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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    Discovery of a mid-infrared protostellar outburst of exceptional amplitude
    (Oxford University Press, 2020-12) Lucas P.W.; Elias J.; Points S.; Guo Z.; Smith L.C.; Stecklum B.; Vorobyov E.; Morris C.; Borissova J.; Kurtev R.; Contreras Peña C.; Medina N.; Minniti D.; Ivanov V.D.; Saito R.K.
    We report the discovery of a mid-infrared outburst in a young stellar object (YSO) with an amplitude close to 8 mag at λ ≈ 4.6 μm. WISEA J142238.82-611553.7 is one of 23 highly variable Wide-field Infrared Survey Explorer (WISE) sources discovered in a search of infrared dark clouds (IRDCs). It lies within the small IRDC G313.671-0.309 (d ≈ 2.6 kpc), seen by the Herschel/Hi-Gal survey as a compact massive cloud core that may have been measurably warmed by the event. Pre-outburst data from Spitzer in 2004 suggest it is a class I YSO, a view supported by observation of weak 2.12 μm H2 emission in an otherwise featureless red continuum spectrum in 2019 (6 mag below the peak in Ks). Spitzer, WISE, and VISTA Variables in the Via Lactea (VVV) data show that the outburst began by 2006 and has a duration >13 yr, with a fairly flat peak from 2010 to 2014. The low pre-outburst luminosity implies a low-mass progenitor. The outburst luminosity of a few × 102 L⊙ is consistent with an accretion rate M ≈ 10-4 M⊙yr-1, comparable to a classical FU Orionis event. The 4.6 μm peak in 2010 implies T = 800-1000 K and a disc radial location R ≈ 4.5 au for the emitting region. The colour evolution suggests subsequent progression outwards. The apparent absence of the hotter matter expected in thermal instability or MRI models may be due to complete obscuration of the innermost disc, e.g. by an edge-on disc view. Alternatively, disc fragmentation/infalling fragment models might more naturally explain a mid-infrared peak, though this is not yet clear. © 2020 The Author(s).
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    Ítem
    Small-scale star formation as revealed by VVVX galactic cluster candidates
    (Oxford University Press, 2020-12) Borissova J.; Kurtev R.; Amarinho N.; Alonso-García J.; Ramírez Alegría S.; Bernal S.; Medina N.; Chené A.-N.; Ivanov V.D.; Lucas P.W.; Minniti D.
    We report a search and analysis of obscured cluster candidates in the 'VISTA Variables in the Via Lactea eXtended (VVVX)' ESO Public Survey area encompassing the region between 229◦.4 < l < 295◦.2 and −4◦.3 < b < 4◦.4 of the southern Galactic disc. We discover and propose 88 new clusters. We improve the completeness of the embedded cluster population in this region, adding small size (linear diameters of 0.2-1.4 pc) and relatively far objects (heliocentric distance between 2 and 4 kpc) to existing catalogues. Nine candidates are proposed to be older open cluster candidates. Three of them (VVVX CL 204, CL 207, CL 208) have sufficient numbers of well-resolved stellar members to allow us to determine some basic cluster parameters. We confirm their nature as older, low-mass open clusters. Photometric analysis of 15 known clusters shows that they have ages above 20 Myr, and masses below 2000 M☉: in general, their proper motions follow the motion of the disc. We outline some groups of clusters, most probably formed within the same dust complex. Broadly, our candidates follow the network of filamentary structure in the remaining dust. Thus, in this part of the southern disc of the Galaxy, we have found recent star formation, producing small size and young clusters, in addition to the well-known, massive young clusters, including NGC 3603, Westerlund 2, and the Carina Nebula Complex. © 2020 The Author(s)