Navegando por Autor "Mello, G. F. Porto de"

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Age and mass of solar twins constrained by lithium abundance
(Astronomy & Astrophysics, 2009) Nascimento Jr, J. D. do; Castro, M.; Meléndez, J.; Bazot, M.; Théado, S.; Mello, G. F. Porto de; Medeiros, José Renan de
Aims. We analyze the non-standard mixing history of the solar twins HIP 55 459, HIP 79 672, HIP 56 948, HIP 73 815, and HIP 100 963, to determine as precisely as possible their mass and age. Methods. We computed a grid of evolutionary models with non-standard mixing at several metallicities with the Toulouse-Geneva code for a range of stellar masses assuming an error bar of ±50 K in Teff. We choose the evolutionary model that reproduces accurately the observed low lithium abundances observed in the solar twins. Results. Our best-fit model for each solar twin provides a mass and age solution constrained by their Li content and Teff determination. HIP 56 948 is the most likely solar-twin candidate at the present time and our analysis infers a mass of 0.994 ± 0.004 M and an age of 4.71 ± 1.39 Gyr. Conclusions. Non-standard mixing is required to explain the low Li abundances observed in solar twins. Li depletion due to additional mixing in solar twins is strongly mass dependent. An accurate lithium abundance measurement and non-standard models provide more precise information about the age and mass more robustly than determined by classical methods alone.
Evolution of the solar activity over time and effects on planetary atmospheres. ii. κ1 ceti, an analog of the sun when life arose on earth∗
(American Astronomical Society, 2010) Ribas, I.; Mello, G. F. Porto de; Ferreira, L. D.; Hebrard, E.; Selsis, F.; Catalan, S.; Garces, A.; Nascimento Jr., J. D. do; Medeiros, José Renan de
The early evolution of Earth's atmosphere and the origin of life took place at a time when physical conditions at the Earth were radically different from its present state. The radiative input from the Sun was much enhanced in the high-energy spectral domain, and in order to model early planetary atmospheres in detail, a knowledge of the solar radiative input is needed. We present an investigation of the atmospheric parameters, state of evolution, and high-energy fluxes of the nearby star κ1 Cet, previously thought to have properties resembling those of the early Sun. Atmospheric parameters were derived from the excitation/ionization equilibrium of Fe I and Fe II, profile fitting of Hα, and the spectral energy distribution. The UV irradiance was derived from Far-Ultraviolet Spectroscopic Explorer and Hubble Space Telescope data, and the absolute chromospheric flux from the Hα line core. From careful spectral analysis and the comparison of different methods, we propose for κ1 Cet the following atmospheric parameters: T eff = 5665 ± 30 K (Hα profile and energy distribution), log g = 4.49 ± 0.05 dex (evolutionary and spectroscopic), and [Fe/H] = +0.10 ± 0.05 (Fe II lines). The UV radiative properties of κ1 Cet indicate that its flux is some 35% lower than the current Sun's between 210 and 300 nm, it matches the Sun's at 170 nm, and increases to at least 2-7 times higher than the Sun's between 110 and 140 nm. The use of several indicators ascribes an age to κ1 Cet in the interval ~0.4-0.8 Gyr and the analysis of the theoretical Hertzsprung-Russell diagram (H-R) suggests a mass ~1.04 M ☉. This star is thus a very close analog of the Sun when life arose on Earth and Mars is thought to have lost its surface bodies of liquid water. Photochemical models indicate that the enhanced UV emission leads to a significant increase in photodissociation rates compared with those commonly assumed of the early Earth. Our results show that reliable calculations of the chemical composition of early planetary atmospheres need to account for the stronger solar photodissociating UV irradiation.
Magnetic field and wind of kappa ceti: toward the planetary habitability of the young sun when life arose on earth
(The American Astronomical Society, 2016-03-20) Nascimento Júnior, José Dias do; Vidotto, A. A.; Petit, P.; Folsom, C.; Castro, M.; Marsden, S. C.; Morin, J.; Mello, G. F. Porto de; Meibom, S.; Jeffers, S. V.; Guinan, E.; Ribas, I.
We report magnetic field measurements for κ1 Cet, a proxy of the young Sun when life arose on Earth. We carry out an analysis of the magnetic properties determined from spectropolarimetric observations and reconstruct the large-scale surface magnetic field to derive the magnetic environment, stellar winds, and particle flux permeating the interplanetary medium around k1 Cet. Our results show a closer magnetosphere and mass-loss rate of M˙ = 9.7 ´ 10-13 M yr-1  , i.e., a factor of 50 times larger than the current solar wind mass-loss rate, resulting in a larger interaction via space weather disturbances between the stellar wind and a hypothetical young-Earth analogue, potentially affecting the planet’s habitability. Interaction of the wind from the young Sun with the planetary ancient magnetic field may have affected the young Earth and its life conditions
Overview of semi-sinusoidal stellar variability with the CoRoT satellite
(Astronomy & Astrophysics, 2013) Medeiros, José Renan de; Lopes, C. E. Ferreira; Leão, Izan de Castro; Martins, Bruno Leonardo Canto; Catelan, M.; Baglin, A.; Vieira, S.; Bravo, J. P.; Cortés, C.; Freitas, D. B. de; Janot-Pacheco, E.; Maciel, S. C.; Melo, C. H. F.; Osorio, Y.; Mello, G. F. Porto de; Valio, A.
Context. To date, the CoRoT space mission has produced more than 124 471 light curves. Classifying these curves in terms of unambiguous variability behavior is mandatory for obtaining an unbiased statistical view on their controlling root-causes. Aims. The present study provides an overview of semi-sinusoidal light curves observed by the CoRoT exo-field CCDs. Methods. We selected a sample of 4206 light curves presenting well-defined semi-sinusoidal signatures. The variability periods were computed based on Lomb-Scargle periodograms, harmonic fits, and visual inspection. Results. Color–period diagrams for the present sample show the trend of an increase of the variability periods as long as the stars evolve. This evolutionary behavior is also noticed when comparing the period distribution in the Galactic center and anti-center directions. These aspects indicate a compatibility with stellar rotation, although more information is needed to confirm their rootcauses. Considering this possibility, we identified a subset of three Sun-like candidates by their photometric period. Finally, the variability period versus color diagram behavior was found to be highly dependent on the reddening correction.

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Navegando por Autor "Mello, G. F. Porto de"