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THE FREQUENCY OF FIELD BLUE-STRAGGLER STARS IN THE THICK DISK AND HALO SYSTEM OF THE GALAXY
Santucci, Rafael M.,Placco, Vinicius M.,Rossi, Silvia,Beers, Timothy C.,Reggiani, Henrique M.,Lee, Young Sun,Xue, Xiang-Xiang,Carollo, Daniela IOP Publishing 2015 The Astrophysical journal Vol.801 No.2
<P>We present an analysis of a new, large sample of field blue-straggler stars (BSSs) in the thick disk and halo system of the Galaxy, based on stellar spectra obtained during the Sloan Digital Sky Survey (SDSS) and the Sloan Extension for Galactic Understanding and Exploration (SEGUE). Using estimates of stellar atmospheric parameters obtained from application of the SEGUE Stellar Parameter Pipeline, we obtain a sample of some 8000 BSSs, which are considered along with a previously selected sample of some 4800 blue horizontal-branch (BHB) stars. We derive the ratio of BSSs to BHB stars, F-BSS/BHB, as a function of Galactocentric distance and distance from the Galactic plane. The maximum value found for F-BSS/BHB is similar to 4.0 in the thick disk (at 3 kpc < vertical bar Z vertical bar < 4 kpc), declining to on the order of similar to 1.5-2.0 in the inner-halo region; this ratio continues to decline to similar to 1.0 in the outer-halo region. We associate a minority of field BSSs with a likely extragalactic origin; at least 5% of the BSS sample exhibit radial velocities, positions, and distances commensurate with membership in the Sagittarius Stream.</P>
CHRONOGRAPHY OF THE MILKY WAY’S HALO SYSTEM WITH FIELD BLUE HORIZONTAL-BRANCH STARS
Santucci, Rafael M.,Beers, Timothy C.,Placco, Vinicius M.,Carollo, Daniela,Rossi, Silvia,Lee, Young Sun,Denissenkov, Pavel,Tumlinson, Jason,Tissera, Patricia B. IOP Publishing 2015 ASTROPHYSICAL JOURNAL LETTERS - Vol.813 No.1
<P>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, <(g - r)(0)>, 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 similar 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 similar to 15 kpc of the Galaxy. Within this central region, the age difference spans a mean color range of about 0.05 mag (similar to 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.</P>
Stability of onshore pipelines in liquefied soils: Overview of computational methods
Castiglia, Massimina,de Magistris, Filippo Santucci,Napolitano, Agostino Techno-Press 2018 Geomechanics & engineering Vol.14 No.4
One of the significant problems in the design of onshore pipelines in seismic areas is their stability in case of liquefaction. Several model tests and numerical analyses allow investigating the behavior of pipelines when the phenomenon of liquefaction occurs. While experimental tests contribute significantly toward understanding the liquefaction mechanism, they are costly to perform compared to numerical analyses; on the other hand, numerical analyses are difficult to execute, because of the complexity of the soil behavior in case of liquefaction. This paper reports an overview of the existing computational methods to evaluate the stability of onshore pipelines in liquefied soils, with particular attention to the development of excess pore water pressures and the floatation of buried structures. The review includes the illustration of the mechanism of floating and the description of the available calculation methods that are classified in static and dynamic approaches. We also highlighted recent trends in numerical analyses. Moreover, for the static condition, referring to the American Petroleum Institute (API) Specification, we computed and compared the uplift safety factors in different cases that might have a relevant practical use.
Spectroscopic Validation of Low-metallicity Stars from RAVE
Placco, Vinicius M.,Beers, Timothy C.,Santucci, Rafael M.,Chanamé,, Julio,Sepú,lveda, Marí,a Paz,Coronado, Johanna,Points, Sean D.,Kaleida, Catherine C.,Rossi, Silvia,Kordopatis, Geo American Astronomical Society 2018 The Astronomical journal Vol.155 No.6
THE FRACTIONS OF INNER- AND OUTER-HALO STARS IN THE LOCAL VOLUME
An, Deokkeun,Beers, Timothy C.,Santucci, Rafael M.,Carollo, Daniela,Placco, Vinicius M.,Lee, Young Sun,Rossi, Silvia IOP Publishing 2015 ASTROPHYSICAL JOURNAL LETTERS - Vol.813 No.2
<P>We obtain a new determination of the metallicity distribution function (MDF) of stars within similar to 5-10 kpc of the Sun, based on recently improved co-adds of ugriz photometry for Stripe. 82 from the Sloan Digital Sky Survey. Our new estimate uses the methodology developed previously by An et al. to study in situ halo stars, but is based on a factor of two larger sample than available before, with much-improved photometric errors and zero-points. The newly obtained MDF can be divided into multiple populations of halo stars, with peak metallicities at [Fe/H] approximate to 1.4 and 1.9, which we associate with the inner-halo and outer-halo populations of the Milky Way, respectively. We find that the kinematics of these stars (based on proper-motion measurements at high Galactic latitude) supports the proposed dichotomy of the halo, as stars with retrograde motions in the rest frame of the Galaxy are generally more metal-poor than stars with prograde motions, consistent with previous claims. In addition, we generate mock catalogs of stars from a simulated Milk Way halo system, and demonstrate for the first time that the chemically and kinematically distinct properties of the inner-and outer-halo populations are qualitatively in agreement with our observations. The decomposition of the observed MDF and our comparison with the mock catalog results suggest that the outer-halo population contributes on the order of similar to 35%-55% of halo stars in the local volume.</P>