Examinando por Autor "Medina, N."

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    An Automated Tool to Detect Variable Sources in the Vista Variables in the Vía Láctea Survey: The VVV Variables (V4) Catalog of Tiles d001 and d002
    (Institute of Physics Publishing, 2018-09) Medina, N.; Borissova, J.; Bayo, A.; Kurtev, R.; Navarro Molina, C.; Kuhn, M.; Kumar, N.; Lucas, P.W.; Catelan, M.; Minniti, D.; Smith, L.C.
    Time-varying phenomena are one of the most substantial sources of astrophysical information, and their study has led to many fundamental discoveries in modern astronomy. We have developed an automated tool to search for and analyze variable sources in the near-infrared K s band using the data from the VISTA Variables in the Vía Láctea (VVV) ESO Public Large Survey. This process relies on the characterization of variable sources using different variability indices calculated from time series generated with point-spread function (PSF) photometry of sources under analysis. In particular, we used two main indices, the total amplitude and the eta index η, to identify variable sources. Once the variable objects are identified, periods are determined with generalized Lomb-Scargle periodograms and the information potential metric. Variability classes are assigned according to a compromise between comparisons with VVV templates and the period of the variability. The automated tool is applied on VVV tiles d001 and d002 and led to the discovery of 200 variable sources. We detected 70 irregular variable sources and 130 periodic ones. In addition, nine open-cluster candidates projected in the region are analyzed, and the infrared variable candidates found around these clusters are further scrutinized by cross-matching their locations against emission star candidates from VPHAS+ survey Hα color cuts. © 2018. The American Astronomical Society. All rights reserved.
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    Near-infrared photometry and spectroscopy of the low Galactic latitude globular cluster 2MASS-GC 03
    (OXFORD UNIV PRESS, 2016-10) Carballo-Bello, Julio A.; Ramírez Alegría, S.; Borissova, J.; Smith, L. C.; Kurtev, R.; Lucas, P. W.; Moni Bidin, Ch.; Alonso-García, J.; Minniti, D.; Palma, T.; Dékány, I.; Medina, N.; Moyano, M.; Villanueva, V.; Kuhn, M. A.
    We present deep near-infrared photometry and spectroscopy of the globular cluster 2MASS-GC 03 projected in the Galactic disc using MMT and Magellan Infrared Spectrograph on the Clay Telescope (Las Campanas Observatory) and VISTA Variables in the Via Lactea Survey data. Most probable cluster member candidates were identified from near-infrared photometry. Out of 10 candidates that were followed-up spectroscopically, 5 have properties of cluster members, from which we calculate <[Fe/H]> = -0.9 +/- 0.2 and a radial velocity of = 78 +/- 12 km s(-1). A distance of 10.8 kpc is estimated from three likely RR Lyrae members. Given that the cluster is currently at a distance of 4.2 kpc from the Galactic Centre, the cluster's long survival time of an estimated 11.3 +/- 1.2 Gyr strengthens the case for its globular-cluster nature. The cluster has a hint of elongation in the direction of the Galactic Centre.
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    New Galactic star clusters discovered in the disc area of the VVVX survey
    (Oxford University Press, 2018-12) Borissova, J.; Ivanov, V.D.; Lucas, P.W.; Kurtev, R.; Alonso-Garcia, J.; Ramírez Alegría, S.; Minniti, D.; Froebrich, D.; Hempel, M.; Medina, N.; Chené, A.-N.; Kuhn, M.A.
    The 'VISTA Variables in the Vía Láctea eXtended (VVVX)' ESO Public Survey is a nearinfrared photometric sky survey that covers nearly 1700 deg2 towards the Galactic disc and bulge. It is well-suited to search for newopen clusters, hidden behind dust and gas. The pipeline processed and calibrated KS-band tile images of 40 per cent of the disc area covered by VVVX was visually inspected for stellar overdensities. Then, we identified cluster candidates by examination of the composite JHKS colour images. The colour-magnitude diagrams of the cluster candidates are constructed. Whenever possible the Gaia DR2 parameters are used to calculate the mean proper motions, radial velocities, reddening and distances. We report the discovery of 120 new infrared clusters and stellar groups. Approximately half of them (47 per cent) are faint, compact, highly reddened, and they seem to be associated with other indicators of recent star formation, such as nearby Young Stellar Objects, Masers, HII regions or bubbles. The preliminary distance determinations allow us to trace the clusters up to 4.5 kpc, but most of the cluster candidates are centred at 2.2 kpc. The mean proper motions of the clusters show that in general they follow the disc motion of the Galaxy. © 2018 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society.
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    YOUNG STELLAR CLUSTERS CONTAINING MASSIVE YOUNG STELLAR OBJECTS IN THE VVV SURVEY
    (IOP PUBLISHING, 2016-09) Borissova, J.; Ramírez Alegría, S.; Alonso, J.; Lucas, P. W.; Kurtev, R.; Medina, N.; Navarro, C.; Kuhn, M.; Gromadzki, M.; Retamales, G.; Fernandez, M. A.; Agurto-Gangas, C.; Chené, A.-N.; Minniti, D.; Contreras Pena, C.; Catelan, M.; Decany, I.; Thompson, M. A.; Morales, E. F. E.; Amigo, P.
    The purpose of this research is to study the connections of the global properties of eight young stellar clusters projected in the Vista Variables in the Via Lactea (VVV) ESO Large Public Survey disk area and their young stellar object (YSO) populations. The analysis is based on the combination of spectroscopic parallax-based reddening and distance determinations with main-sequence and pre-main-sequence ishochrone fitting to determine the basic parameters (reddening, age, distance) of the sample clusters. The lower mass limit estimations show that all clusters are low or intermediate mass (between 110 and 1800 M-circle dot), the slope Gamma of the obtained present-day mass functions of the clusters is close to the Kroupa initial mass function. The YSOs in the cluster's surrounding fields are classified using low resolution spectra, spectral energy distribution fits with theoretical predictions, and variability, taking advantage of multi-epoch VVV observations. All spectroscopically confirmed YSOs (except one) are found to be massive (more than 8 M-circle dot). Using VVV and GLIMPSE color-color cuts we have selected a large number of new YSO candidates, which are checked for variability and 57% are found to show at least low-amplitude variations. In few cases it was possible to distinguish between YSO and AGB classifications on the basis of light curves.