1,485 research outputs found
Accretion of a satellite onto a spherical galaxy. II. Binary evolution and orbital decay
We study the dynamical evolution of a satellite orbiting outside of a
companion spherical galaxy. The satellite is subject to a back-reaction force
resulting from the density fluctuations excited in the primary stellar system.
We evaluate this force using the linear response theory developed in Colpi and
Pallavicini (1997). The force is computed in the reference frame comoving with
the primary galaxy and is expanded in multipoles. To capture the relevant
features of the physical process determining the evolution of the detached
binary, we introduce in the Hamiltonian the harmonic potential as interaction
potential among stars. The dynamics of the satellite is computed
self-consistently. We determine the conditions for tidal capture of a satellite
from an asymptotic free state. If the binary comes to existence as a bound
pair, stability against orbital decay is lost near resonance. The time scale of
binary coalescence is computed as a function of the eccentricity and mass
ratio. In a comparison with Weinberg's perturbative technique we demonstrate
that pinning the center of mass of the galaxy would induce a much larger torque
on the satellite.Comment: 13 pages, Tex,+ 10 .ps figures Submitted to The Astrophysical Journa
The Value of Liquidity
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/72848/1/1080-8620.00026.pd
Dynamical friction and the evolution of satellites in virialized halos: the theory of linear response
The evolution of a small satellite inside a more massive truncated isothermal
spherical halo is studied using both the Theory of Linear Response for
dynamical friction and N-Body simulations. The analytical approach includes the
effects of the gravitational wake, of the tidal deformation and the shift of
the barycenter of the primary, so unifying the local versus global
interpretation of dynamical friction. Sizes, masses, orbital energies and
eccentricities are chosen as expected in hierarchical clustering models. We
find that in general the drag force in self-gravitating backgrounds is weaker
than in uniform media and that the orbital decay is not accompanied by a
significant circularization. We also show that the dynamical friction time
scale is weakly dependent on the initial circularity. We provide a fitting
formula for the decay time that includes the effect of mass and angular
momentum loss. Live satellites with dense cores can survive disruption up to an
Hubble time within the primary, notwithstanding the initial choice of orbital
parameters. Dwarf spheroidal satellites of the Milky Way, like Sagittarius A
and Fornax, have already suffered mass stripping and, with their present
masses, the sinking times exceed 10 Gyr even if they are on very eccentric
orbits.Comment: 27 pages including 9 figures. Accepted for publication in the
Astrophysical Journal. Part 2, issue November 10 1999, Volume 52
GaN/AlN Quantum Dots for Single Qubit Emitters
We study theoretically the electronic properties of -plane GaN/AlN quantum
dots (QDs) with focus on their potential as sources of single polarized photons
for future quantum communication systems. Within the framework of eight-band
k.p theory we calculate the optical interband transitions of the QDs and their
polarization properties. We show that an anisotropy of the QD confinement
potential in the basal plane (e.g. QD elongation or strain anisotropy) leads to
a pronounced linear polarization of the ground state and excited state
transitions. An externally applied uniaxial stress can be used to either induce
a linear polarization of the ground-state transition for emission of single
polarized photons or even to compensate the polarization induced by the
structural elongation.Comment: 6 pages, 9 figures. Accepted at Journal of Physics: Condensed Matte
Toward a validation process for model based safety analysis
International audienceToday, Model Based Safety Analysis processes become more and more widespread to achieve the safety analysis of a system. However and at our knowledge, there is no formal testing approach to ensure that the formal model is compliant with the real system. In the paper, we choose to study AltaRica model. We present a general process to well construct and validate an AltaRica formal model. The focus is made on this validation phase, i.e. verifying the compliance between the model and the real system. For it, the proposed process recommends to build a specification for the AltaRica model. Then, the validation process is transformed to a classical verification problem between an implementation and a specification. We present the first phase of a method to verify the compliance between the model and the specification
Darwin Tames an Andromeda Dwarf: Unraveling the Orbit of NGC 205 Using a Genetic Algorithm
NGC 205, a close satellite of the M31 galaxy, is our nearest example of a
dwarf elliptical galaxy. Photometric and kinematic observations suggest that
NGC 205 is undergoing tidal distortion from its interaction with M31. Despite
earlier attempts, the orbit and progenitor properties of NGC 205 are not well
known. We perform an optimized search for these unknowns by combining a genetic
algorithm with restricted N-body simulations of the interaction. This approach,
coupled with photometric and kinematic observations as constraints, allows for
an effective exploration of the parameter space. We represent NGC 205 as a
static Hernquist potential with embedded massless test particles that serve as
tracers of surface brightness. We explore 3 distinct, initially stable
configurations of test particles: cold rotating disk, warm rotating disk, and
hot, pressure-supported spheroid. Each model reproduces some, but not all, of
the observed features of NGC 205, leading us to speculate that a rotating
progenitor with substantial pressure support could match all of the
observables. Furthermore, plausible combinations of mass and scale length for
the pressure-supported spheroid progenitor model reproduce the observed
velocity dispersion profile. For all 3 models, orbits that best match the
observables place the satellite 11+/-9 kpc behind M31 moving at very large
velocities: 300-500 km/s on primarily radial orbits. Given that the observed
radial component is only 54 km/s, this implies a large tangential motion for
NGC 205, moving from the NW to the SE. These results suggest NGC 205 is not
associated with the stellar arc observed to the NE of NGC 205. Furthermore, NGC
205's velocity appears to be near or greater than its escape velocity,
signifying that the satellite is likely on its first M31 passage.Comment: 34 pages, 20 figures, accepted for publication in the Astrophysical
Journal, A pdf version with high-resolution figures may be obtained from
http://www.ucolick.org/~kirsten/ms.pd
Heliophysics Event Knowledgebase for the Solar Dynamics Observatory and Beyond
The immense volume of data generated by the suite of instruments on SDO
requires new tools for efficient identifying and accessing data that is most
relevant to research investigations. We have developed the Heliophysics Events
Knowledgebase (HEK) to fill this need. The HEK system combines automated data
mining using feature-detection methods and high-performance visualization
systems for data markup. In addition, web services and clients are provided for
searching the resulting metadata, reviewing results, and efficiently accessing
the data. We review these components and present examples of their use with SDO
data.Comment: 17 pages, 4 figure
Debt, Agency, and Management Contracts in REITs: The External Advisor Puzzle
This study investigates why externally advised real estate investment trusts (REITs) underperform their internally managed counterparts. Consistent with previous studies, we find that REITs managed by external advisors underperform internally managed ones by over 7 percent per year. Property-level cash-flow yields are similar between the two managerial forms, but corporate-level expenses and especially interest expenses are responsible for lower levels of cash available to shareholders in externally advised REITs. We document that the higher-interest expenses are due to both higher levels of debt and to higher debt yields for externally advised REITs. We posit that compensating managers based on either assets under management or on property-level cash flows creates incentives for managers to increase the asset base by issuing debt even if the interest costs are unfavorable.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47758/1/11146_2004_Article_254942.pd
Conformational Entropy as a Means to Control the Behavior of Poly(diketoenamine) Vitrimers In and Out of Equilibrium.
Control of equilibrium and non-equilibrium thermomechanical behavior of poly(diketoenamine) vitrimers is shown by incorporating linear polymer segments varying in molecular weight (MW) and conformational degrees of freedom into the dynamic covalent network. While increasing MW of linear segments yields a lower storage modulus at the rubbery plateau after softening above the glass transition (Tg ), both Tg and the characteristic time of stress relaxation are independently governed by the conformational entropy of the embodied linear segments. Activation energies for bond exchange in the solid state are lower for networks incorporating flexible chains; the network topology freezing temperature decreases with increasing MW of flexible linear segments but increases with increasing MW of stiff segments. Vitrimer reconfigurability is therefore influenced not only by the energetics of bond exchange for a given network density, but also the entropy of polymer chains within the network
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