Navegando por Autor "Castro, M."

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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.
Lithium abundance and rotation of seismic solar analogues: solar and stellar connection from Kepler and Hermes observations
(EDP Sciences, 2017-02-02) Beck, P. G.; Nascimento Júnior, José Dias do; Duarte, T.; Salabert, D.; Tkachenko, A.; Mathis, S.; Marthur, S.; García, R. A.; Castro, M.; Pallé, P. L.; Egeland, R.; Montes, D.; Creevey, O.; Andersen, M F.; Kamath, D.; van Winckel, H.
Context. Lithium abundance A(Li) and surface rotation are good diagnostic tools to probe the internal mixing and angular momentum transfer in stars. Aims. We explore the relation between surface rotation, A(Li), and age in a sample of seismic solar-analogue stars, and we study their possible binary nature. Methods. We selected a sample of 18 solar-analogue stars observed by the NASA Kepler satellite for an in-depth analysis. Their seismic properties and surface rotation rates are well constrained from previous studies. About 53 h of high-resolution spectroscopy were obtained to derive fundamental parameters from spectroscopy and A(Li). These values were combined and compared with seismic masses, radii, and ages, as well as with surface rotation periods measured from Kepler photometry. Results. Based on radial velocities, we identify and confirm a total of six binary star systems. For each star, a signal-to-noise ratio of 80 . S=N . 210 was typically achieved in the final spectrum around the lithium line. We report fundamental parameters and A(Li). Using the surface rotation period derived from Kepler photometry, we obtained a well-defined relation between A(Li) and rotation. The seismic radius translates the surface rotation period into surface velocity. With models constrained by the characterisation of the individual mode frequencies for single stars, we identify a sequence of three solar analogues with similar mass ( 1.1 M ) and stellar ages ranging between 1 to 9 Gyr.Within the realistic estimate of 7% for the mass uncertainty, we find a good agreement between the measured A(Li) and the predicted A(Li) evolution from a grid of models calculated with the Toulouse-Geneva stellar evolution code, which includes rotational internal mixing, calibrated to reproduce solar chemical properties. We found a scatter in ages inferred from the global seismic parameters that is too large when compared with A(Li). Conclusions.We present the Li-abundance for a consistent spectroscopic survey of solar-analogue stars with a mass of 1:00 0:15 M that are characterised through asteroseismology and surface rotation rates based on Kepler observations. The correlation between A(Li) and Prot supports the gyrochronological concept for stars younger than the Sun and becomes clearer when the confirmed binaries are excluded. The consensus between measured A(Li) for solar analogues with model grids, calibrated on the Sun’s chemical properties, suggests that these targets share the same internal physics. In this light, the solar Li and rotation rate appear to be normal for a star like the Sun
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
Mass effect on the lithium abundance evolution of open clusters: Hyades, NGC 752, and M67
(EDP Sciences, 2016-03-12) Castro, M.; Duarte, T.; Pace, G.; Nascimento Júnior, José Dias do
Lithium abundances in open clusters provide an e ective way of probing mixing processes in the interior of solar-type stars and convection is not the only mixing mechanism at work. To understand which mixing mechanisms are occurring in low-mass stars, we test non-standard models, which were calibrated using the Sun, with observations of three open clusters of di erent ages, the Hyades, NGC 752, and M67. We collected all available data, and for the open cluster NGC 752, we redetermine the equivalent widths and the lithium abundances. Two sets of evolutionary models were computed, one grid of only standard models with microscopic di usion and one grid with rotation-induced mixing, at metallicity [Fe/H] = 0.13, 0.0, and 0.01 dex, respectively, using the Toulouse-Geneva evolution code. We compare observations with models in a color–magnitude diagram for each cluster to infer a cluster age and a stellar mass for each cluster member. Then, for each cluster we analyze the lithium abundance of each star as a function of mass. The data for the open clusters Hyades, NGC 752, and M67, are compatible with lithium abundance being a function of both age and mass for stars in these clusters. Our models with meridional circulation qualitatively reproduce the general trend of lithium abundance evolution as a function of stellar mass in all three clusters. This study points out the importance of mass dependence in the evolution of lithium abundance as a function of age. Comparison between models with and without rotation-induced mixing shows that the inclusion of meridional circulation is essential to account for lithium depletion in low-mass stars. However, our results suggest that other mechanisms should be included to explain the Li-dip and the lithium dispersion in low-mass stars
Shallow extra mixing in solar twins inferred from Be abundances
(EDP Sciences, 2015-03-10) Maia, M. Tucci; Meléndez, J.; Castro, M.; Asplund, M.; Ramírez, I.; Monroe, T. R.; Nascimento Júnior, José Dias do; Young, D.
Context. Lithium and beryllium are destroyed at different temperatures in stellar interiors. As such, their relative abundances offer excellent probes of the nature and extent of mixing processes within and below the convection zone. Aims. We determine Be abundances for a sample of eight solar twins for which Li abundances have previously been determined. The analyzed solar twins span a very wide range of age, 0.5–8.2 Gyr, which enables us to study secular evolution of Li and Be depletion. Methods. We gathered high-quality UVES/VLT spectra and obtained Be abundances by spectral synthesis of the Be ii 313 nm doublet. Results. The derived beryllium abundances exhibit no significant variation with age. The more fragile Li, however, exhibits a monotonically decreasing abundance with increasing age. Therefore, relatively shallow extra mixing below the convection zone is necessary to simultaneously account for the observed Li and Be behavior in the Sun and solar twins
Solar twins in M 67: evolutionary status and lithium abundance
(Astronomy & Astrophysics, 2011) Castro, M.; Nascimento Jr., J. D. do; Biazzo, K.; Meléndez, J.; Medeiros, José Renan de
Aims. We determine the age and mass of the three best solar twin candidates in open cluster M 67 through lithium evolutionary models. Methods. We computed a grid of evolutionary models with non-standard mixing at metallicity [Fe/H] = 0.01 with the ToulouseGeneva evolution code for a range of stellar masses. We estimated the mass and age of 10 solar analogs belonging to the open cluster M 67. We made a detailed study of the three solar twins of the sample, YPB637, YPB1194, and YPB1787. Results. We obtained a very accurate estimation of the mass of our solar analogs in M 67 by interpolating in the grid of evolutionary models. The three solar twins allowed us to estimate the age of the open cluster, which is 3.87+0.55 −0.66 Gyr, which is better constrained than former estimates. Conclusions. Our results show that the 3 solar twin candidates have one solar mass within the errors and that M 67 has a solar age within the errors, validating its use as a solar proxy. M 67 is an important cluster when searching for solar twins.

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Navegando por Autor "Castro, M."