12,399 research outputs found
On Sasaki-Einstein manifolds in dimension five
We prove the existence of Sasaki-Einstein metrics on certain simply connected
5-manifolds where until now existence was unknown. All of these manifolds have
non-trivial torsion classes. On several of these we show that there are a
countable infinity of deformation classes of Sasaki-Einstein structures.Comment: 18 pages, Exposition was expanded and a reference adde
Hydrodynamic reductions of the heavenly equation
We demonstrate that Pleba\'nski's first heavenly equation decouples in
infinitely many ways into a triple of commuting (1+1)-dimensional systems of
hydrodynamic type which satisfy the Egorov property. Solving these systems by
the generalized hodograph method, one can construct exact solutions of the
heavenly equation parametrized by arbitrary functions of a single variable. We
discuss explicit examples of hydrodynamic reductions associated with the
equations of one-dimensional nonlinear elasticity, linearly degenerate systems
and the equations of associativity.Comment: 14 page
Some Heuristic Semiclassical Derivations of the Planck Length, the Hawking Effect and the Unruh Effect
The formulae for Planck length, Hawking temperature and Unruh-Davies
temperature are derived by using only laws of classical physics together with
the Heisenberg principle. Besides, it is shown how the Hawking relation can be
deduced from the Unruh relation by means of the principle of equivalence; the
deep link between Hawking effect and Unruh effect is in this way clarified.Comment: LaTex file, 6 pages, no figure
Understanding the production of dual BEC with sympathetic cooling
We show, both experimentally and theoretically, that sympathetic cooling of
Rb atoms in the state by evaporatively cooled atoms in the
state can be precisely controlled to produce dual or single
condensate in either state. We also study the thermalization rate between two
species. Our model renders a quantitative account of the observed role of the
overlap between the two clouds and points out that sympathetic cooling becomes
inefficient when the masses are very different. Our calculation also yields an
analytical expression of the thermalization rate for a single species.Comment: 3 figure
On the nature of the most obscured C-rich AGB stars in the Magellanic Clouds
The stars in the Magellanic Clouds with the largest degree of obscuration are
used to probe the highly uncertain physics of stars in the asymptotic giant
branch (AGB) phase of evolution. Carbon stars in particular, provide key
information on the amount of third dredge-up (TDU) and mass loss. We use two
independent stellar evolution codes to test how a different treatment of the
physics affects the evolution on the AGB. The output from the two codes are
used to determine the rates of dust formation in the circumstellar envelope,
where the method used to determine the dust is the same for each case. The
stars with the largest degree of obscuration in the LMC and SMC are identified
as the progeny of objects of initial mass and , respectively. This difference in mass is motivated by the
difference in the star formation histories of the two galaxies, and offers a
simple explanation of the redder infrared colours of C-stars in the LMC
compared to their counterparts in the SMC. The comparison with the Spitzer
colours of C-rich AGB stars in the SMC shows that a minimum surface carbon mass
fraction must have been reached by stars of initial
mass around . Our results confirm the necessity of adopting
low-temperature opacities in stellar evolutionary models of AGB stars. These
opacities allow the stars to obtain mass-loss rates high enough () to produce the amount of dust needed to reproduce the
Spitzer coloursComment: 14 pages, 5 figures, 1 table; accepted for publication in MNRAS Main
Journa
Homogeneous heterotic supergravity solutions with linear dilaton
I construct solutions to the heterotic supergravity BPS-equations on products
of Minkowski space with a non-symmetric coset. All of the bosonic fields are
homogeneous and non-vanishing, the dilaton being a linear function on the
non-compact part of spacetime.Comment: 36 pages; v2 conclusion updated and references adde
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