Examinando por Autor "Gonzalez O.A."

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    Galactic bulge population II Cepheids in the VVV survey: Period-luminosity relations and a distance to the Galactic centre
    (EDP Sciences, 2017-09) Bhardwaj A.; Rejkuba M.; Minniti D.; Surot F.; Valenti E.; Zoccali M.; Gonzalez O.A.; Romaniello M.; Kanbur S.M.; Singh H.P.
    Context. Multiple stellar populations of different ages and metallicities reside in the Galactic bulge that trace its structure and provide clues to its formation and evolution. Aims. We present the near-infrared observations of population II Cepheids in the Galactic bulge from VISTA Variables in the Vía Láctea (VVV) survey. The JHKs photometry together with optical data from Optical Gravitational Lensing Experiment (OGLE) survey provide an independent estimate of the distance to the Galactic centre. The old, metal-poor and low-mass population II Cepheids are also investigated as useful tracers for the structure of the Galactic bulge. Methods. We identify 340 population II Cepheids in the VVV survey Galactic bulge catalogue based on their match with the OGLE-III Catalogue. The single-epoch JH and multi-epoch Ks observations complement the accurate periods and optical (VI) mean-magnitudes from OGLE. The sample consisting of BL Herculis and W Virginis subtypes is used to derive period-luminosity relations after correcting mean-magnitudes for the extinction. Our Ks-band period-luminosity relation, Ks = -2.189(0.056) [log (P)-1] + 11.187(0.032), is consistent with published work for BL Herculis and W Virginis variables in the Large Magellanic Cloud. Results. We present a combined OGLE-III and VVV catalogue with periods, classification, mean magnitudes, and extinction for 264 Galactic bulge population II Cepheids that have good-quality Ks-band light curves. The absolute magnitudes for population II Cepheids and RR Lyraes calibrated using Gaia and Hubble Space Telescope parallaxes, together with calibrated magnitudes for Large Magellanic Cloud population II Cepheids, are used to obtain a distance to the Galactic centre, R0 = 8.34 ± 0.03(stat.) ± 0.41(syst.), which changes by with different extinction laws. While noting the limitation of small number statistics, we find that the present sample of population II Cepheids in the Galactic bulge shows a nearly spheroidal spatial distribution, similar to metal-poor RR Lyrae variables. We do not find evidence of the inclined bar as traced by the metal-rich red-clump stars. Conclusions. Population II Cepheid and RR Lyrae variables follow similar period-luminosity relations and trace the same metal-poor old population in the Galactic bulge. The number density for population II Cepheids is more limited as compared to abundant RR Lyraes but they are bright and exhibit a wide range in period that provides a robust period-luminosity relation for an accurate estimate of the distance to the Galactic centre. © ESO, 2017.
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    The Gaia -ESO Survey: Exploring the complex nature and origins of the Galactic bulge populations
    (EDP Sciences, 2017-05) Rojas-Arriagada A.; Recio-Blanco A.; De Laverny P.; Mikolaitis Š.; Matteucci F.; Spitoni E.; Schultheis M.; Hayden M.; Hill V.; Zoccali M.; Minniti D.; Gonzalez O.A.; Gilmore G.; Randich S.; Feltzing S.; Alfaro E.J.; Babusiaux C.; Bensby T.; Bragaglia A.; Flaccomio E.; Koposov S.E.; Pancino E.; Bayo A.; Carraro G.; Casey A.R.; Costado M.T.; Damiani F.; Donati P.; Franciosini E.; Hourihane A.; Jofré P.; Lardo C.; Lewis J.; Lind K.; Magrini L.; Morbidelli L.; Sacco G.G.; Worley C.C.; Zaggia S.
    Context. As observational evidence steadily accumulates, the nature of the Galactic bulge has proven to be rather complex: the structural, kinematic, and chemical analyses often lead to contradictory conclusions. The nature of the metal-rich bulge - and especially of the metal-poor bulge - and their relation with other Galactic components, still need to be firmly defined on the basis of statistically significant high-quality data samples. Aims. We used the fourth internal data release of the Gaia-ESO survey to characterize the bulge metallicity distribution function (MDF), magnesium abundance, spatial distribution, and correlation of these properties with kinematics. Moreover, the homogeneous sampling of the different Galactic populations provided by the Gaia-ESO survey allowed us to perform a comparison between the bulge, thin disk, and thick disk sequences in the [Mg/Fe] vs. [Fe/H] plane in order to constrain the extent of their eventual chemical similarities. Methods. We obtained spectroscopic data for ∼2500 red clump stars in 11 bulge fields, sampling the area -10° ≥ l ≥ +8° and -10° ≥ b ≥ -4° from the fourth internal data release of the Gaia-ESO survey. A sample of ∼6300 disk stars was also selected for comparison. Spectrophotometric distances computed via isochrone fitting allowed us to define a sample of stars likely located in the bulge region. Results. From a Gaussian mixture models (GMM) analysis, the bulge MDF is confirmed to be bimodal across the whole sampled area. The relative ratio between the two modes of the MDF changes as a function of b, with metal-poor stars dominating at high latitudes. The metal-rich stars exhibit bar-like kinematics and display a bimodality in their magnitude distribution, a feature which is tightly associated with the X-shape bulge. They overlap with the metal-rich end of the thin disk sequence in the [Mg/Fe] vs. [Fe/H] plane. On the other hand, metal-poor bulge stars have a more isotropic hot kinematics and do not participate in the X-shape bulge. Their Mg enhancement level and general shape in the [Mg/Fe] vs. [Fe/H] plane is comparable to that of the thick disk sequence. The position at which [Mg/Fe] starts to decrease with [Fe/H], called the "knee", is observed in the metal-poor bulge at [Fe/H]knee = -0:37 ± 0:09, being 0.06 dex higher than that of the thick disk. Although this difference is inside the error bars, it suggest a higher star formation rate (SFR) for the bulge than for the thick disk. We estimate an upper limit for this difference of Δ[Fe/H]knee = 0:24 dex. Finally, we present a chemical evolution model that suitably fits the whole bulge sequence by assuming a fast (<1 Gyr) intense burst of stellar formation that takes place at early epochs. Conclusions.We associate metal-rich stars with the bar boxy/peanut bulge formed as the product of secular evolution of the early thin disk. On the other hand, the metal-poor subpopulation might be the product of an early prompt dissipative collapse dominated by massive stars. Nevertheless, our results do not allow us to firmly rule out the possibility that these stars come from the secular evolution of the early thick disk. This is the first time that an analysis of the bulge MDF and α-abundances has been performed in a large area on the basis of a homogeneous, fully spectroscopic analysis of high-resolution, high S/N data. © ESO 2017.