2,081 research outputs found
An explicit KO-degree map and applications
The goal of this note is to study the analog in unstable -homotopy theory of the unit map from the motivic sphere spectrum to the
Hermitian K-theory spectrum, i.e., the degree map in Hermitian K-theory. We
show that "Suslin matrices", which are explicit maps from odd dimensional split
smooth affine quadrics to geometric models of the spaces appearing in Bott
periodicity in Hermitian K-theory, stabilize in a suitable sense to the unit
map. As applications, we deduce that for ,
which can be thought of as an extension of Matsumoto's celebrated theorem
describing of a field. These results provide the first step in a program
aimed at computing the sheaf for .Comment: 36 Pages, Final version, to appear Journal of Topolog
Dust sedimentation and self-sustained Kelvin-Helmholtz turbulence in protoplanetary disk mid-planes. I. Radially symmetric simulations
We perform numerical simulations of the Kelvin-Helmholtz instability in the
mid-plane of a protoplanetary disk. A two-dimensional corotating slice in the
azimuthal--vertical plane of the disk is considered where we include the
Coriolis force and the radial advection of the Keplerian rotation flow. Dust
grains, treated as individual particles, move under the influence of friction
with the gas, while the gas is treated as a compressible fluid. The friction
force from the dust grains on the gas leads to a vertical shear in the gas
rotation velocity. As the particles settle around the mid-plane due to gravity,
the shear increases, and eventually the flow becomes unstable to the
Kelvin-Helmholtz instability. The Kelvin-Helmholtz turbulence saturates when
the vertical settling of the dust is balanced by the turbulent diffusion away
from the mid-plane. The azimuthally averaged state of the self-sustained
Kelvin-Helmholtz turbulence is found to have a constant Richardson number in
the region around the mid-plane where the dust-to-gas ratio is significant.
Nevertheless the dust density has a strong non-axisymmetric component. We
identify a powerful clumping mechanism, caused by the dependence of the
rotation velocity of the dust grains on the dust-to-gas ratio, as the source of
the non-axisymmetry. Our simulations confirm recent findings that the critical
Richardson number for Kelvin-Helmholtz instability is around unity or larger,
rather than the classical value of 1/4Comment: Accepted for publication in ApJ. Some minor changes due to referee
report, most notably that the clumping mechanism has been identified as the
streaming instability of Youdin & Goodman (2005). Movies of the simulations
are still available at http://www.mpia.de/homes/johansen/research_en.ph
Effects of Geochemical and Environmental Parameters on Abiotic Organic Chemistry Driven by Iron Hydroxide Minerals
Geological conditions play a significant role in prebiotic/abiotic organic chemistry, especially when reactive minerals are present. Previous studies of the prebiotic synthesis of amino acids and other products in mineral‐containing systems have shown that a diverse array of compounds can be produced, depending on the experimental conditions. However, these previous experiments have not simulated the effects of varying geochemical conditions, in which factors such as pH, iron redox state, or chemical concentrations may vary over time and space in a natural environment. In geochemical systems that contain overlapping gradients, many permutations of individual conditions could exist and affect the outcome of an organic reaction network. We investigated reactions of pyruvate and glyoxylate, two compounds that are central to the emergence of metabolism, in simulated geological gradients of redox, pH, and ammonia concentration. Our results show that the positioning of pyruvate/glyoxylate reactions in this environmental parameter space determines the organic product distribution that results. Therefore, the distribution pattern of amino acids and alpha‐hydroxy acids produced prebiotically in a system reflects the specific reaction conditions, and would be distinct at various locations in an environment depending on local geochemistry. This is significant for origin of life chemistry in which the composition and function of oligomers could be affected by the environmentally driven distribution of monomers available. Also, for astrobiology and planetary science where organic distribution patterns are sometimes considered as a possible biosignature, it is important to consider environmentally driven abiotic organic reactions that might produce similar effects
Kick stability in groups and dynamical systems
We consider a general construction of ``kicked systems''. Let G be a group of
measure preserving transformations of a probability space. Given its
one-parameter/cyclic subgroup (the flow), and any sequence of elements (the
kicks) we define the kicked dynamics on the space by alternately flowing with
given period, then applying a kick. Our main finding is the following stability
phenomenon: the kicked system often inherits recurrence properties of the
original flow. We present three main examples. 1) G is the torus. We show that
for generic linear flows, and any sequence of kicks, the trajectories of the
kicked system are uniformly distributed for almost all periods. 2) G is a
discrete subgroup of PSL(2,R) acting on the unit tangent bundle of a Riemann
surface. The flow is generated by a single element of G, and we take any
bounded sequence of elements of G as our kicks. We prove that the kicked system
is mixing for all sufficiently large periods if and only if the generator is of
infinite order and is not conjugate to its inverse in G. 3) G is the group of
Hamiltonian diffeomorphisms of a closed symplectic manifold. We assume that the
flow is rapidly growing in the sense of Hofer's norm, and the kicks are
bounded. We prove that for a positive proportion of the periods the kicked
system inherits a kind of energy conservation law and is thus superrecurrent.
We use tools of geometric group theory and symplectic topology.Comment: Latex, 40 pages, revised versio
Multiple and Fast: The Accretion of Ordinary Chondrite Parent Bodies
Although petrologic, chemical and isotopic studies of ordinary chondrites and
meteorites in general have largely helped establish a chronology of the
earliest events of planetesimal formation and their evolution, there are
several questions that cannot be resolved via laboratory measurements and/or
experiments only. Here we propose rationale for several new constraints on the
formation and evolution of ordinary chondrite parent bodies (and by extension
most planetesimals) from newly available spectral measurements and
mineralogical analysis of main belt S-type asteroids (83 objects) and
unequilibrated ordinary chondrite meteorites (53 samples). Based on the latter,
we suggest spectral data may be used to distinguish whether an ordinary
chondrite was formed near the surface or in the interior of its parent body. If
these constraints are correct, the suggested implications include that: i)
large groups of compositionally similar asteroids are a natural outcome of
planetesimal formation and, consequently, meteorites within a given class can
originate from multiple parent bodies; ii) the surfaces of large (up to ~200km)
S-type main-belt asteroids expose mostly the interiors of the primordial
bodies, a likely consequence of impacts by small asteroids (D<10km) in the
early solar system (Ciesla et al. 2013); iii) the duration of accretion of the
H chondrite parent bodies was likely short (instantaneous or in less then ~10^5
yr but certainly not as long as 1 Myr); iv) LL-like bodies formed closer to the
Sun than H-like bodies, a possible consequence of radial mixing and size
sorting of chondrules in the protoplanetary disk prior to accretion.Comment: Accepted for publication in Ap
Frontiers of the physics of dense plasmas and planetary interiors: experiments, theory, applications
Recent developments of dynamic x-ray characterization experiments of dense
matter are reviewed, with particular emphasis on conditions relevant to
interiors of terrestrial and gas giant planets. These studies include
characterization of compressed states of matter in light elements by x-ray
scattering and imaging of shocked iron by radiography. Several applications of
this work are examined. These include the structure of massive "Super Earth"
terrestrial planets around other stars, the 40 known extrasolar gas giants with
measured masses and radii, and Jupiter itself, which serves as the benchmark
for giant planets.Comment: Accepted to Physics of Plasmas special issue. Review from
HEDP/HEDLA-08, April 12-15, 200
The secondary eclipse of CoRoT-1b
The transiting planet CoRoT-1b is thought to belong to the pM-class of
planets, in which the thermal emission dominates in the optical wavelengths. We
present a detection of its secondary eclipse in the CoRoT white channel data,
whose response function goes from ~400 to ~1000 nm. We used two different
filtering approaches, and several methods to evaluate the significance of a
detection of the secondary eclipse. We detect a secondary eclipse centered
within 20 min at the expected times for a circular orbit, with a depth of
0.016+/-0.006%. The center of the eclipse is translated in a 1-sigma upper
limit to the planet's eccentricity of ecosomega<0.014. Under the assumption of
a zero Bond Albedo and blackbody emission from the planet, it corresponds to a
T_{CoRoT}=2330 +120-140 K. We provide the equilibrium temperatures of the
planet as a function of the amount of reflected light. If the planet is in
thermal equilibrium with the incident flux from the star, our results imply an
inefficient transport mechanism of the flux from the day to the night sides.Comment: 6 pages, to appear in A&A, submitted 18 march 2009, accepted 7 July
200
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