427 research outputs found
Large-wavelength instabilities in free-surface Hartmann flow at low magnetic Prandtl numbers
We study the linear stability of the flow of a viscous electrically
conducting capillary fluid on a planar fixed plate in the presence of gravity
and a uniform magnetic field. We first confirm that the Squire transformation
for MHD is compatible with the stress and insulating boundary conditions at the
free surface, but argue that unless the flow is driven at fixed Galilei and
capillary numbers, the critical mode is not necessarily two-dimensional. We
then investigate numerically how a flow-normal magnetic field, and the
associated Hartmann steady state, affect the soft and hard instability modes of
free surface flow, working in the low magnetic Prandtl number regime of
laboratory fluids. Because it is a critical layer instability, the hard mode is
found to exhibit similar behaviour to the even unstable mode in channel
Hartmann flow, in terms of both the weak influence of Pm on its neutral
stability curve, and the dependence of its critical Reynolds number Re_c on the
Hartmann number Ha. In contrast, the structure of the soft mode's growth rate
contours in the (Re, alpha) plane, where alpha is the wavenumber, differs
markedly between problems with small, but nonzero, Pm, and their counterparts
in the inductionless limit. As derived from large wavelength approximations,
and confirmed numerically, the soft mode's critical Reynolds number grows
exponentially with Ha in inductionless problems. However, when Pm is nonzero
the Lorentz force originating from the steady state current leads to a
modification of Re_c(Ha) to either a sublinearly increasing, or decreasing
function of Ha, respectively for problems with insulating and conducting walls.
In the former, we also observe pairs of Alfven waves, the upstream propagating
wave undergoing an instability at large Alfven numbers.Comment: 58 pages, 16 figure
Superpartner Solutions of a BPS Monopole in Noncommutative Space
We construct U(2) BPS monopole superpartner solutions in N=2 non-commutative
super Yang-Mills theory. Calculation to the second order in the noncommutative
parameter shows that there is no electric quadrupole moment that is
expected from the magnetic dipole structure of noncommtative U(2) monopole.
This might give an example of the nature of how supersymmetry works not
changing between the commutative and noncommutative theories.Comment: 8 page
Gauge Field Fluctuations and First-Order Phase Transition in Color Superconductivity
We study the gauge field fluctuations in dense quark matter and determine the
temperature of the induced first-order phase transition to the
color-superconducting phase in weak coupling. We find that the local
approximation of the coupling between the gauge potential and the order
parameter, employed in the Ginzburg-Landau theory, has to be modified by
restoring the full momentum dependence of the polarization function of gluons
in the superconducting phase.Comment: 5 pages, 1 figure, Revtex, we have modified our conclusions for the
metallic superconducto
HS 1857+5144 : a hot and young pre-cataclysmic variable
Aims. We report the discovery of a new white dwarf/M dwarf binary, HS 1857+5144, identified in the Hamburg Quasar Survey (HQS).
Methods. Time-resolved optical spectroscopy and photometry were carried out to determine the properties of this new cataclysmic variable progenitor (pre-CV).
Results. The light curves of HS 1857+5144 display a sinusoidal variation with a period of Porb = 383.52 min and peak-to-peak amplitudes of 0.7 mag and 1.1mag in the B-band and R-band, respectively. The large amplitude of the brightness variation results from a reflection effect on the heated inner hemisphere of the companion star, suggesting a very high temperature of the white
dwarf. Our radial velocity study confirms the photometric period as the orbital period of the system. A model atmosphere fit to the spectrum of the white dwarf obtained at minimum light provides limits to its mass and temperature of Mwd 0.6−1.0 M and Twd 70 000−100 000 K, respectively. The detection of He II λ4686 absorption classifies the primary star of HS 1857+5144 as a
DAO white dwarf. Combining the results from our spectroscopy and photometry, we estimate the mass of the companion star and the binary inclination to be Msec 0.15−0.30 M and i 45◦−55◦, respectively.
Conclusions. We classify HS 1857+5144 as one of the youngest pre-CV known to date. The cooling age of the white dwarf suggests that the present system has just emerged from a common envelope phase ∼105 yr ago. HS 1857+5144 will start mass transfer within or below the 2–3 h period gap
Higgs Mechanism in String Theory
In first-quantized string theory, spacetime symmetries are described by inner
automorphisms of the underlying conformal field theory. In this paper we use
this approach to illustrate the Higgs effect in string theory. We consider
string propagation on M^{24,1} \times S^1, where the circle has radius R, and
study SU(2) symmetry breaking as R moves away from its critical value. We find
a gauge-covariant equation of motion for the broken-symmetry gauge bosons and
the would-be Goldstone bosons. We show that the Goldstone bosons can be
eliminated by an appropriate gauge transformation. In this unitary gauge, the
Goldstone bosons become the longitudinal components of massive gauge bosons.Comment: 12 pages, Te
Asymmetric Fermion Superfluid with Inter- and Intra-Species Pairings
We investigate the phase structure of an asymmetric fermion superfluid with
inter- and intra-species pairings. The introduction of the intra-species
pairing mechanism in canonical ensemble changes significantly the phase diagram
and brings in a new state with coexisting inter- and intra-species pairings.
Different from the case with only inter-species pairing, all the fermion
excitations are fully gapped in the region with intra-species pairing.Comment: 5 pages, 4 figure
Electric Dipole Moment of a BPS Monopole
Monopole ``superpartner'' solutions are constructed by acting with finite,
broken supersymmetry transformations on a bosonic N=2 BPS monopole. The terms
beyond first order in this construction represent the backreaction of the the
fermionic zero-mode state on the other fields. Because of the quantum nature of
the fermionic zero-modes, the superpartner solution is necessarily operator
valued. We extract the electric dipole moment operator and show that it is
proportional to the fermion zero-mode angular momentum operator with a
gyroelectric ratio g=2. The magnetic quadrupole operator is shown to vanish
identically on all states. We comment on the usefulness of the monopole
superpartner solution for a study of the long-range spin dependent dynamics of
BPS monopoles.Comment: 8 pages, references and note adde
Relativistic Stars in Randall-Sundrum Gravity
The non-linear behaviour of Randall-Sundrum gravity with one brane is
examined. Due to the non-compact extra dimension, the perturbation spectrum has
no mass gap, and the long wavelength effective theory is only understood
perturbatively. The full 5-dimensional Einstein equations are solved
numerically for static, spherically symmetric matter localized on the brane,
yielding regular geometries in the bulk with axial symmetry. An elliptic
relaxation method is used, allowing both the brane and asymptotic radiation
boundary conditions to be simultaneously imposed. The same data that specifies
stars in 4-dimensional gravity, uniquely constructs a 5-dimensional solution.
The algorithm performs best for small stars (radius less than the AdS length)
yielding highly non-linear solutions. An upper mass limit is observed for these
small stars, and the geometry shows no global pathologies. The geometric
perturbation is shown to remain localized near the brane at high densities, the
confinement interestingly increasing for both small and large stars as the
upper mass limit is approached. Furthermore, the static spatial sections are
found to be approximately conformal to those of AdS. We show that the intrinsic
geometry of large stars, with radius several times the AdS length, is described
by 4-dimensional General Relativity far past the perturbative regime. This
indicates that the non-linear long wavelength effective action remains local,
even though the perturbation spectrum has no mass gap. The implication is that
Randall-Sundrum gravity, with localized brane matter, reproduces relativistic
astrophysical solutions, such as neutron stars and massive black holes,
consistent with observation.Comment: 57 pages, 26 (colour) figures; minor typos corrected, references
added and introduction condense
Linearized gravity as a gauge theory
We discuss linearized gravity from the point of view of a gauge theory. In
(3+1)-dimensions our analysis allows to consider linearized gravity in the
context of the MacDowell-Mansouri formalism. Our observations may be of
particular interest in the strong-weak coupling duality for linearized gravity,
in Randall-Sundrum brane world scenario and in Ashtekar formalism.Comment: Latex, 13 page
Gravitational collapse on the brane: a no-go theorem
We investigate how braneworld gravity affects gravitational collapse and
black hole formation by studying Oppenheimer-Snyder-like collapse on a
Randall-Sundrum type brane. Without making any assumptions about the bulk, we
prove a no-go theorem: the exterior spacetime on the brane cannot be static,
which is in stark contrast with general relativity. We also consider the role
of Kaluza-Klein energy density in collapse, using a toy model.Comment: 5 pages, REVTEX style, 1 figure, main results unchanged, revised to
improve clarity; version accepted by Phys. Rev. Let
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