Espectroscopia de Alta Resolução no Infravermelho de Estrelas Anãs-M, Anãs-G, Subgigantes-G e Gigantes Vermelhas-K Observadas com o APOGEE

Abstract

In this Thesis, we present the first results of a detailed chemical study of M-dwarf stars. We demonstrate that it is possible to obtain elemental abundances of up to thirteen elements (C, O, Na, Mg, Al, Si, K, Ca, Ti, V, Cr, Mn and Fe) from an LTE analysis of high-resolution near-infrared spectra, between 1.5 - 1.7 μm, obtained by the APOGEE survey. Two M-dwarfs with exoplanets detected by the Kepler mission, Kepler-138 and Kepler-186 were analyzed. Our results indicate that both stars have near-solar metallicities, [Fe/H] = -0.09 and -0.08 dex, for Kepler-138 and Kepler-186, respectively. The metallicities obtained for these stars are roughly ∼0.2 dex higher than literature values derived from low-resolution spectra. These systematic differences may be due to contributions from numerous CO and H2O lines, which affect the intensity and the equivalent widths of Na I and Ca I spectral lines, commonly used to determine the metallicities from low-resolution spectra. One interesting result obtained was that the star Kepler-186 exhibits an enrichment of silicon ([Si/Fe] = +0.18), and this element is important in controlling the internal structure of rocky planets. We also have analyzed a sample of 13 M-dwarf stars (which we can be used as a calibration sample), with 11 of these stars in binary systems having hotter primary stars (of spectral types G and K), while the other two M-dwarfs have precise radii measurements from interferometry. Our results indicate that the metallicity scale for the M-dwarf stars obtained in this thesis is very similar to the metallicity scale obtained from optical high-resolution spectra of the G and K primary stars published in the literature. Our metallicity scale for M-dwarfs is also be confirmed by studying two M-dwarf members of the open cluster M 67. We find that the M-dwarfs analyzed in M67 have metallicities very close to solar, which is expected for this cluster based upon numerous studies in the literature. The stellar sample analyzed in M 67 also included: two solar-type G-dwarfs, two G-type subgiants, and two K-type red giants, covering a range in stellar mass between 0.50 and 1.30 M⊙. The metallicity obtained for the eight stars in M 67 is 〈A(Fe)〉 = 7.48 ± 0.02, showing a high degree of homogeneity. Besides iron, we also determined the abundances of the following elements: C, N, O, Na, Mg, Al, Si, K, Ca, Ti, V, Cr, and Mn. The abundances of 12C and 14N exhibit the signature of the first dredge-up in the more evolved stars, with 12C/14N = 2.34 for subgiant stars and 12C/14N = 1.73 for the red giant stars. We also found that diffusion and/or gravitational settling effects may be present in the dwarf stars of M 67 where the abundances of the lighter elements, such as Na, Mg, Al, and Si, are the most affected. Another result obtained for M 67 is that the star 2M08510076+1153115 is a solar twin. The open cluster NGC 2420 was also studied in this thesis. Abundances of 16 chemical elements (C, N, O, Na, Mg, Al, Si, K, Ca, Ti, V, Cr, Mn, Fe, Co, and Ni) were determined in a sample of 12 red giant memebers of this open cluster. The abundances obtained shown systematic differences when compared to some studies in the literature. We obtained a mean metallicity of 〈[Fe/H]〉 = -0.16 ± 0.04. From the 12 stars studied, we identified six as belonging to the red giant branch (RGB) and six belonging to the red clump (RC). The abundances of 12C and 14N are consistent with the first dredge-up, where A(12C) is slightly lower (-0.06 dex) and A(14N) is slightly higher (on average by +0.11 dex). The derived abundance ratio of carbon over nitrogen did not indicate significant differences between stars of these two groups, with mean values 12C/14N = 2.50 ± 0.29 (RGB) and 2.36 ± 0.18 (RC).