8,221 research outputs found
Single grid accelerator for an ion thrustor
A single grid accelerator system for an ion thrustor is discussed. A layer of dielectric material is interposed between this metal grid and the chamber containing an ionized propellant for protecting the grid against sputtering erosion
Nonlinear behavior of geometric phases induced by photon pairs
In this study, we observe the nonlinear behavior of the two-photon geometric
phase for polarization states using time-correlated photons pairs. This phase
manifests as a shift of two-photon interference fringes. Under certain
arrangements, the geometric phase can vary nonlinearly and become very
sensitive to a change in the polarization state. Moreover, it is known that the
geometric phase for identically polarized photons is times larger than
that for one photon. Thus, the geometric phase for two photons can become two
times more sensitive to a state change. This high sensitivity to a change in
the polarization can be exploited for precision measurement of small
polarization variation. We evaluate the signal-to-noise ratio of the
measurement scheme using the nonlinear behavior of the geometric phase under
technical noise and highlight the practical advantages of this scheme.Comment: 10 pages, 10 figure
Diffusion and spectral dimension on Eden tree
We calculate the eigenspectrum of random walks on the Eden tree in two and
three dimensions. From this, we calculate the spectral dimension and the
walk dimension and test the scaling relation (
for an Eden tree). Finite-size induced crossovers are observed, whereby the
system crosses over from a short-time regime where this relation is violated
(particularly in two dimensions) to a long-time regime where the behavior
appears to be complicated and dependent on dimension even qualitatively.Comment: 11 pages, Plain TeX with J-Phys.sty style, HLRZ 93/9
Hydrogen Atom in Relativistic Motion
The Lorentz contraction of bound states in field theory is often appealed to
in qualitative descriptions of high energy particle collisions. Surprisingly,
the contraction has not been demonstrated explicitly even in simple cases such
as the hydrogen atom. It requires a calculation of wave functions evaluated at
equal (ordinary) time for bound states in motion. Such wave functions are not
obtained by kinematic boosts from the rest frame. Starting from the exact
Bethe-Salpeter equation we derive the equal-time wave function of a
fermion-antifermion bound state in QED, i.e., positronium or the hydrogen atom,
in any frame to leading order in alpha. We show explicitly that the bound state
energy transforms as the fourth component of a vector and that the wave
function of the fermion-antifermion Fock state contracts as expected.
Transverse photon exchange contributes at leading order to the binding energy
of the bound state in motion. We study the general features of the
corresponding fermion-antifermion-photon Fock states, and show that they do not
transform by simply contracting. We verify that the wave function reduces to
the light-front one in the infinite momentum frame.Comment: 20 pages, 10 figures; v2: some changes in discussion, accepted for
publication in Phys.Rev.
HCN to HCO^+ Millimeter Line Diagnostics of AGN Molecular Torus I : Radiative Transfer Modeling
We explore millimeter line diagnostics of an obscuring molecular torus
modeled by a hydrodynamic simulation with three-dimensional nonLTE radiative
transfer calculations. Based on the results of high-resolution hydrodynamic
simulation of the molecular torus around an AGN, we calculate intensities of
HCN and HCO^{+} rotational lines as two representative high density tracers.
The three-dimensional radiative transfer calculations shed light on a
complicated excitation state in the inhomogeneous torus, even though a
spatially uniform chemical structure is assumed. Our results suggest that HCN
must be much more abundant than HCO^{+} in order to obtain a high ratio
() observed in some of the nearby galaxies. There is a
remarkable dispersion in the relation between integrated intensity and column
density, indicative of possible shortcomings of HCN(1-0) and HCO^{+}(1-0) lines
as high density tracers. The internal structures of the inhomogeneous molecular
torus down to subparsec scale in external galaxies will be revealed by the
forthcoming Atacama Large Millimeter/submillimeter Array (ALMA). The
three-dimensional radiative transfer calculations of molecular lines with
high-resolution hydrodynamic simulation prove to be a powerful tool to provide
a physical basis for molecular line diagnostics of the central regions of
external galaxies.Comment: 29 pages, 13 figures, Accepted for publication in ApJ, For high
resolution figures see http://alma.mtk.nao.ac.jp/~masako/MS72533v2.pd
The Dog on the Ship: The "Canis Major Dwarf Galaxy" as an Outlying Part of the Argo Star System
Overdensities in the distribution of low latitude, 2MASS giant stars are
revealed by systematically peeling away from sky maps the bulk of the giant
stars conforming to ``isotropic'' density laws generally accounting for known
Milky Way components. This procedure, combined with a higher resolution
treatment of the sky density of both giants and dust allows us to probe to
lower Galactic latitudes than previous 2MASS giant star studies. While the
results show the swath of excess giants previously associated with the
Monoceros ring system in the second and third Galactic quadrants at distances
of 6-20 kpc, we also find a several times larger overdensity of giants in the
same distance range concentrated in the direction of the ancient constellation
Argo. Isodensity contours of the large structure suggest that it is highly
elongated and inclined by about 3 deg to the disk, although details of the
structure -- including the actual location of highest density, overall extent,
true shape -- and its origin, remain unknown because only a fraction of it lies
outside highly dust-obscured, low latitude regions. Nevertheless, our results
suggest that the 2MASS M giant overdensity previously claimed to represent the
core of a dwarf galaxy in Canis Major (l ~ 240 deg) is an artifact of a dust
extinction window opening to the overall density rise to the more significant
Argo structure centered at larger longitude (l ~ 290 +- 10 deg, b ~ -4 +- 2
deg).Comment: 4 pages, 4 figure
Millimeter Interferometric Investigations of the Energy Sources of Three Ultraluminous Infrared Galaxies, UGC 5101, Mrk 273, and IRAS 17208-0014, based on HCN to HCO+ Ratios
We present interferometric observations of three ultraluminous infrared
galaxies (ULIRGs; UGC 5101, Mrk 273, and IRAS 17208-0014) in the 3-mm
wavelength range, using the Nobeyama Millimeter Array. Both the HCN (J=1-0) and
HCO+ (J=1-0) molecular lines were observed simultaneously. HCN emission was
clearly detected at the nuclear positions of these ULIRGs, and HCO+ emission
was detected at the nuclear positions of UGC 5101 and IRAS 17208-0014. The HCN
to HCO+ brightness-temperature ratios toward the nuclei of the three ULIRGs
were derived and compared with those of lower luminosity galaxies known to be
dominated by active galactic nuclei (AGNs) or starbursts. In UGC 5101 and Mrk
273, where there is evidence for obscured AGNs from previous observations at
other wavelengths, we found high HCN/HCO+ ratios (>1.8) that are in the range
found for AGN-dominated galaxies. In IRAS 17208-0014, where the presence of a
powerful obscured AGN has been unclear, the ratio (1.7) is in between the
observed values for starburst- and AGN-dominated galaxies. The high HCN/HCO+
brightness-temperature ratios in UGC 5101 and Mrk 273 could be the consequence
of an HCN abundance enhancement, which is expected from chemical effects of the
central X-ray emitting AGN on the surrounding dense molecular gas. Our proposed
millimeter interferometric method based on HCN/HCO+ ratios may be an effective
tool for unveiling elusive buried AGNs at the cores of ULIRGs, especially
because of the negligible dust extinction at these wavelengths.Comment: 15 pages (emulateapj.sty), 8 figures (figures 1-5 resolution
reduced), Accepted for publication in Astronomical Journal, A PDF file with
high resolution is availble at
http://optik2.mtk.nao.ac.jp/~imanishi/Paper/HCN/HCN.pd
Surface Transitions for Confined Associating Mixtures
Thin films of binary mixtures that interact through isotropic forces and
directionally specific "hydrogen bonding" are considered through Monte Carlo
simulations. We show, in good agreement with experiment, that the single phase
of these mixtures can be stabilized or destabilized on confinement. These
results resolve a long standing controversy, since previous theories suggest
that confinement only stabilizes the single phase of fluid mixtures.Comment: LaTeX document, documentstyle[aps,preprint]{revtex}, psfig.sty,
bibtex, 13 pages, 4 figure
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