6,647 research outputs found
Leptonic dark matter annihilation in the evolving universe: constraints and implications
The cosmic electron and positron excesses have been explained as possible
dark matter (DM) annihilation products. In this work we investigate the
possible effects of such a DM annihilation scenario during the evolution
history of the Universe. We first calculate the extragalactic -ray
background (EGRB), which is produced through the final state radiation of DM
annihilation to charged leptons and the inverse Compton scattering between
electrons/positrons and the cosmic microwave background. The DM halo profile
and the minimal halo mass, which are not yet well determined from the current
N-body simulations, are constrained by the EGRB data from EGRET and Fermi
telescopes. Then we discuss the impact of such leptonic DM models on cosmic
evolution, such as the reionization and heating of intergalactic medium,
neutral Hydrogen 21 cm signal and suppression of structure formation. We show
that the impact on the Hydrogen 21 cm signal might show interesting signatures
of DM annihilation, but the influence on star formation is not remarkable.
Future observations of the 21 cm signals could be used to place new constraints
on the properties of DM.Comment: 24 pages, 6 figures and 2 tables. Improved treatment of the energy
deposition process, the suppression on structure formation is weaker.
Accepted for publication by JCA
Separation and determination of the bioactivity of oosporein from Chaetomium cupreum
Many plants have devised methods of protecting themselves; one of such methods is the use of endophytic fungi. The antagonistic mechanism by endophytic fungi has rarely been revealed. This study investigated Chaetomium cupreum from Macleaya cordata (Willd.) R.Br Herb, which was identified by analysis of morphological characteristics and 28S rDNA sequence. The crystal isolated from culture broth of C. cupreum was identified by X-ray single crystal diffraction as 3,3',6,6'-tetrahydroxy-4,4'- dimethyl-1,1'-bi(cyclohexa-3,6-diene)-2,2',5,5'-tetraone (abbreviated as oosporein). We demonstrated that oosporein had antifungal activity against Rhizoctonia solani, Botrytis cinerea, Pytium ultimum, and antitumor activities against HL-60 and A549. It is toxic to Artemia salina.Keywords: Chaetomium cupreum, toxicity, antifungal activity, oosporein, antitumorAfrican Journal of Biotechnology Vol. 9(36), pp. 5955-5961, 6 September, 201
New symmetries for the Ablowitz-Ladik hierarchies
In the letter we give new symmetries for the isospectral and non-isospectral
Ablowitz-Ladik hierarchies by means of the zero curvature representations of
evolution equations related to the Ablowitz-Ladik spectral problem. Lie
algebras constructed by symmetries are further obtained. We also discuss the
relations between the recursion operator and isospectral and non-isospectral
flows. Our method can be generalized to other systems to construct symmetries
for non-isospectral equations.Comment: 11 page
Dry sliding wear behavior of an extruded Mg–Dy–Zn alloy with long period stacking ordered phase
AbstractThe dry sliding wear behavior of extruded Mg-2Dy-0.5Zn alloy (at.%) was investigated using a pin-on-disk configuration. The friction coefficient and wear rate were measured within a load range 20–760 N at a sliding velocity of 0.785 m/s. Microstructure and wear surface of alloy were examined using scanning electron microscopy. The mechanical properties of alloy were tested at room and elevated temperatures. Five wear mechanisms, namely abrasion, oxidation, delamination, thermal softening and melting dominated the whole wear behavior with increasing applied load. The extruded Mg-2Dy-0.5Zn alloy exhibited the better wear resistance as compared with as-cast Mg97Zn1Y2 alloy under the given conditions through contact surface temperature analysis. The improved wear resistance was mainly related to fine grain size, good thermal stability of long period stacking order (LPSO) phase and excellent higher-temperature mechanical properties
Corrected Navier-Stokes equations for compressible flows
For gas flows, the Navier-Stokes (NS) equations are established by
mathematically expressing conservations of mass, momentum and energy. The
advantage of the NS equations over the Euler equations is that the NS equations
have taken into account the viscous stress caused by the thermal motion of
molecules. The viscous stress arises from applying Isaac Newton's second law to
fluid motion, together with the assumption that the stress is proportional to
the gradient of velocity1. Thus, the assumption is the only empirical element
in the NS equations, and this is actually the reason why the NS equations
perform poorly under special circumstances. For example, the NS equations
cannot describe rarefied gas flows and shock structure. This work proposed a
correction to the NS equations with an argument that the viscous stress is
proportional to the gradient of momentum when the flow is under compression,
with zero additional empirical parameters. For the first time, the NS equations
have been capable of accurately solving shock structure and rarefied gas flows.
In addition, even for perfect gas, the accuracy of the prediction of heat flux
rate is greatly improved. The corrected NS equations can readily be used to
improve the accuracy in the computation of flows with density variations which
is common in nature.Comment: 13 pages, 7 figure
Evaluating the effects of high-throughput structural neuroimaging predictors on whole-brain functional connectome outcomes via network-based vector-on-matrix regression
The joint analysis of multimodal neuroimaging data is critical in the field
of brain research because it reveals complex interactive relationships between
neurobiological structures and functions. In this study, we focus on
investigating the effects of structural imaging (SI) features, including white
matter micro-structure integrity (WMMI) and cortical thickness, on the whole
brain functional connectome (FC) network. To achieve this goal, we propose a
network-based vector-on-matrix regression model to characterize the FC-SI
association patterns. We have developed a novel multi-level dense bipartite and
clique subgraph extraction method to identify which subsets of spatially
specific SI features intensively influence organized FC sub-networks. The
proposed method can simultaneously identify highly correlated
structural-connectomic association patterns and suppress false positive
findings while handling millions of potential interactions. We apply our method
to a multimodal neuroimaging dataset of 4,242 participants from the UK Biobank
to evaluate the effects of whole-brain WMMI and cortical thickness on the
resting-state FC. The results reveal that the WMMI on corticospinal tracts and
inferior cerebellar peduncle significantly affect functional connections of
sensorimotor, salience, and executive sub-networks with an average correlation
of 0.81 (p<0.001).Comment: 20 pages, 5 figures, 2 table
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