2,228 research outputs found
The (In)Stability of Planetary Systems
We present results of numerical simulations which examine the dynamical
stability of known planetary systems, a star with two or more planets. First we
vary the initial conditions of each system based on observational data. We then
determine regions of phase space which produce stable planetary configurations.
For each system we perform 1000 ~1 million year integrations. We examine
upsilon And, HD83443, GJ876, HD82943, 47UMa, HD168443, and the solar system
(SS). We find that the resonant systems, 2 planets in a first order mean motion
resonance, (HD82943 and GJ876) have very narrow zones of stability. The
interacting systems, not in first order resonance, but able to perturb each
other (upsilon And, 47UMa, and SS) have broad regions of stability. The
separated systems, 2 planets beyond 10:1 resonance, (we only examine HD83443
and HD168443) are fully stable. Furthermore we find that the best fits to the
interacting and resonant systems place them very close to unstable regions. The
boundary in phase space between stability and instability depends strongly on
the eccentricities, and (if applicable) the proximity of the system to perfect
resonance. In addition to million year integrations, we also examined stability
on ~100 million year timescales. For each system we ran ~10 long term
simulations, and find that the Keplerian fits to these systems all contain
configurations which may be regular on this timescale.Comment: 37 pages, 49 figures, 13 tables, submitted to Ap
Dark Matter Substructure in Galactic Halos
We use numerical simulations to examine the substructure within galactic and
cluster mass halos that form within a hierarchical universe. Clusters are
easily reproduced with a steep mass spectrum of thousands of substructure
clumps that closely matches observations. However, the survival of dark matter
substructure also occurs on galactic scales, leading to the remarkable result
that galaxy halos appear as scaled versions of galaxy clusters. The model
predicts that the virialised extent of the Milky Way's halo should contain
about 500 satellites with circular velocities larger than Draco and Ursa-Minor
i.e. bound masses > 10^8Mo and tidally limited sizes > kpc. The substructure
clumps are on orbits that take a large fraction of them through the stellar
disk leading to significant resonant and impulsive heating. Their abundance and
singular density profiles has important implications for the existence of old
thin disks, cold stellar streams, gravitational lensing and indirect/direct
detection experiments.Comment: Astrophysical Journal Letters. 4 pages, latex. Simulation images and
movies at http://star-www.dur.ac.uk:80/~moore
Tidal stirring and the origin of dwarf spheroidals in the Local Group
N-Body/SPH simulations are used to study the evolution of dwarf irregular
galaxies (dIrrs) entering the dark matter halo of the Milky Way or M31 on
plunging orbits. We propose a new dynamical mechanism driving the evolution of
gas rich, rotationally supported dIrrs, mostly found at the outskirts of the
Local Group (LG), into gas free, pressure supported dwarf spheroidals (dSphs)
or dwarf ellipticals (dEs), observed to cluster around the two giant spirals.
The initial model galaxies are exponential disks embedded in massive dark
matter halos and reproduce nearby dIrrs. Repeated tidal shocks at the
pericenter of their orbit partially strip their halo and disk and trigger
dynamical instabilities that dramatically reshape their stellar component.
After only 2-3 orbits low surface brightness (LSB) dIrrs are transformed into
dSphs, while high surface brightness (HSB) dIrrs evolve into dEs. This
evolutionary mechanism naturally leads to the morphology-density relation
observed for LG dwarfs. Dwarfs surrounded by very dense dark matter halos, like
the archetypical dIrr GR8, are turned into Draco or Ursa Minor, the faintest
and most dark matter dominated among LG dSphs. If disks include a gaseous
component, this is both tidally stripped and consumed in periodic bursts of
star formation. The resulting star formation histories are in good qualitative
agreement with those derived using HST color-magnitude diagrams for local
dSphs.Comment: 5 pages, 5 figures, to appear on ApJL. Simulation images and movies
can be found at the Local Group web page at
http://pcblu.uni.mi.astro.it/~lucio/LG/LG.htm
Molecular recording of mammalian embryogenesis.
Ontogeny describes the emergence of complex multicellular organisms from single totipotent cells. This field is particularly challenging in mammals, owing to the indeterminate relationship between self-renewal and differentiation, variation in progenitor field sizes, and internal gestation in these animals. Here we present a flexible, high-information, multi-channel molecular recorder with a single-cell readout and apply it as an evolving lineage tracer to assemble mouse cell-fate maps from fertilization through gastrulation. By combining lineage information with single-cell RNA sequencing profiles, we recapitulate canonical developmental relationships between different tissue types and reveal the nearly complete transcriptional convergence of endodermal cells of extra-embryonic and embryonic origins. Finally, we apply our cell-fate maps to estimate the number of embryonic progenitor cells and their degree of asymmetric partitioning during specification. Our approach enables massively parallel, high-resolution recording of lineage and other information in mammalian systems, which will facilitate the construction of a quantitative framework for understanding developmental processes
Vertical pairing of identical particles suspended in the plasma sheath
It is shown experimentally that vertical pairing of two identical
microspheres suspended in the sheath of a radio-frequency (rf) discharge at low
gas pressures (a few Pa), appears at a well defined instability threshold of
the rf power. The transition is reversible, but with significant hysteresis on
the second stage. A simple model, which uses measured microsphere resonance
frequencies and takes into account besides Coulomb interaction between
negatively charged microspheres also their interaction with positive ion wake
charges, seems to explain the instability threshold quite well.Comment: 4 pages, 6 figures. to appear in Phys. Rev. Lett. 86, May 14th (2001
PKC-dependent Phosphorylation of the H1 Histamine Receptor Modulates TRPC6 Activity
Transient receptor potential canonical 6 (TRPC6) is a cation selective, DAG-regulated, Ca2+-permeable channel activated by the agonists of Gq-protein-coupled heptahelical receptors. Dysfunctions of TRPC6 are implicated in the pathogenesis of various cardiovascular and kidney conditions such as vasospasm and glomerulosclerosis. When stimulated by agonists of the histamine H1 receptor (H1R), TRPC6 activity decays to the baseline despite the continuous presence of the agonist. In this study, we examined whether H1R desensitization contributes to regulating the decay rate of TRPC6 activity upon receptor stimulation. We employed the HEK expression system and a biosensor allowing us to simultaneously detect the changes in intracellular diacylglycerol (DAG) and Ca2+ concentrations. We found that the histamine-induced DAG response was biphasic, in which a transient peak was followed by maintained elevated plateau, suggesting that desensitization of H1R takes place in the presence of histamine. The application of PKC inhibitor Gö6983 slowed the decay rate of intracellular DAG concentration. Activation of the mouse H1R mutant lacking a putative PKC phosphorylation site, Ser399, responsible for the receptor desensitization, resulted in a prolonged intracellular DAG increase and greater Mn2+ influx through the TRPC6 channel. Thus, our data support the hypothesis that PKC-dependent H1R phosphorylation leads to a reduced production of intracellular DAG that contributes to TRPC6 activity regulation
Origin and Dynamics of the Mutually Inclined Orbits of Upsilon Andromedae c and d
We evaluate the orbital evolution and several plausible origins scenarios for
the mutually inclined orbits of Upsilon Andromedae c and d. These two planets
have orbital elements that oscillate with large amplitudes and lie close to the
stability boundary. This configuration, and in particular the observed mutual
inclination, demands an explanation. The planetary system may be influenced by
a nearby low-mass star, Upsilon And B, which could perturb the planetary
orbits, but we find it cannot modify two coplanar orbits into the observed
mutual inclination of ~30 deg. However, it could incite ejections or collisions
between planetary companions that subsequently raise the mutual inclination to
>30 deg. Our simulated systems with large mutual inclinations tend to be
further from the stability boundary than Upsilon And, but we are able to
produce similar systems. We conclude that scattering is a plausible mechanism
to explain the observed orbits of Upsilon And c and d, but we cannot determine
whether the scattering was caused by instabilities among the planets themselves
or by perturbations from Upsilon And B. We also develop a procedure to
quantitatively compare numerous properties of the observed system to our
numerical models. Although we only implement this procedure to Upsilon And, it
may be applied to any exoplanetary system.Comment: 19 pages, 5 figures, accepted to Astrophysical Journa
The Structural Evolution of Substructure
We investigate the evolution of substructure in cold dark matter halos using
N-body simulations of tidal stripping of substructure halos (subhalos) within a
static host potential. We find that halos modeled following the Navarro, Frenk
& White (NFW) mass profile lose mass continuously due to tides from the massive
host, leading to the total disruption of satellite halos with small tidal
radii. The structure of stripped NFW halos depends mainly on the fraction of
mass lost, and can be expressed in terms of a simple correction to the original
NFW profile. We apply these results to substructure in the Milky Way, and
conclude that the dark matter halos surrounding its dwarf spheroidal (dSph)
satellites have circular velocity curves that peak well beyond the luminous
radius at velocities significantly higher than expected from the stellar
velocity dispersion. Our modeling suggests that the true tidal radii of dSphs
lie well beyond the putative tidal cutoff observed in the surface brightness
profile, suggesting that the latter are not really tidal in origin but rather
features in the light profile of limited dynamical relevance. For Draco, in
particular, our modeling implies that its tidal radius is much larger than
derived by Irwin & Hatzidimitriou (1995), lending support to the interpretation
of recent Sloan survey data by Odenkirchen et al. (2001). Similarly, our model
suggests that Carina's halo has a peak circular velocity of ~55 km/s, which may
help explain how this small galaxy has managed to retain enough gas to undergo
several bursts of star formation. Our results imply a close correspondence
between the most massive subhalos expected in a CDM universe and the known
satellites of the Milky Way, and suggest that only subhalos with peak circular
velocities below 35 km/s lack readily detectable luminous counterparts.Comment: 28 pages, 14 figures; accepted for publication in the Astrophysical
Journa
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