11,029 research outputs found
On three-dimensional Weyl structures with reduced holonomy
Cartan's list of 3-dimensional Weyl structures with reduced holonomy is
revisited. We show that the only Einstein-Weyl structures on this list
correspond to the structures generated by the solutions of the dKP equation
Why do starless cores appear more flattened than protostellar cores?
We evaluate the intrinsic three dimensional shapes of molecular cores, by
analysing their projected shapes. We use the recent catalogue of molecular line
observations of Jijina et al. and model the data by the method originally
devised for elliptical galaxies. Our analysis broadly supports the conclusion
of Jones et al. that molecular cores are better represented by triaxial
intrinsic shapes (ellipsoids) than biaxial intrinsic shapes (spheroids).
However, we find that the best fit to all of the data is obtained with more
extreme axial ratios () than those derived by Jones et al.
More surprisingly, we find that starless cores have more extreme axial ratios
than protostellar cores -- starless cores appear more `flattened'. This is the
opposite of what would be expected from modeling the freefall collapse of
triaxial ellipsoids. The collapse of starless cores would be expected to
proceed most swiftly along the shortest axis - as has been predicted by every
modeller since Zel'dovich - which should produce more flattened cores around
protostars, the opposite of what is seen.Comment: 7 pages, 3 figure
Einstein--Maxwell--Dilaton metrics from three--dimensional Einstein--Weyl structures
A class of time dependent solutions to Einstein--Maxwell-dilaton
theory with attractive electric force is found from Einstein--Weyl structures
in (2+1) dimensions corresponding to dispersionless Kadomtsev--Petviashvili and
Toda equations. These solutions are obtained from time--like
Kaluza--Klein reductions of solitons.Comment: 12 pages, to be published in Class.Quantum Gra
Surface-acoustic-wave-driven luminescence from a lateral p-n junction
The authors report surface-acoustic-wave-driven luminescence from a lateral
p-n junction formed by molecular beam epitaxy regrowth of a modulation doped
GaAs/AlGaAs quantum well on a patterned GaAs substrate.
Surface-acoustic-wave-driven transport is demonstrated by peaks in the
electrical current and light emission from the GaAs quantum well at the
resonant frequency of the transducer. This type of junction offers high carrier
mobility and scalability. The demonstration of surface-acoustic-wave
luminescence is a significant step towards single-photon applications in
quantum computation and quantum cryptography.Comment: 4 pages, 3 figure
Protein annotation and modelling servers at University College London
The UCL Bioinformatics Group web portal offers several high quality protein structure prediction and function annotation algorithms including PSIPRED, pGenTHREADER, pDomTHREADER, MEMSAT, MetSite, DISOPRED2, DomPred and FFPred for the prediction of secondary structure, protein fold, protein structural domain, transmembrane helix topology, metal binding sites, regions of protein disorder, protein domain boundaries and protein function, respectively. We also now offer a fully automated 3D modelling pipeline: BioSerf, which performed well in CASP8 and uses a fragment-assembly approach which placed it in the top five servers in the de novo modelling category. The servers are available via the group web site at http://bioinf.cs.ucl.ac.uk/
Report of the Working Group on `W Mass and QCD' (Phenomenology Workshop on LEP2 Physics, Oxford, April 1997)
The W Mass and QCD Working Group discussed a wide variety of topics relating
to present and future measurements of M(W) at LEP2, including QCD backgrounds
to W+W- production. Particular attention was focused on experimental issues
concerning the direct reconstruction and threshold mass measurements, and on
theoretical and experimental issues concerning the four jet final state. This
report summarises the main conclusions.Comment: 43 pages LaTeX and 15 encapsulated postscript figures. Uses epsfig
and ioplppt macros. Full Proceedings to be published in Journal of Physics
Extensive dissolution of live pteropods in the Southern Ocean
The carbonate chemistry of the surface ocean is rapidly
changing with ocean acidification, a result of human activities. In the upper layers of the Southern Ocean, aragonite—a metastable form of calcium carbonate with rapid dissolution kinetics—may become undersaturated by 2050 (ref. 2). Aragonite undersaturation is likely to affect aragonite-shelled organisms, which can dominate surface water communities in polar regions. Here we present analyses of specimens of the pteropod Limacina helicina antarctica that were extracted live from the Southern Ocean early in 2008. We sampled from the top 200m of the water column, where aragonite saturation levels were around 1, as upwelled deep water is mixed with surface water containing anthropogenic CO2. Comparing the shell structure with samples from aragonite-supersaturated regions elsewhere under a scanning electron microscope, we found severe levels of shell dissolution in the undersaturated region alone. According to laboratory incubations of intact samples with a range of aragonite saturation levels, eight days of incubation in aragonite saturation levels of 0.94–
1.12 produces equivalent levels of dissolution. As deep-water upwelling and CO2 absorption by surface waters is likely to increase as a result of human activities2,4, we conclude that upper ocean regions where aragonite-shelled organisms are affected by dissolution are likely to expand
Submillimeter Studies of Prestellar Cores and Protostars: Probing the Initial Conditions for Protostellar Collapse
Improving our understanding of the initial conditions and earliest stages of
protostellar collapse is crucial to gain insight into the origin of stellar
masses, multiple systems, and protoplanetary disks. Observationally, there are
two complementary approaches to this problem: (1) studying the structure and
kinematics of prestellar cores observed prior to protostar formation, and (2)
studying the structure of young (e.g. Class 0) accreting protostars observed
soon after point mass formation. We discuss recent advances made in this area
thanks to (sub)millimeter mapping observations with large single-dish
telescopes and interferometers. In particular, we argue that the beginning of
protostellar collapse is much more violent in cluster-forming clouds than in
regions of distributed star formation. Major breakthroughs are expected in this
field from future large submillimeter instruments such as Herschel and ALMA.Comment: 12 pages, 9 figures, to appear in the proceedings of the conference
"Chemistry as a Diagnostic of Star Formation" (C.L. Curry & M. Fich eds.
The role of MYH and microsatellite instability in the development of sporadic colorectal cancer
Biallelic germline mutations in MYH are associated with colorectal neoplasms, which develop through a pathway involving somatic inactivation of APC. In this study, we investigated the incidence of the common MYH mutations in an Australian cohort of sporadic colorectal cancers, the clinicopathological features of MYH cancers, and determined whether inactivation of mismatch repair and base excision repair (BER) were mutually exclusive. The MYH gene was sequenced from lymphocyte DNA of 872 colorectal cancer patients and 478 controls. Two compound heterozygotes were identified in the cancer population and all three cancers from these individuals displayed a prominent infiltration of intraepithelial lymphocytes. In total, 11 heterozygotes were found in the cancer group and five in the control group. One tumour from an individual with biallelic germline mutation of MYH also demonstrated microsatellite instability (MSI) as a result of biallelic hypermethylation of the MLH1 promoter. Although MYH-associated cancers are rare in a sporadic colorectal population, this study shows that these tumours can develop through either a chromosomal or MSI pathway. Tumours arising in the setting of BER or mismatch repair deficiency may share a biological characteristic, which promotes lymphocytic infiltration
Is there a Pronounced Giant Dipole Resonance in ^4He?
A four-nucleon calculation of the total ^4He photodisintegration cross
section is performed. The full final-state interaction is taken into account
for the first time. This is achieved via the method of the Lorentz integral
transform. Semi-realistic NN interactions are employed. Different from the
known partial two-body ^4He(\gamma,n)^3He and ^4He(\gamma,p)^3H cross sections
our total cross section exhibits a pronounced giant resonance. Thus, in
contrast to older data, we predict quite a strong contribution of
the channel at the giant resonance peak energy.Comment: 10 pages, Latex (REVTEX), 4 Postscript figures, to appear in Phys.
Rev. Let
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