CCET - DFTE - Artigos publicados em periódicos

URI Permanente para esta coleçãohttps://repositorio.ufrn.br/handle/1/2977

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Li-rich giant stars under scrutiny: binarity, magnetic activity, and the evolutionary status after Gaia DR2
(Oxford University Press, 2020-09-02) Costa, Jefferson Soares da; Almeida, Leandro de; Castro, Matthieu Sebastien; Nascimento Júnior, José Dias do; Gonçalves, B. F. O
We present a study of the evolutionary state of a few lithium-rich giant stars based on the Gaia Data Release 2 (DR2) parallaxes and photometry. We also investigate the chromospheric activity, the presence of a surface magnetic field, and the radial velocity for our sample stars. We analysed both archive and new data. We gathered archive spectra from several instruments, mainly ELODIE and NARVAL, and we added new data acquired with the spectrograph MUSICOS. We applied the least-squares deconvolution technique to obtain Stokes V and I mean profiles to compute longitudinal magnetic field for a subset. Moreover, for the same subset, we analysed the Ca II H&K emission lines to calculate the S-index. We also derived atmospheric parameters and Li abundances for all 18 stars of our sample. We found that stars previously classified as red giant branch (RGB) may actually be at a different evolutionary state. Furthermore, we identified that most stars in our sample with detection of surface magnetic field show at least moderate rotation velocities, but none the less, we could not detect a magnetic field in two fast rotators. Because of our small sample of magnetic giants, it is difficult to determine if the presence of surface magnetic field and the Li-rich giant phenomena could be somehow linked. The large variation of the radial velocity of part of our sample indicates that some of them might have a binary companion, which may change the way we look at the Li problem in giant stars
Optimized cutting off transit algorithm to study stellar rotation from PLATO mission light curves
(Springer, 2020-06-16) Costa, Jefferson Soares da; Almeida, Leandro de; Anthony, Francys; Mattiuci, Ana Carolina; Castro, Matthieu; Samadi, Reza; Nascimento Júnior, José Dias do
Measuring the stellar rotation of one of the components in eclipsing binaries (EBs) or planetary systems is a challenge task. The difficulty is mainly due to the complexity of analyzing, in the same light curve, the signal from the stellar rotation mixed with the transit signal of a stellar or substellar companion, like a brown dwarf or planet. There are many methods to correct the long-term trend of the light curve. However, the correction often erases the signal of the stellar rotation from spots crossing the visible stellar disk and other weaker signals like planets. In this work, we present the DiffeRencial flUx Method of cuTting Off biNariES (DRUM TONES) to identify the signal of the binary transits and disentangle it from stellar rotation planet signals. We present our technique with applications to EBs from CoRoT, Kepler, Kepler K2 and TESS missions. We also applied our method to simulated synthetic EB from the PLATO mission. Our method shows good agreement in the determination of stellar rotation periods for few observed targets from last space missions, as well it is naturally useful for future European mission, such as PLAnetary Transits and Oscillations of stars (PLATO)
Spectroscopic and asteroseismic analysis of the secondary clump red giant HD 226808
(IOP Publishing, 2020-05-06) Costa, Jefferson Soares da; Moura, Bruno Lustosa de; Beck, Paul G.; Mauro, Maria Pia Di; Almeida, Leandro de; Duarte, Tharcisyo S. S.; Coelho, Hugo R.; Castro, Matthieu Sebastien; Nascimento Júnior, José Dias do
In order to clarify the properties of the secondary clump star HD 226808 (KIC 5307747), we combined four years of data from Kepler space photometry with high-resolution spectroscopy of the High Efficiency and Resolution Mercator Échelle Spectrograph mounted on the Mercator telescope. The fundamental atmospheric parameters, radial velocities, rotation velocities, and elemental abundance for Fe and Li were determined by analyzing line strengths and fitting-line profiles, based on a 1D local thermodynamic equilibrium model atmosphere. Second, we analyzed a photometric light curve obtained by Kepler and we extracted asteroseismic data of this target using Lets Analysis, Use and Report of Asteroseismology, a new seismic tool developed for the study of evolved FGK solar-like stars. We determined the evolutionary status and effective temperature, surface gravity, metallicity, microturbulence, and chemical abundances for Li, Ti, Fe, and Ni for HD 226808, by employing spectroscopy, asteroseismic scaling relations, and evolutionary structure models built in order to match observed data. Our results also show that an accurate synergy between good spectroscopic analysis and asteroseismology can provide a jump toward understanding evolved stars

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