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Search Results: 1 - 10 of 506306 matches for " Huan Y. A. Meng "
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Variability of the Infrared Excess of Extreme Debris Disks
Huan Y. A. Meng,George H. Rieke,Kate Y. L. Su,Valentin D. Ivanov,Leonardo Vanzi,Wiphu Rujopakarn
Physics , 2012, DOI: 10.1088/2041-8205/751/1/L17
Abstract: Debris disks with extremely large infrared excesses (fractional luminosities $> 10^{-2}$) are rare. Those with ages between 30 and 130 Myr are of interest because their evolution has progressed well beyond that of protoplanetary disks (which dissipate with a timescale of order 3 Myr), yet they represent a period when dynamical models suggest that terrestrial planet building may still be progressing through large, violent collisions that could yield large amounts of debris and large infrared excesses. For example, our Moon was formed through a violent collision of two large proto-planets during this age range. We report two disks around the solar-like stars ID8 and HD 23514 in this age range where the 24 {\mu}m infrared excesses vary on timescales of a few years, even though the stars are not variable in the optical. Variations this rapid are difficult to understand if the debris is produced by collisional cascades, as it is for most debris disks. It is possible that the debris in these two systems arises in part from condensates from silicate-rich vapor produced in a series of violent collisions among relatively large bodies. If their evolution is rapid, the rate of detection of extreme excesses would indicate that major collisions may be relatively common in this age range.
Large Impacts around a Solar Analog Star in the Era of Terrestrial Planet Formation
Huan Y. A. Meng,Kate Y. L. Su,George H. Rieke,David J. Stevenson,Peter Plavchan,Wiphu Rujopakarn,Carey M. Lisse,Saran Poshyachinda,Daniel E. Reichart
Physics , 2015, DOI: 10.1126/science.1255153
Abstract: The final assembly of terrestrial planets occurs via massive collisions, which can launch copious clouds of dust that are warmed by the star and glow in the infrared. We report the real-time detection of a debris-producing impact in the terrestrial planet zone around a 35-million year-old solar analog star. We observed a substantial brightening of the debris disk at 3-5 {\mu}m, followed by a decay over a year, with quasi-periodic modulations of the disk flux. The behavior is consistent with the occurrence of a violent impact that produced vapor out of which a thick cloud of silicate spherules condensed that were ground into dust by collisions. These results demonstrate how the time domain can become a new dimension for the study of terrestrial planet formation.
Planetary Collisions outside the Solar System: Time Domain Characterization of Extreme Debris Disks
Huan Y. A. Meng,Kate Y. L. Su,George H. Rieke,Wiphu Rujopakarn,Gordon Myers,Michael Cook,Emery Erdelyi,Chris Maloney,James McMath,Gerald Persha,Saran Poshyachinda,Daniel E. Reichart
Physics , 2015, DOI: 10.1088/0004-637X/805/1/77
Abstract: Luminous debris disks of warm dust in the terrestrial planet zones around solar-like stars are recently found to vary, indicative of ongoing large-scale collisions of rocky objects. We use Spitzer 3.6 and 4.5 {\mu}m time-series observations in 2012 and 2013 (extended to 2014 in one case) to monitor 5 more debris disks with unusually high fractional luminosities ("extreme debris disk"), including P1121 in the open cluster M47 (80 Myr), HD 15407A in the AB Dor moving group (80 Myr), HD 23514 in the Pleiades (120 Myr), HD 145263 in the Upper Sco Association (10 Myr), and the field star BD+20 307 (>1 Gyr). Together with the published results for ID8 in NGC 2547 (35 Myr), this makes the first systematic time-domain investigation of planetary impacts outside the solar system. Significant variations with timescales shorter than a year are detected in five out of the six extreme debris disks we have monitored. However, different systems show diverse sets of characteristics in the time domain, including long-term decay or growth, disk temperature variations, and possible periodicity.
Pressure-induced phase transitions in AgClO4
D. Errandonea,L. Gracia,A. Beltran,A. Vegas,Y. Meng
Physics , 2012, DOI: 10.1103/PhysRevB.84.064103
Abstract: AgClO4 has been studied under compression by x-ray diffraction and density functional theory calculations. Experimental evidence of a structural phase transition from the tetragonal structure of AgClO4 to an orthorhombic barite-type structure has been found at 5.1 GPa. The transition is supported by total-energy calculations. In addition, a second transition to a monoclinic structure is theoretically proposed to take place beyond 17 GPa. The equation of state of the different phases is reported as well as the calculated Raman-active phonons and their pressure evolution. Finally, we provide a description of all the structures of AgClO4 and discuss their relationships. The structures are also compared with those of AgCl in order to explain the structural sequence determined for AgClO4.
Effect for Inhomogeneity by Pb Substitution on (Bi1-yPby)2Sr2Ca1-xYxCu2O8+δSingle Crystals  [PDF]
S. Komaki, Y. Kourogi, T. Murano, A. Furushima, S. Maeda, Y. Sawada, B. Saya, Q. T. Meng, T. Tsuneoka, F. Ichikawa, K. Itoh
Materials Sciences and Applications (MSA) , 2012, DOI: 10.4236/msa.2012.312120
Abstract: Inhomogeneity is one of the most important problems for the investigation of the superconductor-insulator transition. Y and Pb co-doped Bi2Sr2CaCu2O8+δ(PbY-Bi2212) crystals were fabricated by the self-flax method in order to control both carrier density and dimensionality. The Y-content of PbY-Bi2212 crystals was much wide distributed in the same crucible rather than that of only Y doping Bi2212 (Y-Bi2212) crystals. In order to clear the origin of inhomogeneity, crystals were characterized by wavelength-dispersive X-ray spectroscopy (WDS) and X-ray diffraction (XRD). One can estimate the Y-content of cleavage samples from the c-axis lattice parameter obtained by XRD because mainly Y-con- tent affects to c-axis lattice constant for both Y-Bi2212 and PbY-Bi2212 crystals. It found that there are some samples with multi-phases and the ratio of samples with multi-phase for PbY-Bi2212 crystals is larger than that for Y-Bi2212 crystals. The linear relation between Y-content and Pb-content were also found from WDS. Large inhomogeneity of Y-content may be induced from Pb-content inhomogeneity. Transport properties are strongly dependent to Y-content. However they cannot be explained only from the macroscopic chemical compositions of the substitution atom Y and Pb. It may be related to microscopic inhomogeneity.
Omega 3 polyunsaturated fatty acid improves spatial learning and hippocampal Peroxisome Proliferator Activated Receptors (PPARα and PPARγ) gene expression in rats
Hajjar Toktam,Meng Goh Y,Rajion Mohamed A,Vidyadaran Sharmili
BMC Neuroscience , 2012, DOI: 10.1186/1471-2202-13-109
Abstract: Background This study examined the effects of dietary polyunsaturated fatty acids (PUFA) as different n-6: n-3 ratios on spatial learning and gene expression of peroxisome- proliferator-activated receptors (PPARs) in the hippocampus of rats. Thirty male Sprague–Dawley rats were randomly allotted into 3 groups of ten animals each and received experimental diets with different n-6: n-3 PUFA ratios of either 65:1, 22:1 or 4.5:1. After 10 weeks, the spatial memory of the animals was assessed using the Morris Water Maze test. The expression of PPARα and PPARγ genes were determined using real-time PCR. Results Decreasing dietary n-6: n-3 PUFA ratios improved the cognitive performance of animals in the Morris water maze test along with the upregulation of PPARα and PPARγ gene expression. The animals with the lowest dietary n-6: n-3 PUFA ratio presented the highest spatial learning improvement and PPAR gene expression. Conclusion It can be concluded that modulation of n-6: n-3 PUFA ratios in the diet may lead to increased hippocampal PPAR gene expression and consequently improved spatial learning and memory in rats.
Point-contact Andreev reflection spectroscopy of candidate topological superconductor Cu0.25Bi2Se3
X. Chen,C. Huan,Y. S. Hor,C. A. R. Sá de Melo,Z. Jiang
Physics , 2012,
Abstract: We perform a point-contact Andreev reflection spectroscopic study of the topological superconducting material, Cu0.25Bi2Se3, in the ballistic regime using a normal-metal gold tip. We observe distinct point-contact spectra on the superconducting and non-superconducting regions of the crystal surface: the former shows a marked zero-bias conductance peak, indicative of unconventional superconductivity, while the latter exhibits a pseudogap-like feature. In both cases the measured differential conductance spectra exhibit a large linear background, preventing direct quantitative comparison with theory. We attribute this background to inelastic scattering at the tip-sample interface, and compare the background-subtracted spectra with a single-band p-wave model.
Water Content and Superconductivity in Na0.3CoO2*yH2O
J. Cmaidalka,A. Baikalov,Y. Y. Xue,R. L. Meng,C. W. Chu
Physics , 2003, DOI: 10.1016/j.physc.2003.11.008
Abstract: We report here the correlation between the water content and superconductivity in Na0.3CoO2*yH2O under the influences of elevated temperature and cold compression. The x-ray diffraction of the sample annealed at elevated temperatures indicates that intergrowths exist in the compound at equilibrium when 0.6 < y < 1.4. Its low-temperature diamagnetization varies linearly with y, but is insensitive to the intergrowth, indicative of quasi-2D superconductivity. The Tc-onset, especially, shifts only slightly with y. Our data from cold compressed samples, on the other hand, show that the water-loss non-proportionally suppresses the diamagnetization, which is suggestive of weak links.
Apoptosis and DNA Methylation
Huan X. Meng,James A. Hackett,Colm Nestor,Donncha S. Dunican,Monika Madej,James P. Reddington,Sari Pennings,David J. Harrison,Richard R. Meehan
Cancers , 2011, DOI: 10.3390/cancers3021798
Abstract: Epigenetic mechanisms assist in maintaining gene expression patterns and cellular properties in developing and adult tissues. The molecular pathology of disease states frequently includes perturbation of DNA and histone methylation patterns, which can activate apoptotic pathways associated with maintenance of genome integrity. This perspective focuses on the pathways linking DNA methyltransferases and methyl-CpG binding proteins to apoptosis, and includes new bioinformatic analyses to characterize the evolutionary origin of two G/T mismatch-specific thymine DNA glycosylases, MBD4 and TDG.
Ultrafast absorptive and refractive nonlinearities in multi-walled carbon nanotube film
H. I. Elim,W. Ji,G. H. Ma,K. Y. Lim,C. H. Sow,C. H. A. Huan
Physics , 2003, DOI: 10.1063/1.1786371
Abstract: By using femtosecond laser pulses at a wavelength range from 720 to 780 nm, we have observed absorptive and refractive nonlinearities in a film of multi-walled carbon nanotubes grown mainly along the direction perpendicular to the surface of quartz substrate. The Z-scans show that both absorptive and refractive nonlinearities are of negative and cubic nature in the laser irradiance range from a few to 300 GW/cm^2. The magnitude of the third-order nonlinear susceptibility,chi-(3), is of an order of 10^-11 esu. The degenerate pump-probe measurement reveals a relaxation time of 2 ps.
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