14,594 research outputs found
String theory extensions of Einstein-Maxwell fields: the static case
We present a new approach for generation of solutions in the four-dimensional
heterotic string theory with one vector field and in the five-dimensional
bosonic string theory starting from the static Einstein-Maxwell fields. Our
approach allows one to construct the solution classes invariant with respect to
the total subgroup of the three-dimensional charging symmetries of these string
theories. The new generation procedure leads to the extremal
Israel-Wilson-Perjes subclass of string theory solutions in a special case and
provides its natural continuous extension to the realm of non-extremal
solutions. We explicitly calculate all string theory solutions related to
three-dimensional gravity coupled to an effective dilaton field which arises
after an appropriate charging symmetry invariant reduction of the static
Einstein-Maxwell system.Comment: 19 pages in late
Charged Dual String Vacua from Interacting Rotating Black Holes Via Discrete and Nonlinear Symmetries
Using the stationary formulation of the toroidally compactified heterotic
string theory in terms of a pair of matrix Ernst potentials we consider the
four-dimensional truncation of this theory with no U(1) vector fields excited.
Imposing one time-like Killing vector permits us to express the stationary
effective action as a model in which gravity is coupled to a matrix Ernst
potential which, under certain parametrization, allows us to interpret the
matter sector of this theory as a double Ernst system. We generate a web of
string vacua which are related to each other via a set of discrete symmetries
of the effective action (some of them involve S-duality transformations and
possess non-perturbative character). Some physical implications of these
discrete symmetries are analyzed and we find that, in some particular cases,
they relate rotating black holes coupled to a dilaton with no Kalb--Ramond
field, static black holes with non-trivial dilaton and antisymmetric tensor
fields, and rotating and static naked singularities. Further, by applying a
nonlinear symmetry, namely, the so-called normalized Harrison transformation,
on the seed field configurations corresponding to these neutral backgrounds, we
recover the U(1)^n Abelian vector sector of the four-dimensional action of the
heterotic string, charging in this way the double Ernst system which
corresponds to each one of the neutral string vacua, i.e., the stationary and
the static black holes and the naked singularities.Comment: 19 pages in latex, added referenc
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Planar laser induced fluorescence for temperature measurement of optical thermocavitation
Pulsed laser-induced cavitation, has been the subject of many studies describing bubble growth, collapse and ensuing shock waves. To a lesser extent, hydrodynamics of continuous wave (CW) cavitation or thermocavitation have also been reported. However, the temperature field around these bubbles has not been measured, partly because a sensor placed in the fluid would interfere with the bubble dynamics, but also because the short-lived bubble lifetimes (∼70–200 µs) demand high sampling rates which are costly to achieve via infrared (IR) imaging. Planar laser-induced fluorescence (PLIF) provides a non-intrusive alternative technique to costly IR imaging to measure the temperature around laser-induced cavitation bubbles. A 440 nm laser sheet excites rhodamine-B dye to fluoresce while thermocavitation is induced by a CW 810 nm laser. Post-calibration, the fluorescence intensity captured with a high-speed Phantom Miro camera is correlated to temperature field adjacent to the bubble. Using shadowgraphy and PLIF, a significant decrease in sensible heat is observed in the nucleation site– temperature decreases after bubble collapse and the initial heated volume of liquid shrinks. Based on irradiation time and temperature, the provided optical energy is estimated to be converted up to 50% into acoustic energy based on the bubble's size, with larger bubbles converting larger percentages
Charging Interacting Rotating Black Holes in Heterotic String Theory
We present a formulation of the stationary bosonic string sector of the whole
toroidally compactified effective field theory of the heterotic string as a
double Ernst system which, in the framework of General Relativity describes, in
particular, a pair of interacting spinning black holes; however, in the
framework of low--energy string theory the double Ernst system can be
particularly interpreted as the rotating field configuration of two interacting
sources of black hole type coupled to dilaton and Kalb--Ramond fields. We
clarify the rotating character of the --component of the
antisymmetric tensor field of Kalb--Ramond and discuss on its possible torsion
nature. We also recall the fact that the double Ernst system possesses a
discrete symmetry which is used to relate physically different string vacua.
Therefore we apply the normalized Harrison transformation (a charging symmetry
which acts on the target space of the low--energy heterotic string theory
preserving the asymptotics of the transformed fields and endowing them with
multiple electromagnetic charges) on a generic solution of the double Ernst
system and compute the generated field configurations for the 4D effective
field theory of the heterotic string. This transformation generates the
vector field content of the whole low--energy heterotic string
spectrum and gives rise to a pair of interacting rotating black holes endowed
with dilaton, Kalb--Ramond and multiple electromagnetic fields where the charge
vectors are orthogonal to each other.Comment: 15 pages in latex, revised versio
Antisite Domains in Double Perovskite Ferromagnets: Impact on Magnetotransport and Half-metallicity
Several double perovskite materials of the form A_2BB'O_6 exhibit high
ferromagnetic T_c, and significant low field magnetoresistance. They are also a
candidate source of spin polarized electrons. The potential usefulness of these
materials is, however, frustrated by mislocation of the B and B' ions, which do
not organise themselves in the ideal alternating structure. The result is a
strong dependence of physical properties on preparative conditions, reducing
the magnetization and destroying the half-metallicity. We provide the first
results on the impact of spatially correlated antisite disorder, as observed
experimentally, on the ferromagnetic double perovskites. The antisite domains
suppress magnetism and half-metallicity, as expected, but lead to a dramatic
enhancement of the low field magnetoresistance.Comment: 6 pages, pdflatex, EPL styl
Chiral models in dilaton-Maxwell gravity
We study symmetry properties of the Einstein-Maxwell theory nonminimaly
coupled to the dilaton field. We consider a static case with pure electric
(magnetic) Maxwell field and show that the resulting system becomes a nonlinear
sigma-model wich possesses a chiral representation. We construct the
corresponding chiral matrix and establish a representation which is related to
the pair of Ernst-like potentials. These potentials are used for separation of
the symmetry group into the gauge and nongauge (charging) sectors. New
variables, which linearize the action of charging symmetries, are also
established; a solution generation technique based on the use of charging
symmetries is formulated. This technique is used for generation of the
elecricaly (magneticaly) charged dilatonic fields from the static General
Relativity ones.Comment: 9 pages in LaTex; published in Gen. Rel. Grav. 32 (2000) pp 1389-139
The Elephant Trunk Nebula and the Trumpler 37 cluster: Contribution of triggered star formation to the total population of an HII region
Rich young stellar clusters produce HII regions whose expansion into the
nearby molecular cloud is thought to trigger the formation of new stars.
However, the importance of this mode of star formation is uncertain. This
investigation seeks to quantify triggered star formation (TSF) in IC 1396A
(a.k.a., the Elephant Trunk Nebula), a bright rimmed cloud (BRC) on the
periphery of the nearby giant HII region IC 1396 produced by the Trumpler 37
cluster. X-ray selection of young stars from Chandra X-ray Observatory data is
combined with existing optical and infrared surveys to give a more complete
census of the TSF population. Over 250 young stars in and around IC 1396A are
identified; this doubles the previously known population. A spatio-temporal
gradient of stars from the IC 1396A cloud toward the primary ionizing star HD
206267 is found. We argue that the TSF mechanism in IC 1396A is the
radiation-driven implosion process persisting over several million years.
Analysis of the X-ray luminosity and initial mass functions indicates that >140
stars down to 0.1 Msun were formed by TSF. Considering other BRCs in the IC
1396 HII region, we estimate the TSF contribution for the entire HII region
exceeds 14-25% today, and may be higher over the lifetime of the HII region.
Such triggering on the periphery of HII regions may be a significant mode of
star formation in the Galaxy.Comment: Accepted for publication in MNRAS; 28 pages, 18 figure
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