5,266 research outputs found
Ultrastructural Characteristics of the Fetal and Neonatal Rat Urinary Bladder
The embryologic and neonatal development of the normal rat urinary bladder was investigated in Sprague-Dawley rats by light, transmission, and scanning electron microscopy from day 11 of gestation through 21 days of age. The epithelium at day 11 of gestation is composed of small, loosely-connected, rounded cells with occasional short microvilli on their surfaces. The large polygonal cells characteristic of the adult bladder begin to appear by day 15, but the microridges are not apparent until day 17. By day 20, the epithelium appears morphologically similar to the adult bladder. Several morphological features are observed at different times of gestation which are not seen in the normal adult bladder, but they have been found in bladder tumors. During days 12-15 of gestation, most of the luminal lining cells of the bladder epithelium have a single central cilium. Cilia are also occasionally seen at days 11, 16, and 17 of gestation. Occasional cells with long tentacles are present from days 13-16 of gestation. Cells that appear to form bridges between cells are also seen from day 14 of gestation and continue to be observed through day 11 after birth. No cells with distinctive pleomorphic microvilli, a feature of rapidly proliferating bladder epithelial cells in the hyperplastic or tumorous epithelium of the adult, were seen at any time during gestation or after birth. Small foci of superficial layer sloughing occurred at the time of birth, but were rapidly replaced by one day after birth. It is apparent from this study that the bladder epithelium is a rapidly changing, proliferating tissue in utero and continuing for a brief period after birth
High Excitation Molecular Gas in the Magellanic Clouds
We present the first survey of submillimeter CO 4-3 emission in the
Magellanic Clouds. The survey is comprised of 15 6'x6' maps obtained using the
AST/RO telescope toward the molecular peaks of the Large and Small Magellanic
Clouds. We have used these data to constrain the physical conditions in these
objects, in particular their molecular gas density and temperature. We find
that there are significant amounts of molecular gas associated with most of
these molecular peaks, and that high molecular gas temperatures are pervasive
throughout our sample. We discuss whether this may be due to the low
metallicities and the associated dearth of gas coolants in the Clouds, and
conclude that the present sample is insufficient to assert this effect.Comment: 18 pages, 3 figures, 5 tables. To appear in Ap
Oscillator Strengths and Damping Constants for Atomic Lines in the J and H Bands
We have built a line list in the near-infrared J and H bands (1.00-1.34,
1.49-1.80 um) by gathering a series of laboratory and computed line lists.
Oscillator strengths and damping constants were computed or obtained by fitting
the solar spectrum.
The line list presented in this paper is, to our knowledge, the most complete
one now available, and supersedes previous lists.Comment: Accepted, Astrophysical Journal Supplement, tentatively scheduled for
the Sep. 1999 Vol. 124 #1 issue. Text and tables also available at
http://www.iagusp.usp.br/~jorge
Real Space Observations of Magnesium Hydride Formation and Decomposition
The mechanisms of magnesium hydride formation and thermal decomposition are
directly examined using in-situ imaging.Comment: 3 pages, 4 figure
Absolute frequency measurements of 85Rb nF7/2 Rydberg states using purely optical detection
A three-step laser excitation scheme is used to make absolute frequency
measurements of highly excited nF7/2 Rydberg states in 85Rb for principal
quantum numbers n=33-100. This work demonstrates the first absolute frequency
measurements of rubidium Rydberg levels using a purely optical detection
scheme. The Rydberg states are excited in a heated Rb vapour cell and Doppler
free signals are detected via purely optical means. All of the frequency
measurements are made using a wavemeter which is calibrated against a GPS
disciplined self-referenced optical frequency comb. We find that the measured
levels have a very high frequency stability, and are especially robust to
electric fields. The apparatus has allowed measurements of the states to an
accuracy of 8.0MHz. The new measurements are analysed by extracting the
modified Rydberg-Ritz series parameters.Comment: 12 pages, 5 figures, submitted to New. J. Phy
Light emission from a scanning tunneling microscope: Fully retarded calculation
The light emission rate from a scanning tunneling microscope (STM) scanning a
noble metal surface is calculated taking retardation effects into account. As
in our previous, non-retarded theory [Johansson, Monreal, and Apell, Phys. Rev.
B 42, 9210 (1990)], the STM tip is modeled by a sphere, and the dielectric
properties of tip and sample are described by experimentally measured
dielectric functions. The calculations are based on exact diffraction theory
through the vector equivalent of the Kirchoff integral. The present results are
qualitatively similar to those of the non-retarded calculations. The light
emission spectra have pronounced resonance peaks due to the formation of a
tip-induced plasmon mode localized to the cavity between the tip and the
sample. At a quantitative level, the effects of retardation are rather small as
long as the sample material is Au or Cu, and the tip consists of W or Ir.
However, for Ag samples, in which the resistive losses are smaller, the
inclusion of retardation effects in the calculation leads to larger changes:
the resonance energy decreases by 0.2-0.3 eV, and the resonance broadens. These
changes improve the agreement with experiment. For a Ag sample and an Ir tip,
the quantum efficiency is 10 emitted photons in the visible
frequency range per tunneling electron. A study of the energy dissipation into
the tip and sample shows that in total about 1 % of the electrons undergo
inelastic processes while tunneling.Comment: 16 pages, 10 figures (1 ps, 9 tex, automatically included); To appear
in Phys. Rev. B (15 October 1998
Light scattering from disordered overlayers of metallic nanoparticles
We develop a theory for light scattering from a disordered layer of metal
nanoparticles resting on a sample. Averaging over different disorder
realizations is done by a coherent potential approximation. The calculational
scheme takes into account effects of retardation, multipole excitations, and
interactions with the sample. We apply the theory to a system similar to the
one studied experimentally by Stuart and Hall [Phys. Rev. Lett. {\bf 80}, 5663
(1998)] who used a layered Si/SiO/Si sample. The calculated results agree
rather well with the experimental ones. In particular we find conspicuous
maxima in the scattering intensity at long wavelengths (much longer than those
corresponding to plasmon resonances in the particles). We show that these
maxima have their origin in interference phenomena in the layered sample.Comment: 19 pages, 12 figure
Maximum-Reward Motion in a Stochastic Environment: The Nonequilibrium Statistical Mechanics Perspective
We consider the problem of computing the maximum-reward motion in a reward field in an online setting. We assume that the robot has a limited perception range, and it discovers the reward field on the fly. We analyze the performance of a simple, practical lattice-based algorithm with respect to the perception range. Our main result is that, with very little perception range, the robot can collect as much reward as if it could see the whole reward field, under certain assumptions. Along the way, we establish novel connections between this class of problems and certain fundamental problems of nonequilibrium statistical mechanics . We demonstrate our results in simulation examples
Deep, Wide-field CCD Photometry for the Open Cluster NGC3532
We present the results of a deep, wide-field CCD survey for the open cluster
NGC~3532. Our new photometry effectively covers a one square
degree area and reaches an unprecedented depth of to reveal that
NGC~3532 is a rich open cluster that harbors a large number of faint, low-mass
stars. We employ a number of methods to reduce the impact of field star
contamination in the cluster color-magnitude diagrams, including supplementing
our photometry with data from the 2MASS catalog. These efforts allow
us to define a robust sample of candidate main sequence stars suitable for a
purely empirical determination of the cluster's parameters by comparing them to
the well-established Hyades main sequence. Our results confirm previous
findings that NGC~3532 lies fairly near to the Sun [;
~pc] and has an extremely low reddening for its location near
the Galactic plane []. Moreover, an age of \,Myr
has been derived for the cluster by fitting a set of overshooting isochrones to
the well-populated upper main-sequence. This new photometry also extends faint
enough to reach the cluster white dwarf sequence, as confirmed by our
photometric recovery of eight spectroscopically identified members of the
cluster. Using the location of these eight members, along with the latest
theoretical cooling tracks, we have identified additional white dwarf
stars in the color-magnitude diagram that have a high probability
of belonging to NGC~3532. The age we derive from fitting white dwarf isochrones
to the locus of these stars, \,Myr, is consistent with the age
derived from the turnoff. Our analysis of the photometry also includes an
estimation of the binary star fraction, as well as a determination of the
cluster's luminosity and mass functions.Comment: Accepted for publication in AJ. 55 pages, 21 figures. High-quality
version with complete data tables can be downloaded from
http://www.phys.lsu.edu/~jclem/NGC3532
- …