457 research outputs found
Optimal Monetary Policy in a Channel System of Interest-Rate Control
This paper studies optimal interest-rate policies when the central bank operates a channel system of interest-rate control. We conduct our analysis in a dynamic general equilibrium model with infinitely-lived agents who are subject to idiosyncratic trading shocks which generate random liquidity needs. In response to these shocks agents either borrow against collateral or deposit money at the central bank at the specified rates. We show that it is optimal to have a strictly positive interest-rate corridor if the opportunity cost of holding collateral is strictly positive and that the optimal corridor is strictly decreasing in the collateral's real returnOptimal Monetary Policy, Channel System, Interest Rate Rule, Essential Money
Bars in Cuspy Dark Halos
We examine the bar instability in models with an exponential disk and a cuspy
NFW-like dark matter (DM) halo inspired by cosmological simulations. Bar
evolution is studied as a function of numerical resolution in a sequence of
models spanning 10K to 100M DM particles - including a multi-mass model with an
effective resolution of 10G. The goal is to find convergence in dynamical
behaviour. We characterize the bar growth, the buckling instability, pattern
speed decay through resonant transfer of angular momentum, and possible
destruction of the DM halo cusp. Overall, most characteristics converge in
behaviour in detail for halos containing more than 10M particles. Notably, the
formation of the bar does not destroy the density cusp in this case. These
higher resolution simulations clearly illustrate the importance of discrete
resonances in transporting angular momentum from the bar to the halo.Comment: 6 pages, 5 figures, IAU Symposium 254 submission. The animations
referenced by the paper can be found at
http://www.cita.utoronto.ca/~dubinski/IAU25
Gas Feedback on Stellar Bar Evolution
We analyze evolution of live disk-halo systems in the presence of various gas
fractions, f_gas less than 8% in the disk. We addressed the issue of angular
momentum (J) transfer from the gas to the bar and its effect on the bar
evolution. We find that the weakening of the bar, reported in the literature,
is not related to the J-exchange with the gas, but is caused by the vertical
buckling instability in the gas-poor disks and by a steep heating of a stellar
velocity dispersion by the central mass concentration (CMC) in the gas-rich
disks. The gas has a profound effect on the onset of the buckling -- larger
f_gas brings it forth due to the more massive CMCs. The former process leads to
the well-known formation of the peanut-shaped bulges, while the latter results
in the formation of progressively more elliptical bulges, for larger f_gas. The
subsequent (secular) evolution of the bar differs -- the gas-poor models
exhibit a growing bar while gas-rich models show a declining bar whose vertical
swelling is driven by a secular resonance heating. The border line between the
gas-poor and -rich models lies at f_gas ~ 3% in our models, but is
model-dependent and will be affected by additional processes, like star
formation and feedback from stellar evolution. The overall effect of the gas on
the evolution of the bar is not in a direct J transfer to the stars, but in the
loss of J by the gas and its influx to the center that increases the CMC. The
more massive CMC damps the vertical buckling instability and depopulates orbits
responsible for the appearance of peanut-shaped bulges. The action of resonant
and non-resonant processes in gas-poor and gas-rich disks leads to a converging
evolution in the vertical extent of the bar and its stellar dispersion
velocities, and to a diverging evolution in the bulge properties.Comment: 12 pages, 12 figures, accepted for publication by the Astrophysical
Journal. Minor corrections following the referee repor
Merger of Massive Black Holes using N-Body Simulations with Post-Newtonian Corrections
We present preliminary results from self-consistent, high resolution direct
{\it N}-body simulations of massive black hole binaries in mergers of galactic
nuclei. The dynamics of the black hole binary includes the full Post-Newtonian
corrections (up to 2.5PN) to its equations of motion. We show that massive
black holes starting at separations of 100 pc can evolve down to
gravitational-wave-induced coalescence in less than a Hubble time. The
binaries, in our models, often form with very high eccentricity and, as a
result, reach separations of 50 Schwarzschild radius with eccentricities which
are clearly distinct from zero -- even though gravitational wave emission damps
the eccentricity during the inspiral. These deviations from exact circular
orbits, at such small separations, may have important consequences for LISA
data analysis.Comment: 8 pages, 6 figures, proceedings to the 7th LISA Symposium, Barcelona,
16-20 June 2008. Submitted to Journal of Physics: Conference Serie
Star cluster evolution in barred disc galaxies. I. Planar periodic orbits
The dynamical evolution of stellar clusters is driven to a large extent by
their environment. Several studies so far have considered the effect of tidal
fields and their variations, such as, e.g., from giant molecular clouds,
galactic discs, or spiral arms. In this paper we will concentrate on a tidal
field whose effects on star clusters have not yet been studied, namely that of
bars. We present a set of direct N-body simulations of star clusters moving in
an analytic potential representing a barred galaxy. We compare the evolution of
the clusters moving both on different planar periodic orbits in the barred
potential and on circular orbits in a potential obtained by axisymmetrising its
mass distribution. We show that both the shape of the underlying orbit and its
stability have strong impact on the cluster evolution as well as the morphology
and orientation of the tidal tails and the sub-structures therein. We find that
the dissolution time-scale of the cluster in our simulations is mainly
determined by the tidal forcing along the orbit and, for a given tidal forcing,
only very little by the exact shape of the gravitational potential in which the
cluster is moving.Comment: 15 pages, 17 figures, 5 tables; accepted for publication in MNRAS.
Complementary movies can be be found at this http URL
http://lam.oamp.fr/research/dynamique-des-galaxies/scientific-results/star-cluster-evolution
The regeneration of stellar bars by tidal interactions. Numerical simulations of fly-by encounters
We study the regeneration of stellar bars triggered by a tidal interaction,
using numerical simulations of either purely stellar or stellar+gas disc
galaxies. We find that interactions which are sufficiently strong to regenerate
the bar in the purely stellar models do not lead to a regeneration in the
dissipative models, owing to the induced gas inflow in those models. In models
in which the bar can be regenerated, we find a tight correlation between the
strength and the pattern speed of the induced bar. This relation can be
explained by a significant radial redistribution of angular momentum in the
disc due to the interaction, similar to the processes and correlations found
for isolated barred spirals. We furthermore show that the regenerated bars show
the same dynamical properties as their isolated counterparts.Comment: 18 pages, 26 figures, accepted for publication in MNRA
- …