1,356 research outputs found
Lateral current density fronts in asymmetric double-barrier resonant-tunneling structures
We present a theoretical analysis and numerical simulations of lateral
current density fronts in bistable resonant-tunneling diodes with Z-shaped
current-voltage characteristics. The bistability is due to the charge
accumulation in the quantum well of the double-barrier structure. We focus on
asymmetric structures in the regime of sequential incoherent tunneling and
study the dependence of the bistability range, the front velocity and the front
width on the structure parameters. We propose a sectional design of a structure
that is suitable for experimental observation of front propagation and discuss
potential problems of such measurements in view of our theoretical findings. We
point out the possibility to use sectional resonant-tunneling structures as
controllable three-terminal switches.Comment: to appear in J.Appl.Phy
The location, clustering, and propagation of massive star formation in giant molecular clouds
Massive stars are key players in the evolution of galaxies, yet their
formation pathway remains unclear. In this work, we use data from several
galaxy-wide surveys to build an unbiased dataset of ~700 massive young stellar
objects (MYSOs), ~200 giant molecular clouds (GMCs), and ~100 young (<10 Myr)
optical stellar clusters (SCs) in the Large Magellanic Cloud. We employ this
data to quantitatively study the location and clustering of massive star
formation and its relation to the internal structure of GMCs. We reveal that
massive stars do not typically form at the highest column densities nor centers
of their parent GMCs at the ~6 pc resolution of our observations. Massive star
formation clusters over multiple generations and on size scales much smaller
than the size of the parent GMC. We find that massive star formation is
significantly boosted in clouds near SCs. Yet, whether a cloud is associated
with a SC does not depend on either the cloud's mass or global surface density.
These results reveal a connection between different generations of massive
stars on timescales up to 10 Myr. We compare our work with Galactic studies and
discuss our findings in terms of GMC collapse, triggered star formation, and a
potential dichotomy between low- and high-mass star formation.Comment: 13 pages, 7 figures, in pres
Classical and quantum three-dimensional integrable systems with axial symmetry
We study the most general form of a three dimensional classical integrable
system with axial symmetry and invariant under the axis reflection. We assume
that the three constants of motion are the Hamiltonian, , with the standard
form of a kinetic part plus a potential dependent on the position only, the
-component of the angular momentum, , and a Hamiltonian-like constant,
, for which the kinetic part is quadratic in the momenta. We find
the explicit form of these potentials compatible with complete integrability.
The classical equations of motion, written in terms of two arbitrary potential
functions, is separated in oblate spheroidal coordinates. The quantization of
such systems leads to a set of two differential equations that can be presented
in the form of spheroidal wave equations.Comment: 17 pages, 3 figure
Gravitational instabilities in Kerr space-times
In this paper we consider the possible existence of unstable axisymmetric
modes in Kerr space times, resulting from exponentially growing solutions of
the Teukolsky equation. We describe a transformation that casts the radial
equation that results upon separation of variables in the Teukolsky equation,
in the form of a Schr\"odinger equation, and combine the properties of the
solutions of this equations with some recent results on the asymptotic
behaviour of spin weighted spheroidal harmonics to prove the existence of an
infinite family of unstable modes. Thus we prove that the stationary region
beyond a Kerr black hole inner horizon is unstable under gravitational linear
perturbations. We also prove that Kerr space-time with angular momentum larger
than its square mass, which has a naked singularity, is unstable.Comment: 9 pages, 4 figures, comments, references and calculation details
added, asymptotic expansion typos fixe
Investigations on T violation and CPT symmetry in the neutral kaon system -- a pedagogical approach --
During the recent years experiments with neutral kaons have yielded
remarkably sensitive results which are pertinent to such fundamental phenomena
as CPT invariance (protecting causality), time-reversal invariance violation,
coherence of wave functions, and entanglement of kaons in pair states. We
describe the phenomenological developments and the theoretical conclusions
drawn from the experimental material. An outlook to future experimentation is
indicated.Comment: 41 pages, 9 figures. See arXiv:hep-ph/0603075 for an enlarged versio
Dust composition and mass-loss return from the luminous blue variable R71 in the LMC
We present an analysis of mid-and far-infrared (IR) spectrum and spectral
energy distribution (SED) of the LBV R71 in the LMC.This work aims to
understand the overall contribution of high-mass LBVs to the total dust-mass
budget of the interstellar medium (ISM) of the LMC and compare this with the
contribution from low-mass asymptotic giant branch (AGB) stars. As a case
study, we analyze the SED of R71. We compiled all the available photometric and
spectroscopic observational fluxes from various telescopes for a wide
wavelength range (0.36 -- 250\,m). We determined the dust composition from
the spectroscopic data, and derived the ejected dust mass, dust mass-loss rate,
and other dust shell properties by modeling the SED of R71. We noted nine
spectral features in the dust shell of R71 by analyzing Spitzer spectroscopic
data. Among these, we identified three new crystalline silicate features. We
computed our model spectrum by using 3D radiative transfer code MCMax. Our
model calculation shows that dust is dominated by amorphous silicates, with
some crystalline silicates, metallic iron, and a very tiny amount of polycyclic
aromatic hydrocarbon (PAH) molecules. The presence of both silicates and PAHs
indicates that the dust has a mixed chemistry. We derived a dust mass of 0.01
M, from which we arrive at a total ejected mass of 5
M. This implies a time-averaged dust mass-loss rate of
2.510 M\,yr with an explosion about 4000 years
ago. We assume that the other five confirmed dusty LBVs in the LMC loose mass
at a similar rate, and estimate the total contribution to the mass budget of
the LMC to be 10 M\,yr, which is comparable to
the contribution by all the AGB stars in the LMC. Based on our analysis on R71,
we speculate that LBVs as a class may be an important dust source in the ISM of
the LMC.Comment: 10 pages, 6 figures, 2 table
Westbrook's Molecular Gun: Discovery of Near-IR Micro-Structures in AFGL 618
We present high-sensitivity near-IR images of a carbon-rich proto-planetary
nebula, AFGL 618, obtained with the Subaru Telescope. These images have
revealed ``bullets'' and ``horns'' extending farther out from the edges of the
previously known bipolar lobes. The spatial coincidence between these near-IR
micro-structures and the optical collimated outflow structure, together with
the detection of shock-excited, forbidden IR lines of atomic species, strongly
suggests that these bullets and horns represent the locations from which
[\ion{Fe}{2}] IR lines arise. We have also discovered CO clumps moving at km s at the positions of the near-IR bullets by re-analyzing the
existing CO interferometry data. These findings indicate that
the near-IR micro-structures represent the positions of shocked surfaces at
which fast-moving molecular clumps interface with the ambient circumstellar
shell.Comment: 2 figures. To appear in the ApJ Letter
Polarized Spectroscopy Studies of Single Molecules of Porphycenes
Using ambient atmosphere instead of pure nitrogen environment enabled efficient recording of room temperature fluorescence from single molecules of porphycenes, chromophores with a high triplet formation efficiency. Double hydrogen transfer between two chemically identical trans tautomers has been demonstrated for parent porphycene and three alkyl derivatives by the analysis of spatial patterns of the emission obtained after raster scanning the sample excited with an appropriately polarized laser beam. Because of tautomerization, fluorescence in porphycenes is due to two nearly orthogonal transition dipole moments. This property allows the spatial orientation of the single molecule chromophores to be determined using radially and azimuthally polarized laser beams as excitation sources
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