Navegando por Autor "Costa, A. D."

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Kepler rapidly rotating giant stars
(American Astronomical Society, 2015) Costa, A. D.; Martins, B. L. Canto; Bravo, J. P.; Paz-Chinchón, F.; Chagas, M. L. das; Leão, I. C.; Oliveira, G. Pereira de; Silva, R. Rodrigues da; Roque, S.; Oliveira, L. L. A. de; Silva, D. Freire da; Medeiros, José Renan de
Rapidly rotating giant stars are relatively rare and may represent important stages of stellar evolution, resulting from stellar coalescence of close binary systems or accretion of substellar companions by their hosting stars. In the present Letter, we report 17 giant stars observed in the scope of the Kepler space mission exhibiting rapid rotation behavior. For the first time, the abnormal rotational behavior for this puzzling family of stars is revealed by direct measurements of rotation, namely from photometric rotation period, exhibiting a very short rotation period with values ranging from 13 to 55 days. This finding points to remarkable surface rotation rates, up to 18 times the rotation of the Sun. These giants are combined with six others recently listed in the literature for mid-infrared (IR) diagnostics based on Wide-field Infrared Survey Explorer information, from which a trend for an IR excess is revealed for at least one-half of the stars, but at a level far lower than the dust excess emission shown by planet-bearing main-sequence stars.
New Suns in the Cosmos II: differential rotation in Kepler Sun-like stars
(Royal Astronomical Society, 2016) Chagas, M. L. das; Bravo, J. P.; Costa, A. D.; Lopes, C. E. Ferreira; Sobrinho, R. Silva; Paz-Chinchón, F.; Leão, I. C.; Valio, A.; Freitas, D. B. de; Martins, B. L. Canto; Lanza, A. F.; Medeiros, José Renan de
The present study reports the discovery of Sun-like stars, namely main-sequence stars with Teff, log g and rotation periods Prot similar to solar values, presenting evidence of surface differential rotation (DR). An autocorrelation of the time series was used to select stars presenting photometric signal stability from a sample of 881 stars with light curves collected by the Kepler space-borne telescope, in which we have identified 17 stars with stable signals. A simple two-spot model together with a Bayesian information criterion were applied to these stars in the search for indications of DR; in addition, for all 17 stars, it was possible to compute the spot rotation period P, the mean values of the individual spot rotation periods and their respective colatitudes, and the relative amplitude of the DR.
New suns in the cosmos. iii. multifractal signature analysis
(American Astronomical Society, 2016) Freitas, D. B. de; Nepomuceno, M. M. F.; Moraes Junior, P. R. V. de; Lopes, C. E. F.; Chagas, M. L. das; Bravo, J. P.; Costa, A. D.; Martins, B. L. Canto; Medeiros, José Renan de; Leão, I. C.
In the present paper, we investigate the multifractality signatures in hourly time series extracted from the CoRoTspacecraft database. Our analysis is intended to highlight the possibility that astrophysical time series can be members of a particular class of complex and dynamic processes, which require several photometric variability diagnostics to characterize their structural and topological properties. To achieve this goal, we search for contributions due to a nonlinear temporal correlation and effects caused by heavier tails than the Gaussian distribution, using a detrending moving average algorithm for one-dimensional multifractal signals (MFDMA). We observe that the correlation structure is the main source of multifractality, while heavy-tailed distribution plays a minor role in generating the multifractal effects. Our work also reveals that the rotation period of stars is inherently scaled by the degree of multifractality. As a result, analyzing the multifractal degree of the referred series, we uncover an evolution of multifractality from shorter to larger periods.
New suns in the cosmos. IV. The multifractal nature of stellar magnetic activity in kepler cool stars
(The American Astronomical Society, 2017) Freitas, D. B. de; Nepomuceno, M. M. F.; Souza, M. Gomes de; Leão, Isan Castro; Chagas, M. L. das; Costa, A. D.; Martins, Bruno Leonardo Canto; Medeiros, José Renan de
In the present study, we investigate the multifractal nature of a long-cadence time series observed by the Kepler mission for a sample of 34 M dwarf stars and the Sun in its active phase. Using the Multifractal Detrending Moving Average algorithm, which enables the detection of multifractality in nonstationary time series, we define a set of multifractal indices based on the multifractal spectrum profile as a measure of the level of stellar magnetic activity. This set of indices is given by the (A, ${\rm{\Delta }}\alpha $, C, H)-quartet, where A, ${\rm{\Delta }}\alpha $, and C are related to geometric features from the multifractal spectrum and the global Hurst exponent H describes the global structure and memorability of time series dynamics. As a test, we measure these indices and compare them with a magnetic index defined as S ph and verify the degree of correlation among them. First, we apply the Poincaré plot method and find a strong correlation between the $\langle {S}_{\mathrm{ph}}\rangle $ index and one of the descriptors that emerges from this method. As a result, we find that this index is strongly correlated with long-term features of the signal. From the multifractal perspective, the $\langle {S}_{\mathrm{ph}}\rangle $ index is also strongly linked to the geometric properties of the multifractal spectrum except for the H index. Furthermore, our results emphasize that the rotation period of stars is scaled by the H index, which is consistent with Skumanich's relationship. Finally, our approach suggests that the H index may be related to the evolution of stellar angular momentum and a star's magnetic properties.

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Navegando por Autor "Costa, A. D."