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      • BRIGHT METAL-POOR STARS FROM THE HAMBURG/ESO SURVEY. II. A CHEMODYNAMICAL ANALYSIS

        Beers, Timothy C.,Placco, Vinicius M.,Carollo, Daniela,Rossi, Silvia,Lee, Young Sun,Frebel, Anna,Norris, John E.,Dietz, Sarah,Masseron, Thomas American Astronomical Society 2017 The Astrophysical journal Vol.835 No.1

        <P>We obtain estimates of stellar atmospheric parameters for a previously published sample of 1777 relatively bright (9 < B < 14) metal-poor candidates from the Hamburg/ESO Survey. The original Frebel et al. analysis of these stars was able to derive estimates of [Fe/H] and [C/Fe] only for a subset of the sample, due to limitations in the methodology then available. A new spectroscopic analysis pipeline has been used to obtain estimates of T-eff, log g, [Fe/H], and [C/Fe] for almost the entire data set. This sample is very local-about 90% of the stars are located within 0.5 kpc of the Sun. We consider the chemodynamical properties of these stars in concert with a similarly local sample of stars from a recent analysis of the Bidelman and MacConnell 'weak metal' candidates by Beers et al. We use this combined sample to identify possible members of the halo stream of stars suggested by Helmi et al. and Chiba & Beers, as well as stars that may be associated with stripped debris from the putative parent dwarf of the globular cluster Omega Centauri, suggested to exist by previous authors. We identify a clear increase in the cumulative frequency of carbon-enhanced metal-poor (CEMP) stars with declining metallicity, as well as an increase in the fraction of CEMP stars with distance from the Galactic plane, consistent with previous results. We also identify a relatively large number of CEMP stars with kinematics consistent with the metal-weak thick-disk population, with possible implications for its origin.</P>

      • THE STELLAR METALLICITY DISTRIBUTION FUNCTION OF THE GALACTIC HALO FROM SDSS PHOTOMETRY

        An, Deokkeun,Beers, Timothy C.,Johnson, Jennifer A.,Pinsonneault, Marc H.,Lee, Young Sun,Bovy, Jo,Ivezić,, Ž,eljko,Carollo, Daniela,Newby, Matthew IOP Publishing 2013 The Astrophysical journal Vol.763 No.1

        <P>We explore the stellar metallicity distribution function of the Galactic halo based on SDSS ugriz photometry. A set of stellar isochrones is calibrated using observations of several star clusters and validated by comparisons with medium-resolution spectroscopic values over a wide range of metal abundance. We estimate distances and metallicities for individual main-sequence stars in the multiply scanned SDSS Stripe 82, at heliocentric distances in the range 5-8 kpc and |b| > 35 degrees, and find that the in situ photometric metallicity distribution has a shape that matches that of the kinematically selected local halo stars from Ryan & Norris. We also examine independent kinematic information from proper-motion measurements for high Galactic latitude stars in our sample. We find that stars with retrograde rotation in the rest frame of the Galaxy are generally more metal poor than those exhibiting prograde rotation, which is consistent with earlier arguments by Carollo et al. that the halo system comprises at least two spatially overlapping components with differing metallicity, kinematics, and spatial distributions. The observed photometric metallicity distribution and that of Ryan & Norris can be described by a simple chemical evolution model by Hartwick (or by a single Gaussian distribution); however, the suggestive metallicity-kinematic correlation contradicts the basic assumption in this model that the Milky Way halo consists primarily of a single stellar population. When the observed metallicity distribution is deconvolved using two Gaussian components with peaks at [Fe/H] approximate to -1.7 and -2.3, the metal-poor component accounts for approximate to 20%-35% of the entire halo population in this distance range.</P>

      • SCISCIESCOPUS

        Galactic Archeology with the AEGIS Survey: The Evolution of Carbon and Iron in the Galactic Halo

        Yoon, Jinmi,Beers, Timothy C.,Dietz, Sarah,Lee, Young Sun,Placco, Vinicius M.,Costa, Gary Da,Keller, Stefan,Owen, Christopher I.,Sharma, Mahavir American Astronomical Society 2018 The Astrophysical journal Vol.861 No.2

        <P>Understanding the evolution of carbon and iron in the Milky Way's halo is of importance because these two elements play crucial roles in constraining star formation, Galactic assembly, and chemical evolution in the early universe. Here we explore the spatial distributions of the carbonicity, [C/Fe], and metallicity, [Fe/H], of the halo system based on medium-resolution (R similar to 1300)spectroscopy of similar to 58,000 stars in the southern hemisphere from the AAOmega Evolution of Galactic Structure (AEGIS) survey. The AEGIS carbonicity map exhibits a positive gradient with distance, as similarly found for the Sloan Digital Sky Survey carbonicity map of Lee et al. The metallicity map confirms that [Fe/H] decreases with distance from the inner halo to the outer halo. We also explore the formation and chemical evolution history of the halo by considering the populations of carbon-enhanced metalpoor (CEMP) stars present in the AEGIS sample. The cumulative and differential frequency of CEMP-no stars (as classified by their characteristically lower levels of absolute carbon abundance, A(C) <= 7.1, for subgiants and giants) increases with decreasing metallicity and is substantially higher than previous determinations for CEMP stars as a whole. In contrast, that of CEMP-s stars (with higher A(C)) remains almost flat, at a value of similar to 10% in the range -4.0 less than or similar to [Fe/H] less than or similar to -2.0. The distinctly different behaviors of the CEMP-no and CEMP-s stars relieve the tension with population synthesis models assuming a binary mass-transfer origin, which previously struggled to account for the higher reported frequencies of CEMP stars, taken as a whole, at low metallicity.</P>

      • 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>

      • KCI등재

        A Review of the Role of Bioreactors for iPSCs-Based Tissue-Engineered Articular Cartilage

        Reina-Mahecha Alejandro,Beers Martine J.,van der Veen Hugo C.,Zuhorn Inge S.,van Kooten Theo G.,Sharma Prashant K. 한국조직공학과 재생의학회 2023 조직공학과 재생의학 Vol.20 No.7

        BACKGROUND: Osteoarthritis (OA) is the most common degenerative joint disease without an ultimate treatment. In a search for novel approaches, tissue engineering (TE) has shown great potential to be an effective way for hyaline cartilage regeneration and repair in advanced stages of OA. Recently, induced pluripotent stem cells (iPSCs) have been appointed to be essential stem cells for degenerative disease treatment because they allow a personalized medicine approach. For clinical translation, bioreactors in combination with iPSCs-engineerd cartilage could match patients needs, serve as platform for large-scale patient specific cartilage production, and be a tool for patient OA modelling and drug screening. Furthermore, to minimize in vivo experiments and improve cell differentiation and cartilage extracellular matrix (ECM) deposition, TE combines existing approaches with bioreactors. METHODS: This review summarizes the current understanding of bioreactors and the necessary parameters when they are intended for cartilage TE, focusing on the potential use of iPSCs. RESULTS: Bioreactors intended for cartilage TE must resemble the joint cavity niche. However, recreating human synovial joints is not trivial because the interactions between various stimuli are not entirely understood. CONCLUSION: The use of mechanical and electrical stimulation to differentiate iPSCs, and maintain and test chondrocytes are key stimuli influencing hyaline cartilage homeostasis. Incorporating these stimuli to bioreactors can positively impact cartilage TE approaches and their possibility for posterior translation into the clinics.

      • KCI등재

        Diverse Chemo-Dynamical Properties of Nitrogen-Rich Stars Identified from Low-Resolution Spectra

        김창민,이영선,Timothy C. Beers,김영광 한국천문학회 2023 Journal of The Korean Astronomical Society Vol.56 No.1

        The second generation of stars in the globular clusters (GCs) of the Milky Way (MW) exhibit unusually high N, Na, or Al, compared to typical Galactic halo stars at similar metallicities. The halo field stars enhanced with such elements are believed to have originated in disrupted GCs or escaped from existing GCs. We identify such stars in the metallicity range -3.0 < [Fe/H] < 0.0 from a sample of ~36,800 giant stars observed in the Sloan Digital Sky Survey and Large Sky Area Multi-Object Fiber Spectroscopic Telescope survey, and present their dynamical properties. The N-rich population (NRP) and N-normal population (NNP) among our giant sample do not exhibit similarities in either in their metallicity distribution function (MDF) or dynamical properties. We find that, even though the MDF of the NRP looks similar to that of the MW's GCs in the range of [Fe/H] < -1.0, our analysis of the dynamical properties does not indicate similarities between them in the same metallicity range, implying that the escaped members from existing GCs may account for a small fraction of our N-rich stars, or the orbits of the present GCs have been altered by the dynamical friction of the MW. We also find a significant increase in the fraction of N-rich stars in the halo field in the very metal-poor (VMP; [Fe/H] < -2.0) regime, comprising up to ~20% of the fraction of the N-rich stars below [Fe/H] = -2.5, hinting that partially or fully destroyed VMP GCs may have in some degree contributed to the Galactic halo. A more detailed dynamical analysis of the NRP reveals that our sample of N-rich stars do not share a single common origin. Although a substantial fraction of the N-rich stars seem to originate from the GCs formed in situ, more than 60% of them are not associated with those of typical Galactic populations, but probably have extragalactic origins associated with Gaia Sausage/Enceladus, Sequoia, and Sagittarius dwarf galaxies, as well as with presently unrecognized progenitors.

      • 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>

      • FORMATION AND EVOLUTION OF THE DISK SYSTEM OF THE MILKY WAY: [α/Fe] RATIOS AND KINEMATICS OF THE SEGUE G-DWARF SAMPLE

        Lee, Young Sun,Beers, Timothy C.,An, Deokkeun,Ivezić,, Ž,eljko,Just, Andreas,Rockosi, Constance M.,Morrison, Heather L.,Johnson, Jennifer A.,Schö,nrich, Ralph,Bird, Jonathan,Yanny, Brian IOP Publishing 2011 The Astrophysical journal Vol.738 No.2

        <P>We employ measurements of the [alpha/Fe] ratio derived from low-resolution (R similar to 2000) spectra of 17,277 G-type dwarfs from the SEGUE survey to separate them into likely thin-and thick-disk subsamples. Both subsamples exhibit strong gradients of orbital rotational velocity with metallicity, of opposite signs, -20 to -30 km s(-1) dex(-1) for the thin-disk and + 40 to + 50 km s(-1) dex(-1) for the thick-disk population. The rotational velocity is uncorrelated with Galactocentric distance for the thin-disk subsample and exhibits a small trend for the thick-disk subsample. The rotational velocity decreases with distance from the plane for both disk components, with similar slopes (-9.0 +/- 1.0 km s(-1) kpc(-1)). Thick-disk stars exhibit a strong trend of orbital eccentricity with metallicity (about -0.2 dex(-1)), while the eccentricity does not change with metallicity for the thin-disk subsample. The eccentricity is almost independent of Galactocentric radius for the thin-disk population, while a marginal gradient of the eccentricity with radius exists for the thick-disk population. Both subsamples possess similar positive gradients of eccentricity with distance from the Galactic plane. The shapes of the eccentricity distributions for the thin-and thick-disk populations are independent of distance from the plane, and include no significant numbers of stars with eccentricity above 0.6. Among several contemporary models of disk evolution that we consider, radial migration appears to have played an important role in the evolution of the thin-disk population, but possibly less so for the thick disk, relative to the gas-rich merger or disk heating scenarios. We emphasize that more physically realistic models and simulations need to be constructed in order to carry out the detailed quantitative comparisons that our new data enable.</P>

      • A comparative study of wet etching and contacts on ( 2 ¯ 01 ) and (010) oriented β-Ga<sub>2</sub>O<sub>3</sub>

        Jang, Soohwan,Jung, Sunwoo,Beers, Kimberly,Yang, Jiancheng,Ren, Fan,Kuramata, A.,Pearton, S.J.,Baik, Kwang Hyeon Elsevier 2018 Journal of alloys and compounds Vol.731 No.-

        <P><B>Abstract</B></P> <P>We report on the effect of β-Ga<SUB>2</SUB>O<SUB>3</SUB> crystal orientation on wet etching and Ohmic contact formation. The photochemical etching rate in KOH solutions of ( 2 ¯ 01 ) oriented, n-type bulk single crystals grown by the edge-defined film-fed growth method is ∼3–4 times higher than for the (010) planes. The activation energy for etching was 0.498 eV and 0.424 eV for ( 2 ¯ 01 ) and (010) orientations, respectively, suggesting the etching is reaction-limited with the same rate-limiting step. Ti (200 Å)/Au (1500 Å) metallization deposited on the two different orientations and annealed at 450 °C showed Ohmic current-voltage (<I>I-V</I>) behavior for ( 2 ¯ 01 ) but rectifying characteristics for (010). For (010) Ga<SUB>2</SUB>O<SUB>3</SUB>, there exists 2 types of surfaces having Ga and O atomic densities of 0.58 and 0.87 × 10<SUP>15</SUP> cm<SUP>−2</SUP>, respectively. By contrast, for ( 2 ¯ 01 ) Ga<SUB>2</SUB>O<SUB>3</SUB> surfaces, there exist 2 types of surface, with each type terminated with only Ga or O. If the surface is terminated with O, the dangling bond densities of O are 1.78 and 2.68 × 10<SUP>15</SUP> cm<SUP>−2</SUP>, respectively. We found that ( 2 ¯ 01 ) –oriented Ga<SUB>2</SUB>O<SUB>3</SUB> is etched at higher rates and is easier to form Ohmic contacts than (010) Ga<SUB>2</SUB>O<SUB>3</SUB>. The higher density of oxygen dangling bonds on the ( 2 ¯ 01 ) plane correlates with the faster etch rates and pronounced Ohmic behavior from deposited metals.</P> <P><B>Highlights</B></P> <P> <UL> <LI> We report on the effect of β-Ga<SUB>2</SUB>O<SUB>3</SUB> crystal orientation on wet etching and Ohmic contact formation. </LI> <LI> The photochemical etching rate in KOH solutions of ( 2 ¯ 01 ) oriented, n-type bulk single crystals grown by the edge-defined film-fed growth method is ∼3–4 times higher than for the (010) planes. </LI> <LI> Ti (200 Å)/Au (1500 Å) metallization deposited on the two different orientations and annealed at 450 °C showed Ohmic current-voltage (<I>I-V</I>) behavior for ( 2 ¯ 01 ) but rectifying characteristics for (010). </LI> <LI> We found that ( 2 ¯ 01 ) –oriented Ga<SUB>2</SUB>O<SUB>3</SUB> is etched at higher rates and is easier to form Ohmic contacts to than (010) Ga<SUB>2</SUB>O<SUB>3</SUB>. The higher density of oxygen dangling bonds on the ( 2 ¯ 01 ) plane correlates with the faster etch rates and pronounced Ohmic behavior from deposited metals. </LI> </UL> </P>

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