ESTRELAS DO CAMPO DO HALO GALÁCTICO: UM ESTUDO DAS ABUNDÂNCIAS DE CARBONO, NITROGÊNIO E OXIGÊNIO

Abstract

The metal-poor stars composition is an important diagnosis in the matter of galactic chemical evolution. The peculiar abundance of carbon, nitrogen, and oxygen in cold stars reveal details about the first generations of these celestial bodies (population III), as well as different aspects regarding the possible nucleosynthesis sites of these elements. The F, G, and K halo stars are especially important to rebuild the Milky Way history, due to their long lives. The supernovae collapse and the nucleosynthesis in one of the stars in a binary system, along with the mass transfer caused by it, are some of the main astrophysical sites for carbon and nitrogen forma- -tion. In this context, progenitor models of supermassive objects explain the oxygen overabundance observed in the Galactic halo. In this study, we determined the C, N, and O abundances through the spectral synthesis technique, in high-resolution spectra. For such, we used atmosphere models in Local Thermodynamic Equilibrium (LTE) with the MOOG code (Sneden et al.1973), modified by Sobeck et al.(2011). The determination of C, N, and O abundances was motivated by the discovery of a CEMP-s star (Roriz et al.2017) and a sodium-enriched star (pereira et al.2019), referring to a sample that consistsof 35 stars of the halo, which were obser- -ved with the spectrometer FEROS (Fiber-fed Extended Range Optical Spectrograph). The following transitions of the molecular bands were used to determine carbon and nitrogen abundance: CH at 4300 ANGSTROM (system A2Delta-X2prod) and CN at 3800 ANGSTROM (system B2Sigma(+)-X2Sigma(+)). To verify the oxygen abundance, we used the forbidden line at 6300 ANGSTROM [OI] after verifying if the telluric absorption lines were contaminated. According to the results obtained, no overabundance of carbon or nitrogen was observed. Oxygen presented overabundance in the range of metallicity studied (-0.9<[Fe/H]<-2.9). The [C/Fe], [N/Fe] and [O/Fe] abundance ratios are similar to those already present in the existing literature on stars with same metallicity. For the abundance ratios of [C/Fe], we found values between -0.77dex and 0.23dex, whereas, for the abundance ratios of [N/Fe], it values obtained were between -0.76 dex and 1.32 dex. We also determined the [C/H], [N/H], and [O/H] abundance ratios, which presented similar tendencies to those found in the existing literature, for the same metallicity range studied in this thesis.