2,203 research outputs found
Event-based personal retrieval
People who work in a research, academic or business environment often have personal information collections which are large enough to need retrieval aids. A major difference between personal information retrieval and normal document retrieval is that the items to be retrieved are often associated with events in the searcher's life and can be retrieved by their relationship to other events as well as by content. This paper describes some of the background to event-based retrieval and then describes a prototype graphical event-based retrieval system
The Future Evolution of White Dwarf Stars Through Baryon Decay and Time Varying Gravitational Constant
Motivated by the possibility that the fundamental ``constants'' of nature
could vary with time, this paper considers the long term evolution of white
dwarf stars under the combined action of proton decay and variations in the
gravitational constant. White dwarfs are thus used as a theoretical laboratory
to study the effects of possible time variations, especially their implications
for the future history of the universe. More specifically, we consider the
gravitational constant to vary according to the parametric relation , where the time scale is the same order as
the proton lifetime. We then study the long term fate and evolution of white
dwarf stars. This treatment begins when proton decay dominates the stellar
luminosity, and ends when the star becomes optically thin to its internal
radiation.Comment: 12 pages, 10 figures, accepted to Astrophysics and Space Scienc
On operad structures of moduli spaces and string theory
Recent algebraic structures of string theory, including homotopy Lie
algebras, gravity algebras and Batalin-Vilkovisky algebras, are deduced from
the topology of the moduli spaces of punctured Riemann spheres. The principal
reason for these structures to appear is as simple as the following. A
conformal field theory is an algebra over the operad of punctured Riemann
surfaces, this operad gives rise to certain standard operads governing the
three kinds of algebras, and that yields the structures of such algebras on the
(physical) state space naturally.Comment: 33 pages (An elaboration of minimal area metrics and new references
are added
A Short Review on Jet Identification
Jets can be used to probe the physical properties of the high energy density
matter created in collisions at the Relativistic Heavy Ion Collider (RHIC).
Measurements of strong suppression of inclusive hadron distributions and
di-hadron correlations at high have already provided evidence for
partonic energy loss. However, these measurements suffer from well-known
geometric biases due to the competition of energy loss and fragmentation. These
biases can be avoided if the jets are reconstructed independently of their
fragmentation details - quenched or unquenched. In this paper, we discuss
modern jet reconstruction algorithms (cone and sequential recombination) and
their corresponding background subtraction techniques required by the high
multiplicities of heavy ion collisions. We review recent results from the STAR
experiment at RHIC on direct jet reconstruction in central Au+Au collisions at
GeV.Comment: Proceedings for the invited talk of Hot Quarks 2008, Estes Park, CO
18-23 August 200
Five Glutamic Acid Residues in the C-Terminal Domain of the ChlD Subunit Play a Major Role in Conferring Mg2+ Cooperativity upon Magnesium Chelatase
Magnesium chelatase catalyzes the first committed step in chlorophyll biosynthesis by inserting a Mg2+ ion into protoporphyrin IX in an ATP-dependent manner. The cyanobacterial (Synechocystis) and higher-plant chelatases exhibit a complex cooperative response to free magnesium, while the chelatases from Thermosynechococcus elongatus and photosynthetic bacteria do not. To investigate the basis for this cooperativity, we constructed a series of chimeric ChlD proteins using N-terminal, central, and C-terminal domains from Synechocystis and Thermosynechococcus. We show that five glutamic acid residues in the C-terminal domain play a major role in this process
Suppression of inhomogeneous broadening in rf spectroscopy of optically trapped atoms
We present a novel method for reducing the inhomogeneous frequency broadening
in the hyperfine splitting of the ground state of optically trapped atoms. This
reduction is achieved by the addition of a weak light field, spatially
mode-matched with the trapping field and whose frequency is tuned in-between
the two hyperfine levels. We experimentally demonstrate the new scheme with Rb
85 atoms, and report a 50-fold narrowing of the rf spectrum
The ChlD subunit links the motor and porphyrin binding subunits of magnesium chelatase
Magnesium chelatase initiates chlorophyll biosynthesis, catalysing the MgATP2- dependent insertion of a Mg2+ ion into protoporphyin IX. The catalytic core of this large enzyme complex consists of three subunits: Bch/ChlI, Bch/ChlD and Bch/ChlH (in bacteriochlorophyll and chlorophyll producing species respectively). The D and I subunits are members of the AAA+ (ATPases associated with various cellular activities) superfamily of enzymes, and they form a complex that binds to H, the site of metal ion insertion. In order to investigate the physical coupling between ChlID and ChlH in vivo and in vitro , ChlD was FLAG-tagged in the cyanobacterium Synechocystis sp. PCC 6803 and co-immunoprecipitation experiments showed interactions with both ChlI and ChlH. Co-production of recombinant ChlD and ChlH in Escherichia coli yielded a ChlDH. Quantitative analysis using microscale thermophoresis (MST) showed magnesium-dependent binding ( K d 331 ± 58 nM) between ChlD and H. The physical basis for a ChlD-H interaction was investigated using chemical crosslinking coupled with mass spectrometry (XL-MS), together with modifications that either truncate ChlD or modify single residues. We found that the C-terminal integrin I domain of ChlD governs association with ChlH, the Mg2+ dependence of which also mediates the cooperative response of the Synechocystis chelatase to magnesium. Our work, showing the interaction site between the AAA+ motor and the chelatase domain of magnesium chelatase, will be essential for understanding how free energy from the hydrolysis of ATP on the AAA+ ChlI subunit is transmitted via the bridging subunit ChlD to the active site on ChlH
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