The information about the chemical compositions of stars is encoded in their spectra. Accurate determinations of these compositions are crucial for our understanding of stellar nucleosynthesis and Galactic chemical evolution. The determination of elemental abundances in stars requires models for the stellar atmospheres and the processes of line formation. Nearly all spectroscopic analyses of late-type stars carried out today are based on one-dimensional (1D), hydrostatic model atmospheres and on the assumption of local thermodynamic equilibrium (LTE). This approach can lead to large systematic errors in the predicted stellar atmospheric structures and line-strengths, and, hence, in the derived stellar abundances. In this thesis, examples of departures from LTE and from hydrostatic equilibrium are explored. The effects of background line opacities (line-blocking) due to atomic lines on the statistical equilibrium of Fe are investigated in late-type stars. Accounting for this line opacity is important at solar metallicity, where line-blocking significantly reduces the rates of radiatively induced ionizations of Fe. On the contrary, the effects of line-blocking in metal-poor stars are insignificant. In metal-poor stars, the dominant uncertainty in the statistical equilibrium of Fe is the treatment of inelastic H+Fe collisions. Substantial departures of Fe abundances from LTE are found at low metallicities: about 0.3 dex with efficient H+Fe collisions and about 0.5 dex without. The impact of three-dimensional (3D) hydrodynamical model atmospheres on line formation in red giant stars is also investigated. Inhomogeneities and correlated velocity fields in 3D models and differences between the mean 3D stratifications and corresponding 1D model atmospheres can significantly affect the predicted line strengths and derived abundances, in particular at very low metallicities. In LTE, the differences between 3D and 1D abundances of C, N, and O derived from CH, NH, and OH weak low-excitation lines are in the range -0.5 dex to -1.0 dex at [Fe/H]=-3. Large negative corrections (about -0.8 dex) are also found in LTE for weak low-excitation neutral Fe lines. We also investigate the impact of 3D hydrodynamical model stellar atmospheres on the determination of elemental abundances in the carbon-rich, hyper iron-poor stars HE 0107-5240 and HE 1327-2326. The lower temperatures of the line-forming regions of the 3D models compared with 1D models cause changes in the predicted spectral line strengths. In particular we find the 3D abundances of C, N, and O to be lower by about -0.8 dex (or more) than estimated from a 1D analysis. The 3D abundance of Fe is decreased but only by -0.2 dex. Departures from LTE for Fe might actually be very large for these stars and dominate over the effects due to granulation.

I developed a spectrum synthesis method to investigate the physical properties of hot OB main sequence (MS) stars, which are often rapid rotators. The code realistically simulates the surface conditions of rapidly rotating stars, considering the rotationally-deformed stellar shape and gravity darkening effect. Comparing the synthesized absorption line profiles with the observed spectra of the member stars of 19 young Galactic clusters, I determined the projected rotational velocities of 496 stars. The average surface temperature and gravity for most of these objects were then derived from H$\gamma$ line fits. The polar gravity of each star was estimated as a good indicator of the evolutionary status of the star. The derived parameters show that massive rotators spin down during their MS phase. The He abundance data (measured by fitting the He I lines) also favor the theoretical prediction that rotationally-induced mixing can gradually enrich the surface helium abundance through the MS phase. A quicker spin-down is evident in the lower mass star group where a large portion of helium peculiar stars are found. This correlation implies that strong stellar magnetic fields may exist on the surface of these lower mass stars. The same method is also applied to interferometric observations from the CHARA Array of a nearby rapid rotator, Regulus. By combining results from spectroscopic and interferometric analysis, the shape, space orientation, mass, and surface temperature distribution of Regulus are firmly determined. This analysis provides the first evidence of the gravity darkening phenomenon among stars that are not components of an eclipsing binary system. The surprisingly high luminosity determined for Regulus appears to agree with the theoretical prediction that rapid rotator can become more luminous as rotationally-induced mixing brings fresh hydrogen down to the core. Finally I present an extension of the model that simulates the shape, velocity, and temperature variations of a star experiencing nonradial pulsation. I simulated and analyzed the line profile variations in the spectra of $\epsilon$ Per, a B0.7~III star with strong evidence of nonradial pulsation (NRP). A comparison of the model simulations and observations indicates that the pulsations of $\epsilon$ Per have a corresponding local temperature variation that is out of phase with the radial oscillation (a non-adiabatic phase lag).

Há uma concentração de aglomerados globulares moderadamente pobres em metais no bojo galáctico, e muitos deles mostram um Ramo Horizontal Azul (BHB). Essas características juntas apontam para uma idade antiga. Para entender melhor a origem desses aglomerados, o estudo de seu padrão de abundâncias pode ajudar a identificar o tipo das primeiras supernovas nas partes centrais da Galáxia. O NGC 6522 na janela do Baade é um representante desta classe de aglomerados. Análises de abundância de estrelas individuais nesses aglomerados confirmaram sua metalicidade de [Fe/H] -1.0, enriquecimento em elementos-$\\alpha$ e detectaram uma variação nas abundâncias dos elementos pesados de processo-s. Entre os maiores enriquecimentos em Y e Ba, a explicação usual da transferência de massa de uma companheira do ramo assintótico das gigantes pode não se aplicar, e um enriquecimento por estrelas massivas de alta rotação foi sugerido. A fim de estudar melhor as abundâncias em NGC 6522, obtivemos um programa com o FLAMES em 2012, a partir da qual, com os dados de UVES, mostramos que o enriquecimento em elementos-s ainda poderia ser acomodado com o modelo de transferência de massa de uma estrela companheira. Além disso, obtivemos novos dados com o FLAMES em 2016. No presente trabalho analisamos outras 6 estrelas observadas em alta resolução com UVES, e 32 estrelas em resolução média-alta observadas com GIRAFFE, onde foram selecionadas por suas velocidades radiais em torno de -14,3 km/s com uma abrangência de ±15 km/s. Os parâmetros atmosféricos e as abundâncias dos elementos leves C, N, O, elementos de Z ímpar Na e Al, elementos-$alpha$ Mg, Si, Ca, Ti, elementos de pico de ferro Mn, Cu, Zn, elementos de processo-s Y, Zr, Ba, La, Ce, Nd e o elemento de processo-r Eu são derivados para a amostra UVES e preliminarmente para a amostra GIRAFFE. Entre as estrelas UVES, duas delas mostram um enriquecimento significativo nos elementos do processo-s e uma com alto valor nas razões [Y/Ba] e [Zr/Ba], sugerindo um enriquecimento por estrelas massivas de alta rotação. / There is a concentration of moderately metal-poor globular clusters in the Galactic bulge, and many of them show a Blue Horizontal Branch (BHB). These characteristics together point to an old age. In order to better understand the origin of these clusters, the study of their abundance pattern can help identifying the kind of the earliest supernovae in the central parts of the Galaxy. NGC 6522 in Baades Window is a representative of this class of clusters. Abundance analyses of individual stars in this clusters have confirmed its metallicity of [Fe/H]-1.0, enhanced -elements, and detected a variation in the abundances of the s-process heavy elements. Among the highest enhancements of Y and Ba, the usual explanation of mass transfer from a companion in the Asymptotic Giant Branch might not apply, and an enrichment by early fast-rotating massive stars was suggested. In order to further study the abundances in NGC 6522 we obtained a run with FLAMES- UVES in 2012, from which with the UVES data we have shown that the enhancement in s-elements could still be accommodated with the companion transfer model. We further obtained new data with FLAMES-UVES in 2016. In the present work we analyze another 6 stars observed at high resolution with UVES, and 32 stars at medium-high resolution observed with GIRAFFE. The latter were selected from their radial velocities of -14.3±15 km/s. The abundances of the light elements C, N, O, odd-Z elements Na, Al, -elements Mg, Si, Ca, Ti, iron-peak elements Mn, Cu, Zn, s-process elements Y, Zr, Ba, La, Ce, Nd and r-process element Eu are derived. Among the UVES stars, two of them show a significant enrichment in s-process elements and one of them show high [Y/Ba] and [Zr/Ba] ratios, suggesting that an early enrichment by fast rotating massive stars is a probable scenario.

Há uma concentração de aglomerados globulares moderadamente pobres em metais no bojo galáctico, e muitos deles mostram um Ramo Horizontal Azul (BHB). Essas características juntas apontam para uma idade antiga. Para entender melhor a origem desses aglomerados, o estudo de seu padrão de abundâncias pode ajudar a identificar o tipo das primeiras supernovas nas partes centrais da Galáxia. O NGC 6522 na janela do Baade é um representante desta classe de aglomerados. Análises de abundância de estrelas individuais nesses aglomerados confirmaram sua metalicidade de [Fe/H] -1.0, enriquecimento em elementos-$\\alpha$ e detectaram uma variação nas abundâncias dos elementos pesados de processo-s. Entre os maiores enriquecimentos em Y e Ba, a explicação usual da transferência de massa de uma companheira do ramo assintótico das gigantes pode não se aplicar, e um enriquecimento por estrelas massivas de alta rotação foi sugerido. A fim de estudar melhor as abundâncias em NGC 6522, obtivemos um programa com o FLAMES em 2012, a partir da qual, com os dados de UVES, mostramos que o enriquecimento em elementos-s ainda poderia ser acomodado com o modelo de transferência de massa de uma estrela companheira. Além disso, obtivemos novos dados com o FLAMES em 2016. No presente trabalho analisamos outras 6 estrelas observadas em alta resolução com UVES, e 32 estrelas em resolução média-alta observadas com GIRAFFE, onde foram selecionadas por suas velocidades radiais em torno de -14,3 km/s com uma abrangência de ±15 km/s. Os parâmetros atmosféricos e as abundâncias dos elementos leves C, N, O, elementos de Z ímpar Na e Al, elementos-$alpha$ Mg, Si, Ca, Ti, elementos de pico de ferro Mn, Cu, Zn, elementos de processo-s Y, Zr, Ba, La, Ce, Nd e o elemento de processo-r Eu são derivados para a amostra UVES e preliminarmente para a amostra GIRAFFE. Entre as estrelas UVES, duas delas mostram um enriquecimento significativo nos elementos do processo-s e uma com alto valor nas razões [Y/Ba] e [Zr/Ba], sugerindo um enriquecimento por estrelas massivas de alta rotação. / There is a concentration of moderately metal-poor globular clusters in the Galactic bulge, and many of them show a Blue Horizontal Branch (BHB). These characteristics together point to an old age. In order to better understand the origin of these clusters, the study of their abundance pattern can help identifying the kind of the earliest supernovae in the central parts of the Galaxy. NGC 6522 in Baades Window is a representative of this class of clusters. Abundance analyses of individual stars in this clusters have confirmed its metallicity of [Fe/H]-1.0, enhanced -elements, and detected a variation in the abundances of the s-process heavy elements. Among the highest enhancements of Y and Ba, the usual explanation of mass transfer from a companion in the Asymptotic Giant Branch might not apply, and an enrichment by early fast-rotating massive stars was suggested. In order to further study the abundances in NGC 6522 we obtained a run with FLAMES- UVES in 2012, from which with the UVES data we have shown that the enhancement in s-elements could still be accommodated with the companion transfer model. We further obtained new data with FLAMES-UVES in 2016. In the present work we analyze another 6 stars observed at high resolution with UVES, and 32 stars at medium-high resolution observed with GIRAFFE. The latter were selected from their radial velocities of -14.3±15 km/s. The abundances of the light elements C, N, O, odd-Z elements Na, Al, -elements Mg, Si, Ca, Ti, iron-peak elements Mn, Cu, Zn, s-process elements Y, Zr, Ba, La, Ce, Nd and r-process element Eu are derived. Among the UVES stars, two of them show a significant enrichment in s-process elements and one of them show high [Y/Ba] and [Zr/Ba] ratios, suggesting that an early enrichment by fast rotating massive stars is a probable scenario.