Open clusters have long been objects of interest in astronomy. As a good approximation of essentially pure stellar populations, they have proved very useful for studies in a wide range of astrophysically interesting questions, including stellar evolution and atmosphere, the chemical and dynamical evolution of our Galaxy, and the structure of our Galaxy. Of fundamental importance to our understanding of open clusters, as well as many other questions in astrophysics, is the accurate determination of ages. Currently there are two main techniques for independently determining the ages of stellar populations: main sequence evolution theory (via cluster isochrones) and white dwarf cooling theory. Open clusters provide the ideal environment for the calibration of these two important clocks, as well as the unique opportunity to directly compare and refine our understanding of both theories. Here I present a photometric study of six open clusters, including both ground-based data, and new, deep photometric data from the Hubble Space Telescope. From the former I derive main sequence turn off ages, while the latter will be used to search for faint cluster white dwarfs. From these data I measure a white dwarf age for each cluster and directly compare these ages with those I find from the main sequence turn off age. For this analysis I employ a new Bayesian statistical technique that has been developed by our group. Additionally, I use this new technique to explore the feasibility of a new method to determine cluster white dwarf ages from the hot (bright) white dwarfs alone, and its first successful application to the Hyades. / text

Open clusters are important tools for studying of the Galactic disk and for understanding stellar evolution. Stars of clusters have constant age, composition and distance. This makes them to be excellent laboratories for investigations of stellar and Galactic chemical evolution. The main aim of the study is the analysis of mixing tracers and extra mixing evidences in photospheres of evolved stars. Extra-mixing processes may modify the surface abundances of 12^C/13^C and 12^C/14^N ratios. This study aims to test theoretical models. The secondary goal is a contribution to the study of present properties of the Galactic disc. The 12^C/13^C ratios in helium-core-burning clump stars for the clusters with turn-off masses lower than 2 solar masses are in agreement with the Cool-bottom processing (CBP) model and the Thermohaline mixing (TH) model. The observed 12^C/13^C ratios of the helium-core-burning stars in the open cluster NGC 6134 support evidences of larger then theoretically predicted extra-mixing in stars heavier than 2.5 solar masses. In two open clusters the 12^C/13^C ratios in core-helium-burning stars are lower than in the first ascent giants, located above the RGB luminosity bump. This could be caused by the material mixing during a very violent helium flash event. Almost all element to iron ratios show essentially flat radial abundance distributions across Galactic disk. The radial distributions of [O/Fe] and [Mg/Fe] have a tendency to increase towards larger... [to full text] / Padrikieji spiečiai naudojami astrofizikoje, tiriant Galaktikos disko cheminę raidą bei žvaigždžių evoliuciją. Spiečiaus žvaigždžių amžius, atstumas nuo Saulės, pirminė cheminė sudėtis yra apytiksliai vienodi, tad juos galima laikyti kosminėmis laboratorijomis. Pagrindinis disertacijos tikslas skirtas tirti maišymosi procesus evoliucionavusiose žvaigždėse. Siekiama įvertinti kaip maišymosi procesai paveikia 12^C/13^C ir 12^C/14^N gausų santykius ir palyginti su teoriniais maišymosi modeliais. Kitas tikslas skirtas prisidėti prie Galaktikos disko cheminės sudėties bei evoliucijos tyrimo. 12^C/13^C ir 12^C/14^N gausų santykiai nustatyti penkių spiečių evoliucionavusių žvaigždžių atmosferose. Helį centre deginančiose žvaigždėse kurių žvaigždžių posūkio iš pagrindinės sekos masės (M_toff) yra mažesnės nei 2 saulės masės, 12^C/13^C santykiai gerai sutampa su termohalinio (TH) bei šaltojo žemutinių sluoksnių (CBP) papildomo maišymosi modeliais, o tokių spiečių kurių M_toff yra didesnės nei 2,5 saulės masių papildomas maišymasis yra didesnis nei numato teoriniai modeliai. Dviejuose padrikuosiuose spiečiuose anglies izotopų gausų santykiai helį centre deginančių žvaigždžių atmosferose yra mažesni nei žvaigždėse, perkopusiose šviesio mazgą raudonųjų milžinių sekoje. Tai galėtų būti dėl galimo papildomo medžiagos maišymosi helio žybsnio metu. Beveik visų tirtų elementų radialinis pasiskirstymas Galaktikoje yra iš esmės plokščias, bet [O/Fe] ir [Mg/Fe] gausos turi tendenciją augti... [toliau žr. visą tekstą]

Padrikieji spiečiai naudojami astrofizikoje, tiriant Galaktikos disko cheminę raidą bei žvaigždžių evoliuciją. Spiečiaus žvaigždžių amžius, atstumas nuo Saulės, pirminė cheminė sudėtis yra apytiksliai vienodi, tad juos galima laikyti kosminėmis laboratorijomis. Pagrindinis disertacijos tikslas skirtas tirti maišymosi procesus evoliucionavusiose žvaigždėse. Siekiama įvertinti kaip maišymosi procesai paveikia 12^C/13^C ir 12^C/14^N gausų santykius ir palyginti su teoriniais maišymosi modeliais. Kitas tikslas skirtas prisidėti prie Galaktikos disko cheminės sudėties bei evoliucijos tyrimo. 12^C/13^C ir 12^C/14^N gausų santykiai nustatyti penkių spiečių evoliucionavusių žvaigždžių atmosferose. Helį centre deginančiose žvaigždėse kurių žvaigždžių posūkio iš pagrindinės sekos masės (M_toff) yra mažesnės nei 2 saulės masės, 12^C/13^C santykiai gerai sutampa su termohalinio (TH) bei šaltojo žemutinių sluoksnių (CBP) papildomo maišymosi modeliais, o tokių spiečių kurių M_toff yra didesnės nei 2,5 saulės masių papildomas maišymasis yra didesnis nei numato teoriniai modeliai. Dviejuose padrikuosiuose spiečiuose anglies izotopų gausų santykiai helį centre deginančių žvaigždžių atmosferose yra mažesni nei žvaigždėse, perkopusiose šviesio mazgą raudonųjų milžinių sekoje. Tai galėtų būti dėl galimo papildomo medžiagos maišymosi helio žybsnio metu. Beveik visų tirtų elementų radialinis pasiskirstymas Galaktikoje yra iš esmės plokščias, bet [O/Fe] ir [Mg/Fe] gausos turi tendenciją augti... [toliau žr. visą tekstą] / Open clusters are important tools for studying of the Galactic disk and for understanding stellar evolution. Stars of clusters have constant age, composition and distance. This makes them to be excellent laboratories for investigations of stellar and Galactic chemical evolution. The main aim of the study is the analysis of mixing tracers and extra mixing evidences in photospheres of evolved stars. Extra-mixing processes may modify the surface abundances of 12^C/13^C and 12^C/14^N ratios. This study aims to test theoretical models. The secondary goal is a contribution to the study of present properties of the Galactic disc. The 12^C/13^C ratios in helium-core-burning clump stars for the clusters with turn-off masses lower than 2 solar masses are in agreement with the Cool-bottom processing (CBP) model and the Thermohaline mixing (TH) model. The observed 12^C/13^C ratios of the helium-core-burning stars in the open cluster NGC 6134 support evidences of larger then theoretically predicted extra-mixing in stars heavier than 2.5 solar masses. In two open clusters the 12^C/13^C ratios in core-helium-burning stars are lower than in the first ascent giants, located above the RGB luminosity bump. This could be caused by the material mixing during a very violent helium flash event. Almost all element to iron ratios show essentially flat radial abundance distributions across Galactic disk. The radial distributions of [O/Fe] and [Mg/Fe] have a tendency to increase towards larger... [to full text]

Massive OB stars are very influential objects in the ecology of galaxies like our own. Current catalogues of Galactic OB stars are heavily biased towards bright (g < 13) objects, only typically including fainter objects when found in prominent star clusters (Garmany et al., 1982; Reed, 2003; Maíz-Apellániz et al., 2004). Exploitation of the VST Photometric Hα Survey (VPHAS+) allows us to build a robust catalogue of photometrically-selected OB stars across the entire Southern Galactic plane, both within clusters and in the field, down to ~20th magnitude in g. For the first time, a complete accounting of the OB star runaway phenomenon becomes possible. Along with making the primary selection using VPHAS+ colours, I have performed Markov-Chain Monte Carlo fitting of the spectral energy distributions of the selected stars by combining VPHAS+ u, g, r, i with published J, H, K photometry. This gives rough constraints on effective temperature and distance, whilst delivering much more precise reddening parameters A0 and RV - allowing us to build a much richer picture of how extinction and extinction laws vary across the Galactic Plane. My thesis begins with a description of the method of photometric selection of OB star candidates and its validation across a 2 square degree field including the well- known young massive star cluster Westerlund 2 (Mohr-Smith et al., 2015)1. Following on from this I present spectroscopy with AAOmega of 283 candidates identified by our method, which confirms that ~ 94% of the sample are the expected O and early B stars. I then develop this method further and apply it to a Galactic Plane strip of 42 square-degrees that runs from the Carina Arm tangent region to the much studied massive cluster in NGC 3603. A new aspect I attend to in this expansion of method is tightening up the uniform photometric calibration of the data, paying particular attention to the always-challenging u band. This leads to a new and reliable catalogue of 5915 OB stars. As well as increasing the numbers of identified massive stars in this large region of the sky by nearly an order of magnitude, a more complete picture of massive star formation in the Carina Arm has emerged. I have found a broad over-density of O stars around the highly luminous cluster NGC 3603 and have uncovered two new candidate OB clusters/associations. I have also paired up the ionization sources of a number of HII regions catalogued by the RMS survey. It is also shown that the OB star scale- height can serve as a roughly standard ruler, leading to the result that the OB star layer shows the onset of warping at RG ~ 10kpc. My results confirm that this entire region requires a non-standard (3.5 < RV < 4.0) reddening law for distances greater than ~2kpc. The methods developed in this study are ready to roll out across the rest of the VPHAS+ footprint that has been observed to date. This extension will take in a strip ~ ±2 degrees across the entire Southern Galactic mid-plane (a sky area of over 700 square degrees), within which we expect to find the majority of massive OB stars. This will result in the largest catalogue of Galactic OB stars to date.

Comprendre le processus de formation stellaire est un objectif majeur en astronomie. Sur ce sujet les observations ne donnent que très peu d'information, et les modèles numériques sont donc naturellement privilégiés. De tels modèles s'attachent à suivre la dynamique du gaz, sous l'effet de processus physique variés, ce qui nécessite un temps de calcul très important et ne permet pas de modéliser l'évolution au delà de 0.2 Myr environ. Or les résultats observationnels sont essentiellement issus du champ galactique proche, des amas évolués, voire des regions jeunes ou associations d'étoiles, dont l'âge peut varier de 1 Myr à quelques Gyr. Par conséquent, il est nécessaire pour comparer les résultats des modèles aux observations de comprendre ce qu'il se passe durant cet intervalle de temps. La formation stellaire tend à produire des étoiles en groupes, à partir de l'effondrement gravitationnel d'un nuage moléculaire turbulent. A mesure que les étoiles se forment, le gaz est éjecté et l'évolution est dominée par les interactions gravitationnelles. Suivre l'évolution sous l'effet de ces interactions est couramment utilisé afin de contraindre les modèles et de mieux comprendre l'origine des populations stellaires observées. Les étoiles se forment en sous-groupes ou structures hiérarchisées, qui peuvent ensuite fusionner pour donner des amas stellaires proche des amas ouverts, ou au contraire finir en associations distinctes. Dans ma thèse, je me suis intéressé à l'évolution dynamique de petits groupes d'étoiles, jusqu'alors peu étudiés par rapport aux groupes à 1000 ou 10^4 étoiles. J'ai simulé l'évolution de groupes à N < 100, dans le but d'en étudier la dynamique d'un point de vue statistique, grâce notamment au grand nombre de simulations effectuées, et afin d'identifier les signatures observationnelles propres à une situation initiale donnée. A partir d'un grand nombre de configurations initiales (avec N=20, 50, 100, un rayon typique de 0.025 pc à 1 pc) et 500 simulations par configurations, j'ai étudié l'évolution dynamique de groupes composés d'étoiles de même masse ou comprenant un spectre de masse, et sans population de binaire initiale. L'évolution de tels groupes s'est révélée similaire à celle de groupes plus grands, mais avec une phase d'effondrement plus rapide et surtout moins prononcée. Je décris le comportement moyen menant à une lente expansion de l'amas, ainsi qu'une voie d'évolution très différente, apparaissant dans 17% des cas étudiés, où l'amas est complètement dispersé suite à l'éjection d'une binaire centrale serrée. J'ai également recherché dans quelle mesure les données en densité et en vitesse 3D pouvaient permettre d'identifier l'état dynamique initial d'un groupe. L'utilisation de ces seules données suffisait dans certain cas à déterminer la densité initiale, mais elles devraient être complétées par des données concernant la population de binaire. Ce travail pourra être mis en application pour étudier l'origine dynamique d'association ou de groupes stellaires connus. Enfin, j'ai effectué un grand nombre de simulations numériques dans le but de reproduire l'état observé de l'amas eta Chamaeleontis par pure évolution dynamique à partir de conditions initiales standards. Cette association présente des caractéristiques d'amas évolué, telle que son spectre de masse pauvre en objets de faible masse et l'absence de binaires larges. Je montre que ces propriétés ne peuvent pas être reproduites uniquement par la dynamique, et sont donc les traces d'un processus de formation non standard. / Understanding the star formation process is a key issue in astronomy. Since direct observation provide only very limited information, this issue is investigated by models. Such models need to take into account complex physical processes while following the gas dynamics, so that simulations need a lot of time to run and do not follow the star formation process for longer than 0.2 Myr. The best known observational results concerns the field population, evolved open clusters or younger clusters or associations, which are between 1 Myr and a few Gyr old. Therefore in order to compare the results from models to known observations, we need to bridge the gap between the two. Star formation appears to produce groups of stars from the collapse of turbulent molecular clouds. As stars form, the gas is progressively ejected from the cluster, and the evolution is dominated by gravitational interactions. Following the dynamical evolution of a group of star using N-Body codes is a standard way used to constraint the models and understand the origin of the different populations. Star formation may produce sub-structure or small groups that merge to form bigger entities, or end up as loose association. In my thesis I focused on the dynamics of small groups, that have not been investigated as thoroughly as 1000 or 10^4 star groups. I performed N-Body simulations of small stellar groups, with N<100, in order to study their dynamics using a statistical approach, made possible by running a large number of simulations, and to find some observational signatures of given initial conditions. This approach enable to take full account of stochastic effects due to dynamical interactions. Using a large number of initial configurations (with N=20, 50, 100, a typical radius from 0.025 pc to 1 pc) and a sample of 500 simulations per configuration, I looked at equal mass groups as well as groups having a mass spectrum, without any binary initially. Such small groups show similar evolution to bigger groups, but with faster and less pronounced collapse phase. I described the average behaviour of slow expansion of the cluster, and an alternative evolution, occurring with 17% probability, that ended in the complete dissolution of the group due to ejection of a central binary. Searching for a way to identify the initial configuration from observational measure, I looked at the complementarity of density and 3D velocity and was able to show that it could be sufficient in some cases to determine the initial density. Further investigations are needed to take into account the information on the binary population and will be used to investigate the formation of known associations or young regions. Finally, I ran a large number of simulations, aiming at reproducing the observed state of the eta Chamaeleontis from standard initial conditions and pure dynamical evolution. This association properties are consistent with a dynamical evolved cluster, namely low-mass object poor and having only tight binaries. I showed that these properties cannot be reproduced with pure dynamical evolution from standard initial mass function and binary population, meaning that its particular features must have been pristine.

Nosso grupo tem dedicado atenção especial em manter o Novo Catálogo de Aglomerados Abertos e Candidatos Opticamente Visíveis (Dias et.al. [1] denominado comumente DAML02), que vem sendo continuamente atualizado com novos resultados vindos da literatura. Além de manter o catálogo, nosso grupo está constantemente produzindo novos resultados como movimentos próprios médios e determinação de probabilidades de pertinência de estrelas aos aglomerados estudados (Dias et. al. [2] [3] [4]), também descobrindo novos aglomerados abertos (Alessi et. al. [5]) contribuindo para o conhecimento da amostra de aglomerados abertos conhecida na nossa Galáxia. Neste trabalho nós apresentamos os primeiros resultados de um estudo cinemático e fotométrico de uma amostra de aglomerados abertos com distância e idade desconhecidas. Trata-se de uma amostra de 850 aglomerados originalmente listados na versão 2.3 do catálogo DAML02. As probabilidades de pertinência das estrelas na região de cada aglomerado foram determinadas através de métodos estatísticos conhecidamente eficazes como (Vasilevskis e Rach [6], Sanders [7] e Zhao e He [8] aplicados aos movimentos próprios individuais UCAC2 (Zacharias et.al.[9]). Conseqüentemente, os movimentos próprios médios dos aglomerados também foram estimados. Utilizando apenas as estrelas membro e dados fotométricos 2MASS (Skrutskie et.al.[10]) construímos os diagramas cor-magnitude para cada aglomerado que nos permitiram estimar parâmetros fundamentais dos aglomerados como distâncias, excessos de cor nas bandas do infravermelho e idades. A determinação desses parâmetros envolve o ajuste de isócronas a seqüência principal do aglomerado e a partir de valores iniciais ajustamos essas curvas para a obtenção dos valores de distâncias, excessos de cor e idades. Desenvolvemos com o objetivo de fornecer esses valores preliminares, um programa que usa apenas as estrelas com Tipo Espectral identificado no SIMBAD. Portanto, neste trabalho determinamos movimentos próprios e probabilidades de pertinência para 319 aglomerados. Desses, 32 apresentaram solução para os parâmetros fundamentais (distância, excesso de cor e idade) e segundo nossas análises para os casos inéditos, 11 aglomerados encontram-se na vizinhança solar. Os erros envolvidos estimados são totalmente coerentes com os erros do catálogo DAML02. / Our group has dedicate a special attention to maintain the New Catalogue of Optically Visible Open Clusters and Candidates (Dias et al.[1] hereinafter DAML02) that is being continuously updated with new results from the literature. Besides maintaining the catalogue, our group is actively producing new results such as the mean absolute proper motion and membership determination (Dias et. al. [2] [3] [4]) and discovering new open clusters (Alessi et al.[5]) contributing to complete the knowledge of the hole sample of open clusters in the Galaxy. In this work we present the first results of the kinematic and photometric study of the open clusters with unknown distance and age in DAML02. The membership probabilities of the stars in the region of each cluster were determined applying the statistical method of Zhao e He [8] using the individual stellar UCAC2 (Zacharias et.al.[9]) proper motions. Consequently, the mean absolute proper motion of the clusters were also estimated. Using the investigated stars we were able to construct the colourmagnitude diagrams that allowed us to derive preliminary estimates of the fundamental parameters of the clusters such as reddening, distance and ages where the 2MASS (Strutskie et.al.[10]) photometric data were considered.

We present an empirical assessment of the use of broad-band optical colours as age indicators for unresolved extragalactic clusters and investigate stochastic sampling effects on integrated colours. We use the integrated properties of Galactic open clusters (OCs) as models for unresolved extragalactic clusters. The population synthesis code Starburst99 (SB99) and four optical colours were used to estimate how well we can recover the ages of 62 well-studied Galactic OCs with published ages. We provide a method for estimating the ages of unresolved clusters and for reliably determining the uncertainties in the age estimates. Our results support earlier conclusions based on comparisons to synthetic clusters, namely the (U - B) colour is critical to the estimation of the ages of star-forming regions. We compare the observed optical colours with those obtained from SB99 using the published ages and get good agreement. The scatter in the (B - V)observed - (B - V)model is larger for lower luminosity clusters, perhaps due to stochastic effects.

The detection of variable stars in open clusters is an essential component of testing stellar structure and evolution theories. The ability to detect low-amplitude variability among cluster members is directly related to the quality of the photometric results. Point Spread Function (PSF) fitting is the best method available for measuring accurate magnitudes within crowded fields of stars, while high-precision differential photometry is the preferred technique for removing the effects of atmospheric extinction and variable seeing. In the search for new variable stars among hundreds or thousands of stars, the Robust Median Statistic (RoMS) is proven more effective for finding low-amplitude variables than the traditional error curve approach. A reputable computer program called DAOPHOT was used to perform PSF fitting, whereas programs, CLUSTER and RoMS, were created to carry out high-precision differential photometry and calculate the RoMS, respectively, on the open clusters NGC 225, NGC 559, NGC 6811, NGC 6940, NGC 7142, and NGC 7160. Twenty-two new variables and eighty-seven suspected variable stars were discovered, and time-series data of the new variables are presented.

One phase of formation of medium- and low-mass stars is the optically visible phase known as the pre-main-sequence or PMS phase. In order to further the understanding of this phase, more of these PMS objects need to be identified and classified. Previous techniques have used photometry to identify possible PMS objects by their characteristic Hα emission. Once identified these objects can be studied spectroscopically yielding complete PMS classification. This study develops a method to locate these emission objects that overcomes two limitations of previous techniques. The first limitation is the need for the creation of reddening maps. It is eliminated by the creation of a reddening free Hα wide/narrow index for the selection of emission objects. The second limitation is the requirement of the creation of mosaics to study the entire region of interest. This limitation is overcome by the construction of a wide-angle observation facility. This makes it possible to obtain the entire region of interest in a single frame. Once tested to ensure the validity of the method, the wide-angle Hα wide/narrow procedure is applied to several young open clusters. The development of the index and the results of its application to the clusters are presented. Also, an examination into how the results can be used to address some of the questions currently surrounding the PMS is included. Finally, a guideline for the implementation of the method into future studies is discussed.

In this dissertation, we present three complementary studies of the processes that drive planetary migration. The first is a radial-velocity survey in search of giant planets in adolescent (<1 >Gyr) open clusters. While several different mechanisms may act to drive giant planets inward, only some mechanisms will excite high eccentricities while doing so. Measuring the eccentricities of young hot Jupiters in these clusters (at a time before the orbits have had a chance to circularize due to tidal friction with their host stars) will allow us to identify which mechanisms are most important. Through this survey, we detect the first 3 hot Jupiters in open clusters (and at least 4 long-period planets), and we measure the occurrence rate of hot Jupiters in clusters to be similar to that of the field (~1%). We determine via analyses of hot Jupiter eccentricities and outer companions in these systems that high eccentricity migration mechanisms (those requiring the presence of a third body) are important for migration. The second project, an adaptive optics imaging survey for stellar companions to known hot Jupiter hosts, aims to determine the role that stellar companions in particular play in giant planet migration. Through a preliminary analysis, we derive a lower limit on the binary frequency of 45% (greater than that of the typical field star), and we find that the presence of a companion is correlated with misalignment of the spin-orbit angle of the planetary system, as would be expected for stellar Kozai-Lidov migration: at least 74% of misaligned systems reside in binaries. We thus conclude that among high eccentricity migration mechanisms, those requiring a stellar companion play a significant role. Finally, we describe simulations of measurements of the planet population expected to be discovered by TESS, and use these to demonstrate that a strong constraint on the obliquity distribution of small planets can be derived using only TESS photometry, Gaia astrometry, and vsin(i) measurements of the host stars. This obliquity distribution will be a key piece of evidence to help detemine the likely formation and migration histories of small planets, and can contribute to the assessment of the potential for Earth-like planets to harbor life.