47,310 research outputs found
Collapse, outflows and fragmentation of massive, turbulent and magnetized prestellar barotropic cores
Stars and more particularly massive stars, have a drastic impact on galaxy
evolution. Yet the conditions in which they form and collapse are still not
fully understood. In particular, the influence of the magnetic field on the
collapse of massive clumps is relatively unexplored, it is thus of great
relevance in the context of the formation of massive stars to investigate its
impact. We perform high resolution, MHD simulations of the collapse of hundred
solar masses, turbulent and magnetized clouds, using the adaptive mesh
refinement code RAMSES. We compute various quantities such as mass
distribution, magnetic field and angular momentum within the collapsing core
and study the episodic outflows and the fragmentation that occurs during the
collapse. The magnetic field has a drastic impact on the cloud evolution. We
find that magnetic braking is able to substantially reduce the angular momentum
in the inner part of the collapsing cloud. Fast and episodic outflows are being
launched with typical velocities of the order of 3-5 km s although the
highest velocities can be as high as 30-40 km s. The fragmentation in
several objects, is reduced in substantially magnetized clouds with respect to
hydrodynamical ones by a factor of the order of 1.5-2. We conclude that
magnetic fields have a significant impact on the evolution of massive clumps.
In combination with radiation, magnetic fields largely determine the outcome of
massive core collapse. We stress that numerical convergence of MHD collapse is
a challenging issue. In particular, numerical diffusion appears to be important
at high density therefore possibly leading to an over-estimation of the number
of fragments.Comment: accepted for publication in A&
A heterotic sigma model with novel target geometry
We construct a (1,2) heterotic sigma model whose target space geometry
consists of a transitive Lie algebroid with complex structure on a Kaehler
manifold. We show that, under certain geometrical and topological conditions,
there are two distinguished topological half--twists of the heterotic sigma
model leading to A and B type half--topological models. Each of these models is
characterized by the usual topological BRST operator, stemming from the
heterotic (0,2) supersymmetry, and a second BRST operator anticommuting with
the former, originating from the (1,0) supersymmetry. These BRST operators
combined in a certain way provide each half--topological model with two
inequivalent BRST structures and, correspondingly, two distinct perturbative
chiral algebras and chiral rings. The latter are studied in detail and
characterized geometrically in terms of Lie algebroid cohomology in the
quasiclassical limit.Comment: 83 pages, no figures, 2 references adde
Dissipative production of a maximally entangled steady state
Entangled states are a key resource in fundamental quantum physics, quantum
cryp-tography, and quantum computation [1].To date, controlled unitary
interactions applied to a quantum system, so-called "quantum gates", have been
the most widely used method to deterministically create entanglement [2]. These
processes require high-fidelity state preparation as well as minimizing the
decoherence that inevitably arises from coupling between the system and the
environment and imperfect control of the system parameters. Here, on the
contrary, we combine unitary processes with engineered dissipation to
deterministically produce and stabilize an approximate Bell state of two
trapped-ion qubits independent of their initial state. While previous works
along this line involved the application of sequences of multiple
time-dependent gates [3] or generated entanglement of atomic ensembles
dissipatively but relied on a measurement record for steady-state entanglement
[4], we implement the process in a continuous time-independent fashion,
analogous to optical pumping of atomic states. By continuously driving the
system towards steady-state, the entanglement is stabilized even in the
presence of experimental noise and decoherence. Our demonstration of an
entangled steady state of two qubits represents a step towards dissipative
state engineering, dissipative quantum computation, and dissipative phase
transitions [5-7]. Following this approach, engineered coupling to the
environment may be applied to a broad range of experimental systems to achieve
desired quantum dynamics or steady states. Indeed, concurrently with this work,
an entangled steady state of two superconducting qubits was demonstrated using
dissipation [8].Comment: 25 pages, 5 figure
Equation of state of a superfluid Fermi gas in the BCS-BEC crossover
We present a theory for a superfluid Fermi gas near the BCS-BEC crossover,
including pairing fluctuation contributions to the free energy similar to that
considered by Nozieres and Schmitt-Rink for the normal phase. In the strong
coupling limit, our theory is able to recover the Bogoliubov theory of a weakly
interacting Bose gas with a molecular scattering length very close to the known
exact result. We compare our results with recent Quantum Monte Carlo
simulations both for the ground state and at finite temperature. Excellent
agreement is found for all interaction strengths where simulation results are
available.Comment: 7 pages, 4 figures, published version in Europhysics Letters, a long
preprint with details will appear soo
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