Estrelas gigantes ricas em lítio: um estudo da anomalia química e do campo magnético desses objetos

Abstract

According to the standard theory of stellar evolution, low-mass stars (spectral type K and G) must reach the beginning of the main sequence (ZAMS) with lithium (Li) abundance near to the meteoritical value, which is ∼ 3.3 dex, and hold this abundance approximately constant until they reach the first dredge-up zone in the Red Giant Branch (RGB). After completion of the dilution processes, already in the red clump, such stars should present a relatively low Li abundance (< 1.5 dex). However, approximately 1-2% of all observed K and G giant stars have an abnormally high Li abundance (≥ 1.5 dex). It is possible to find in the literature many attempts to reconcile theory and observation, but none of them is capable of explaining all the scenarios in which those chemical anomalies occur. Our work aims at presenting a new study of lithium-rich G and K spectral type stars and at analyzing the possible existence of particular characteristics to the lithium-rich stars that present detected magnetic field. We have assembled a sample of 20 giant stars — taken from Charbonnel e Balachandran (2000), Kumar, Reddy e Lambert (2011), and Lèbre et al. (2009) — and we computed the atmospheric parameters and Li abundances for those stars. We also computed the longitudinal magnetic field for a sub-sample of stars with observed high-resolution spectra available at PolarBase (Petit et al., 2014). In order to get those results, we have used the Least-Squares Deconvolution (LSD) technique (Donati et al., 1997) and the spectral analysis tool iSpec (Blanco-Cuaresma et al., 2014). Concerning the evolutionary states, we have used parallaxes recently provided by ESA’s Gaia space observatory. We obtained results for the atmospheric parameters and the Li abundance using the same procedure for all stars. Thus, we have the reliability in comparing stars that possibly had their spectra treated in different ways and that were observed by different instruments. We obtained relations between Li abundance, rotation velocity, and presence of a magnetic field according to what is predicted in the literature. We conclude that each star needs to be analyzed individually and with a more refined spectroscopy such that the real nature of its Li abundance is unraveled. The 12C/13C isotopic ratio, and the C/N elemental ratio, need to be investigated so that we can determine precisely the position of some stars (of our sample) on the H-R diagram. The influence of the magnetic field on the Li abundance is still not clear, since we cannot rule out the existence of a non-superficial field acting inside the convective zone and altering mixing mechanisms.