1,227 research outputs found
Geometric and homological finiteness in free abelian covers
We describe some of the connections between the Bieri-Neumann-Strebel-Renz
invariants, the Dwyer-Fried invariants, and the cohomology support loci of a
space X. Under suitable hypotheses, the geometric and homological finiteness
properties of regular, free abelian covers of X can be expressed in terms of
the resonance varieties, extracted from the cohomology ring of X. In general,
though, translated components in the characteristic varieties affect the
answer. We illustrate this theory in the setting of toric complexes, as well as
smooth, complex projective and quasi-projective varieties, with special
emphasis on configuration spaces of Riemann surfaces and complements of
hyperplane arrangements.Comment: 30 pages; to appear in Configuration Spaces: Geometry, Combinatorics
and Topology (Centro De Giorgi, 2010), Edizioni della Normale, Pisa, 201
AGN Feedback Compared: Jets versus Radiation
Feedback by Active Galactic Nuclei is often divided into quasar and radio
mode, powered by radiation or radio jets, respectively. Both are fundamental in
galaxy evolution, especially in late-type galaxies, as shown by cosmological
simulations and observations of jet-ISM interactions in these systems. We
compare AGN feedback by radiation and by collimated jets through a suite of
simulations, in which a central AGN interacts with a clumpy, fractal galactic
disc. We test AGN of and erg/s, considering jets
perpendicular or parallel to the disc. Mechanical jets drive the more powerful
outflows, exhibiting stronger mass and momentum coupling with the dense gas,
while radiation heats and rarifies the gas more. Radiation and perpendicular
jets evolve to be quite similar in outflow properties and effect on the cold
ISM, while inclined jets interact more efficiently with all the disc gas,
removing the densest in Myr, and thereby reducing the amount of
cold gas available for star formation. All simulations show small-scale inflows
of M/yr, which can easily reach down to the Bondi radius of
the central supermassive black hole (especially for radiation and perpendicular
jets), implying that AGN modulate their own duty cycle in a feedback/feeding
cycle.Comment: 21 pages, 15 figures, 2 table
Haydeeite: a spin-1/2 kagome ferromagnet
The mineral haydeeite, alpha-MgCu3(OD)6Cl2, is a S=1/2 kagome ferromagnet
that displays long-range magnetic order below TC=4.2 K with a strongly reduced
moment. Our inelastic neutron scattering data show clear spin-wave excitations
that are well described by a Heisenberg Hamiltonian with ferromagnetic
nearest-neighbor exchange J1=-38 K and antiferromagnetic exchange Jd=+11 K
across the hexagons of the kagome lattice. These values place haydeeite very
close to the quantum phase transition between ferromagnetic order and
non-coplanar twelve-sublattice cuboc2 antiferromagnetic order. Diffuse dynamic
short-range ferromagnetic correlations observed above TC persist well into the
ferromagnetically ordered phase with a behavior distinct from critical
scattering
On the Indeterministic Nature of Star Formation on the Cloud Scale
Molecular clouds are turbulent structures whose star formation efficiency
(SFE) is strongly affected by internal stellar feedback processes. In this
paper we determine how sensitive the SFE of molecular clouds is to randomised
inputs in the star formation feedback loop, and to what extent relationships
between emergent cloud properties and the SFE can be recovered. We introduce
the yule suite of 26 radiative magnetohydrodynamic (RMHD) simulations of a
10,000 solar mass cloud similar to those in the solar neighbourhood. We use the
same initial global properties in every simulation but vary the initial mass
function (IMF) sampling and initial cloud velocity structure. The final SFE
lies between 6 and 23 percent when either of these parameters are changed. We
use Bayesian mixed-effects models to uncover trends in the SFE. The number of
photons emitted early in the cluster's life and the length of the cloud provide
are the strongest predictors of the SFE. The HII regions evolve following an
analytic model of expansion into a roughly isothermal density field. The more
efficient feedback is at evaporating the cloud, the less the star cluster is
dispersed. We argue that this is because if the gas is evaporated slowly, the
stars are dragged outwards towards surviving gas clumps due to the
gravitational attraction between the stars and gas. While star formation and
feedback efficiencies are dependent on nonlinear processes, statistical models
describing cloud-scale processes can be constructed.Comment: 24 pages, 16 figures, 6 tables. Accepted to MNRAS, version updated
with published titl
Carotenoids and Antioxidant Nutrients following Burn Injury a
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/72213/1/j.1749-6632.1993.tb26193.x.pd
Aphid genotypes vary in their response to the presence of fungal endosymbionts in host plants
Genetic variation for fitness-relevant traits may be maintained in natural populations by fitness differences that depend on environmental conditions.
For herbivores, plant quality and variation in chemical plant defences can maintain genetic variation in performance. Apart from plant secondary
compounds, symbiosis between plants and endosymbiotic fungi (endophytes) can produce herbivore-toxic compounds. We show that there is significant variation among aphid genotypes in response to endophytes by comparing life-history traits of 37 clones of the bird cherry-oat aphid Rhopalosiphum padi feeding on endophyte-free and endophyte-infected tall fescue Lolium arundinaceum.
Clonal variation for life-history traits was large, and most clones performed better on endophyte-free plants. However, the clones differed in
the relative performance across the two environments, resulting in significant
genotype · environment interactions for all reproductive traits. These findings suggest that natural variation in prevalence of endophyte infection can contribute to the maintenance of genetic diversity in aphid populations
Optimized spectrally selective steady-state free precession sequences for cartilage imaging at ultra-high fields
Object: Fat suppressed 3D steady-state free precession (SSFP) sequences are of special interest in cartilage imaging due to their short repetition time in combination with high signal-to-noise ratio. At low-to-high fields (1.5-2.0T), spectral spatial (spsp) radio frequency (RF) pulses perform superiorly over conventional saturation of the fat signal (FATSAT pulses). However, ultra-high fields (7.0T and more) may offer alternative fat suppression techniques as a result of the increased chemical shift. Materials and methods: Application of a single, frequency selective, RF pulse is compared to spsp excitation for water (or fat) selective imaging at 7.0T. Results: For SSFP, application of a single frequency selective RF pulse for selective water or fat excitation performs beneficially over the commonly applied spsp RF pulses. In addition to the overall improved fat suppression, the application of single RF pulses leads to decreased power depositions, still representing one of the major restrictions in the design and application of many pulse sequences at ultra-high fields. Conclusion: The ease of applicability and implementation of single frequency selective RF pulses at ultra-high-fields might be of great benefit for a vast number of applications where fat suppression is desirable or fat-water separation is needed for quantification purpose
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