We present new H (1.51.8 mu m) photometric and K-1 (1.92.2 mu m) spectroscopic observations of the young exoplanet HD 95086 b obtained with the Gemini Planet Imager. The Hband magnitude has been significantly improved relative to previous measurements, whereas the lowresolution K-1 (lambda/delta lambda approximate to 66) spectrum is featureless within the measurement uncertainties and presents a monotonically increasing pseudocontinuum consistent with a cloudy atmosphere. By combining these new measurements with literature L' photometry, we compare the spectral energy distribution (SED) of the planet to other young planetarymass companions, field brown dwarfs, and to the predictions of grids of model atmospheres. HD 95086 b is over a magnitude redder in K-1 - L' color than 2MASS J120733463932539 b and HR 8799 c and d, despite having a similar L' magnitude. Considering only the near-infrared measurements, HD 95086 b is most analogous to the brown dwarfs 2MASS J2244316+204343 and 2MASS J21481633+4003594, both of which are thought to have dusty atmospheres. Morphologically, the SED of HD 95086 b is best fit by low temperature (T-eff = 8001300 K), low surface gravity spectra from models which simulate high photospheric dust content. This range of effective temperatures is consistent with field L/T transition objects, but the spectral type of HD 95086 b is poorly constrained between early L and late T due to its unusual position the colormagnitude diagram, demonstrating the difficulty in spectral typing young, low surface gravity substellar objects. As one of the reddest such objects, HD 95086 b represents an important empirical benchmark against which our current understanding of the atmospheric properties of young extrasolar planets can be tested.

Through the cosmic matter cycle, the chemical evolution of the Milky Way is imprinted in the elemental abundance patterns of late-type stars (spectral types F to K). Due to their long lifetimes ( 1 Hubble time), these stars are of particular importance when it comes to studying the build-up of elements during the early times of our Galaxy. The chemical composition of the atmospheric layers of such stars is believed to resemble the gas from which they were formed. However, recent observations in globular clusters seem to contradict this assumption. The observations indicate that processes are at work that alter the surface compositions in these stars. The combined effect of processes responsible for an exchange of material between the stellar interior and atmosphere during the main sequence lifetime of the star, is referred to as atomic diffusion. Yet, the extent to which these processes alter surface abundances is still debated. By comparing abundances in unevolved and evolved stars all drawn from the same stellar population, any surface abundance anomalies can be traced. The anomalies, if found, can be compared to theoretical predictions from stellar structure models including atomic diffusion. Globular clusters provide stellar populations suitable to conduct such a comparison. In this thesis, the results of three independent analyses of two globular clusters, NGC 6752 and M4, at different metallicities are presented. The comparison between observations and models yields constraints on the models and finally a better understanding of the physical processes at work inside stars.

Within this past decade, advances in spectropolarimetric analyses allowed mapping surface characteristics of nearby non-resolved stars with unique characteristics. In this study, we attempt mapping of the magnetic field structure and surface distribution of elements for such a star, the magnetic Ap star phi Dra. Using high-precision photometric data from TESS satellite, we improve its rotation period. Spectrum synthesis calculations allow compilation of a list of lines present in its spectrum. The resulting synthetic spectrum and observed NARVAL spectra are used to re-estimate element abundances. Least-squares deconvolved (LSD) intensity and circular polarisation profiles are then computed from a sample of 1260 metal lines. To determine which element(s) are most suitable for in-depth analysis, variability of LSD profiles is studied qualitatively for Fe, Cr and Si. The longitudinal magnetic field of phi Dra is calculated from LSD circular polarisation profiles. Stellar magnetic field maps and distributions of Fe concentration are derived through Zeeman Doppler Imaging (ZDI). The resulting maps of this study show five areas of high Fe concentrations, in the Northern stellar hemisphere. The magnetic field topology of phi Dra resulting from our analysis is that of an offset dipole with small quadrupole contributions. Our abundance and magnetic maps suggest correlation between high concentrations of Fe and high magnetic field strength. The field is primarily radial in 4 out of 5 such regions, contrary to theoretical expectations.