PARÂMETROS ATMOSFÉRICOS E CAMPOS MAGNÉTICOS DE ESTRELAS ANÃS M OBSERVADAS PELO LEVANTAMENTO APOGEE

Abstract

This thesis presents a spectroscopic analysis of M dwarf stars using high-resolution (R$\sim$22.500) near-infrared (1,514--1,696 $\mu$m) spectra from the SDSS/APOGEE survey. Our methodology adopts spectrum synthesis with LTE MARCS model atmospheres, along with the APOGEE DR17 line list, and the radiative transfer code Turbospectrum. We studied 48 M dwarf stars from the Hyades open cluster and obtained a median metallicity of [M/H]=0.09$\pm$0.03 dex, in good agreement with optical results for Hyades red-giants, and which is chemically homogeneous within the abundance uncertainties. We determined for these stars a median radius inflation between 1.6$\pm$2.3$\%$ and 2.4$\pm$2.3$\%$, which can be explained by a stellar spot coverage of up to 40$\%$. We also derived stellar magnetic fields for a sample containing 62 M-dwarf members of the Pleiades open cluster (with effective temperatures between 3400 K $\lesssim$ T$_{\rm eff} \lesssim$4000 K), using Fe I lines that are sensitive to magnetic fields. These calculations used the radiative transfer code Synmast, and a methodology based on Monte Carlo and Markov Chain (MCMC). The obtained mean magnetic fields vary between $\sim$1.0 and $\sim$4.2 kG, with most of the stars of the sample being in the saturated regime of magnetic fields. The studied M dwarfs present a median radius inflation ranging from +3.0$\%$ to +7.0$\%$, depending on the isochrone model selected. There is a positive correlation between magnetic field strength and radius inflation, as well as with stellar spot coverage, correlations that together indicate that stellar spots generated by strong magnetic fields might be the mechanism that drives radius inflation in these stars. We also derived atmospheric parameters for a sample containing 34 planet-hosting M dwarf stars. The oxygen abundances and metallicities obtained for the studied M dwarf stars show that [O/M] versus [M/H] are in good agreement with galactic evolution models for the solar neighborhood. We derived planetary radii for 47 exoplanets orbiting these stars. Most of the studied planets are Super-Earths; the planet radius distribution has a peak at 1.2 –- 1.4 R$_{\oplus}$, followed by a decline for larger planetary radii, and a drop at 1.8 –- 2.0 R$_{\oplus}$, corresponding to the radius valley. The slope of the radius valley with orbital periods and insolation shows that planetesimal impacts may be the dominant mechanism for the creation of the radius valley for this sample. We separated our sample of exoplanets into multi-planetary systems, and systems containing only one detected planet. We identified that the first group shows a positive linear relation between stellar metallicities and planetary radii, while the latter presents two regimes, with Earth-sized and Super-Earths planets orbiting stars with [M/H]$<$0, and planets of different sizes, including sub-Neptunes orbiting stars with higher metallicities. Finally, we found that the exoplanets members of multi-planetary systems of the sample orbit on average lower-metallicity stars ($<$[M/H]$>$=$-$0.29±0.16) than single exoplanets ($<$[M/H]$>$=$-$0.02±0.18). We also derive mean magnetic fields for 29 M dwarf stars of this sample. Differently from what we found for the Pleiades cluster, the mean magnetic fields of this sample are in the unsaturated regime, presenting values ranging between $\sim$0.2 to $\sim$1.5 kG. We studied habitability for 43 exoplanets that orbit these stars and found that only the exoplanets Kepler-186f and TOI-700d are inside their respective habitable zones, with the other studied exoplanets presenting equilibrium temperatures and insolation levels too high to maintain liquid water on the surface. We evaluated the minimum planetary magnetic field that would be necessary for the exoplanets to maintain a present-day Earth magnetosphere and found values for Kepler-186f and TOI-700d of respectively 0.65 and 3.02 G. Considering a young Earth magnetosphere of 3.4 Gyr ago when there was already life on Earth, the minimum planetary magnetic fields are respectively 0.05 and 0.24 G. These results suggest that these two exoplanets might be able to protect their atmospheres from stellar winds driven by stellar magnetic fields, and are very interesting for habitability studies.