Estrelas gigantes ricas em lítio: um estudo da anomalia química e do campo magnético desses objetos
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 fir...
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| Formato: | Dissertação |
| Idioma: | por |
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| Endereço do item: | https://repositorio.ufrn.br/jspui/handle/123456789/26558 |
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| Resumo: | 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. |
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