701 research outputs found
Global Spinors and Orientable Five-Branes
Fermion fields on an M-theory five-brane carry a representation of the double
cover of the structure group of the normal bundle. It is shown that, on an
arbitrary oriented Lorentzian six-manifold, there is always an Sp(2) twist that
allows such spinors to be defined globally. The vanishing of the arising
potential obstructions does not depend on spin structure in the bulk, nor does
the six-manifold need to be spin or spin-C. Lifting the tangent bundle to such
a generalised spin bundle requires picking a generalised spin structure in
terms of certain elements in the integral and modulo-two cohomology of the
five-brane world-volume in degrees four and five, respectively.Comment: 18 pages, LaTeX; v2: version to appear in JHE
Thermodynamic metrics and optimal paths
A fundamental problem in modern thermodynamics is how a molecular-scale
machine performs useful work, while operating away from thermal equilibrium
without excessive dissipation. To this end, we derive a friction tensor that
induces a Riemannian manifold on the space of thermodynamic states. Within the
linear-response regime, this metric structure controls the dissipation of
finite-time transformations, and bestows optimal protocols with many useful
properties. We discuss the connection to the existing thermodynamic length
formalism, and demonstrate the utility of this metric by solving for optimal
control parameter protocols in a simple nonequilibrium model.Comment: 5 page
Syk tyrosine kinase is critical for B cell antibody responses and memory B cell survival
Signals from the BCR are required for Ag-specific B cell recruitment into the immune response. Binding of Ag to the BCR induces phosphorylation of immune receptor tyrosine-based activation motifs in the cytoplasmic domains of the CD79a and CD79b signaling subunits, which subsequently bind and activate the Syk protein tyrosine kinase. Earlier work with the DT40 chicken B cell leukemia cell line showed that Syk was required to transduce BCR signals to proximal activation events, suggesting that Syk also plays an important role in the activation and differentiation of primary B cells during an immune response. In this study, we show that Syk-deficient primary mouse B cells have a severe defect in BCR-induced activation, proliferation, and survival. Furthermore, we demonstrate that Syk is required for both T-dependent and T-independent Ab responses, and that this requirement is B cell intrinsic. In the absence of Syk, Ag fails to induce differentiation of naive B cells into germinal center B cells and plasma cells. Finally, we show that the survival of existing memory B cells is dependent on Syk. These experiments demonstrate that Syk plays a critical role in multiple aspects of B cell Ab responses
The Serre spectral sequence of a noncommutative fibration for de Rham cohomology
For differential calculi on noncommutative algebras, we construct a twisted
de Rham cohomology using flat connections on modules. This has properties
similar, in some respects, to sheaf cohomology on topological spaces. We also
discuss generalised mapping properties of these theories, and relations of
these properties to corings. Using this, we give conditions for the Serre
spectral sequence to hold for a noncommutative fibration. This might be better
read as giving the definition of a fibration in noncommutative differential
geometry. We also study the multiplicative structure of such spectral
sequences. Finally we show that some noncommutative homogeneous spaces satisfy
the conditions to be such a fibration, and in the process clarify the
differential structure on these homogeneous spaces. We also give two explicit
examples of differential fibrations: these are built on the quantum Hopf
fibration with two different differential structures.Comment: LaTeX, 33 page
Loop Groups, Kaluza-Klein Reduction and M-Theory
We show that the data of a principal G-bundle over a principal circle bundle
is equivalent to that of a \hat{LG} = U(1) |x LG bundle over the base of the
circle bundle. We apply this to the Kaluza-Klein reduction of M-theory to IIA
and show that certain generalized characteristic classes of the loop group
bundle encode the Bianchi identities of the antisymmetric tensor fields of IIA
supergravity. We further show that the low dimensional characteristic classes
of the central extension of the loop group encode the Bianchi identities of
massive IIA, thereby adding support to the conjectures of hep-th/0203218.Comment: 26 pages, LaTeX, utarticle.cls, v2:clarifications and refs adde
On the integral cohomology of smooth toric varieties
Let be a smooth, not necessarily compact toric variety. We show
that a certain complex, defined in terms of the fan , computes the
integral cohomology of , including the module structure over the
homology of the torus. In some cases we can also give the product. As a
corollary we obtain that the cycle map from Chow groups to integral Borel-Moore
homology is split injective for smooth toric varieties. Another result is that
the differential algebra of singular cochains on the Borel construction of
is formal.Comment: 10 page
The chameleon groups of Richard J. Thompson: automorphisms and dynamics
The automorphism groups of several of Thompson's countable groups of
piecewise linear homeomorphisms of the line and circle are computed and it is
shown that the outer automorphism groups of these groups are relatively small.
These results can be interpreted as stability results for certain structures of
PL functions on the circle. Machinery is developed to relate the structures on
the circle to corresponding structures on the line
A Survey of High Contrast Stellar Flares Observed by Chandra
The X-ray light curves of pre-main sequence stars can show variability in the
form of flares altering a baseline characteristic activity level; the largest
X-ray flares are characterized by a rapid rise to more than 10 times the
characteristic count rate, followed by a slower quasi-exponential decay.
Analysis of these high-contrast X-ray flares enables the study of the innermost
magnetic fields of pre-main sequence stars. We have scanned the ANCHORS
database of Chandra observations of star-forming regions to extend the study of
flare events on pre-main sequence stars both in sky coverage and in volume. We
developed a sample of 30 high-contrast flares out of the 14,000 stars of
various ages and masses available in ANCHORS at the start of our study.
Applying methods of time-resolved spectral analysis, we obtain the
temperatures, confining magnetic field strengths, and loop lengths of these
bright, energetic flares. The results of the flare analysis are compared to the
2MASS and Spitzer data available for the stars in our sample. We find that the
longest flare loop lengths (of order several stellar radii) are only seen on
stars whose IR data indicates the presence of disks. This suggests that the
longest flares may stretch all the way to the disk. Such long flares tend to be
more tenuous than the other large flares studied. A wide range of loop lengths
are observed, indicating that different types of flares may occur on disked
young stellar objects.Comment: 38 pages, 8 figures, 4 table
Locating the Accretion Footprint on a Herbig Ae Star: MWC 480
Accretion is a fundamental process which establishes the dynamics of the protoplanetary disk and the final properties of the forming star. In solar-type stars, the star–disk coupling is determined by the magnetic field structure, which is responsible for funneling material from the disk midplane to higher latitudes on the star. Here, we use pan-chromatic data for the Herbig Ae star MWC 480 to address whether similar processes occur in intermediatemass stars. MWC 480 has X-ray emission typical of actively accreting Herbig Ae stars, but with ∼10× more photoelectric absorption than expected from optical and FUV data. We consider three sources for the absorption: the disk, absorption in a wind or jet, and accretion. While we detect the disk in scattered light in a re-analysis of archival Hubble Space Telescope data, the data are consistent with grazing illumination of the dust disk.We find that MWC 480’s disk is stratified, geometrically thin, and is not responsible for the observed photoelectric absorption. MWC 480 drives a bipolar jet, but with a mass-loss rate that is low compared to other Herbig Ae stars, where the outflow is more favorably oriented and enhanced photoelectric absorption is not seen. This excludes a jet or wind origin for the enhanced photoelectric absorption. We compare MWC 480’s Ovi emission with other Herbig Ae stars. The distribution of the emission in inclination, and lack of a correlation of profile shape and system inclination excludes equatorially confined accretion for the FUSE Herbig Ae stars. The photoelectric absorption data further suggest that the accretion footprint on MWC 480 and other Herbig Ae stars is located at high-temperate, rather than polar, latitudes. These findings support the presence of funneled accretion in MWC 480 and Herbig Ae stars, strengthening the parallel to T Tauri stars
Representation theory of finite W algebras
In this paper we study the finitely generated algebras underlying
algebras. These so called 'finite algebras' are constructed as Poisson
reductions of Kirillov Poisson structures on simple Lie algebras. The
inequivalent reductions are labeled by the inequivalent embeddings of
into the simple Lie algebra in question. For arbitrary embeddings a coordinate
free formula for the reduced Poisson structure is derived. We also prove that
any finite algebra can be embedded into the Kirillov Poisson algebra of a
(semi)simple Lie algebra (generalized Miura map). Furthermore it is shown that
generalized finite Toda systems are reductions of a system describing a free
particle moving on a group manifold and that they have finite symmetry. In
the second part we BRST quantize the finite algebras. The BRST cohomology
is calculated using a spectral sequence (which is different from the one used
by Feigin and Frenkel). This allows us to quantize all finite algebras in
one stroke. Explicit results for and are given. In the last part
of the paper we study the representation theory of finite algebras. It is
shown, using a quantum version of the generalized Miura transformation, that
the representations of finite algebras can be constructed from the
representations of a certain Lie subalgebra of the original simple Lie algebra.
As a byproduct of this we are able to construct the Fock realizations of
arbitrary finite algebras.Comment: 62 pages, THU-92/32, ITFA-28-9
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