30,184 research outputs found
Radiative transitions of the helium atom in highly magnetized neutron star atmospheres
Recent observations of thermally emitting isolated neutron stars revealed
spectral features that could be interpreted as radiative transitions of He in a
magnetized neutron star atmosphere. We present Hartree-Fock calculations of the
polarization-dependent photoionization cross sections of the He atom in strong
magnetic fields ranging from 10^12 G to 10^14 G. Convenient fitting formulae
for the cross sections are given as well as related oscillator strengths for
various bound-bound transitions. The effects of finite nucleus mass on the
radiative absorption cross sections are examined using perturbation theory.Comment: 14 pages, 7 figures. Minor changes. MNRAS in pres
Nonlinear robust controller design for multi-robot systems with unknown payloads
This work is concerned with the control problem of a multi-robot system handling a payload with unknown mass properties. Force constraints at the grasp points are considered. Robust control schemes are proposed that cope with the model uncertainty and achieve asymptotic path tracking. To deal with the force constraints, a strategy for optimally sharing the task is suggested. This strategy basically consists of two steps. The first detects the robots that need help and the second arranges that help. It is shown that the overall system is not only robust to uncertain payload parameters, but also satisfies the force constraints
Dynamical evolution and leading order gravitational wave emission of Riemann-S binaries
An approximate strategy for studying the evolution of binary systems of
extended objects is introduced. The stars are assumed to be polytropic
ellipsoids. The surfaces of constant density maintain their ellipsoidal shape
during the time evolution. The equations of hydrodynamics then reduce to a
system of ordinary differential equations for the internal velocities, the
principal axes of the stars and the orbital parameters. The equations of motion
are given within Lagrangian and Hamiltonian formalism. The special case when
both stars are axially symmetric fluid configurations is considered. Leading
order gravitational radiation reaction is incorporated, where the quasi-static
approximation is applied to the internal degrees of freedom of the stars. The
influence of the stellar parameters, in particular the influence of the
polytropic index , on the leading order gravitational waveforms is studied.Comment: 31 pages, 7 figures, typos correcte
Human African trypanosomiasis : the current situation in endemic regions and the risks for non-endemic regions from imported cases
Human African trypanosomiasis (HAT) is caused by Trypanosoma brucei
gambiense and T. b. rhodesiense and caused devastating epidemics during the 20th
century. Due to effective control programs implemented in the last two decades, the
number of reported cases has fallen to a historically low level. Although fewer than
977 cases were reported in 2018 in endemic countries, HAT is still a public health
problem in endemic regions until it is completely eliminated. In addition, almost 150
confirmed HAT cases were reported in non-endemic countries in the last three
decades. The majority of non-endemic HAT cases were reported in Europe, United
States and South Africa, due to historical alliances, economic links or geographic
proximity to disease endemic countries. Furthermore, with the implementation of the
“Belt and Road” project, sporadic imported HAT cases have been reported in China
as a warning sign of tropical diseases prevention. In this paper, we explore and
interpret the data on HAT incidence and find no positive correlation between the
number of HAT cases from endemic and non-endemic countries.This data will
provide useful information for better understanding the imported cases of HAT
globally in the post-elimination phase
Post-Newtonian Models of Binary Neutron Stars
Using an energy variational method, we calculate quasi-equilibrium
configurations of binary neutron stars modeled as compressible triaxial
ellipsoids obeying a polytropic equation of state. Our energy functional
includes terms both for the internal hydrodynamics of the stars and for the
external orbital motion. We add the leading post-Newtonian (PN) corrections to
the internal and gravitational energies of the stars, and adopt hybrid orbital
terms which are fully relativistic in the test-mass limit and always accurate
to PN order. The total energy functional is varied to find quasi-equilibrium
sequences for both corotating and irrotational binaries in circular orbits. We
examine how the orbital frequency at the innermost stable circular orbit
depends on the polytropic index n and the compactness parameter GM/Rc^2. We
find that, for a given GM/Rc^2, the innermost stable circular orbit along an
irrotational sequence is about 17% larger than the innermost secularly stable
circular orbit along the corotating sequence when n=0.5, and 20% larger when
n=1. We also examine the dependence of the maximum neutron star mass on the
orbital frequency and find that, if PN tidal effects can be neglected, the
maximum equilibrium mass increases as the orbital separation decreases.Comment: 53 pages, LaTex, 9 figures as 10 postscript files, accepted by Phys.
Rev. D, replaced version contains updated reference
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