19,691 research outputs found
The Gaia inertial reference frame and the tilting of the Milky Way disk
While the precise relationship between the Milky Way disk and the symmetry
planes of the dark matter halo remains somewhat uncertain, a time-varying disk
orientation with respect to an inertial reference frame seems probable.
Hierarchical structure formation models predict that the dark matter halo is
triaxial and tumbles with a characteristic rate of ~2 rad/Hubble time (~30
muas/yr). These models also predict a time-dependent accretion of gas, such
that the angular momentum vector of the disk should be misaligned with that of
the halo. These effects, as well as tidal effects of the LMC, will result in
the rotation of the angular momentum vector of the disk population with respect
to the quasar reference frame. We assess the accuracy with which the positions
and proper motions from Gaia can be referred to a kinematically non-rotating
system, and show that the spin vector of the transformation from any rigid
self-consistent catalog frame to the quasi-inertial system defined by quasars
should be defined to better than 1 muas/yr. Determination of this inertial
frame by Gaia will reveal any signature of the disk orientation varying with
time, improve models of the potential and dynamics of the Milky Way, test
theories of gravity, and provide new insights into the orbital evolution of the
Sagittarius dwarf galaxy and the Magellanic Clouds.Comment: 16 pages; accepted for publication in Ap
Possible origins of macroscopic left-right asymmetry in organisms
I consider the microscopic mechanisms by which a particular left-right (L/R)
asymmetry is generated at the organism level from the microscopic handedness of
cytoskeletal molecules. In light of a fundamental symmetry principle, the
typical pattern-formation mechanisms of diffusion plus regulation cannot
implement the "right-hand rule"; at the microscopic level, the cell's
cytoskeleton of chiral filaments seems always to be involved, usually in
collective states driven by polymerization forces or molecular motors. It seems
particularly easy for handedness to emerge in a shear or rotation in the
background of an effectively two-dimensional system, such as the cell membrane
or a layer of cells, as this requires no pre-existing axis apart from the layer
normal. I detail a scenario involving actin/myosin layers in snails and in C.
elegans, and also one about the microtubule layer in plant cells. I also survey
the other examples that I am aware of, such as the emergence of handedness such
as the emergence of handedness in neurons, in eukaryote cell motility, and in
non-flagellated bacteria.Comment: 42 pages, 6 figures, resubmitted to J. Stat. Phys. special issue.
Major rewrite, rearranged sections/subsections, new Fig 3 + 6, new physics in
Sec 2.4 and 3.4.1, added Sec 5 and subsections of Sec
Generalized Centrifugal Force Model for Pedestrian Dynamics
A spatially continuous force-based model for simulating pedestrian dynamics
is introduced which includes an elliptical volume exclusion of pedestrians. We
discuss the phenomena of oscillations and overlapping which occur for certain
choices of the forces. The main intention of this work is the quantitative
description of pedestrian movement in several geometries. Measurements of the
fundamental diagram in narrow and wide corridors are performed. The results of
the proposed model show good agreement with empirical data obtained in
controlled experiments.Comment: 10 pages, 14 figures, accepted for publication as a Regular Article
in Physical Review E. This version contains minor change
Direct imaging of extra-solar planets in star forming regions: Lessons learned from a false positive around IM Lup
Most exoplanet imagers consist of ground-based adaptive optics coronagraphic
cameras which are currently limited in contrast, sensitivity and astrometric
precision, but advantageously observe in the near-IR (1- 5{\mu}m). Because of
these practical limitations, our current observational aim at detecting and
characterizing planets puts heavy constraints on target selection, observing
strategies, data reduction, and follow-up. Most surveys so far have thus
targeted young systems (1-100Myr) to catch the putative remnant thermal
radiation of giant planets, which peaks in the near-IR. They also favor systems
in the solar neighborhood (d<80pc), which eases angular resolution requirements
but also ensures a good knowledge of the distance and proper motion, which are
critical to secure the planet status, and enable subsequent characterization.
Because of their youth, it is very tempting to target the nearby star forming
regions, which are typically twice as far as the bulk of objects usually combed
for planets by direct imaging. Probing these interesting reservoirs sets
additional constraints that we review in this paper by presenting the planet
search that we initiated in 2008 around the disk-bearing T Tauri star IM Lup
(Lupus star forming region, 140-190pc). We show and discuss why age
determination, the choice of evolutionary model for the central star and the
planet, precise knowledge of the host star proper motion, relative or absolute
astrometric accuracy, and patience are the key ingredients for exoplanet
searches around more distant young stars. Unfortunately, most of the time,
precision and perseverance are not paying off: we discovered a candidate
companion around IM Lup in 2008, which we report here to be an unbound
background object. We nevertheless review in details the lessons learned from
our endeavor, and additionally present the best detection limits ever
calculated for IM Lup.Comment: 8 pages, 3 figures, 3 tables, accepted to A&
A Numerical Treatment of Melt/Solid Segregation: Size of the Eucrite Parent Body and Stability of the Terrestrial Low-Velocity Zone
Crystal sinking to form cumulates and melt percolation toward segregation in magma pools can be treated with modifications of Stokes' and Darcy's laws, respectively. The velocity of crystals and melt depends, among other things, on the force of gravity (g) driving the separations and the cooling time of the environment. The increase of g promotes more efficient differentiation, whereas the increase of cooling rate limits the extent to which crystals and liquid can separate. The rate at which separation occurs is strongly dependent on the proportion of liquid that is present. As a result, cumulate formation is a process with a negative feedback; the more densely aggregated the crystals become, the slower the process can proceed. In contrast, melt accumulation is a process with a positive feedback; partial accumulation of melt leads to more rapid accumulation of subsequent melt. This positive feedback can cause melt accumulation to run rapidly to completion once a critical stability limit is passed. The observation of cumulates and segregated melts among the eucrite meteorites is used as a basis for calculating the g (and planet size) required to perform these differentiations. The eucrite parent body was probably at least 10-100 km in radius. The earth's low velocity zone (LVZ) is shown to be unstable with respect to draining itself of excess melt if the melt forms an interconnecting network. A geologically persistent LVZ with a homogeneous distribution of melt can be maintained with melt fractions only on the order of 0.1% or less
Lensing Magnification: Implications for Counts of Submillimeter Galaxies and SZ Clusters
We study lensing magnification of source galaxies by intervening galaxy
groups and clusters using a halo model. Halos are modeled with truncated NFW
profiles with ellipticity added to their lensing potential and propagated to
observable lensing statistics. We present the formalism to calculate observable
effects due to a distribution of halos of different masses at different
redshifts along the l ine of sight. We calculate the effects of magnification
on the number counts of high-redshift galaxies. Using BLAST survey data for
submillimeter galaxies (SMGs), we find that magnification affects the steep,
high flux par t of the counts by about 60%. The effect becomes much stronger if
the intrinsic distribution is signi ficantly steeper than observed. We also
consider the effect of this high-redshift galaxy population on contaminating
the Sunyaev-Zel'dovich (SZ) signal of massive clusters using the halo model
approach. We find that for the majority of clusters expected to be detected
with ongoing SZ surveys, there is significant contamination from the Poisson
noise due to background SMGs. This contr ibution can be comparable to the SZ
increment for typical clusters and can also contaminate the SZ decrement of low
mass clusters. Thus SZ observations, especially for the increment part of the
SZ spectrum, need to include careful modeling of this irreducible contamination
for mass estimation. Lensing further enhances the contamination, especially
close to the cores of massive clusters and for very disturbed clusters with
large magnification cross-section.Comment: 21 pages, 14 figures, Submitted to MNRA
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