ANÁLISES ESPECTROSCÓPICAS DE ESTRELAS HOSPEDEIRAS DE PLANETAS KEPLER 2 E DE ESTRELAS DO AGLOMERADO ABERTO NGC 6705: CARACTERIZAÇÃO E ATIVIDADE ESTELAR, RAIOS PLANETÁRIOS E ABUNDÂNCIAS QUÍMICAS

Abstract

In this thesis we present a spectroscopic study of 201 stars of F-, G-, and K-type stars that host exoplanets discovered by the Kepler and Kepler 2 (K2) mission. We analyzed two samples, one containing 81 stars from K2 and 22 from Kepler, whose spectra were obtained with the Hydra spectrograph (R ~18,500) mounted on the WIYN 3.5 m telescope, covering the spectral range between 6050 to 6350 Å, and another, containing 86 K2 stars, whose Echelle spectra were obtained by the California Planet Search (CPS) program, observed with the HIRES spectrograph (R ~ 60,000) at the Keck I 10 m telescope and spanning the spectral range between approximately 3600 to 8000 Å. We determined the effective temperatures (Teff), surface gravities (log g), and metallicities ([Fe/H]) by adopting the spectroscopic methodology based on measurements of equivalent widths and the excitation and ionization equilibria of the Fe I and Fe II lines. The calculations were carried out in ETL, using the Kurucz atmospheric models and the qoyllur-quipu (q2) package which uses the MOOG radiative transfer code. We constructed a Fe I and II line list adequate for the analysis of Hydra spectra, consisting of 25 lines of Fe I and 5 lines of Fe II. The line list used to analyze HIRES spectra contains 61 lines of Fe I and 13 lines of Fe II, noting that this list does not include lines which are deemed as being sensitive to magnetic fields. Stellar masses and radii were derived by combining the stellar parameters (Teff and [Fe/H]) with Gaia DR3 parallaxes and V magnitudes using isochrones methods and q2 code. The measured stellar radii have a median internal precision of 4.2% for the Hydra sample and 4.6% for the HIRES sample, leading to a median internal uncertainty of 4.4% and 2.6% in the derived planetary radii, respectively. The distribution of planetary radii of the Hydra and HIRES samples for confirmed small planets revealed the well-known radius gap, with a dearth os planets with radii around Rplanet ~ 1.9 R⊕, what would not be observed if our results had lower internal precision. For the Hydra sample, relationships between planetary radius, orbital period, and metallicity were explored. We found that the metallicity of K2 stars increases with planetary radius, becoming more pronounced in the transition from smaller planets (<4.4 R⊕) to larger planets (>4.4 R⊕). Furthermore, it was observed that the median metallicity of K2 stars hosting planets with P < 10 days is slightly metal-rich, while for planets with P ≥ 10 days it is slightly metal-poor. For the HIRES sample, chromospheric activity was measured in 725 spectra corresponding to 144 stars from the Ca II H (λ3968.47 Å) and K (λ3933.66 Å) lines using the Mt. Wilson S_HK index; the S_HK values were also converted to the chromospheric activity indicator log R'_HK. Values of log R'_HK were investigated as a function of stellar rotational period (Prot), and we found that chromospheric activity decreases with increasing Prot, although there is a large scatter in log R'_HK (~0.5 dex) for a given Prot. The distribution of the R' HK indices in our sample of dwarf stars showed a lack of F and G stars with intermediate activity levels, as found by previous studies: the "Vaughan-Preston gap". Finally, for the Hydra and HIRES samples, we investigated the effect of stellar activity on the determination of stellar parameters, including magnetic field sensitive Fe I lines in the spectroscopic analysis. No significant differences were found between the analyses with and without magnetic field sensitive lines. However, it is noted that the most active stars (log R'_HK > -5.0) show a larger scatter in the differences in Teff and [Fe/H] between the two analyses. In the second part of this thesis we determined the chemical abundances of 16 elements (C, N, O, Na, Mg, Al, Si, K, Ca, Ti, V, Cr, Mn, Fe, Co, and Ni) in a sample of 11 red giant stars from the Galactic disk cluster NGC 6705. The analysis was based on near-infrared spectra (1.51 - 1.70 μm) from the APOGEE survey using the spectral synthesis technique. The synthetic spectra were calculated in LTE using MARCS models with spherical geometry with the Turbospectrum radiative transfer code. The effective temperatures of the sample stars were determined using photometric calibrations, and the surface gravities were obtained from fundamental equations. Our results suggest that seven stars in the sample may belong to the red giant branch, while four may belong to the red clump. We find a mean cluster metallicity of ⟨[Fe/H]⟩ = +0.13 ± 0.04, indicating that NGC 6705 is metal-rich, as expected for a young cluster in the inner disk of the Galaxy. The red giants of NGC 6705 exhibit low 12C abundances (⟨[12C/Fe]⟩ = -0.16) and enhanced 14N abundances (⟨[14N/Fe]⟩ = +0.51), which are key signatures of the first dredge-up on the red giant branch. NGC 6705 also provides valuable insights for studying stellar mixing, given its turn-off mass of M ~ 3.3 M⊙. An interesting result from this detailed study of chemical abundances is the identification of an additional signature of dredge-up in the Na abundances, which are enriched with ⟨[Na/Fe]⟩ = +0.29, along with a very small non-LTE correction. The 16O and Al abundances are found to be near-solar. All derived mixing-sensitive abundances are consistent with stellar models of approximately 3.3 M⊙ evolving along the red giant branch and onto the red clump. Previous results in the literature have suggested that the young inner-disk open cluster NGC 6705 is α-enhanced. We have investigated this possibility via an independent chemical abundance analysis and found that the mean α-element abundance relative to iron is ⟨[α/Fe]⟩ = -0.03 ± 0.05, which is not at odds with expectations from general Galactic disc abundance trends. Previous results in the literature have found the young inner-disc open cluster NGC 6705 to be mildly α-enhanced. We examined this possibility via an independent chemical abundance analysis for 11 red-giant members of NGC 6705. The analysis is based on near-infrared APOGEE spectra and relies on LTE calculations using spherical model atmospheres and radiative transfer. We find a mean cluster metallicity of [Fe/H] = +0.13 ± 0.04, indicating that NGC 6705 is metal-rich, as may be expected for a young inner-disc cluster. The mean α-element abundance relative to iron is ⟨[α/Fe]⟩ = -0.03 ± 0.05, which is not at odds with expectations from general Galactic abundance trends. NGC 6705 also provides important probes for studying stellar mixing, given its turn-off mass of M ~ 3.3 solar masses. Its red giants have low 12C abundances ([12C/Fe] = -0.16) and enhanced 14N abundances ([14N/Fe] = +0.51), which are key signatures of the first dredge-up on the red giant branch. An additional signature of dredge-up was found in the Na abundances, which are enhanced by [Na/Fe] = +0.29, with a very small non-LTE correction. The 16O and Al abundances are found to be near-solar. All of the derived mixing-sensitive abundances are in agreement with stellar models of approximately 3.3 solar masses evolving along the red giant branch and onto the red clump. As found in young open clusters with similar metallicities, NGC 6705 exhibits a mild excess in the s-process element cerium with [Ce/Fe] = +0.13 ± 0.07. Host-stars; Chemical abundances; Stellar activity; Exoplanets; Open cluster