As estrelas azuis tardias (blue straggler stars ou estrelas BS) são estrelas de sequência principal que apresentam um aparente atraso evolutivo em relação às suas vizinhanças. Elas foram identificadas inicialmente na sequência principal de aglomerados globulares acima do ponto de turnoff no diagrama HR. Desde então, têm sido encontradas em todos os ambientes estelares: aglomerados abertos e globulares, galáxias anãs próximas e entre as estrelas de campo na Galáxia. O principal objetivo deste trabalho é construir uma grande amostra de estrelas BS no campo galáctico. Para tanto, métodos de separação desses objetos de estrelas BHB foram comparados com critérios de seleção que envolvem parâmetros atmosféricos estimados pelo SEGUE Stellar Parameter Pipeline (SSPP). Tal procedimento permitiu incluir também estrelas com magnitudes g maiores que 18, antes excluídas pelos métodos tradicionais para objetos com razão sinal-ruído menores que 9. Os métodos apresentados neste trabalho permitiram a seleção de uma amostra de 8001 candidatas a estrelas BS que foram analisadas cinematicamente, através de suas velocidades radiais. Verificou-se que aproximadamente um quinto dessa amostra (cerca de 1500 objetos) possui características que a associa à corrente de Sagitário, sugerindo uma origem extragaláctica para tais objetos. / Blue straggler stars (BSSs) are main sequence stars that exhibit an apparent evolutionary delay over the region where they are found in the HR diagram. They were initially identified in the main sequence of globular clusters above the turnoff point. Since then, they have been found in many different stellar environments: globular and open clusters, near dwarf galaxies, and also in the field of the Galaxy. The main goal of this work is to build a large sample of BSSs in the galactic field region. In order to accomplish this task, classical methods of separation of BSS from BHB stars were compared with proposed restrictions based on their atmospheric parameters, which are estimated by the SEGUE Stellar Parameter Pipeline (SSPP). The latter procedure allowed to include stars with magnitudes g greater than 18, not handled by traditional methods when signal-noise ratio is smaller than 9. The selection methods gathered a sample of 8001 BS stars. These stars were kinematically analyzed through their radial velocities. The results suggest that many of them (about 1500) can have extragalactic origin, associated with Sagittarius stream.

Thesis (Ph. D.)--Ohio State University, 2005. / Title from first page of PDF file. Document formatted into pages; contains xii, 190 p.; also includes graphics (some col.). Includes bibliographical references (p. 181-190). Available online via OhioLINK's ETD Center.

As estrelas azuis tardias (blue straggler stars ou estrelas BS) são estrelas de sequência principal que apresentam um aparente atraso evolutivo em relação às suas vizinhanças. Elas foram identificadas inicialmente na sequência principal de aglomerados globulares acima do ponto de turnoff no diagrama HR. Desde então, têm sido encontradas em todos os ambientes estelares: aglomerados abertos e globulares, galáxias anãs próximas e entre as estrelas de campo na Galáxia. O principal objetivo deste trabalho é construir uma grande amostra de estrelas BS no campo galáctico. Para tanto, métodos de separação desses objetos de estrelas BHB foram comparados com critérios de seleção que envolvem parâmetros atmosféricos estimados pelo SEGUE Stellar Parameter Pipeline (SSPP). Tal procedimento permitiu incluir também estrelas com magnitudes g maiores que 18, antes excluídas pelos métodos tradicionais para objetos com razão sinal-ruído menores que 9. Os métodos apresentados neste trabalho permitiram a seleção de uma amostra de 8001 candidatas a estrelas BS que foram analisadas cinematicamente, através de suas velocidades radiais. Verificou-se que aproximadamente um quinto dessa amostra (cerca de 1500 objetos) possui características que a associa à corrente de Sagitário, sugerindo uma origem extragaláctica para tais objetos. / Blue straggler stars (BSSs) are main sequence stars that exhibit an apparent evolutionary delay over the region where they are found in the HR diagram. They were initially identified in the main sequence of globular clusters above the turnoff point. Since then, they have been found in many different stellar environments: globular and open clusters, near dwarf galaxies, and also in the field of the Galaxy. The main goal of this work is to build a large sample of BSSs in the galactic field region. In order to accomplish this task, classical methods of separation of BSS from BHB stars were compared with proposed restrictions based on their atmospheric parameters, which are estimated by the SEGUE Stellar Parameter Pipeline (SSPP). The latter procedure allowed to include stars with magnitudes g greater than 18, not handled by traditional methods when signal-noise ratio is smaller than 9. The selection methods gathered a sample of 8001 BS stars. These stars were kinematically analyzed through their radial velocities. The results suggest that many of them (about 1500) can have extragalactic origin, associated with Sagittarius stream.

Cette thèse est une étude de l'origine des étoiles chaudes à grande vitesse et/ou à grande distance du plan galactique, à l'aide des données du satellite Hipparcos. L'existence de ce type d'étoiles montre une incohérence entre le temps de vie assigné par la théorie de l'évolution stellaire aux étoiles de type B et A de Population I et celui beaucoup plus long des mécanismes responsables de l'augmentation de la dispersion des vitesses. Dans un premier temps, l'effet de la rotation stellaire sur les magnitudes absolues tirées des parallaxes trigonométriques est analysé sur un échantillon d'étoiles A normales de séquence principale. Aucune variation significative n'est observée. Une méthode de détermination des vitesses radiales est développée pour les spectres observés avec le spectrographe Élodie (OHP). Elle permet le calcul de la vitesse radiale avec une précision de l'ordre de 1-2 km/s sans limitation due au vsini de l'étoile observée. Dans un second temps, un échantillon de 35000 étoiles de type B-A-F tirées du catalogue Hipparcos est analysé. 316 étoiles à grande vitesse sont identifiées et les différentes hypothèses expliquant leur origine sont discutées. En particulier, la contribution des blue stragglers est étudiée à l'aide d'un modèle de synthèse de populations. Il est montré que les blue stragglers du disque mince représentent une source d'étoiles à grande vitesse essentiellement dans les étoiles de type précoce (B et A) alors que les blue stragglers du disque épais concourent aux étoiles de type A tardif et F.

La consommation d’énergie est une préoccupation importante pour les systèmes de traitement Big Data à grande échelle, ce qui entraîne un coût monétaire énorme. En raison de l’hétérogénéité du matériel et des conflits entre les charges de travail simultanées, les stragglers (i.e., les tâches qui sont relativement plus lentes que les autres tâches) peuvent augmenter considérablement le temps d’exécution et la consommation d’énergie du travail. Par conséquent, l’atténuation des stragglers devient une technique importante pour améliorer les performances des systèmes de traitement Big Data à grande échelle. Typiquement, il se compose de deux phases: la détection de stragglers et la manipulation de stragglers. Dans la phase de détection, les tâches lentes (par exemple, les tâches avec une vitesse ou une progression inférieure à la moyenne) sont marquées en tant que stragglers. Ensuite, les stragglers sont traités en utilisant la technique d’exécution spéculative. Avec cette technique, une copie du straggler détecté est lancée en parallèle avec le straggler dans l’espoir qu’il puisse finir plus tôt, réduisant ainsi le temps d’exécution du straggler. Bien qu’un grand nombre d’études aient été proposées pour améliorer les performances des applications Big Data en utilisant la technique d’exécution spéculative, peu d’entre elles ont étudié l’efficacité énergétique de leurs solutions.Dans le cadre de cette thèse, nous commençons par caractériser l’impact de l’atténuation des stragglers sur la performance et la consommation d’énergie des systèmes de traitement de Big Data. Nous observons que l’efficacité énergétique des techniques actuelles d’atténuation des stragglers pourrait être considérablement améliorée. Cela motive une étude détaillée de ses deux phases: détection de straggler et traitement de straggler. En ce qui concerne la détection de straggler, nous introduisons un cadre novateur pour caractériser et évaluer de manière exhaustive les mécanismes de détection de straggler. En conséquence, nous proposons un nouveau mécanisme énergétique de détection de straggler. Ce mécanisme de détection est implémenté dans Hadoop et se révèle avoir une efficacité énergétique plus élevée par rapport aux mécanismes les plus récentes. En ce qui concerne le traitement de straggler, nous présentons une nouvelle méthode pour répartir des copies spéculatives, qui prend en compte l’impact de l’hétérogénéité des ressources sur la performance et la consommation d’énergie. Enfin, nous introduisons un nouveau mécanisme éconergétique pour gérer les stragglers. Ce mécanisme fournit plus de ressources disponibles pour lancer des copies spéculatives, en utilisant une approche de réservation dynamique de ressources. Il est démontré qu’elle améliore considérablement l’efficacité énergétique en utilisant une simulation. / Energy consumption is an important concern for large-scale Big Data processing systems, which results in huge monetary cost. Due to the hardware heterogeneity and contentions between concurrent workloads, stragglers (i.e., tasks performing relatively slower than other tasks) can severely increase the job’s execution time and energy consumption. Consequently, straggler mitigation becomes an important technique to improve the performance of large-scale Big Data processing systems. Typically, it consists of two phases: straggler detection and straggler handling. In the detection phase, slow tasks (e.g., tasks with speed or progress below the average) are marked as stragglers. Then, stragglers are handled using the speculative execution technique. With this technique, a copy of the detected straggler is launched in parallel with the straggler with the expectation that it can finish earlier, thus, reduce the straggler’s execution time. Although a large number of studies have been proposed to improve the performance of Big Data applications using speculative execution technique, few of them have studied the energy efficiency of their solutions. Addressing this lack, we conduct an experimental study to fully understand the impact of straggler mitigation techniques on the performance and the energy consumption of Big Data processing systems. We observe that current straggler mitigation techniques are not energy efficient. As a result, this promotes further studies aiming at higher energy efficiency for straggler mitigation. In terms of straggler detection, we introduce a novel framework for comprehensively characterizing and evaluating straggler detection mechanisms. Accordingly, we propose a new energy-driven straggler detection mechanism. This straggler detection mechanism is implemented into Hadoop and is demonstrated to have higher energy efficiency compared to the state-of-the-art mechanisms. In terms of straggler handling, we present a new speculative copy allocation method, which takes into consideration the impact of resource heterogeneity on performance and energy consumption. Finally, an energy efficient straggler handling mechanism is introduced. This mechanism provides more resource availability for launching speculative copies, by adopting a dynamic resource reservation approach. It is demonstrated, via a trace-driven simulation, to bring a high improvement in energy efficiency.

The orbital parameters of binaries at intermediate periods (10(2)-10(3) d) are difficult to measure with conventional methods and are very incomplete. We have undertaken a new survey, applying our pulsation timing method to Kepler light curves of 2224 main-sequence A/F stars and found 341 non-eclipsing binaries. We calculate the orbital parameters for 317 PB1 systems (single-pulsator binaries) and 24 PB2s (double-pulsators), tripling the number of intermediate-mass binaries with full orbital solutions. The method reaches down to small mass ratios q approximate to 0.02 and yields a highly homogeneous sample. We parametrize the mass-ratio distribution using both inversion and Markov-Chain Monte Carlo forward-modelling techniques, and find it to be skewed towards low-mass companions, peaking at q approximate to 0.2. While solar-type primaries exhibit a brown dwarf desert across short and intermediate periods, we find a small but statistically significant (2.6 sigma) population of extreme-mass-ratio companions (q < 0.1) to our intermediate-mass primaries. Across periods of 100-1500 d and at q > 0.1, we measure the binary fraction of current A/F primaries to be 15.4 per cent +/- 1.4 per cent, though we find that a large fraction of the companions (21 per cent +/- 6 per cent) are white dwarfs in post-mass-transfer systems with primaries that are now blue stragglers, some of which are the progenitors of Type Ia supernovae, barium stars, symbiotics, and related phenomena. Excluding these white dwarfs, we determine the binary fraction of original A/F primaries to be 13.9 per cent +/- 2.1 per cent over the same parameter space. Combining our measurements with those in the literature, we find the binary fraction across these periods is a constant 5 per cent for primaries M-1 < 0.8 M-circle dot, but then increases linearly with log M-1, demonstrating that natal discs around more massive protostars M-1 greater than or similar to M-1(circle dot) become increasingly more prone to fragmentation. Finally, we find the eccentricity distribution of the main-sequence pairs to be much less eccentric than the thermal distribution.

京都大学 / 0048 / 新制・課程博士 / 博士(情報学) / 甲第21280号 / 情博第674号 / 新制||情||116(附属図書館) / 京都大学大学院情報学研究科システム科学専攻 / (主査)教授 田中 利幸, 教授 山下 信雄, 准教授 増山 博之, 教授 笠原 正治 / 学位規則第4条第1項該当 / Doctor of Informatics / Kyoto University / DFAM

<p> The conditions ideally suited for blue straggler star formation are for the most part unknown, though there is mounting evidence to suggest that the preferred blue straggler formation pathway, whether it be via the coalescence of a primordial binary system or through the collision of two single main-sequence stars, depends largely on the cluster environment. In this thesis we are trying to isolate the preferred blue straggler formation mechanism(s) operating in various globular cluster environments by comparing relative blue straggler frequencies to global cluster properties.</p> <p> We define a series of selection rules to isolate the blue stragglers from main-sequence turn-off and extended horizontal branch stars in the colour-magnitude diagrams of 57 globular clusters taken from HST images of their central cores. The boundary conditions were defined using only the main-sequence turn-off as a point of reference, and are hence applied consistently from cluster to cluster. We chose to count only those stars found within one core radius of the cluster center in an effort to obtain a sample that is approximately representative of a uniform cluster environment where, ideally, a single blue straggler formation mechanism is predominantly operating. Relative frequencies of blue straggler stars are then found using the red giant branch for normalization and are subsequently analyzed. We confirm the anticorrelation between relative blue straggler frequency and total integrated cluster luminosity previously observed by Piotto et al. (2004), and find a new anticorrelation between relative blue straggler frequency and the central velocity dispersion, as well as a weak anticorrelation with the half-mass relaxation time. We find no other statistically significant trends. Observational implications pertaining to blue straggler formation mechanisms are then discussed.</p> <p> We present a very simple, semi-analytic model designed in an attempt to reproduce the observed trends in the core. Using estimates for the collisional timescales, we find that only a small percentage of the blue stragglers produced are a direct result of collisions. The majority of the blue stragglers created in our model are thus products of mass-transfer in tight, low-mass binary systems. We were surprised to find a reasonably good agreement between the data and our predictions, given the simplicity of our model. Our results suggest that the binary fraction could be a crucial parameter in shaping blue straggler populations, and hence better and more abundant observations of binary systems and their numbers could prove an important step in gaining a better understanding of blue straggler formation mechanisms.</p> / Thesis / Master of Science (MSc)