Caracterização estelar e abundâncias químicas de anãs M hospedeiras de exoplanetas a partir de espectros APOGEE

Abstract

The characterization of exoplanetary atmospheres, one of the objectives of the JWST, depends on precise knowledge of their host stars, especially M dwarfs, whose chemical compositions are difficult to determine. This thesis addresses this need through a detailed, homogeneous spectroscopic analysis of four planet-hosting M dwarfs that are high-priority targets for JWST: TOI-1685, GJ 436, TOI-2445, and GJ 3470. From high-resolution near-infrared (H-band) spectra from the APOGEE survey, atmospheric parameters and chemical abundances for twelve elements were derived using spectral synthesis with MARCS models and the Turbospectrum code. The analysis focused on rock-forming elements and the C/O ratio, which is crucial for planetary formation. The new stellar parameters enabled refinement of the associated exoplanet parameters. As a significant contribution, the results of this work were used to enable the prediction of the mass (3.276+0.448 −0.419 M⊕) and internal structure of TOI-2445 b using stellar chemistry as a reference. The results indicate a dense, rocky world with a core mass fraction similar to Earth’s. The derived stellar C/O ratios (ranging from 0.526 to 0.638) provide the primordial chemical context for the interpretation of JWST data. Placing these results within a broader population context enabled the investigation of potential chemical trends in M dwarfs. A comparison between stars with and without planets revealed no statistically significant differences in the C/O ratio, indicating that any effects associated with planetary formation remain inconclusive within this sample. Conversely, a robust difference was identified in the slope of the C/O versus [Fe/H] relationship between M and FGK dwarfs with exoplanets, suggesting that the local population of M dwarfs has a distinct chemical history, possibly associated with diverse Galactic origins. Finally, this Thesis presents an uncertainty Atlas for determining chemical abundances in M dwarfs with effective temperatures between 3200 and 4000 K, constructed from synthetic spectra in the 1.51–1.69 µm range. The Atlas systematically quantifies the impact of variations in atmospheric parameters, signal-to-noise ratio, and pseudo-continuum definition, providing a valuable tool for realistic uncertainty estimation in future H-band spectroscopic analyses.