648 research outputs found
Lattice Stellar Dynamics
We describe a technique for solving the combined collisionless Boltzmann and
Poisson equations in a discretised, or lattice, phase space. The time and the
positions and velocities of `particles' take on integer values, and the forces
are rounded to the nearest integer. The equations of motion are symplectic. In
the limit of high resolution, the lattice equations become the usual
integro-differential equations of stellar dynamics. The technique complements
other tools for solving those equations approximately, such as -body
simulation, or techniques based on phase-space grids. Equilibria are found in a
variety of shapes and sizes. They are true equilibria in the sense that they do
not evolve with time, even slowly, unlike existing -body approximations to
stellar systems, which are subject to two-body relaxation. They can also be
`tailor-made' in the sense that the mass distribution is constrained to be
close to some pre-specified function. Their principal limitation is the amount
of memory required to store the lattice, which in practice restricts the
technique to modeling systems with a high degree of symmetry. We also develop a
method for analysing the linear stability of collisionless systems, based on
lattice equilibria as an unperturbed model.Comment: Accepted for publication in Monthly Notices. 18 pages, compressed
PostScript, also available from http://www.cita.utoronto.ca/~syer/papers
Made-to-measure N-body systems
We describe an algorithm for constructing N-body realisations of equilibrium
stellar systems. The algorithm complements existing orbit-based modelling
techniques using linear programming or other optimization algorithms. The
equilibria are constructed by integrating an N-body system while slowly
adjusting the masses of the particles until the time-averaged density field and
other observables converge to a prescribed value. The procedure can be arranged
to maximise a linear combination of the entropy of the system and the
statistic for the observables. The equilibria so produced may be useful as
initial conditions for N-body simulations or for modelling observations of
individual galaxies.Comment: 8 pages, tex, figures included, accepted by MNRAS, also available at
http://www.mpa-garching.mpg.de/~syer/papers
Dark Matter from Early Decays
Two leading dark matter candidates from supersymmetry and other theories of
physics beyond the standard model are WIMPs and weak scale gravitinos. If the
lightest stable particle is a gravitino, then a WIMP will decay into it with a
natural lifetime of order a month ~ M_{pl}^2/M_{weak}^3. We show that if the
bulk of dark matter today came from decays of neutral particles with lifetimes
of order a year or smaller, then it could lead to a reduction in the amount of
small scale substructure, less concentrated halos and constant density cores in
the smallest mass halos. Such beneficial effects may therefore be realized
naturally, as discussed by Cembranos, Feng, Rajaraman, and Takayama, in the
case of supersymmetry.Comment: Matches version accepted for publication in PRD. Added a paragraph to
Sec V. 9 pages, 3 figure
Dynamical Origin of Extrasolar Planet Eccentricity Distribution
We explore the possibility that the observed eccentricity distribution of
extrasolar planets arose through planet-planet interactions, after the initial
stage of planet formation was complete. Our results are based on ~3250
numerical integrations of ensembles of randomly constructed planetary systems,
each lasting 100 Myr. We find that for a remarkably wide range of initial
conditions the eccentricity distributions of dynamically active planetary
systems relax towards a common final equilibrium distribution, well described
by the fitting formula dn ~ e exp[-1/2 (e/0.3)^2] de. This distribution agrees
well with the observed eccentricity distribution for e > 0.2, but predicts too
few planets at lower eccentricities, even when we exclude planets subject to
tidal circularization. These findings suggest that a period of large-scale
dynamical instability has occurred in a significant fraction of newly formed
planetary systems, lasting 1--2 orders of magnitude longer than the ~1 Myr
interval in which gas-giant planets are assembled. This mechanism predicts no
(or weak) correlations between semimajor axis, eccentricity, inclination, and
mass in dynamically relaxed planetary systems. An additional observational
consequence of dynamical relaxation is a significant population of planets
(>10%) that are highly inclined (>25deg) with respect to the initial symmetry
plane of the protoplanetary disk; this population may be detectable in
transiting planets through the Rossiter-McLaughlin effect.Comment: Accepted to ApJ, conclusions updated to reflect the current
observational constraint
Apsidal Alignment in Upsilon Andromedae
One of the parameters fitted by Doppler radial velocity measurements of
extrasolar planetary systems is omega, the argument of pericenter of a given
planet's orbit referenced to the plane of the sky. Curiously, the omega's of
the outer two planets orbiting Upsilon Andromedae are presently nearly
identical: Delta-omega = omega_D - omega_C = 4.8 deg +/- 4.8 deg (1 sigma).
This observation is least surprising if planets C and D occupy orbits that are
seen close to edge-on (sin i_C, sin i_D > 0.5) and whose mutual inclination
Theta does not exceed 20 deg. In this case, planets C and D inhabit a secular
resonance in which Delta-omega librates about 0 deg with an amplitude of 30 deg
and a period of 4000 yr. The resonant configuration spends about one-third of
its time with |Delta-omega| 40 deg, either Delta-omega
circulates or the system is unstable. This instability is driven by the Kozai
mechanism which couples the eccentricity of planet C to Theta to drive the
former quantity to values approaching unity. Our expectation that Theta < 20
deg suggests that planets C and D formed in a flattened, circumstellar disk,
and may be tested by upcoming astrometric measurements with the FAME satellite.Comment: Refereed version, accepted by AJ, to appear in September 2001 issu
The Rotation Period of the Planet-Hosting Star HD 189733
We present synoptic optical photometry of HD 189733, the chromospherically
active parent star of one of the most intensively studied exoplanets. We have
significantly extended the timespan of our previously reported observations and
refined the estimate of the stellar rotation period by more than an order of
magnitude: days. We derive a lower limit on the
inclination of the stellar rotation axis of 56\arcdeg (with 95% confidence),
corroborating earlier evidence that the stellar spin axis and planetary orbital
axis are well aligned.Comment: To appear in A
Unstable Disk Galaxies. I. Modal Properties
I utilize the Petrov-Galerkin formulation and develop a new method for
solving the unsteady collisionless Boltzmann equation in both the linear and
nonlinear regimes. In the first order approximation, the method reduces to a
linear eigenvalue problem which is solved using standard numerical methods. I
apply the method to the dynamics of a model stellar disk which is embedded in
the field of a soft-centered logarithmic potential. The outcome is the full
spectrum of eigenfrequencies and their conjugate normal modes for prescribed
azimuthal wavenumbers. The results show that the fundamental bar mode is
isolated in the frequency space while spiral modes belong to discrete families
that bifurcate from the continuous family of van Kampen modes. The population
of spiral modes in the bifurcating family increases by cooling the disk and
declines by increasing the fraction of dark to luminous matter. It is shown
that the variety of unstable modes is controlled by the shape of the dark
matter density profile.Comment: Accepted for publication in The Astrophysical Journa
The Dark Matter at the End of the Galaxy
Dark matter density profiles based upon Lambda-CDM cosmology motivate an
ansatz velocity distribution function with fewer high velocity particles than
the Maxwell-Boltzmann distribution or proposed variants. The high velocity tail
of the distribution is determined by the outer slope of the dark matter halo,
the large radius behavior of the Galactic dark matter density. N-body
simulations of Galactic halos reproduce the high velocity behavior of this
ansatz. Predictions for direct detection rates are dramatically affected for
models where the threshold scattering velocity is within 30% of the escape
velocity.Comment: 10 pages, 5 figure
Thermodynamics of MHD flows with axial symmetry
We present strategies based upon extremization principles, in the case of the
axisymmetric equations of magnetohydrodynamics (MHD). We study the equilibrium
shape by using a minimum energy principle under the constraints of the MHD
axisymmetric equations. We also propose a numerical algorithm based on a
maximum energy dissipation principle to compute in a consistent way the
equilibrium states. Then, we develop the statistical mechanics of such flows
and recover the same equilibrium states giving a justification of the minimum
energy principle. We find that fluctuations obey a Gaussian shape and we make
the link between the conservation of the Casimirs on the coarse-grained scale
and the process of energy dissipation
Evidence for Quasar Activity Triggered by Galaxy Mergers in HST Observations of Dust-reddened Quasars
We present Hubble ACS images of thirteen dust reddened Type-1 quasars
selected from the FIRST/2MASS Red Quasar Survey. These quasars have high
intrinsic luminosities after correction for dust obscuration (-23.5 > M_B >
-26.2 from K-magnitude). The images show strong evidence of recent or ongoing
interaction in eleven of the thirteen cases, even before the quasar nucleus is
subtracted. None of the host galaxies are well fit by a simple elliptical
profile. The fraction of quasars showing interaction is significantly higher
than the 30% seen in samples of host galaxies of normal, unobscured quasars.
There is a weak correlation between the amount of dust reddening and the
magnitude of interaction in the host galaxy, measured using the Gini
coefficient and the Concentration index. Although few host galaxy studies of
normal quasars are matched to ours in intrinsic quasar luminosity, no evidence
has been found for a strong dependence of merger activity on host luminosity in
samples of the host galaxies of normal quasars. We thus believe that the high
merger fraction in our sample is related to their obscured nature, with a
significant amount of reddening occurring in the host galaxy. The red quasar
phenomenon seems to have an evolutionary explanation, with the young quasar
spending the early part of its lifetime enshrouded in an interacting galaxy.
This might be further indication of a link between AGN and starburst galaxies.Comment: 18 pages, 6 low resolution figures, accepted for publication in Ap
- …