The Experts below are selected from a list of 189 Experts worldwide ranked by ideXlab platform
Y Y Kovalev - One of the best experts on this subject based on the ideXlab platform.
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sub milliarcsecond imaging of quasars and active galactic nuclei iv fine scale Structure
The Astronomical Journal, 2005Co-Authors: Y Y Kovalev, K I Kellermann, M L Lister, D C Homan, R C Vermeulen, M H Cohen, E Ros, M Kadler, A P LobanovAbstract:We have examined the compact Structure in 250 flat-spectrum extragalactic radio sources using interferometric fringe visibilities obtained with the Very Long Baseline Array (VLBA) at 15 GHz. With projected baselines out to 440 Mλ, we are able to investigate source Structure on typical angular scales as small as 0.05 mas. This scale is similar to the resolution of the VLBI Space Observatory Programme data obtained on longer baselines at a lower frequency and with somewhat poorer accuracy. For 171 sources in our sample, more than half of the total flux density seen by the VLBA remains unresolved on the longest baselines. There are 163 sources in our list with a median correlated flux density at 15 GHz in excess of 0.5 Jy on the longest baselines; these will be useful as fringe finders for short-wavelength VLBA observations. The total flux densities recovered in the VLBA images at 15 GHz are generally close to the values measured around the same epoch at the same frequency with the RATAN-600 and University of Michigan Radio Astronomy Observatory telescopes. We have modeled the core of each source with an elliptical Gaussian component. For about 60% of the sources, we have at least one observation in which the core component appears unresolved (generally smaller than 0.05 mas) in one direction, usually transverse to the direction into which the jet extends. BL Lac objects are on average more compact than quasars, while active galaxies are on average less compact. Also, in an active galaxy the sub-milliarcsecond core component tends to be less dominant. Intraday variability (IDV) sources typically have a more compact, more core-Dominated Structure on sub-milliarcsecond scales than non-IDV sources, and sources with a greater amplitude of intraday variations tend to have a greater unresolved VLBA flux density. The objects known to be GeV gamma-ray-loud appear to have a more compact VLBA Structure than the other sources in our sample. This suggests that the mechanisms for the production of gamma-ray emission and for the generation of compact radio synchrotron–emitting features are related. The brightness temperature estimates and lower limits for the cores in our sample typically range between 1011 and 1013 K, but they extend up to 5 × 1013 K, apparently in excess of the equipartition brightness temperature or the inverse Compton limit for stationary synchrotron sources. The largest component speeds are observed in radio sources with high observed brightness temperatures, as would be expected from relativistic beaming. Longer baselines, which can be obtained by space VLBI observations, will be needed to resolve the most compact high brightness temperature regions in these sources.
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sub milliarcsecond imaging of quasars and active galactic nuclei iv fine scale Structure
arXiv: Astrophysics, 2005Co-Authors: Y Y Kovalev, K I Kellermann, M L Lister, D C Homan, R C Vermeulen, M H Cohen, E Ros, M Kadler, A P LobanovAbstract:We have used VLBA fringe visibility data obtained at 15 GHz to examine the compact Structure in 250 extragalactic radio sources. For 171 sources in our sample, more than half of the total flux density seen by the VLBA remains unresolved on the longest baselines. There are 163 sources in our list with a median correlated flux density at 15 GHz in excess of 0.5 Jy on the longest baselines. For about 60% of the sources, we have at least one observation in which the core component appears unresolved (generally smaller than 0.05 mas) in one direction, usually transverse to the direction into which the jet extends. BL Lacs are on average more compact than quasars, while active galaxies are on average less compact. Also, in an active galaxy the sub-milliarcsecond core component tends to be less dominant. IDV sources typically have a more compact, more core-Dominated Structure on sub-milliarcsecond scales than non-IDV sources, and sources with a greater amplitude of intra-day variations tend to have a greater unresolved VLBA flux density. The objects known to be GeV gamma-ray loud appear to have a more compact VLBA Structure than the other sources in our sample. This suggests that the mechanisms for the production of gamma-ray emission and for the generation of compact radio synchrotron emitting features are related. The brightness temperature estimates and lower limits for the cores in our sample typically range between 10^11 and 10^13 K, but they extend up to 5x10^13 K, apparently in excess of the equipartition brightness temperature, or the inverse Compton limit for stationary synchrotron sources. The largest component speeds are observed in radio sources with high observed brightness temperatures, as would be expected from relativistic beaming (abridged).
A P Lobanov - One of the best experts on this subject based on the ideXlab platform.
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sub milliarcsecond imaging of quasars and active galactic nuclei iv fine scale Structure
The Astronomical Journal, 2005Co-Authors: Y Y Kovalev, K I Kellermann, M L Lister, D C Homan, R C Vermeulen, M H Cohen, E Ros, M Kadler, A P LobanovAbstract:We have examined the compact Structure in 250 flat-spectrum extragalactic radio sources using interferometric fringe visibilities obtained with the Very Long Baseline Array (VLBA) at 15 GHz. With projected baselines out to 440 Mλ, we are able to investigate source Structure on typical angular scales as small as 0.05 mas. This scale is similar to the resolution of the VLBI Space Observatory Programme data obtained on longer baselines at a lower frequency and with somewhat poorer accuracy. For 171 sources in our sample, more than half of the total flux density seen by the VLBA remains unresolved on the longest baselines. There are 163 sources in our list with a median correlated flux density at 15 GHz in excess of 0.5 Jy on the longest baselines; these will be useful as fringe finders for short-wavelength VLBA observations. The total flux densities recovered in the VLBA images at 15 GHz are generally close to the values measured around the same epoch at the same frequency with the RATAN-600 and University of Michigan Radio Astronomy Observatory telescopes. We have modeled the core of each source with an elliptical Gaussian component. For about 60% of the sources, we have at least one observation in which the core component appears unresolved (generally smaller than 0.05 mas) in one direction, usually transverse to the direction into which the jet extends. BL Lac objects are on average more compact than quasars, while active galaxies are on average less compact. Also, in an active galaxy the sub-milliarcsecond core component tends to be less dominant. Intraday variability (IDV) sources typically have a more compact, more core-Dominated Structure on sub-milliarcsecond scales than non-IDV sources, and sources with a greater amplitude of intraday variations tend to have a greater unresolved VLBA flux density. The objects known to be GeV gamma-ray-loud appear to have a more compact VLBA Structure than the other sources in our sample. This suggests that the mechanisms for the production of gamma-ray emission and for the generation of compact radio synchrotron–emitting features are related. The brightness temperature estimates and lower limits for the cores in our sample typically range between 1011 and 1013 K, but they extend up to 5 × 1013 K, apparently in excess of the equipartition brightness temperature or the inverse Compton limit for stationary synchrotron sources. The largest component speeds are observed in radio sources with high observed brightness temperatures, as would be expected from relativistic beaming. Longer baselines, which can be obtained by space VLBI observations, will be needed to resolve the most compact high brightness temperature regions in these sources.
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sub milliarcsecond imaging of quasars and active galactic nuclei iv fine scale Structure
arXiv: Astrophysics, 2005Co-Authors: Y Y Kovalev, K I Kellermann, M L Lister, D C Homan, R C Vermeulen, M H Cohen, E Ros, M Kadler, A P LobanovAbstract:We have used VLBA fringe visibility data obtained at 15 GHz to examine the compact Structure in 250 extragalactic radio sources. For 171 sources in our sample, more than half of the total flux density seen by the VLBA remains unresolved on the longest baselines. There are 163 sources in our list with a median correlated flux density at 15 GHz in excess of 0.5 Jy on the longest baselines. For about 60% of the sources, we have at least one observation in which the core component appears unresolved (generally smaller than 0.05 mas) in one direction, usually transverse to the direction into which the jet extends. BL Lacs are on average more compact than quasars, while active galaxies are on average less compact. Also, in an active galaxy the sub-milliarcsecond core component tends to be less dominant. IDV sources typically have a more compact, more core-Dominated Structure on sub-milliarcsecond scales than non-IDV sources, and sources with a greater amplitude of intra-day variations tend to have a greater unresolved VLBA flux density. The objects known to be GeV gamma-ray loud appear to have a more compact VLBA Structure than the other sources in our sample. This suggests that the mechanisms for the production of gamma-ray emission and for the generation of compact radio synchrotron emitting features are related. The brightness temperature estimates and lower limits for the cores in our sample typically range between 10^11 and 10^13 K, but they extend up to 5x10^13 K, apparently in excess of the equipartition brightness temperature, or the inverse Compton limit for stationary synchrotron sources. The largest component speeds are observed in radio sources with high observed brightness temperatures, as would be expected from relativistic beaming (abridged).
Diane K. Stoecker - One of the best experts on this subject based on the ideXlab platform.
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The role of mixotrophic protists in the biological carbon pump
Biogeosciences, 2014Co-Authors: A Mitra, K. J. Flynn, A. Calbet, P J Hansen, T Berge, John A. Raven, Jo Ann M. Burkholder, Edna Granéli, Patricia M. Glibert, Diane K. StoeckerAbstract:The traditional view of the planktonic food web describes consumption of inorganic nutrients by photoautotrophic phytoplankton, which in turn supports zooplankton and ultimately higher trophic levels. Pathways centred on bacteria provide mechanisms for nutrient recycling. This Structure lies at the foundation of most models used to explore biogeochemical cycling, functioning of the biological pump, and the impact of climate change on these processes. We suggest an alternative new paradigm, which sees the bulk of the base of this food web supported by protist plankton communities that are mixotrophic - combining phototrophy and phagotrophy within a single cell. The photoautotrophic eukaryotic plankton and their heterotrophic microzooplankton grazers dominate only during the developmental phases of ecosystems (e. g. spring bloom in temperate systems). With their flexible nutrition, mixotrophic protists dominate in more-mature systems (e. g. temperate summer, established eutrophic systems and oligotrophic systems); the more-stable water columns suggested under climate change may also be expected to favour these mixotrophs. We explore how such a predominantly mixotrophic Structure affects microbial trophic dynamics and the biological pump. The mixotroph-Dominated Structure differs fundamentally in its flow of energy and nutrients, with a shortened and potentially more efficient chain from nutrient regeneration to primary production. Furthermore, mixotrophy enables a direct conduit for the support of primary production from bacterial production. We show how the exclusion of an explicit mixotrophic component in studies of the pelagic microbial communities leads to a failure to capture the true dynamics of the carbon flow. In order to prevent a misinterpretation of the full implications of climate change upon biogeochemical cycling and the functioning of the biological pump, we recommend inclusion of multi-nutrient mixotroph models within ecosystem studies.
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The role of mixotrophic protists in the biological carbon pump
Biogeosciences Discussions, 2013Co-Authors: A Mitra, K. J. Flynn, A. Calbet, P J Hansen, T Berge, John A. Raven, Edna Granéli, Patricia M. Glibert, J. M. Burkholder, Diane K. StoeckerAbstract:Abstract. The traditional view of the planktonic foodweb describes consumption of inorganic nutrients by photo-autotrophic phytoplankton, which in turn supports zooplankton and ultimately higher trophic levels. Pathways centred on bacteria provide mechanisms for nutrient recycling. This Structure lies at the foundation of most models used to explore biogeochemical cycling, functioning of the biological pump, and the impact of climate change on these processes. We suggest an alternative paradigm, which sees the bulk of the base of this foodweb supported by protist plankton (phytoplankton and microzooplankton) communities that are mixotrophic – combining phototrophy and phagotrophy within a~single cell. The photoautotrophic eukaryotic plankton and their heterotrophic microzooplankton grazers dominate only within immature environments (e.g., spring bloom in temperate systems). With their flexible nutrition, mixotrophic protists dominate in more mature systems (e.g., temperate summer, established eutrophic systems and oligotrophic systems); the more stable water columns suggested under climate change may also be expected to favour these mixotrophs. We explore how such a predominantly mixotrophic Structure affects microbial trophic dynamics and the biological pump. The mixotroph Dominated Structure differs fundamentally in its flow of energy and nutrients, with a shortened and potentially more efficient chain from nutrient regeneration to primary production. Furthermore, mixotrophy enables a direct conduit for the support of primary production from bacterial production. We show how the exclusion of an explicit mixotrophic component in studies of the pelagic microbial communities leads to a failure to capture the true dynamics of the carbon flow. In order to prevent a misinterpretation of the full implications of climate change upon biogeochemical cycling and the functioning of the biological pump, we recommend inclusion of multi-nutrient mixotroph models within ecosystem studies.
Javad Kadkhodapour - One of the best experts on this subject based on the ideXlab platform.
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The relationships between deformation mechanisms and mechanical properties of additively manufactured porous biomaterials.
Journal of the mechanical behavior of biomedical materials, 2016Co-Authors: Javad Kadkhodapour, H Montazerian, A. Ch. Darabi, A. Zargarian, Siegfried SchmauderAbstract:Abstract Modulating deformation mechanism through manipulating morphological parameters of scaffold internal pore architecture provides potential to tailor the overall mechanical properties under physiological loadings. Whereas cells sense local strains, cell differentiation is also impressed by the elastic deformations. In this paper, Structure-property relations were developed for Ti6–Al–4V scaffolds designed based on triply periodic minimal surfaces. 10 mm cubic scaffolds composed of 5×5×5 unit cells formed of F-RD (bending Dominated) and I-WP (stretching Dominated) architectures were additively manufactured at different volume fractions and subjected to compressive tests. The first stages of deformation for stretching Dominated Structure, was accompanied by bilateral layer-by-layer failure of unit cells owing to the buckling of micro-struts, while for bending Dominated Structure, namely F-RD, global shearing bands appeared since the shearing failure of struts in the internal architecture. Promoted mechanical properties were found for stretching Dominated Structure since the global orientation of struts were parallel to loading direction while inclination of struts diminished specific properties for bending Dominated Structure. Moreover, elastic–plastic deformation was computationally studied by applying Johnson-Cook damage model to the voxel-based models in FE analysis. Scaling analysis was performed for mechanical properties with respect to the relative density thereby failure mechanism was correlated to the constants of power law describing mechanical properties.
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Additively manufactured scaffolds: localization modes and failure pattern
2016Co-Authors: Javad KadkhodapourAbstract:Development of Structure-property correlations through characterizing deformation behavior in regular scaffolds has led to a more comprehensive understanding of architectural impacts on mechanical properties. Triply periodic minimal surface (TPMS) architectures have shown to provide superior conditions meeting the concurrent biological and mechanical requirements in unit cell design. Hence, in this paper, mechanical properties for additively manufactured Ti6Al4V scaffolds comprised of bending Dominated TPMS based architectures were evaluated versus stretching Dominated ones. To this, 10mm cubic scaffolds composed of 5×5×5 unit cells with F-RD (as bending Dominated) and I-WP (as stretching Dominated) architectures were designed at different volume fractions and compressive tests were performed on the scaffolds. The first stages of deformation for stretching Dominated Structure, was accompanied by bilateral layer-by-layer failure of unit cells owing to the buckling of micro-struts, while for bending Dominated Structure, namely F-RD, global shearing bands appeared since the shearing failure of struts in the internal architecture. Promoted mechanical properties was found for stretching Dominated Structure since the global orientation of struts in line with loading direction while inclination of struts diminished specific properties for bending Dominated Structure. Moreover, elastic-plastic deformation was computationally studied by applying Johnson-Cook damage model to the voxel-based models in FE analysis.
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Additive manufacturing and mechanical characterization of graded porosity scaffolds designed based on triply periodic minimal surface architectures
Journal of the Mechanical Behavior of Biomedical Materials, 2016Co-Authors: Maziar Afshar, A. Pourkamali Anaraki, H Montazerian, Javad KadkhodapourAbstract:Since the advent of additive manufacturing techniques, triply periodic minimal surfaces have emerged as a novel tool for designing porous scaffolds. Whereas scaffolds are expected to provide multifunctional performance, spatially changing pore patterns have been a promising approach to integrate mechanical characteristics of different architectures into a unique scaffold. Smooth morphological variations are also frequently seen in nature particularly in bone and cartilage Structures and can be inspiring for designing of artificial tissues. In this study, we carried out experimental and numerical procedures to uncover the mechanical properties and deformation mechanisms of linearly graded porosity scaffolds for two different mathematically defined pore Structures. Among TPMS-based scaffolds, P and D surfaces were subjected to gradient modeling to explore the mechanical responses for stretching and bending Dominated deformations, respectively. Moreover, the results were compared to their corresponding uniform porosity Structures. Mechanical properties were found to be by far greater for the stretching Dominated Structure (P-Surface). For bending Dominated architecture (D-Surface), although there was no global fracture for uniform Structures, graded Structure showed a brittle fracture at 0.08 strain. A layer by layer deformation mechanism for stretching Dominated Structure was observed. For bending Dominated scaffolds, deformation was accompanied by development of 45° shearing bands. Finite element simulations were also performed and the results showed a good agreement with the experimental observations.
A Mitra - One of the best experts on this subject based on the ideXlab platform.
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The role of mixotrophic protists in the biological carbon pump
Biogeosciences, 2014Co-Authors: A Mitra, K. J. Flynn, A. Calbet, P J Hansen, T Berge, John A. Raven, Jo Ann M. Burkholder, Edna Granéli, Patricia M. Glibert, Diane K. StoeckerAbstract:The traditional view of the planktonic food web describes consumption of inorganic nutrients by photoautotrophic phytoplankton, which in turn supports zooplankton and ultimately higher trophic levels. Pathways centred on bacteria provide mechanisms for nutrient recycling. This Structure lies at the foundation of most models used to explore biogeochemical cycling, functioning of the biological pump, and the impact of climate change on these processes. We suggest an alternative new paradigm, which sees the bulk of the base of this food web supported by protist plankton communities that are mixotrophic - combining phototrophy and phagotrophy within a single cell. The photoautotrophic eukaryotic plankton and their heterotrophic microzooplankton grazers dominate only during the developmental phases of ecosystems (e. g. spring bloom in temperate systems). With their flexible nutrition, mixotrophic protists dominate in more-mature systems (e. g. temperate summer, established eutrophic systems and oligotrophic systems); the more-stable water columns suggested under climate change may also be expected to favour these mixotrophs. We explore how such a predominantly mixotrophic Structure affects microbial trophic dynamics and the biological pump. The mixotroph-Dominated Structure differs fundamentally in its flow of energy and nutrients, with a shortened and potentially more efficient chain from nutrient regeneration to primary production. Furthermore, mixotrophy enables a direct conduit for the support of primary production from bacterial production. We show how the exclusion of an explicit mixotrophic component in studies of the pelagic microbial communities leads to a failure to capture the true dynamics of the carbon flow. In order to prevent a misinterpretation of the full implications of climate change upon biogeochemical cycling and the functioning of the biological pump, we recommend inclusion of multi-nutrient mixotroph models within ecosystem studies.
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The role of mixotrophic protists in the biological carbon pump
Biogeosciences Discussions, 2013Co-Authors: A Mitra, K. J. Flynn, A. Calbet, P J Hansen, T Berge, John A. Raven, Edna Granéli, Patricia M. Glibert, J. M. Burkholder, Diane K. StoeckerAbstract:Abstract. The traditional view of the planktonic foodweb describes consumption of inorganic nutrients by photo-autotrophic phytoplankton, which in turn supports zooplankton and ultimately higher trophic levels. Pathways centred on bacteria provide mechanisms for nutrient recycling. This Structure lies at the foundation of most models used to explore biogeochemical cycling, functioning of the biological pump, and the impact of climate change on these processes. We suggest an alternative paradigm, which sees the bulk of the base of this foodweb supported by protist plankton (phytoplankton and microzooplankton) communities that are mixotrophic – combining phototrophy and phagotrophy within a~single cell. The photoautotrophic eukaryotic plankton and their heterotrophic microzooplankton grazers dominate only within immature environments (e.g., spring bloom in temperate systems). With their flexible nutrition, mixotrophic protists dominate in more mature systems (e.g., temperate summer, established eutrophic systems and oligotrophic systems); the more stable water columns suggested under climate change may also be expected to favour these mixotrophs. We explore how such a predominantly mixotrophic Structure affects microbial trophic dynamics and the biological pump. The mixotroph Dominated Structure differs fundamentally in its flow of energy and nutrients, with a shortened and potentially more efficient chain from nutrient regeneration to primary production. Furthermore, mixotrophy enables a direct conduit for the support of primary production from bacterial production. We show how the exclusion of an explicit mixotrophic component in studies of the pelagic microbial communities leads to a failure to capture the true dynamics of the carbon flow. In order to prevent a misinterpretation of the full implications of climate change upon biogeochemical cycling and the functioning of the biological pump, we recommend inclusion of multi-nutrient mixotroph models within ecosystem studies.