The purpose of this dissertation is to study the Red Supergiant stars (RSGs) in the supermassive young cluster Westerlund 1 through a photometric and spectral analysis. Due to its characteristics, Westerlund 1 is one of the most interesting young massive clusters in the Milky Way and has an impressive population of massive stars, with four RSGs among them. This represents a rare oportunity to study this type of stars in a cluster with well known distance and reddening. Apart from contributing to the general knowledge about RSGs, which parameters are poorly known due to their scarcity, an analysis of these four stars will contribute to solve the puzzle of the formation history of Westerlund 1. For the analysis presented here, photometric data of the four RSGs in Westerlund 1 were used, along with near infrared spectra covering the region from ~8400 Å to ~8900 Å, obtained at two different epochs with the 1.60 m telescope at Observatório do Pico dos Dias (OPD) and the Southern Astrophysical Research Telescope (SOAR). Magnitudes obtained with JHK band photometry were used to calculate effective temperatures and bolometric corrections at the K band through different methods involving the color indices (V-K)_0 and (J-K)_0, in order to determine the luminosities and locate the four RSGs in a Hertzsprung-Russell diagram (HRD). A spectral analysis was performed with the aim of estimating spectral types and the same stellar parameters calculated with the photomety. First, a visual comparison of the depth and strength of the main spectral features (TiO and VO bandheads, and CaT and Fe I lines) was carried out to classify the stars by spectral type. For this purpose, spectra of the four RSGs in the cluster were compared to a group of spectra from well studied reference stars, some of them also RSGs. Then, given their strong dependence on Teff, equivalent widths (EWs) of Fe I lines were measured in all the spectra available. By comparing the EWs measured on spectra of the four RSGs and the reference stars, the Teff\'s of the most similar reference stars were assigned to the RSGs in the cluster. With the Teff\'s obtained through the spectral analysis, new points were added to the initial HRD. The final objective of the present work was to estimate the initial masses and ages of the RSGs in Westerlund 1. This was accomplished by superimposing up to date evolutionary tracks and isochrones to the HRD. Masses and ages were determined for the cases with and without rotation, obtaining an average age for the RSGs of 8 Myr, thus doubling the age of the host cluster determined from the pre-main sequence (PMS). In the final part, it is presented a brief discussion about the discrepancy of the age of Westerlund 1 measured with the PMS isochrones and the age of the RSGs, as well as the uncertainties it raises regarding the formation history of the cluster. / O objetivo desta dissertação é estudar as estrelas Supergigantes Vermelhas (RSGs) no aglomerado jovem supermassivo Westerlund 1 através de uma análise fotométrica e espectral. Devido às suas características, Westerlund 1 é um dos aglomerados jovens mais interessantes da Via Láctea e tem uma população importante de estrelas massivas, com quatro RSGs entre elas. Isso representa uma oportunidade rara para estudar este tipo de estrelas em um aglomerado com a distância e avermelhamento bem conhecidos. Além de contribuir para o conhecimento geral sobre RSGs, cujos parâmetros são pouco conhecidos devido à sua escassez, uma análise dessas quatro estrelas contribuirá para resolver o enigma da história da formação de Westerlund 1. Para a análise apresentada aqui, foram utilizados dados fotométricos das quatro RSGs em Westerlund 1, juntamente com espectros no infravermelho próximo cobrindo a região de ~8400 Å a ~8900 Å, obtidos em duas épocas diferentes com o telescópio de 1.60 m do Observatório do Pico dos Dias (OPD) e o Southern Astrophysical Research Telescope (SOAR). Magnitudes obtidas com a fotometria nos filtros JHK foram usadas para calcular temperaturas efetivas e correções bolométricas na banda K através de diferentes métodos envolvendo os índices de cor (V-K)_0 e (J-K)_0, a fim de determinar as luminosidades e localizar as quatro RSGs em um diagrama de Hertzsprung-Russell (HRD). Uma análise espectral foi realizada com o objetivo de estimar os tipos espectrais e os mesmos parâmetros estelares calculados com a fotometria. Primeiramente, uma comparação visual da profundidade e força das principais características espectrais (as bandas moleculares de TiO e VO e as linhas de CaT e Fe I) foi realizada para classificar as estrelas por tipo espectral. Para este propósito, os espectros das quatro RSGs no aglomerado foram comparados com um grupo de espectros de estrelas de referência bem estudadas, algumas delas também RSGs. Depois, dada a sua forte dependência com a Teff, as larguras equivalentes (EWs) das linhas de Fe I foram medidas em todos os espectros disponíveis. Comparando as EWs medidas nos espectros das quatro RSGs e nos das estrelas de referência, as Teff das estrelas de referência mais similares foram atribuídas às RSGs do aglomerado. Com as Teff obtidas através da análise espectral, novos pontos foram adicionados ao HRD inicial. O objetivo final do presente trabalho foi estimar as massas iniciais e as idades das RSGs em Westerlund 1. Isto foi realizado através da sobreposição no HRD de caminhos evolutivos e de isócronas. Massas e idades foram determinadas para os casos com e sem rotação, obtendo-se uma média de idade para as RSGs de 8 milhões de anos, duplicando assim a idade do aglomerado ao que as RSGs pertencem, e que foi medida através de estrelas de pré-sequência principal (PMS). Na parte final, é apresentada uma breve discussão sobre a discrepância da idade de Westerlund 1 medida com as isócronas da PMS e a idade das RSGs, bem como as incertezas que isso suscita em relação à história da formação do aglomerado.

The purpose of this dissertation is to study the Red Supergiant stars (RSGs) in the supermassive young cluster Westerlund 1 through a photometric and spectral analysis. Due to its characteristics, Westerlund 1 is one of the most interesting young massive clusters in the Milky Way and has an impressive population of massive stars, with four RSGs among them. This represents a rare oportunity to study this type of stars in a cluster with well known distance and reddening. Apart from contributing to the general knowledge about RSGs, which parameters are poorly known due to their scarcity, an analysis of these four stars will contribute to solve the puzzle of the formation history of Westerlund 1. For the analysis presented here, photometric data of the four RSGs in Westerlund 1 were used, along with near infrared spectra covering the region from ~8400 Å to ~8900 Å, obtained at two different epochs with the 1.60 m telescope at Observatório do Pico dos Dias (OPD) and the Southern Astrophysical Research Telescope (SOAR). Magnitudes obtained with JHK band photometry were used to calculate effective temperatures and bolometric corrections at the K band through different methods involving the color indices (V-K)_0 and (J-K)_0, in order to determine the luminosities and locate the four RSGs in a Hertzsprung-Russell diagram (HRD). A spectral analysis was performed with the aim of estimating spectral types and the same stellar parameters calculated with the photomety. First, a visual comparison of the depth and strength of the main spectral features (TiO and VO bandheads, and CaT and Fe I lines) was carried out to classify the stars by spectral type. For this purpose, spectra of the four RSGs in the cluster were compared to a group of spectra from well studied reference stars, some of them also RSGs. Then, given their strong dependence on Teff, equivalent widths (EWs) of Fe I lines were measured in all the spectra available. By comparing the EWs measured on spectra of the four RSGs and the reference stars, the Teff\'s of the most similar reference stars were assigned to the RSGs in the cluster. With the Teff\'s obtained through the spectral analysis, new points were added to the initial HRD. The final objective of the present work was to estimate the initial masses and ages of the RSGs in Westerlund 1. This was accomplished by superimposing up to date evolutionary tracks and isochrones to the HRD. Masses and ages were determined for the cases with and without rotation, obtaining an average age for the RSGs of 8 Myr, thus doubling the age of the host cluster determined from the pre-main sequence (PMS). In the final part, it is presented a brief discussion about the discrepancy of the age of Westerlund 1 measured with the PMS isochrones and the age of the RSGs, as well as the uncertainties it raises regarding the formation history of the cluster. / O objetivo desta dissertação é estudar as estrelas Supergigantes Vermelhas (RSGs) no aglomerado jovem supermassivo Westerlund 1 através de uma análise fotométrica e espectral. Devido às suas características, Westerlund 1 é um dos aglomerados jovens mais interessantes da Via Láctea e tem uma população importante de estrelas massivas, com quatro RSGs entre elas. Isso representa uma oportunidade rara para estudar este tipo de estrelas em um aglomerado com a distância e avermelhamento bem conhecidos. Além de contribuir para o conhecimento geral sobre RSGs, cujos parâmetros são pouco conhecidos devido à sua escassez, uma análise dessas quatro estrelas contribuirá para resolver o enigma da história da formação de Westerlund 1. Para a análise apresentada aqui, foram utilizados dados fotométricos das quatro RSGs em Westerlund 1, juntamente com espectros no infravermelho próximo cobrindo a região de ~8400 Å a ~8900 Å, obtidos em duas épocas diferentes com o telescópio de 1.60 m do Observatório do Pico dos Dias (OPD) e o Southern Astrophysical Research Telescope (SOAR). Magnitudes obtidas com a fotometria nos filtros JHK foram usadas para calcular temperaturas efetivas e correções bolométricas na banda K através de diferentes métodos envolvendo os índices de cor (V-K)_0 e (J-K)_0, a fim de determinar as luminosidades e localizar as quatro RSGs em um diagrama de Hertzsprung-Russell (HRD). Uma análise espectral foi realizada com o objetivo de estimar os tipos espectrais e os mesmos parâmetros estelares calculados com a fotometria. Primeiramente, uma comparação visual da profundidade e força das principais características espectrais (as bandas moleculares de TiO e VO e as linhas de CaT e Fe I) foi realizada para classificar as estrelas por tipo espectral. Para este propósito, os espectros das quatro RSGs no aglomerado foram comparados com um grupo de espectros de estrelas de referência bem estudadas, algumas delas também RSGs. Depois, dada a sua forte dependência com a Teff, as larguras equivalentes (EWs) das linhas de Fe I foram medidas em todos os espectros disponíveis. Comparando as EWs medidas nos espectros das quatro RSGs e nos das estrelas de referência, as Teff das estrelas de referência mais similares foram atribuídas às RSGs do aglomerado. Com as Teff obtidas através da análise espectral, novos pontos foram adicionados ao HRD inicial. O objetivo final do presente trabalho foi estimar as massas iniciais e as idades das RSGs em Westerlund 1. Isto foi realizado através da sobreposição no HRD de caminhos evolutivos e de isócronas. Massas e idades foram determinadas para os casos com e sem rotação, obtendo-se uma média de idade para as RSGs de 8 milhões de anos, duplicando assim a idade do aglomerado ao que as RSGs pertencem, e que foi medida através de estrelas de pré-sequência principal (PMS). Na parte final, é apresentada uma breve discussão sobre a discrepância da idade de Westerlund 1 medida com as isócronas da PMS e a idade das RSGs, bem como as incertezas que isso suscita em relação à história da formação do aglomerado.

Surface brightness asymmetries are a very common feature of stars. Among other effects they cause a difference between the projected centre of mass and the photocentre. The evolution of those surface features makes this difference time-dependent. In some cases the displacement can be a non-negligible fraction of the star radius R, and if R>1 AU, of the parallax. We investigate the impact of surface brightness asymmetries on the Gaia astrometric solution and on the data processing flow. In particular we derive analytical expressions for the change in the derived astrometric parameters for a single-star, with respect to the parameters for a uniformly-bright star, as a function of the characteristics of the surface brightness asymmetries. These predictions are confirmed by the results of the processing of simulated astrometric Gaia data where a photocentre motion caused by surface brightness asymmetries has been added using a Gaussian Markovian model.<p>In the case of a red supergiant star, the average photocentre shift is about 0.1 AU. Such a photocentric noise translates in a 10% inaccuracy on the parallax (independently of the distance), which becomes larger than the statistical error on the parallax derived from the data reduction for stars that are up to about 4 kpc away. For the most nearby stars, we derive an inaccuracy on the parallax that can be 10 times its statistical error. Finally we estimate that up to about 4000 stars among red supergiants and bright giants may have astrometric parameters that are inaccurate at levels bigger than expected because of the surface brightness asymmetries. In the determination of this number, a crucial role is played by the Gaia observable magnitude range. The fact that Gaia will not observe stars brighter than 5.6 in the Gaia G band means that the closest stars will not be observed. Yet, the impact of the surface brightness asymmetries is proportional to their angular size, meaning that the stars whose astrometric accuracy would be most affected are not observed.<p>Various non-Gaussian spot models (as applicable in the case of magnetic spots) have been implemented and analytical predictions for the effects of such magnetic spots are computed for the most representative classes of magnetic stars.<p>Another effect of the presence of surface brightness asymmetries is their impact on Gaia data processing flow. The quality of the fit of the data is evaluated with the F2 parameter that is a transformation of χ2 such that it has a unit normal distribution when the model is adequate and it is independent of the number of measurements. If the goodness-of-fit F2 of the single-star solution is not good enough (F2>3), a chain of solution of growing complexity is tried until a satisfactory one (with F2<3) is obtained. If no good solution is found, a so-called stochastic solution is computed where a "cosmic" error is added to the data in order to obtain a single-star solution with F2=0. We show that the photocentre noise induces an increase in the goodness-of-fit parameter, causing this chain of solutions to be entered. Depending on the characteristics of the photocentre noise, a variable fraction of the stars in our simulations end up with a non-single-star solution. Yet, we show that these (orbital) solutions are not acceptable because non-significant or non-physical. Finally, an important fraction of stars is assigned a stochastic solution with a cosmic noise matching well the photocentric noise.<p><p>/<p><p>Les asymétries de brillance de surface sont une caractéristique commune des étoiles. Parmi d'autres effets, elles provoquent une différence entre la projection du centre de masse et le photocentre. L'évolution de ces structures de surface rend cette différence variable avec le temps. Dans certains cas, le déplacement du photocentre peut être une fraction non négligeable du rayon de l'étoile R et, si R>1 UA, de la parallaxe. Nous examinons l'impact des asymétries de brillance de surface sur la solution astrométrique de Gaia et sur le processus de traitement des données. En particulier nous dérivons des expressions analytiques pour le changement des paramètres astrométriques déerivées pour une étoile simple, par rapport aux paramètres pour une étoile uniformément lumineuse, en fonction des caractéristiques des asymétries de brillance de surface. Ces prévisions sont confirmées par les résultats de simulations du traitement des données astrométriques de Gaia, auxquelles des mouvements du photocentre causés par des asymétries de brillance de surface ont été ajoutés en utilisant un modèle gaussien markovien.<p><p>Dans le cas d'une étoile super-géante rouge, le décalage moyen du photocentre est d'environ 0.1 UA. Un bruit photocentrique de cette amplitude se traduit dans une imprécision de 10% sur la parallaxe (indépendamment de la distance), qui peut devenir plus grande que l'erreur statistique sur la parallaxe déerivée par la réduction des données, pour les étoiles plus proches d'environ 4 kpc. Pour les étoiles les plus proches, nous évaluons une imprécision sur la parallaxe qui peut être 10 fois leur erreur statistique. Finalement, nous estimons que jusqu'à environ 4000 étoiles parmi les super-géantes rouges et géantes brillantes peuvent avoir des paramètres astrométriques inexactes à des niveaux plus grands que prévu en raison des asymétries de brillance de surface. Dans la détermination de ce nombre, la gamme de magnitudes observables par Gaia joue un rôle crucial. Le fait que Gaia n'observera pas les étoiles plus brillantes que 5.6 mag (en bande Gaia) signifie que les étoiles les plus proches ne seront pas observées. Pourtant, l'impact des asymétries de brillance de surface est proportionnel à leur taille angulaire, ce qui signifie que les étoiles dont la précision astrométrique seraient la plus affecté ne seront pas observées.<p>Différents modèles de taches ont été réalisés et des prédictions analytiques pour les effets de ces taches magnétiques sont calculés pour les classes les plus représentatives des étoiles magnétiques. <p>Un autre effet de la présence des asymétries de brillance de surface est leur impact sur le traitement des données de Gaia. La qualité de l'ajustement des données est évaluée avec le paramètre F2 qui est une transformation de χ2 telle qu'il ait une distribution normale lorsque le modèle est adéquat. Si la qualité de l'ajustement F2 de la solution étoile-simple n'est pas acceptable (F2>3), une chaîne de solutions de complexité croissante est essayée jusqu'à ce qu'une solution satisfaisante (avec F2<3) soit obtenue. Si aucune solution satisfaisante n'est trouvée, une solution dite stochastique est calculée où une erreur "cosmique" est ajoutée aux données afin d'obtenir une solution étoile-simple avec F2=0. Nous montrons que le bruit du photocentre induit une augmentation de F2, ce qui provoque l'activation de cette chaîne de solutions. Selon les caractéristiques du bruit du photocentre, une solution étoile-non-simple est obtenue pour une fraction variable des étoiles dans nos simulations. Nous montrons que ces solutions (orbitales) ainsi obtenues ne sont pas acceptables car non significatives ou non-physiques. Enfin, une fraction importante d'étoiles se voient attribuer une solution stochastique avec un bruit cosmique correspondant au bruit photocentrique. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished