289 research outputs found
Binary Capture Rates for Massive Protostars
The high multiplicity of massive stars in dense, young clusters is
established early in their evolution. The mechanism behind this remains
unresolved. Recent results suggest that massive protostars may capture
companions through disk interactions with much higher efficiency than their
solar mass counterparts. However, this conclusion is based on analytic
determinations of capture rates and estimates of the robustness of the
resulting binaries. We present the results of coupled n-body and SPH
simulations of star-disk encounters to further test the idea that disk-captured
binaries contribute to the observed multiplicity of massive stars.Comment: 4 pages, 3 figures, accepted to ApJ
Saari's homographic conjecture for planar equal-mass three-body problem in Newton gravity
Saari's homographic conjecture in N-body problem under the Newton gravity is
the following; configurational measure \mu=\sqrt{I}U, which is the product of
square root of the moment of inertia I=(\sum m_k)^{-1}\sum m_i m_j r_{ij}^2 and
the potential function U=\sum m_i m_j/r_{ij}, is constant if and only if the
motion is homographic. Where m_k represents mass of body k and r_{ij}
represents distance between bodies i and j. We prove this conjecture for planar
equal-mass three-body problem.
In this work, we use three sets of shape variables. In the first step, we use
\zeta=3q_3/(2(q_2-q_1)) where q_k \in \mathbb{C} represents position of body k.
Using r_1=r_{23}/r_{12} and r_2=r_{31}/r_{12} in intermediate step, we finally
use \mu itself and \rho=I^{3/2}/(r_{12}r_{23}r_{31}). The shape variables \mu
and \rho make our proof simple
Exact results for nonlinear ac-transport through a resonant level model
We obtain exact results for the transport through a resonant level model
(noninteracting Anderson impurity model) for rectangular voltage bias as a
function of time. We study both the transient behavior after switching on the
tunneling at time t = 0 and the ensuing steady state behavior. Explicit
expressions are obtained for the ac-current in the linear response regime and
beyond for large voltage bias. Among other effects, we observe current ringing
and PAT (photon assisted tunneling) oscillations.Comment: 7 page
The Effects of Clumps in Explaining X-ray Emission Lines from Hot Stars
It is now well established that stellar winds of hot stars are fragmentary
and that the X-ray emission from stellar winds has a strong contribution from
shocks in winds. Chandra high spectral resolution observations of line profiles
of O and B stars have shown numerous properties that had not been expected.
Here we suggest explanations by considering the X-rays as arising from bow
shocks that occur where the stellar wind impacts on spherical clumps in the
winds. We use an accurate and stable numerical hydrodynamical code to obtain
steady-state physical conditions for the temperature and density structure in a
bow shock. We use these solutions plus analytic approximations to interpret
some major X-ray features: the simple power-law distribution of the observed
emission measure derived from many hot star X-ray spectra and the wide range of
ionization stages that appear to be present in X-ray sources throughout the
winds. Also associated with the adiabatic cooling of the gas around a clump is
a significant transverse velocity for the hot plasma flow around the clumps,
and this can help to understand anomalies associated with observed line widths,
and the differences in widths seen in stars with high and low mass-loss rates.
The differences between bow shocks and the planar shocks that are often used
for hot stars are discussed. We introduce an ``on the shock'' (OTSh)
approximation that is useful for interpreting the X-rays and the consequences
of clumps in hot star winds and elsewhere in astronomy.Comment: to appear in the Astrophysical Journa
Euler configurations and quasi-polynomial systems
In the Newtonian 3-body problem, for any choice of the three masses, there
are exactly three Euler configurations (also known as the three Euler points).
In Helmholtz' problem of 3 point vortices in the plane, there are at most three
collinear relative equilibria. The "at most three" part is common to both
statements, but the respective arguments for it are usually so different that
one could think of a casual coincidence. By proving a statement on a
quasi-polynomial system, we show that the "at most three" holds in a general
context which includes both cases. We indicate some hard conjectures about the
configurations of relative equilibrium and suggest they could be attacked
within the quasi-polynomial framework.Comment: 21 pages, 6 figure
Homoclinic crossing in open systems: Chaos in periodically perturbed monopole plus quadrupolelike potentials
The Melnikov method is applied to periodically perturbed open systems modeled
by an inverse--square--law attraction center plus a quadrupolelike term. A
compactification approach that regularizes periodic orbits at infinity is
introduced. The (modified) Smale-Birkhoff homoclinic theorem is used to study
transversal homoclinic intersections. A larger class of open systems with
degenerated (nonhyperbolic) unstable periodic orbits after regularization is
also briefly considered.Comment: 19 pages, 15 figures, Revtex
Near-adiabatic parameter changes in correlated systems: Influence of the ramp protocol on the excitation energy
We study the excitation energy for slow changes of the hopping parameter in
the Falicov-Kimball model with nonequilibrium dynamical mean-field theory. The
excitation energy vanishes algebraically for long ramp times with an exponent
that depends on whether the ramp takes place within the metallic phase, within
the insulating phase, or across the Mott transition line. For ramps within
metallic or insulating phase the exponents are in agreement with a perturbative
analysis for small ramps. The perturbative expression quite generally shows
that the exponent depends explicitly on the spectrum of the system in the
initial state and on the smoothness of the ramp protocol. This explains the
qualitatively different behavior of gapless (e.g., metallic) and gapped (e.g.,
Mott insulating) systems. For gapped systems the asymptotic behavior of the
excitation energy depends only on the ramp protocol and its decay becomes
faster for smoother ramps. For gapless systems and sufficiently smooth ramps
the asymptotics are ramp-independent and depend only on the intrinsic spectrum
of the system. However, the intrinsic behavior is unobservable if the ramp is
not smooth enough. This is relevant for ramps to small interaction in the
fermionic Hubbard model, where the intrinsic cubic fall-off of the excitation
energy cannot be observed for a linear ramp due to its kinks at the beginning
and the end.Comment: 24 pages, 6 figure
Chaos in Static Axisymmetric Spacetimes I : Vacuum Case
We study the motion of test particle in static axisymmetric vacuum spacetimes
and discuss two criteria for strong chaos to occur: (1) a local instability
measured by the Weyl curvature, and (2) a tangle of a homoclinic orbit, which
is closely related to an unstable periodic orbit in general relativity. We
analyze several static axisymmetric spacetimes and find that the first
criterion is a sufficient condition for chaos, at least qualitatively. Although
some test particles which do not satisfy the first criterion show chaotic
behavior in some spacetimes, these can be accounted for the second criterion.Comment: More comments for the quantitative estimation of chaos are added, and
some inappropriate terms are changed. This will appear on Class. Quant. Gra
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