentally determined wavenumbers are presented. The effect of including the wavenumbers, line oscillator strengths, and lowest energy levels of the VO BX (0, 0) band are calculated and show the expected absorption features observed in the spectra of Mira variables. In the VO B-X (0, 0) band the line oscillator strengths range from about 0.05 to 3.

Model stellar atmospheres are fundamental tools for understanding stellar observations from interferometry, microlensing, eclipsing binaries and planetary transits. However, the calculations also include assumptions, such as the geometry of the model. We use intensity profiles computed for both plane-parallel and spherically symmetric model atmospheres to determine fitting coefficients in the BVRIHK, CoRot and Kepler wavebands for limb darkening using several different fitting laws, for gravity-darkening and for interferometric angular diameter corrections. Comparing predicted variables for each geometry, we find that the spherically symmetric model geometry leads to different predictions for surface gravities log g < 3. In particular, the most commonly used limb-darkening laws produce poor fits to the intensity profiles of spherically symmetric model atmospheres, which indicates the need for more sophisticated laws. Angular diameter corrections for spherically symmetric models range from 0.67 to 1, compared to the much smaller range from 0.95 to 1 for plane-parallel models.

Classical Cepheids are key probes of both stellar astrophysics and cosmology as standard candles and pulsating variable stars. It is important to understand Cepheids in unprecedented detail in preparation for upcoming Gaia, James Webb Space Telescope (JWST) and extremely-large telescope observations. Cepheid eclipsing binary stars are ideal tools for achieving this goal, however there are currently only three known systems. One of those systems, OGLE-LMC-CEP1812, raises new questions about the evolution of classical Cepheids because of an apparent age discrepancy between the Cepheid and its red giant companion. We show that the Cepheid component is actually the product of a stellar merger of two main sequence stars that has since evolved across the Hertzsprung gap of the HR diagram. This post-merger product appears younger than the companion, hence the apparent age discrepancy is resolved. We discuss this idea and consequences for understanding Cepheid evolution.

In this paper, we present the analysis of incoherent non-thermal radio emission from a sample of hot magnetic stars, ranging from early-B to early-A spectral type. Spanning a wide range of stellar parameters and wind properties, these stars display a commonality in their radio emission which presents new challenges to the wind scenario as originally conceived. It was thought that relativistic electrons, responsible for the radio emission, originate in current sheets formed, where the wind opens the magnetic field lines. However, the true mass-loss rates from the cooler stars are too small to explain the observed non-thermal broad-band radio spectra. Instead, we suggest the existence of a radiation belt located inside the inner magnetosphere, similar to that of Jupiter. Such a structure explains the overall indifference of the broad-band radio emissions on wind mass-loss rates. Further, correlating the radio luminosities from a larger sample of magnetic stars with their stellar parameters, the combined roles of rotation and magnetic properties have been empirically determined. Finally, our sample of early-type magnetic stars suggests a scaling relationship between the non-thermal radio luminosity and the electric voltage induced by the magnetosphere's co-rotation, which appears to hold for a broader range of stellar types with dipole-dominated magnetospheres (like the cases of the planet Jupiter and the ultracool dwarf stars and brown dwarfs). We conclude that well-ordered and stable rotating magnetospheres share a common physical mechanism for supporting the generation of non-thermal electrons.

The physics pertaining to the asymptotic giant branch (AGB) phase of stellar evolution has been studied for many years. However, the mechanics behind many characteristics displayed at this stage are still not fully understood. As a member of the Long Period Variable class of stars, AGB stars are invaluable in creating three-dimensional maps of the Milky Way, the Magellanic System and other galaxies with resolved stellar populations. Variable stars can be used to determine radial distances from Earth using their periodic luminosity variations. As this type of star has unknown qualities, models of AGB populations need to be calibrated with observed data. Previous research has derived a best-fitting model using the TRILEGAL code (a TRIdimensional modeL of thE GALaxy). This model was calibrated against single-epoch luminosity functions (LFs) calculated from resolved stellar populations in the Small Magellanic Cloud (SMC). With multi-epoch data now available from the VISTA survey of the Magellanic Clouds (VMC), this best-fitting model can now be compared with the LFs as they vary with time. Firstly, statistical tests are completed to measure the extent of the LF variation between epochs and from the mean LF for both the full VMC AGB catalogue and for the oxygen-rich, carbon-rich and extreme AGB classes. Statistical tests are then performed to measure the similarity between the LFs from different epochs and the simulated LFs, again for the entire sample and the three classes. This investigation shows that, while the current best-fitting model is a good approximation of many individual epochs’ AGB LFs in the SMC to within 3σ, inclusion of multi-epoch data would make for a more robust analysis. In order to do this, it would be desirable to have more epochs with deeper and regular observations that could cover full lightcurves of some of the sources. There also seems to be a statistical difference between the inner and outer areas of the SMC, perhaps due to tidal disruptions. It would be interesting to see the results of a similar study using the LMC, which is less affected by the gravitational influence of its smaller companion. / <p>This thesis was written under the supervision of Maria-Rosa Cioni at the Leibniz Institute for Astrophysics in Potsdam. The presentation was held online due to the COVID-19 pandemic.</p>

O tópico abordado nesta tese é a perda de massa através de ventos coronais magnetizados em estrelas de baixa massa. Ventos estelares têm sido estudados extensivamente há vários anos, tendo inicialmente como foco o vento solar. Atualmente, sabe-se que o campo magnético é essencial na aceleração e aquecimento dos ventos coronais. Apesar do conhecimento detalhado que temos da estrutura magnética do Sol, pouco se sabe sobre a configuração do campo magnético em outras estrelas. Nesta tese, é investigada a estrutura do campo magnético nas coroas de estrelas do tipo solar na Seqüência Principal e de suas predecessoras na pré Seqüência Principal através de simulações numéricas magneto-hidrodinâmicas tri-dimensionais. Aqui, consideramos de forma auto-consistente a interação entre o vento e o campo magnético e vice-versa. Dessa forma, pela interação entre forças magnéticas e forças do vento, consegue-se determinar a configuração do campo magnético e a estrutura dos ventos coronais. Realizamos um estudo de ventos de estrelas do tipo solar e a dependência dos mesmos com o parâmetro beta do plasma (a razão entre as densidades de energia térmica e magnética). Este é o primeiro estudo a realizar tal análise resolvendo as equações tri-dimensionais da magneto-hidrodinâmica ideal. Em nossas simulações, adotamos um parâmetro de aquecimento descrito por gamma, que é responsável pela aceleração térmica do vento. Então, nós analisamos ventos com intensidades de campo magnético nos pólos no intervalo de B0 = 1 a 20 G e mostramos que a estrutura do vento apresenta características que são similares à do vento coronal do Sol. No estado estacionário, a topologia do campo magnético obtida é similar para todos os casos estudados, apresentando uma configuração do tipo helmet streamer, com zonas de linhas fechadas e abertas de campo magnético co-existindo. Intensidades mais altas de campo levam a ventos mais acelerados e mais quentes. O aumento na intensidade do campo gera também uma zona morta maior no vento, i.e., os loops fechados que previnem que a matéria escape da coroa em latitudes menores que ~45 graus se estendem a maiores distâncias da estrela. Além disso, mostramos também que a força de Lorentz gera naturalmente um vento que é dependente da latitude. Ao aumentar a densidade da coroa mantendo B0 = 20 G, mostramos que o sistema volta a apresentar ventos menos acelerados e mais frios. Para um valor fixo de gamma, mostramos que o parâmetro essencial na determinação do perfil de velocidade do vento é o parâmetro beta calculado na base da coroa. Dessa forma, acredita-se que haja um grupo de ventos magnetizados que apresenta a mesma velocidade terminal independentemente das densidades de energia térmica ou magnética, desde que o parâmetro beta seja o mesmo. No entanto, essa degenerescência pode ser removida ao se comparar outros parâmetros físicos do vento, tal como a taxa de perda de massa. Nós também analisamos a influência do gamma nos nossos resultados e mostramos que ele é importante na determinação da estrutura do vento. Além disso, investigamos ventos magnetizados de estrelas de baixa massa da pré Seqüência Principal. Em particular, analisamos sob quais circunstâncias tais estrelas apresentam estruturas magnéticas alongadas (e.g., helmet streamers, proeminências do tipo slingshot, etc). Focamos especialmente em estrelas do tipo T Tauri fracas, uma vez que o tênue disco de acreção, quando presente ao redor de tais estrelas, não deve causar forte influência na estrutura do vento estelar e nem na do campo magnético coronal. Nós mostramos que o parâmetro beta do plasma é um fator decisivo na configuração do campo magnético do vento estelar. Usando parâmetros iniciais adequados ao que se é observado para tais estrelas, nós mostramos que a configuração do campo magnético pode variar entre uma configuração semelhante à de um dipolo e uma configuração com linhas fortemente colimadas em torno do eixo polar e streamers fechados ao redor do equador (configuração de multi-componentes para o campo magnético). Mostramos que as estruturas alongadas do campo magnético somente estão presentes se o parâmetro beta do plasma na base da coroa é beta0 << 1. Usando nossos modelos magneto-hidrodinâmicos, auto-consistentes, tri-dimensionais, estimamos para ventos de estrelas da pré Seqüência Principal a escala temporal de migração planetária devido a forças de arraste exercidas pelo vento em um planeta tipo hot-Jupiter (i.e., um planeta gigante que orbita muito próximo da estrela). Nosso modelo sugere que os ventos estelares de coroas com multi-componentes de campo magnético não têm influências significativas na migração de hot-Jupiters. / The subject of this thesis is the mass loss of low-mass stars through magnetized coronal winds. Stellar winds have been a topic of extensive research in Astrophysics for a long time, and their first investigations focused on the solar wind. Nowadays, we know that the magnetic field plays a crucial role in the acceleration and heating of coronal winds. Despite of the knowledge of the fine structure of the solar magnetic field, much less information is known regarding the configuration of the magnetic field in other stars. In this thesis, we investigate the structure of the magnetic field in the coronae of solar-like stars and young stars by means of three-dimensional magnetohydrodynamical numerical simulations. We self-consistently take into consideration the interaction of the outflowing wind with the magnetic field and vice versa. Hence, from the interplay between magnetic forces and wind forces, we are able to determine the configuration of the magnetic field and the structure of the coronal winds. We investigate solar-like stellar winds and their dependence on the plasma-beta parameter (the ratio between thermal and magnetic energy densities). This is the first study to perform such analysis solving the fully ideal three-dimensional magnetohydrodynamics equations. We adopt in our simulations a heating parameter described by gamma, which is responsible for the thermal acceleration of the wind. We analyze winds with polar magnetic field intensities ranging from B0 = 1 to 20 G and we show that the wind structure presents characteristics that are similar to the solar coronal wind. The steady-state magnetic field topology for all cases is similar, presenting a configuration of helmet streamer-type, with zones of closed field lines and open field lines coexisting. Higher magnetic field intensities lead to faster and hotter winds. The increase of the field intensity generates a larger ``dead zone\'\' in the wind, i.e., the closed loops that inhibit matter to escape from latitudes lower than 45 degrees extend farther away from the star. The Lorentz force leads naturally to a latitude-dependent wind. We show that by increasing the density and maintaining B0 = 20 G, the system recovers to slower and cooler winds. For a fixed gamma, we show that the key parameter in determining the wind velocity profile is the beta-parameter at the coronal base. Therefore, there is a group of magnetized flows that would present the same terminal velocity despite of its thermal and magnetic energy densities, as long as the plasma-beta parameter is the same. This degeneracy, however, can be removed if we compare other physical parameters of the wind, such as the mass-loss rate. We also analyze the influence of gamma in our results and we show that it is also important in determining the wind structure. We further investigate magnetized stellar winds of low-mass pre-main-sequence stars. In particular we analyze under which circumstances these stars present elongated magnetic features (e.g., helmet streamers, slingshot prominences, etc). We focus on weak-lined T Tauri stars, as the presence of the tenuous accretion disk is not expected to have strong influence on the structure of the stellar wind neither on the coronal magnetic field. We show that the plasma-beta parameter is a decisive factor in defining the magnetic configuration of the stellar wind. Using initial parameters within the observed range for these stars, we show that the coronal magnetic field configuration can vary between a dipole-like configuration and a configuration with strong collimated polar lines and closed streamers at the equator (multicomponent configuration for the magnetic field). We show that elongated magnetic features will only be present if the plasma-beta parameter at the coronal base is beta0 << 1. Using our self-consistent three-dimensional magnetohydrodynamical model, we estimate for the stellar winds of pre-main-sequence stars the timescale of planet migration due to drag forces exerted by the stellar wind on a hot-Jupiter (i.e., on a giant planet that orbits very close to the star). Our model suggests that the stellar wind of these multicomponent coronae are not expected to have significant influence on the migration of hot-Jupiters.