Examinando por Autor "Guo, Zhen"

Mostrando 1 - 3 de 3
  • Miniatura
    Ítem
    Analysis of physical processes in eruptive YSOs with near-infrared spectra and multiwavelength light curves
    (Royal Astronomical Society, 2021) Guo, Zhen; Lucas, P W; Contreras Peña, C; Smith, L C; Morris, C; Kurtev, R G; Borissova, J; Alonso-García, J; Minniti, D; Chené, A-N; Kumar, M S N; Caratti O Garatti, A; Froebrich, D; Stimson, W H
    The decade-long Vista Variables in the Via Lactea (VVV) survey has detected numerous highly variable young stellar objects (YSOs). We present a study of 61 highly variable VVV YSOs (ΔKs = 1–5 mag), combining near-infrared spectra from Magellan and ESO Very Large Telescope with VVV and NEOWISE light curves to investigate physical mechanisms behind eruptive events. Most sources are spectroscopically confirmed as eruptive variables (typically Class I YSOs) but variable extinction is also seen. Among them, magnetically controlled accretion, identified by H I recombination emission (usually accompanied by CO emission), is observed in 46 YSOs. Boundary layer accretion, associated with FU Ori-like outbursts identified by CO overtone and H2O absorption, is observed only in longer duration events (≥5 yr total duration). However, even in long duration events, the magnetically controlled accretion mode predominates, with amplitudes similar to the boundary layer mode. Shorter (100–700 d) eruptive events usually have lower amplitudes and these events are generally either periodic accretors or multiple time-scale events, wherein large photometric changes occur on time-scales of weeks and years. We find that the ratio of amplitudes in Ks and W2 can distinguish between variable accretion and variable extinction. Several YSOs are periodic or quasi-periodic variables. We identify examples of periodic accretors and extinction-driven periodicity among them (with periods up to 5 yr) though more data are needed to classify some cases. The data suggest that dynamic interactions with a companion may control the accretion rate in a substantial proportion of eruptive systems, although star–disc interactions should also be considered.
  • Miniatura
    Ítem
    Assessing the Stellar Population and the Environment of an HII Region on the Far Side of the Galaxy
    (American Astronomical Society (Aas), 2021) Chené, André-Nicolas; Benjamin, Robert A.; Ramírez-Alegría, Sebastian; Borissova, Jura; Kurtev, Radostin; Moni Bidin, Christian; Mauro, Francesco; Lucas, Phil; Guo, Zhen; Smith, Leigh C.; Gonzalez-Fernandez, Carlos; Ivanov, Valentin D.; Minniti, Dante; Anderson, Loren. D.; Armentrout, William. P.; Gonzalez, Danilo; Herrero, Artemio; Peña Ramírez, Karla
    We have investigated the stellar and interstellar content of the distant star formation region IRAS 17591-2228 (WISE H ii region GAL 007.47+0.06). It is associated with a water maser, whose parallax distance is $d={20.4}_{-2.2}^{+2.8}$ kpc, supported by independent measurements of proper motion and radial velocity. It is projected in the same direction as an extremely red (J−KS ∼ 6 mag) group of stars, and a shell of mid-infrared emission. We qualify the group of stars as a cluster candidate, VVV CL177. Its radius spans between 0farcm45 and 1' and contains at least two young stellar objects with an extreme extinction near ${A}_{{\rm{V}}}\sim 40$ mag. Yet more analysis will be required to determine if it is a real single cluster associated with the water maser. The 13CO emissions at the radial velocity of the maser corresponds to the mid-infrared emission.
  • Miniatura
    Ítem
    Variable stars in the VVV globular clusters II. NGC6441, NGC6569, NGC6626 (M 28), NGC6656 (M 22), 2MASS-GC 02, and Terzan 10
    (European Southern Observatory (ESO), 2021) Alonso-García, Javier; Smith, Leigh C.; Catelan, Márcio; Minniti, Dante; Navarrete, Camila; Borissova, Jura; Carballo-Bello, Julio A.; Contreras Ramos, Rodrigo; Fernández-Trincado, José G.; Ferreira Lopes, Carlos E.; Gran, Felipe; Garro, Elisa R.; Geisler, Doug; Guo, Zhen; Hempel, Maren; Kerins, Eamonn; Lucas, Philip W.; Palma, Tali; Peña Ramírez, Karla; Ramírez Alegría, Sebastián; Saito, Roberto K.
    Context. The Galactic globular clusters (GGCs) located in the inner regions of the Milky Way suffer from high extinction that makes their observation challenging. High densities of field stars in their surroundings complicate their study even more. The VISTA Variables in the Via Lactea (VVV) survey provides a way to explore these GGCs in the near-infrared where extinction effects are highly diminished. Aims. We conduct a search for variable stars in several inner GGCs, taking advantage of the unique multi-epoch, wide-field, near-infrared photometry provided by the VVV survey. We are especially interested in detecting classical pulsators that will help us constrain the physical parameters of these GGCs. In this paper, the second of a series, we focus on NGC 6656 (M 22), NGC 6626 (M 28), NGC 6569, and NGC 6441; these four massive GGCs have known variable sources, but quite different metallicities. We also revisit 2MASS-GC 02 and Terzan 10, the two GGCs studied in the first paper of this series. Methods. We present an improved method and a new parameter that efficiently identify variable candidates in the GGCs. We also use the proper motions of those detected variable candidates and their positions in the sky and in the color-magnitude diagrams to assign membership to the GGCs. Results. We identify and parametrize in the near-infrared numerous variable sources in the studied GGCs, cataloging tens of previously undetected variable stars. We recover many known classical pulsators in these clusters, including the vast majority of their fundamental mode RR Lyrae. We use these pulsators to obtain distances and extinctions toward these objects. Recalibrated period–luminosity–metallicity relations for the RR Lyrae bring the distances to these GGCs to a closer agreement with those reported by Gaia, except for NGC 6441, which is an uncommon Oosterhoff III GGC. Recovered proper motions for these GGCs also agree with those reported by Gaia, except for 2MASS-GC 02, the most reddened GGC in our sample, where the VVV near-infrared measurements provide a more accurate determination of its proper motions.