Navegando por Autor "Freitas, D. B. de"

Agora exibindo 1 - 13 de 13

Incidence of planet candidates in open clusters and a planet confirmation
(EDPSCIENCE, 2018-12-10) Martins, Bruno Leonardo Canto; Leão, I. C.; Alves, S.; Oliveira, G. Pereira de; Cortés, C.; Brucalassi, A.; Melo, C. H. F.; Freitas, D. B. de; Pasquini, L.; Medeiros, J. R. de
Context. Detecting exoplanets in clusters of different ages is a powerful tool for understanding a number of open questions, such as how the occurrence rate of planets depends on stellar metallicity, on mass, or on stellar environment. Aims. We present the first results of our HARPS long-term radial velocity (RV) survey which aims to discover exoplanets around intermediate-mass (between ∼2 and 6 M ) evolved stars in open clusters. Methods. We selected 826 bona fide HARPS observations of 114 giants from an initial list of 29 open clusters and computed the halfpeak to peak variability of the HARPS RV measurements, namely ∆RV=2, for each target, to search for the best planet-host candidates. We also performed time series analyses for a few targets for which we have enough observations to search for orbital solutions. Results. Although we attempted to rule out the presence of binaries on the basis of previous surveys, we detected 14 new binary candidates in our sample, most of them identified from a comparison between HARPS and CORAVEL data. We also suggest 11 new planet-host candidates based on a relation between the stellar surface gravity and ∆RV=2. Ten of the candidates are less than 3 M , showing evidence of a low planet occurrence rate for massive stars. One of the planet-host candidates and one of the binary candidates show very clear RV periodic variations, allowing us to confirm the discovery of a new planet and to compute the orbital solution for the binary. The planet is IC 4651 9122b, with a minimum mass of m sin i = 6:3 MJ and a semimajor axis a = 2:0 AU. The binary companion is NGC 5822 201B, with a very low minimum mass of m sin i = 0:11 M and a semimajor axis a = 6:5 AU, which is comparable to the Jupiter distance to the Sun.
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.
New suns in the cosmos?
(IOP Publishing, 2013) Freitas, D. B. de; Leao, Izan de Castro; Lopes, C. E. Ferreira; Paz-Chinchon, F.; Martins, Bruno Leonardo Canto; Alves, S.; Medeiros, José Renan de; Catelan, M.
The present work reports on the discovery of three stars that we have identified to be rotating Sun-like stars, based on rotational modulation signatures inferred from light curves from the CoRoT mission’s Public Archives. In our analysis, we performed an initial selection based on the rotation period and position in the period–Teff diagram. This revealed that the stars CoRoT IDs 100746852, 102709980, and 105693572 provide potentially good matches to the Sun with a similar rotation period. To refine our analysis, we applied a novel procedure, taking into account the fluctuations of the features associated with photometric modulation at different time intervals and the fractality traces that are present in the light curves of the Sun and of these “New Sun” candidates alike. In this sense, we computed the so-called Hurst exponent for the referred stars, for a sample of 14 CoRoT stars with sub- and super-solar rotational periods, and for the Sun itself in its active and quiet phases. We found that the Hurst exponent can provide a strong discriminant of Sun-like behavior, going beyond what can be achieved with solely the rotation period itself. In particular, we find that CoRoT ID 105693572 is the star that most closely matches the solar rotation properties as far as the latter’s imprints on light curve behavior are concerned. The stars CoRoT IDs 100746852 and 102709980 have significant smaller Hurst exponents than the Sun, notwithstanding their similarity in rotation periods.
A non-extensive approach to the stellar rotational evolution – I: F- and G-type stars
(Oxford University Press, 2013) Freitas, D. B. de; Medeiros, José Renan de
The pioneering study by Skumanich showed that the rotational velocity of G-type mainsequence (MS) stars decreases with stellar age according to v sin i ∝ t −1/2. This relationship is consistent with simple theories of angular momentum loss from rotating stars, where an ionized wind is coupled to the star by a magnetic field. The present study introduces a new approach to the study of stellar rotational braking in unsaturated F- and G-type stars limited in age and mass, connecting angular momentum loss by magnetic stellar wind with Tsallis non-extensive statistical mechanics. As a result, we show that the rotation–age relationship can be well reproduced using a non-extensive approach from Tsallis non-extensive models. Here, the index q, which is related to the degree of non-extensivity, can be associated with the dynamo process and to magnetic field geometry, offering relevant information on the level of stellar magnetic braking for F- and G-type MS stars.
On the incidence of wise infrared excess among solar analog, twin, and sibling stars
(The American Astronomical Society, 2017) Costa, A. D. da; Martins, Bruno Leonardo Canto; Leão, Izan Castro; Lima Jr, J. E.; Silva, D. Freire da; Freitas, D. B. de; Medeiros, José Renan de
This study presents a search for infrared (IR) excess in the 3.4, 4.6, 12, and 22 μm bands in a sample of 216 targets, composed of solar sibling, twin, and analog stars observed by the Wide-field Infrared Survey Explorer (WISE) mission. In general, an IR excess suggests the existence of warm dust around a star. We detected 12 μm and/or 22 μm excesses at the 3σ level of confidence in five solar analog stars, corresponding to a frequency of 4.1% of the entire sample of solar analogs analyzed, and in one out of 29 solar sibling candidates, confirming previous studies. The estimation of the dust properties shows that the sources with IR excesses possess circumstellar material with temperatures that, within the uncertainties, are similar to that of the material found in the asteroid belt in our solar system. No photospheric flux excess was identified at the W1 (3.4 μm) and W2 (4.6 μm) WISE bands, indicating that, in the majority of stars of the present sample, no detectable dust is generated. Interestingly, among the 60 solar twin stars analyzed in this work, no WISE photospheric flux excess was detected. However, a null-detection excess does not necessarily indicate the absence of dust around a star because different causes, including dynamic processes and instrument limitations, can mask its presence.
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.
Rotation period distribution of CoRoT and Kepler Sun-like stars
(Astronomy & Astrophysics, 2015) Leão, Izan de Castro; Pasquini, L.; Lopes, C. E. Ferreira; Neves, V.; Valcarce, A. A. R.; Oliveira, L. L. A. de; Silva, D. Freire da; Freitas, D. B. de; Martins, Bruno Leonardo Canto; Janot-Pacheco, E.; Baglin, A.; Medeiros, José Renan de
Aims. We study the distribution of the photometric rotation period (Prot), which is a direct measurement of the surface rotation at active latitudes, for three subsamples of Sun-like stars: one from CoRoT data and two from Kepler data. For this purpose, we identify the main populations of these samples and interpret their main biases specifically for a comparison with the solar Prot. Methods. Prot and variability amplitude (A) measurements were obtained from public CoRoT and Kepler catalogs, which were combined with public data of physical parameters. Because these samples are subject to selection effects, we computed synthetic samples with simulated biases to compare with observations, particularly around the location of the Sun in the Hertzsprung-Russel (HR) diagram. Publicly available theoretical grids and empirical relations were used to combine physical parameters with Prot and A. Biases were simulated by performing cutoffs on the physical and rotational parameters in the same way as in each observed sample. A crucial cutoff is related with the detectability of the rotational modulation, which strongly depends on A. Results. The synthetic samples explain the observed Prot distributions of Sun-like stars as having two main populations: one of young objects (group I, with ages younger than ~1 Gyr) and another of main-sequence and evolved stars (group II, with ages older than ~1 Gyr). The proportions of groups I and II in relation to the total number of stars range within 64–84% and 16–36%, respectively. Hence, young objects abound in the distributions, producing the effect of observing a high number of short periods around the location of the Sun in the HR diagram. Differences in the Prot distributions between the CoRoT and Kepler Sun-like samples may be associated with different Galactic populations. Overall, the synthetic distribution around the solar period agrees with observations, which suggests that the solar rotation is normal with respect to Sun-like stars within the accuracy of current data.
Stellar cycles from photometric data: CoRoT stars
(EDP Sciences, 2015) Lopes, C. E. Ferreira; Leão, Izan de Castro; Freitas, D. B. de; Martins, Bruno Leonardo Canto; Catelan, M.; Medeiros, José Renan de
Context. Until a few years ago, the amplitude variation in the photometric data had been explored to a limited extent mainly because of time resolution and photometric sensitivity limitations. This investigation is now possible thanks to the Kepler and CoRoT databases which provide a unique set of data for studying the nature of stellar variability cycles. Aims. The present study characterizes the amplitude variation in a sample of main-sequence stars with light curves collected using CoRoT exofield CCDs. Methods. We analyze potential stellar activity cycles by studying the variability amplitude over small boxes. The cycle periods and amplitudes were computed based on the Lomb-Scargle periodogram, harmonic fits, and visual inspection. As a first application of our approach, we considered the photometric data for 16 CoRoT FGK main sequence stars, revisited during the IRa01, LRa01 and LRa06 CoRoT runs. Results. The 16 CoRoT stars appear to follow the empirical relations between activity cycle periods (Pcyc) and the rotation period (Prot) found by previous works. In addition to the so-called A (active) and I (inactive) sequences previously identified, there is a possible third sequence, here named S (short-cycles) sequence. However, recovery fractions estimated from simulations suggest that only a half of our sample has confident cycle measurements. Therefore, more study is needed to verify our results, and Kepler data will clearly be useful for such a study. Overall, our procedure provides a key tool for exploring the CoRoT and Kepler databases to identify and characterize stellar cycle variability.
The rotational behavior of kepler stars with planets
(American Astronomical Society, 2015) Paz-Chinchón, F.; Bravo, J. P.; Freitas, D. B. de; Lopes, C. E. Ferreira; Alves, S.; Catelan, M.; Martins, B. L. Canto; Medeiros, José Renan de; Leão, I. C.
We analyzed the host stars of the present sample of confirmed planets detected by Kepler and Kepler Objects of Interest to compute new photometric rotation periods and to study the behavior of their angular momentum. Lomb–Scargle periodograms and wavelet maps were computed for 3807 stars. For 540 of these stars, we were able to detect rotational modulation of the light curves at a significance level of greater than 99%. For 63 of these 540 stars, no rotation measurements were previously available in the literature. According to the published masses and evolutionary tracks of the stars in this sample, the sample is composed of M- to F-type stars (with masses of 0.48–1.53 M $_{}$) with rotation periods that span a range of 2–89 days. These periods exhibit an excellent agreement with those previously reported (for the stars for which such values are available), and the observed rotational period distribution strongly agrees with theoretical predictions. Furthermore, for the 540 sources considered here, the stellar angular momentum provides an important test of Kraft's relation based on the photometric rotation periods. Finally, this study directly contributes in a direct approach to our understanding of how angular momentum is distributed between the host star and its (detected) planetary system; the role of angular momentum exchange in such systems is an unavoidable piece of the stellar rotation puzzle.
The variability behaviour of CoRoT M-giant stars
(Astronomy & Astrophysics, 2015) Lopes, C. E. Ferreira; Neves, V.; Leão, I. C.; Freitas, D. B. de; Martins, B. L. Canto; Costa, A. D. da; Paz-Chinchón, F.; Chagas, M. L. das; Baglin, A.; Janot-Pacheco, E.; Medeiros, José Renan de
Context. For six years the Convection, Rotation, and planetary Transits (CoRoT) space mission has been acquiring photometric data from more than 100 000 point sources towards and directly opposite the inner and outer regions of the Galaxy. The high temporal resolution of the CoRoT data, combined with the wide time span of the observations, enabled the study of short- and long-time variations in unprecedented detail. Aims. The aim of this work is to study the variability and evolutionary behaviour of M-giant stars using CoRot data. Methods. From the initial sample of 2534 stars classified as M giants in the CoRoT databases, we selected 1428 targets that exhibit well defined variability, by visual inspection. Then, we defined three catalogues: C1 – stars with Teff< 4200 K and LCs displaying semi-sinusoidal signatures; C2 – rotating variable candidates with Teff> 4200 K; C3 – long-period variable candidates (with LCs showing a variability period up to the total time span of the observations). The variability period and amplitude of C1 stars were computed using Lomb-Scargle and harmonic fit methods. Finally, we used C1 and C3 stars to study the variability behaviour of M-giant stars. Results. The trends found in the V−I vs. J−K colour–colour diagram are in agreement with standard empirical calibrations for M giants. The sources located towards the inner regions of the Galaxy are distributed throughout the diagram, while the majority of the stars towards the outer regions of the Galaxy are spread between the calibrations of M giants and the predicted position for carbon stars. The stars classified as supergiants follow a different sequence from the one found for giant stars. We also performed a Kolmogorov-Smirnov (KS) test of the period and amplitude of stars towards the inner and outer regions of the Galaxy. We obtained a low probability that the two samples came from the same parent distribution. The observed behaviour of the period-amplitude and period-effective temperature (Teff) diagrams are, in general, in agreement with those found for Kepler sources and ground based photometry, with pulsation being the dominant cause responsible for the observed modulation. We also conclude that short-time variations on M-giant stars do not exist or are very rare, and the few cases we found are possibly related to biases or background stars.
A wavelet analysis of photometric variability in Kepler white dwarf stars
(Oxford University Press (OUP), 2019-01-12) Martins, Bruno Leonardo Canto; Lira, S. R. de; Bravo, J. P.; Leão, I. C.; Costa, A. D. da; Freitas, D. B. de; Medeiros, J. R. de
This work describes a wavelet analysis of 14 Kepler white dwarf stars, in order to confirm their photometric variability behaviour and to search for periodicities in these targets. From the observed Kepler light curves we obtained the wavelet local and global power spectra. Through this procedure, one can perform an analysis in the time–frequency domain rich in detail, and so obtain a new perspective on the time evolution of the periodicities present in these stars. We identified a photometric variability behaviour in 10 white dwarfs, corresponding to period variations of ∼2 h to 18 d: among these stars, three are new candidates and seven, earlier identified from other studies, are confirmed.

SIGAA

Navegando por Autor "Freitas, D. B. de"