401 research outputs found
Quantum phase transition in an atomic Bose gas with a Feshbach resonance
We show that in an atomic Bose gas near a Feshbach resonance a quantum phase
transition occurs between a phase with only a molecular Bose-Einstein
condensate and a phase with both an atomic and a molecular Bose-Einstein
condensate. We show that the transition is characterized by an Ising order
parameter. We also determine the phase diagram of the gas as a function of
magnetic field and temperature: the quantum critical point extends into a line
of finite temperature Ising transitions.Comment: 4 pages, 2 figure
Fast rotating stars resulting from binary evolution will often appear to be single
Rapidly rotating stars are readily produced in binary systems. An accreting
star in a binary system can be spun up by mass accretion and quickly approach
the break-up limit. Mergers between two stars in a binary are expected to
result in massive, fast rotating stars. These rapid rotators may appear as Be
or Oe stars or at low metallicity they may be progenitors of long gamma-ray
bursts.
Given the high frequency of massive stars in close binaries it seems likely
that a large fraction of rapidly rotating stars result from binary interaction.
It is not straightforward to distinguish a a fast rotator that was born as a
rapidly rotating single star from a fast rotator that resulted from some kind
of binary interaction. Rapidly rotating stars resulting from binary interaction
will often appear to be single because the companion tends to be a low mass,
low luminosity star in a wide orbit. Alternatively, they became single stars
after a merger or disruption of the binary system during the supernova
explosion of the primary.
The absence of evidence for a companion does not guarantee that the system
did not experience binary interaction in the past. If binary interaction is one
of the main causes of high stellar rotation rates, the binary fraction is
expected to be smaller among fast rotators. How this prediction depend on
uncertainties in the physics of the binary interactions requires further
investigation.Comment: 2 pages, 1 figure, to be published in the proceedings of IAU 272
"Active OB stars: structure, evolution, mass loss and critical limit", Paris
19-23 July 201
Cosmological Multi-Black Hole Solutions
We present simple, analytic solutions to the Einstein-Maxwell equation, which
describe an arbitrary number of charged black holes in a spacetime with
positive cosmological constant . In the limit , these
solutions reduce to the well known Majumdar-Papapetrou (MP) solutions. Like the
MP solutions, each black hole in a solution has charge equal
to its mass , up to a possible overall sign. Unlike the limit,
however, solutions with are highly dynamical. The black holes move
with respect to one another, following natural trajectories in the background
deSitter spacetime. Black holes moving apart eventually go out of causal
contact. Black holes on approaching trajectories ultimately merge. To our
knowledge, these solutions give the first analytic description of coalescing
black holes. Likewise, the thermodynamics of the solutions is
quite interesting. Taken individually, a black hole is in thermal
equilibrium with the background deSitter Hawking radiation. With more than one
black hole, because the solutions are not static, no global equilibrium
temperature can be defined. In appropriate limits, however, when the black
holes are either close together or far apart, approximate equilibrium states
are established.Comment: 15 pages (phyzzx), UMHEP-380 (minor referencing error corrected
General Brane Geometries from Scalar Potentials: Gauged Supergravities and Accelerating Universes
We find broad classes of solutions to the field equations for d-dimensional
gravity coupled to an antisymmetric tensor of arbitrary rank and a scalar field
with non-vanishing potential. Our construction generates these configurations
from the solution of a single nonlinear ordinary differential equation, whose
form depends on the scalar potential. For an exponential potential we find
solutions corresponding to brane geometries, generalizing the black p-branes
and S-branes known for the case of vanishing potential. These geometries are
singular at the origin with up to two (regular) horizons. Their asymptotic
behaviour depends on the parameters of the model. When the singularity has
negative tension or the cosmological constant is positive we find
time-dependent configurations describing accelerating universes. Special cases
give explicit brane geometries for (compact and non-compact) gauged
supergravities in various dimensions, as well as for massive 10D supergravity,
and we discuss their interrelation. Some examples lift to give new solutions to
10D supergravity. Limiting cases with a domain wall structure preserve part of
the supersymmetries of the vacuum. We also consider more general potentials,
including sums of exponentials. Exact solutions are found for these with up to
three horizons, having potentially interesting cosmological interpretation. We
give several additional examples which illustrate the power of our techniques.Comment: 54 pages, 6 figures. Uses JHEP3. Published versio
Dirichlet-Branes and Ramond-Ramond Charges
We show that Dirichlet-branes, extended objects defined by mixed
Dirichlet-Neumann boundary conditions in string theory, break half of the
supersymmetries of the type~II superstring and carry a complete set of electric
and magnetic Ramond-Ramond charges. We also find that the product of the
electric and magnetic charges is a single Dirac unit, and that the quantum of
charge takes the value required by string duality. This is strong evidence that
the Dirchlet-branes are intrinsic to type II string theory and are the
Ramond-Ramond sources required by string duality. We also note the existence of
a previously overlooked 9-form potential in the IIa string, which gives rise to
an effective cosmological constant of undetermined magnitude.Comment: LaTeX, 10 pages. Minor typos corrected in eq. 8, 9, 13. References
added to [11
Symmetric Potentials of Gauged Supergravities in Diverse Dimensions and Coulomb Branch of Gauge Theories
A class of conformally flat and asymptotically anti-de Sitter geometries
involving profiles of scalar fields is studied from the point of view of gauged
supergravity. The scalars involved in the solutions parameterise the
SL(N,R)/SO(N) submanifold of the full scalar coset of the gauged supergravity,
and are described by a symmetric potential with a universal form. These
geometries descend via consistent truncation from distributions of D3-branes,
M2-branes, or M5-branes in ten or eleven dimensions. We exhibit analogous
solutions asymptotic to AdS_6 which descend from the D4-D8-brane system. We
obtain the related six-dimensional theory by consistent reduction from massive
type IIA supergravity. All our geometries correspond to states in the Coulomb
branch of the dual conformal field theories. We analyze linear fluctuations of
minimally coupled scalars and find both discrete and continuous spectra, but
always bounded below.Comment: Latex, 38 pages, minor correction
The Operator Product Expansion of the Lowest Higher Spin Current at Finite N
For the N=2 Kazama-Suzuki(KS) model on CP^3, the lowest higher spin current
with spins (2, 5/2, 5/2,3) is obtained from the generalized GKO coset
construction. By computing the operator product expansion of this current and
itself, the next higher spin current with spins (3, 7/2, 7/2, 4) is also
derived. This is a realization of the N=2 W_{N+1} algebra with N=3 in the
supersymmetric WZW model. By incorporating the self-coupling constant of lowest
higher spin current which is known for the general (N,k), we present the
complete nonlinear operator product expansion of the lowest higher spin current
with spins (2, 5/2, 5/2, 3) in the N=2 KS model on CP^N space. This should
coincide with the asymptotic symmetry of the higher spin AdS_3 supergravity at
the quantum level. The large (N,k) 't Hooft limit and the corresponding
classical nonlinear algebra are also discussed.Comment: 62 pages; the footnotes added, some redundant appendices removed, the
presentations in the whole paper improved and to appear in JHE
Anisotropic Four-Dimensional NS-NS String Cosmology
An anisotropic (Bianchi type I) cosmology is considered in the
four-dimensional NS-NS sector of low-energy effective string theory coupled to
a dilaton and an axion-like -field within a de Sitter-Einstein frame
background. The time evolution of this Universe is discussed in both the
Einstein and string frames.Comment: Revtex, 5 pages, 3 figure
Charged Nariai Black Holes With a Dilaton
The Reissner-Nordstrom-de Sitter black holes of standard Einstein-Maxwell
theory with a cosmological constant have no analogue in dilatonic theories with
a Liouville potential. The only exception are the solutions of maximal mass,
the Charged Nariai solutions. We show that the structure of the solution space
of the Dilatonic Charged Nariai black holes is quite different from the
non-dilatonic case. Its dimensionality depends on the exponential coupling
constants of the dilaton. We discuss the possibility of pair creating such
black holes on a suitable background. We find conditions for the existence of
Charged Nariai solutions in theories with general dilaton potentials, and
consider specifically a massive dilaton.Comment: 20 pages, LaTeX, 4 figures, submitted to Phys. Rev.
Type 0 T-Duality and the Tachyon Coupling
We consider the T-duality relations between Type 0A and 0B theories, and show
that this constraints the possible couplings of the tachyon to the RR-fields.
Due to the `doubling' of the RR sector in Type 0 theories, we are able to
introduce a democratic formulation for the Type 0 effective actions, in which
there is no Chern-Simons term in the effective action. Finally we discuss how
to embed Type II solutions into Type 0 theories.Comment: some misprints corrected and a reference adde
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