Examinando por Autor "Carollo, Daniela"

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    Chronography of the milky way's halo system with field blue horizontal-branch stars
    (Institute of Physics Publishing, 2015-11) Santucci, Rafael M.; Beers, Timothy C.; Placco, Vinicius M.; Carollo, Daniela; Rossi, Silvia; Lee, Young Sun; Denissenkov, Pavel; Tumlinson, Jason; Tissera, Patricia B.
    In a pioneering effort, Preston et al. reported that the colors of blue horizontal-branch (BHB) stars in the halo of the Galaxy shift with distance, from regions near the Galactic center to about 12 kpc away, and interpreted this as a correlated variation in the ages of halo stars, from older to younger, spanning a range of a few Gyrs. We have applied this approach to a sample of some 4700 spectroscopically confirmed BHB stars selected from the Sloan Digital Sky Survey to produce the first "chronographic map" of the halo of the Galaxy. We demonstrate that the mean de-reddened g - r color, increases outward in the Galaxy from -0.22 to -0.08 (over a color window spanning [-0.3: 0.0]) from regions close to the Galactic center to ∼40 kpc, independent of the metallicity of the stars. Models of the expected shift in the color of the field BHB stars based on modern stellar evolutionary codes confirm that this color gradient can be associated with an age difference of roughly 2-2.5 Gyr, with the oldest stars concentrated in the central ∼15 kpc of the Galaxy. Within this central region, the age difference spans a mean color range of about 0.05 mag (∼0.8 Gyr). Furthermore, we show that chronographic maps can be used to identify individual substructures, such as the Sagittarius Stream, and overdensities in the direction of Virgo and Monoceros, based on the observed contrast in their mean BHB colors with respect to the foreground/background field population. © 2015. The American Astronomical Society. All rights reserved..
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    Evidence for the Third Stellar Population in the Milky Way's Disk
    (Institute of Physics Publishing, 2019-12) Carollo, Daniela; Chiba, Masashi; Ishigaki, Miho; Freeman, Ken; Beers, Timothy C.; Lee, Young Sun; Tissera, Patricia; Battistini, Chiara; Primas, Francesca
    The Milky Way is a unique laboratory in which stellar properties can be measured and analyzed in detail. In particular, stars in the older populations encode information on the mechanisms that led to the formation of our Galaxy. In this article, we analyze the kinematics, spatial distribution, and chemistry of a large number of stars in the solar neighborhood, where all of the main Galactic components are well represented. We find that the thick disk comprises two distinct and overlapping stellar populations with different kinematic properties and chemical compositions. The metal-weak thick disk (MWTD) contains two-times less metal content than the canonical thick disk, and exhibits enrichment of light elements typical of the oldest stellar populations of the Galaxy. The rotational velocity of the MWTD around the Galactic center is ∼150 km s-1, corresponding to a rotational lag of 30 km s-1 relative to the canonical thick disk (∼180 km s-1), with a velocity dispersion of 60 km s-1. This stellar population likely originated from the merger of a dwarf galaxy during the early phases of our Galaxy's assembly, or it is a precursor disk, formed in the inner Galaxy and brought into the solar neighborhood by bar instability or spiral-arm formation mechanisms. © 2019. The American Astronomical Society. All rights reserved..
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    The Origin of the Milky Way's Halo Age Distribution
    (Institute of Physics Publishing, 2018-05) Carollo, Daniela; Tissera, Patricia B.; Beers, Timothy C.; Gudin, Dmitrii; Gibson, Brad K.; Freeman, Ken C.; Monachesi, Antonela
    We present an analysis of the radial age gradients for the stellar halos of five Milky Way (MW) mass-sized systems simulated as part of the Aquarius Project. The halos show a diversity of age trends, reflecting their different assembly histories. Four of the simulated halos possess clear negative age gradients, ranging from approximately -7 to -19 Myr kpc-1, shallower than those determined by recent observational studies of the Milky Way's stellar halo. However, when restricting the analysis to the accreted component alone, all of the stellar halos exhibit a steeper negative age gradient with values ranging from -8 to -32 Myr kpc-1, closer to those observed in the Galaxy. Two of the accretion-dominated simulated halos show a large concentration of old stars in the center, in agreement with the Ancient Chronographic Sphere reported observationally. The stellar halo that best reproduces the current observed characteristics of the age distributions of the Galaxy is that formed principally by the accretion of small satellite galaxies. Our findings suggest that the hierarchical clustering scenario can reproduce the MW's halo age distribution if the stellar halo was assembled from accretion and the disruption of satellite galaxies with dynamical masses less than ∼109.5 M, and a minimal in situ contribution. © 2018. The American Astronomical Society..