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| 2. |
- Teodoro, M., et al.
(författare)
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He II lambda 4686 IN eta CARINAE : COLLAPSE OF THE WIND-WIND COLLISION REGION DURING PERIASTRON PASSAGE
- 2012
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 746:1, s. 73-
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Tidskriftsartikel (refereegranskat)abstract
- The periodic spectroscopic events in eta Carinae are now well established and occur near the periastron passage of two massive stars in a very eccentric orbit. Several mechanisms have been proposed to explain the variations of different spectral features, such as an eclipse by the wind-wind collision (WWC) boundary, a shell ejection from the primary star or accretion of its wind onto the secondary. All of them have problems explaining all the observed phenomena. To better understand the nature of the cyclic events, we performed a dense monitoring of eta Carinae with five Southern telescopes during the 2009 low-excitation event, resulting in a set of data of unprecedented quality and sampling. The intrinsic luminosity of the He II lambda 4686 emission line (L similar to 310 L-circle dot) just before periastron reveals the presence of a very luminous transient source of extreme UV radiation emitted in the WWC region. Clumps in the primary's wind probably explain the flare-like behavior of both the X-ray and He II lambda 4686 light curves. After a short-lived minimum, He II lambda 4686 emission rises again to a new maximum, when X-rays are still absent or very weak. We interpret this as a collapse of the WWC onto the surface of the secondary star, switching off the hard X-ray source and diminishing the WWC shock cone. The recovery from this state is controlled by the momentum balance between the secondary's wind and the clumps in the primary's wind.
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| 4. |
- Yi, Chuixiang, et al.
(författare)
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Climate control of terrestrial carbon exchange across biomes and continents
- 2010
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Ingår i: Environmental Research Letters. - : IOP Publishing. - 1748-9326. ; 5:3
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Tidskriftsartikel (refereegranskat)abstract
- Understanding the relationships between climate and carbon exchange by terrestrial ecosystems is critical to predict future levels of atmospheric carbon dioxide because of the potential accelerating effects of positive climate-carbon cycle feedbacks. However, directly observed relationships between climate and terrestrial CO2 exchange with the atmosphere across biomes and continents are lacking. Here we present data describing the relationships between net ecosystem exchange of carbon (NEE) and climate factors as measured using the eddy covariance method at 125 unique sites in various ecosystems over six continents with a total of 559 site-years. We find that NEE observed at eddy covariance sites is (1) a strong function of mean annual temperature at mid-and high-latitudes, (2) a strong function of dryness at mid-and low-latitudes, and (3) a function of both temperature and dryness around the mid-latitudinal belt (45 degrees N). The sensitivity of NEE to mean annual temperature breaks down at similar to 16 degrees C (a threshold value of mean annual temperature), above which no further increase of CO2 uptake with temperature was observed and dryness influence overrules temperature influence.
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| 5. |
- Huang, L. M., et al.
(författare)
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Tuning magnetic properties of In2O3 by control of intrinsic defects
- 2010
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Ingår i: Europhysics letters. - : IOP Publishing. - 0295-5075 .- 1286-4854. ; 89:4
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Tidskriftsartikel (refereegranskat)abstract
- The electronic structure and magnetic properties of In2O3 with four kinds of intrinsic point defects (O vacancy, In interstitial, O interstitial, and In vacancy) have been theoretically studied using the density functional theory. The defect energy states of the O vacancy and In interstitial are close to the bottom of conduction band and act as shallow donors, while the defect energy states of the In vacancy and O interstitial are just above the top of the valence band and act as shallow acceptors. Without addition of any magnetic ions, all the hole states are completely spin polarized, while the electron states display no spin polarization. This implies that semiconducting In2O3 can display magnetic ordering, purely due to the intrinsic defects. However, the formation energies for neutral p-type defects are too high to be thermodynamically stable at reasonable temperatures. Nevertheless, it is shown that negative charging can greatly decrease the formation energies of p-type defects, simultaneously removing the local magnetic moments. We conlcude that V-In''' and O-I '' will be the dominant compensating defects as In2O3 is doped with TM ions, such as Sn, Mo, V and Cr. This result is consistent with the general view that the p-type defect is a key feature to mediate ferromagnetic coupling between transition metal ions of dilute concentration in metal oxides.
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| 6. |
- Pereira, Henrique M., et al.
(författare)
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Scenarios for Global Biodiversity in the 21st Century
- 2010
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 330:6010, s. 1496-1501
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Forskningsöversikt (refereegranskat)abstract
- Quantitative scenarios are coming of age as a tool for evaluating the impact of future socioeconomic development pathways on biodiversity and ecosystem services. We analyze global terrestrial, freshwater, and marine biodiversity scenarios using a range of measures including extinctions, changes in species abundance, habitat loss, and distribution shifts, as well as comparing model projections to observations. Scenarios consistently indicate that biodiversity will continue to decline over the 21st century. However, the range of projected changes is much broader than most studies suggest, partly because there are major opportunities to intervene through better policies, but also because of large uncertainties in projections.
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| 7. |
- Araújo, M.C., et al.
(författare)
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Predicting the mechanical behavior of amorphous polymeric materials under strain through multi-scale simulation
- 2014
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Ingår i: Applied Surface Science. - : Elsevier BV. - 0169-4332. ; 306, s. 37-46
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Tidskriftsartikel (refereegranskat)abstract
- Polymeric materials have become the reference material for high reliability and performance applications. However, their performance in service conditions is difficult to predict, due in large part to their inherent complex morphology, which leads to non-linear and anisotropic behavior, highly dependent on the thermomechanical environment under which it is processed. In this work, a multiscale approach is proposed to investigate the mechanical properties of polymeric-based material under strain. To achieve a better understanding of phenomena occurring at the smaller scales, the coupling of a finite element method (FEM) and molecular dynamics (MD) modeling, in an iterative procedure, was employed, enabling the prediction of the macroscopic constitutive response. As the mechanical response can be related to the local microstructure, which in turn depends on the nano-scale structure, this multiscale approach computes the stress-strain relationship at every analysis point of the macro-structure by detailed modeling of the underlying micro- and meso-scale deformation phenomena. The proposed multiscale approach can enable prediction of properties at the macroscale while taking into consideration phenomena that occur at the mesoscale, thus offering an increased potential accuracy compared to traditional methods.
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| 8. |
- Qian, Zhao, et al.
(författare)
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Excellent Catalytic Effects of Graphene Nanofibers on Hydrogen Release of Sodium alanate
- 2012
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 116:20, s. 10861-10866
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Tidskriftsartikel (refereegranskat)abstract
- One of the most technically challenging barriers to the widespread commercialization of hydrogen-fueled devices and vehicles remains hydrogen storage. More environmentally friendly and effective nonmetal catalysts are required to improve hydrogen sorption. In this paper, through a combination of experiment and theory, we evaluate and explore the catalytic effects of layered graphene nanofibers toward hydrogen release of light metal hydrides such as sodium alanate. Graphene nanofibers, especially the helical kind, are found to considerably improve hydrogen release from NaAlH4, which is of significance for the further enhancement of this practical material for environmentally friendly and effective hydrogen storage applications. Using density functional theory, we find that carbon sheet edges, regardless of whether they are of zigzag or armchair type, can weaken Al-H bonds in sodium alanate, which is believed to be due to a combination of NaAlH4 destabilization and dissociation product stabilization. The helical form of graphene nanofibers, with larger surface area and curved configuration, appears to benefit the functionalization of carbon sheet edges. We believe that our combined experimental and theoretical study will stimulate more explorations of other microporous or mesoporous nanomaterials with an abundance of exposed carbon edges in the application of practical complex light metal hydride systems.
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