1,068 research outputs found
Kompaneets equation for neutrinos: Application to neutrino heating in supernova explosions
We derive a `Kompaneets equation' for neutrinos, which describes how the
distribution function of neutrinos interacting with matter deviates from a
Fermi-Dirac distribution with zero chemical potential. To this end, we expand
the collision integral in the Boltzmann equation of neutrinos up to the second
order in energy transfer between matter and neutrinos. The distortion of the
neutrino distribution function changes the rate at which neutrinos heat matter,
as the rate is proportional to the mean square energy of neutrinos, .
For electron-type neutrinos the enhancement in over its thermal value
is given approximately by
where is the bulk velocity of nucleons, while for the other neutrino
species the enhancement is , where is the
kinetic energy of nucleons divided by the thermal energy. This enhancement has
a significant implication for supernova explosions, as it would aid
neutrino-driven explosions.Comment: 14 pages, 1 figure, matched to published versio
Proto-clusters in the Lambda CDM Universe
We compare the highly clustered populations of very high redshift galaxies
with proto-clusters identified numerically in a standard CDM universe
() simulation. We evolve 256^3 dark matter
particles in a comoving box of side 150h^{-1}Mpc. By the present day there are
63 cluster sized objects of mass in excess of 10^{14}h^{-1}Mo in this box. We
trace these clusters back to higher redshift finding that their progenitors at
z=4--5 are extended regions of typically 20--40 Mpc (comoving) in size, with
dark halos of mass in excess of 10^{12}h^{-1}Mo and are overdense by typically
1.3--13 times the cosmological mean density. Comparison with the observation of
Lyman alpha emitting (LAEs) galaxies at z=4.86 and at z=4.1 indicates that the
observed excess clustering is consistent with that expected for a proto-cluster
region if LAEs typically correspond to massive dark halos of more than
10^{12}h^{-1}Mo. We give a brief discussion on the relation between high
redshift concentration of massive dark halos and present day rich clusters of
galaxies.Comment: 4 pages, 5 figures, Accepted for publication in ApJ Letter
Properties of holographic dark energy at the Hubble length
We consider holographic cosmological models of dark energy in which the
infrared cutoff is set by the Hubble's radius. We show that any interacting
dark energy model, regardless of its detailed form, can be recast as a non
interacting model in which the holographic parameter evolves slowly
with time. Two specific cases are analyzed. We constrain the parameters of both
models with observational data, and show that they can be told apart at the
perturbative level.Comment: 4 pages, 6 figures. Contribution to the Proceedings ERE201
Polydisperse suspensions : erosion, deposition, and flow capacity
Deposition from particle-laden flows is often described in terms of the capacity and competence of the flow, but robust definitions of these terms have proved elusive. In this paper we provide a mathematical modelling framework within which erosion and deposition of polydisperse sediment, and thus flow capacity and competence, can be rigorously defined. This framework explicitly captures the coupling between the suspension and an active layer of sediment at the top of the bed, and is capable of describing both depositional and erosional flows over both erodible and non-erodible beds. Crucially, the capacity of a flow is shown to depend on the erosional and depositional history because these processes determine the composition of the active layer. This dependence is explored within models of bidisperse and polydisperse suspensions. It is further demonstrated that monodisperse representations of suspended sediment transport may severely underpredict actual flow capacity. The polydisperse model is validated against recent experimental studies of the evolution of suspended material in waning turbulent flows, and is used to demonstrate that loss of capacity is the principal driver of sediment deposition
Gate-induced band ferromagnetism in an organic polymer
We propose that a chain of five-membered rings (polyaminotriazole) should be
ferromagnetic with an appropriate doping that is envisaged to be feasible with
an FET structure. The ferromagnetism is confirmed by a spin density functional
calculation, which also shows that ferromagnetism survives the Peierls
instability. We explain the magnetism in terms of Mielke and Tasaki's flat-band
ferromagnetism with the Hubbard model. This opens a new possibility of band
ferromagnetism in purely organic polymers.Comment: 4 pages, 7 figure
Evidence for Acquisition in Nature of a Chromosomal 2,4-Dichlorophenoxyacetic Acid/(alpha)-Ketoglutarate Dioxygenase Gene by Different \u3ci\u3eBurkholderia\u3c/i\u3e spp.
We characterized the gene required to initiate the degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) by the soil bacterium Burkholderia sp. strain TFD6, which hybridized to the tfdA gene of the canonical 2,4-D catabolic plasmid pJP4 under low-stringency conditions. Cleavage of the ether bond of 2,4-D by cell extracts of TFD6 proceeded by an (alpha)-ketoglutarate-dependent reaction,characteristic of TfdA (F. Fukumori and R. P. Hausinger, J. Bacteriol. 175:2083-2086, 1993). The TFD6 tfdA gene was identified in a recombinant plasmid which complemented a tfdA transposon mutant of TFD6 created by chromosomal insertion of Tn5. The plasmid also expressed TfdA activity in Escherichia coli DH5(alpha), as evidenced by enzyme assays with cell extracts. Sequence analysis of the tfdA gene and flanking regions from strain TFD6 showed 99.5% similarity to a tfdA gene cloned from the chromosome of a different Burkholderia species (strain RASC) isolated from a widely separated geographical area. This chromosomal gene has 77.2% sequence identity to tfdA from plasmid pJP4 (Y. Suwa, W. E. Holben, and L. J. Forney, abstr. Q-403, in Abstracts of the 94th General Meeting of the American Society for Microbiology 1994.). The tfdA homologs cloned from strains TFD6 and RASC are the first chromosomally encoded 2,4-D catabolic genes to be reported. The occurrence of highly similar tfdA genes in different bacterial species suggests that this chromosomal gene can be horizontally transferred
Flat-Band Ferromagnetism in Organic Polymers Designed by a Computer Simulation
By coupling a first-principles, spin-density functional calculation with an
exact diagonalization study of the Hubbard model, we have searched over various
functional groups for the best case for the flat-band ferromagnetism proposed
by R. Arita et al. [Phys. Rev. Lett. {\bf 88}, 127202 (2002)] in organic
polymers of five-membered rings. The original proposal (poly-aminotriazole) has
turned out to be the best case among the materials examined, where the reason
why this is so is identified here. We have also found that the ferromagnetism,
originally proposed for the half-filled flat band, is stable even when the band
filling is varied away from the half-filling. All these make the ferromagnetism
proposed here more experimentally inviting.Comment: 11 pages, 13figure
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