Navegando por Autor "Alves, S."

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A catalog of rotational and radial velocities for evolved stars
(Astronomy & Astrophysics, 2014) Medeiros, José Renan de; Alves, S.; Udry, S.; Andersen, J.; Nordström, B.; Mayor, M.
Rotational and radial velocities have been measured for 1589 evolved stars of spectral types F, G, and K and luminosity classes IV, III, II, and Ib, based on observations carried out with the CORAVEL spectrometers. The precision in radial velocity is better than 0.30 km s−1 per observation, whereas rotational velocity uncertainties are typically 1.0 km s−1 for subgiants and giants and 2.0 km s−1 for class II giants and Ib supergiants.
Chromospheric activity of stars with planets
(Astronomy & Astrophysics, 2011) Martins, Bruno Leonardo Canto; Chagas, M. L. das; Alves, S.; Leão, Izan de Castro; Souza Neto, L. P. de; Medeiros, José Renan de
Context. Signatures of chromospheric activity enhancement have been found for a dozen stars, pointing to a possible star-planet interaction. Nevertheless in the coronal activity regime, there is no conclusive observational evidence of such an interaction. Does star-planet interaction manifest itself only for a few particular cases, without having a major effect on stars with planets in general? Aims. We aim to add additional observational constraints to support or reject the major effects of star-planet interactions in stellar activity, based on Ca II chromospheric emission flux. Methods. We performed a statistical analysis of Ca II emission flux of stars with planets, as well as a comparison between Ca II and X-ray emission fluxes, searching for dependencies on planetary parameters. Results. In the present sample of stars with planets, there are no significant correlations between chromospheric activity indicator log(RHK) and planetary parameters. Furthermore, the distribution of the chromospheric activity indicator for stars without planets is indistinguishable from the one with planets.
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?
(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.
On the rotational behaviour of parent stars of extrasolar planets
(Oxford University Press, 2009) Alves, S.; Nascimento Junior, Jose Ribamar Silva do; Medeiros, José Renan de
We analysed the behaviour of the rotational velocity in the parent stars of extrasolar planets. Projected rotational velocity v sin i and angular momentum were combined with stellar and planetary parameters, for a unique sample of 147 stars, amounting to 184 extrasolar planets, including 25 multiple systems. Indeed, for the present working sample we considered only stars with planets detected by the radial-velocity procedure. Our analysis shows that the v sin i distribution of stars with planets along the HR diagram follows the well-established scenario for the rotation of intermediate to low main-sequence stars, with a sudden decline in rotation near 1.2 M⊙. The decline occurs around Teff∼ 6000 K, corresponding to the late-F spectral region. A statistical comparison of the distribution of the rotation of stars with planets and a sample of stars without planets indicates that the v sin i distribution for these two families of stars is drawn from the same population distribution function. We also found that the angular momentum of extrasolar planet parent stars follows, at least qualitatively, Kraft's relation J∝ (M/M⊙)α. The stars without detected planets show a clear trend of angular momentum deficit compared to the stars with planets, in particular for masses higher than about 1.25 M⊙. Stars with the largest mass planets tend to have angular momentum comparable to or higher than the Sun.
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.

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Navegando por Autor "Alves, S."