Browsing by Author "De Medeiros, J. R."
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- ItemChemical abundances and kinematics of 257 G-, K-type field giants. Setting a base for further analysis of giant-planet properties orbiting evolved stars(OXFORD UNIV PRESS, 2015) Adibekyan, V. Zh.; Benamati, L.; Santos, N. C.; Alves, S.; Lovis, C.; Udry, S.; Israelian, G.; Sousa, S. G.; Tsantaki, M.; Mortier, A.; Sozzetti, A.; De Medeiros, J. R.We performed a uniform and detailed abundance analysis of 12 refractory elements (Na, Mg, Al, Si, Ca, Ti, Cr, Ni, Co, Sc, Mn, and V) for a sample of 257 G-and K-type evolved stars from the CORALIE planet search programme. To date, only one of these stars is known to harbour a planetary companion. We aimed to characterize this large sample of evolved stars in terms of chemical abundances and kinematics, thus setting a solid base for further analysis of planetary properties around giant stars. This sample, being homogeneously analysed, can be used as a comparison sample for other planet-related studies, as well as for different type of studies related to stellar and Galaxy astrophysics. The abundances of the chemical elements were determined using an local thermodynamic equilibrium (LTE) abundance analysis relative to the Sun, with the spectral synthesis code MOOG and a grid of Kurucz ATLAS9 atmospheres. To separate the Galactic stellar populations, both a purely kinematical approach and a chemical method were applied. We confirm the overabundance of Na in giant stars compared to the field FGKdwarfs. This enhancement might have a stellar evolutionary character, but departures from LTE may also produce a similar enhancement. Our chemical separation of stellar populations also suggests a 'gap' in metallicity between the thick-disc and high-a metal-rich stars, as previously observed in dwarfs sample from HARPS. The present sample, as most of the giant star samples, also suffers from the B - V colour cut-off, which excludes low-log g stars with high metallicities, and high-log g star with low [Fe/H]. For future studies of planet occurrence dependence on stellar metallicity around these evolved stars, we suggest to use a subsample of stars in a 'cut-rectangle' in the log g-[Fe/H] diagram to overcome the aforementioned issue.
- ItemConstraints on Helium Enhancement in the Globular Cluster M4 (NGC 6121) : The Horizontal Branch Test(2014) Valcarce Bravo, Aldo Alfonso Raúl; Catelan, Márcio; Alonso García, J.; Cortes, C.; De Medeiros, J. R.
- ItemDetermination of the spectroscopic stellar parameters for 257 field giant stars(OXFORD UNIV PRESS, 2015) Alves, S.; Benamati, L.; Santos, N. C.; Adibekyan, V. Zh; Sousa, S. G.; Israelian, G.; De Medeiros, J. R.; Lovis, C.; Udry, S.The study of stellar parameters of planet-hosting stars, such as metallicity and chemical abundances, help us to understand the theory of planet formation and stellar evolution. Here, we present a catalogue of accurate stellar atmospheric parameters and iron abundances for a sample of 257 K and G field evolved stars that are being surveyed for planets using precise radial-velocity measurements as part of the Coralie programme to search for planets around giants. The analysis was done using a set of high-resolution and high-signal-to-noise Ultraviolet and Visible Echelle Spectrograph spectra. The stellar parameters were derived using Fe I and II ionization and excitation equilibrium methods. To take into account possible effects related to the choice of the lines on the derived parameters, we used three different iron line-list sets in our analysis, and the results differ among themselves by a small factor for most of stars. For those stars with previous literature parameter estimates, we found very good agreement with our own values. In the present catalogue, we are providing new precise spectroscopic measurements of effective temperature, surface gravity, microturbulence, and metallicity for 190 stars for which it has not been found or published in previous articles.
- ItemFundamental properties of nearby stars and the consequences on ΔY/ΔZ(2013) Valcarce, Aldo A. R.; Catelan, Márcio; De Medeiros, J. R.Context. One of the greatest difficulties in astrophysics is the determination of the fundamental stellar parameters, one of which is the initial mass fraction of helium (Y). However, given that Y can be measured spectroscopically in only a small percentage of stars, a linear relationship is assumed between Y and the mass fraction of metals (Z) from a canonical perspective of the chemical evolution of the galaxies. This Y-Z relation is generally represented as Y = Yp + ΔY/ΔZ × Z, with the value of the helium-to-metal enrichment ratio (ΔY/ΔZ) assumed as a constant. However, there is no fundamental reason for every star to have a Y value on a linear scale with Z. Indeed, different ΔY/ΔZ values may be expected in different populations which have undergone different chemical enrichment histories. Aims: In this paper a new method for determining the fundamental stellar parameters of nearby stars is presented that uses at the same time Mbol, Teff, and log 1. One of these parameters is Y, which is used to determine the validity of the Y-Z relation. Methods: A new set of evolutionary tracks is created using the PGPUC stellar evolution code, which includes 7 masses (0.5 ≤ M/M⊙ ≤ 1.1), 7 helium abundances (0.230 ≤ Y ≤ 0.370), and 12 metallicities (1.6 × 10-4 ≤ Z ≤ 6.0 × 10-2) for solar-scaled chemical compositions ([α/Fe] = 0.0). The suggested method is tested using two different spectroscopic databases of nearby main sequence stars with precise parallaxes, and spectroscopic measurements of [Fe/H], Teff and 1. Results: The proposed method is compared to other techniques used to determine the fundamental stellar parameters, where one assumes an age of 5 Gyr for all nearby stars. This comparison demonstrates that the hypothesis regarding constant age leads to an underestimation of the Y value, especially for low metallicities. In addition, the suggested method is limited to masses above 0.60 M⊙ and requires high-precision measurements of spectroscopic surface gravities in order to obtain reliable results. Finally, estimating masses and Ages assuming a Y-Z relation rather than a free Y value may induce average errors of approximately 0.02 M⊙ and 2 Gyr, respectively....
- ItemRotation period distribution of CoRoT and Kepler Sun-like stars(2015) Leao, I. C.; Pasquini, L.; Ferreira Lopes, C. E.; Neves, V.; Valcarce Bravo, Aldo Alfonso Raúl; De Oliveira, L. L. A.; Freire da Silva, D.; De Freitas, D. B.; Canto Martins, B. L.; Janot Pacheco, E.; Baglin, A.; De Medeiros, J. R.
- ItemStellar cycles from photometric data : CoRoT stars(2015) Ferreira Lopes, C. E.; Leao, I. C.; De Freitas, D. B.; Canto Martins, B. L.; Catelan, Márcio; De Medeiros, J. R.
- ItemStellar parameters for stars of the CoRoT exoplanet field(2015) Cortés, C.; Maciel, S. C.; Vieira, S.; Ferreira Lopes, C. E.; Leão, I. C.; De Oliveira, G. P.; Correia, C.; Canto Martins, B. L.; Catelan, Márcio; De Medeiros, J. R.
- ItemThe rotational behavior of Kepler Stars with Planets(2015) Chinchón, F. Paz; Leão, I. C.; Bravo, J. P.; Freitas, D. B. de; Ferreira Lopes, C. E.; Alves, S.; Catelan, Márcio; Canto Martins, B. L.; De Medeiros, J. R.
- ItemThe VVV Infrared Variability Catalog (VIVA-I)(2020) Ferreira Lopes, E. C.; Cross, N. J. G.; Catelan, M.; Minniti, D.; Hempel, M.; Lucas, W. P.; Angeloni, R.; Jablonsky, F.; Braga, F. V.; Leao, C. I.; Herpich, F. R.; Alonso-Garcia, J.; Papageorgiou, A.; Pichara, K.; Saito, K. R.; Bradley, A.; Beamin Muhlenbrock Juan Carlos; Cortes, C.; De Medeiros, J. R.; Russell, ChristopherThanks to the VISTA Variables in the Via Lactea (VVV) ESO Public Survey it is now possible to explore a large number of objects in those regions. This paper addresses the variability analysis of all VVV point sources having more than 10 observations in VVVDR4 using a novel approach. In total, the near-IR light curves of 288,378,769 sources were analysed using methods developed in the New Insight Into Time Series Analysis project. As a result, we present a complete sample having 44, 998, 752 variable star candidates (VVV-CVSC), which include accurate individual coordinates, near-IR magnitudes (ZYJHKs), extinctions A(Ks), variability indices, periods, amplitudes, among other parameters to assess the science. Unfortunately, a side effect of having a highly complete sample, is also having a high level of contamination by non-variable (contamination ratio of non-variables to variables is slightly over 10:1). To deal with this, we also provide some flags and parameters that can be used by the community to de-crease the number of variable candidates without heavily decreasing the completeness of the sample. In particular, we cross-identified 339,601 of our sources with Simbad and AAVSO databases, which provide us with information for these objects at other wavelegths. This sub-sample constitutes a unique resource to study the corresponding near-IR variability of known sources as well as to assess the IR variability related with X-ray and Gamma-Ray sources. On the other hand, the other 99.5% sources in our sample constitutes a number of potentially new objects with variability information for the heavily crowded and reddened regions of the Galactic Plane and Bulge. The present results also provide an important queryable resource to perform variability analysis and to characterize ongoing and future surveys like TESS and LSST.
- ItemThe WFCAM multiwavelength Variable Star Catalog (Corrigendum)(2015) Ferreira Lopes, C. E.; Dékány, I.; Catelan, Marcio; Cross, N. J. G.; Angeloni, R.; Leão, I. C.; De Medeiros, J. R.