2,516 research outputs found
Infrared Spectroscopy of Naphthalene Aggregation and Cluster Formation in Argon Matrices
Fourier-transform mid-infrared absorption spectra of mixed argon/naphthalene matrices at 5 K are shown with ratios of argon-to-naphthalene that vary from 1000 to 0. These spectra show the changes as naphthalene clustering and aggregation occurs, with moderate spectral shifts affecting the C-H vibrational modes and relatively small or no shifts to the C-C and C-C-C vibrational modes. The possible contribution of homogeneous naphthalene clusters to the interstellar unidentified infrared bands is discussed. The contribution of polycyclic aromatic hydrocarbon (PAH) clusters to the 7.7 micron emission plateau and the blue shading of the 12.7 micron emission band are identified as promising candidates for future research. In addition, since PAH clusters are model components of Jupiter and Titan's atmospheres, the information presented here may also be applicable to the spectroscopy of these objects
Large-Scale Regular Morphological Patterns in the Radio Jet of NGC 6251
We report on large-scale, regular morphological patterns found in the radio
jet of the nearby radio galaxy NGC 6251. Investigating morphological properties
of this radio jet from the nucleus to a radial distance of 300 arcsec
( 140 kpc) mapped at 1662 MHz and 4885 MHz by Perley, Bridle, &
Willis, we find three chains, each of which consists of five radio knots. We
also find that eight radio knots in the first two chains consist of three small
sub-knots (the triple-knotty substructures). We discuss the observational
properties of these regular morphological patterns.Comment: 8 figures, 15 pages, accepted for publication in A
Computer-assisted polyp matching between optical colonoscopy and CT colonography: a phantom study
Potentially precancerous polyps detected with CT colonography (CTC) need to
be removed subsequently, using an optical colonoscope (OC). Due to large
colonic deformations induced by the colonoscope, even very experienced
colonoscopists find it difficult to pinpoint the exact location of the
colonoscope tip in relation to polyps reported on CTC. This can cause unduly
prolonged OC examinations that are stressful for the patient, colonoscopist and
supporting staff.
We developed a method, based on monocular 3D reconstruction from OC images,
that automatically matches polyps observed in OC with polyps reported on prior
CTC. A matching cost is computed, using rigid point-based registration between
surface point clouds extracted from both modalities. A 3D printed and painted
phantom of a 25 cm long transverse colon segment was used to validate the
method on two medium sized polyps. Results indicate that the matching cost is
smaller at the correct corresponding polyp between OC and CTC: the value is 3.9
times higher at the incorrect polyp, comparing the correct match between polyps
to the incorrect match. Furthermore, we evaluate the matching of the
reconstructed polyp from OC with other colonic endoluminal surface structures
such as haustral folds and show that there is a minimum at the correct polyp
from CTC.
Automated matching between polyps observed at OC and prior CTC would
facilitate the biopsy or removal of true-positive pathology or exclusion of
false-positive CTC findings, and would reduce colonoscopy false-negative
(missed) polyps. Ultimately, such a method might reduce healthcare costs,
patient inconvenience and discomfort.Comment: This paper was presented at the SPIE Medical Imaging 2014 conferenc
Synthetic Observations of Simulated Radio Galaxies I: Radio and X-ray Analysis
We present an extensive synthetic observational analysis of numerically-
simulated radio galaxies designed to explore the effectiveness of conventional
observational analyses at recovering physical source properties. These are the
first numerical simulations with sufficient physical detail to allow such a
study. The present paper focuses on extraction of magnetic field properties
from nonthermal intensity information. Synchrotron and inverse-Compton
intensities provided meaningful information about distributions and strengths
of magnetic fields, although considerable care was called for. Correlations
between radio and X-ray surface brightness correctly revealed useful dynamical
relationships between particles and fields. Magnetic field strength estimates
derived from the ratio of X-ray to radio intensity were mostly within about a
factor of two of the RMS field strength along a given line of sight. When
emissions along a given line of sight were dominated by regions close to the
minimum energy/equipartition condition, the field strengths derived from the
standard power-law-spectrum minimum energy calculation were also reasonably
close to actual field strengths, except when spectral aging was evident.
Otherwise, biases in the minimum- energy magnetic field estimation mirrored
actual differences from equipartition. The ratio of the inverse-Compton
magnetic field to the minimum-energy magnetic field provided a rough measure of
the actual total energy in particles and fields in most instances, within an
order of magnitude. This may provide a practical limit to the accuracy with
which one may be able to establish the internal energy density or pressure of
optically thin synchrotron sources.Comment: 43 pages, 14 figures; accepted for publication in ApJ, v601 n2
February 1, 200
Molecular Hydrogen Formation on Low Temperature Surfaces in Temperature Programmed Desorption Experiments
The study of the formation of molecular hydrogen on low temperature surfaces
is of interest both because it allows to explore elementary steps in the
heterogeneous catalysis of a simple molecule and because of the applications in
astrochemistry. Here we report results of experiments of molecular hydrogen
formation on amorphous silicate surfaces using temperature-programmed
desorption (TPD). In these experiments beams of H and D atoms are irradiated on
the surface of an amorphous silicate sample. The desorption rate of HD
molecules is monitored using a mass spectrometer during a subsequent TPD run.
The results are analyzed using rate equations and the activation energies of
the processes leading to molecular hydrogen formation are obtained from the TPD
data. We show that a model based on a single isotope provides the correct
results for the activation energies for diffusion and desorption of H atoms.
These results can thus be used to evaluate the formation rate of H_2 on dust
grains under the actual conditions present in interstellar clouds.Comment: 30 pages, 1 table, 6 figures. Published versio
Structure of the X-ray Emission from the Jet of 3C 273
We present images from five observations of the quasar 3C 273 with the
Chandra X-ray Observatory. The jet has at least four distinct features which
are not resolved in previous observations. The first knot in the jet (A1) is
very bright in X-rays. Its X-ray spectrum is well fitted with a power law with
alpha = 0.60 +/- 0.05. Combining this measurement with lower frequency data
shows that a pure synchrotron model can fit the spectrum of this knot from
1.647 GHz to 5 keV (over nine decades in energy) with alpha = 0.76 +/- 0.02,
similar to the X-ray spectral slope. Thus, we place a lower limit on the total
power radiated by this knot of 1.5e43 erg/s; substantially more power may be
emitted in the hard X-ray and gamma-ray bands.
Knot A2 is also detected and is somewhat blended with knot B1. Synchrotron
emission may also explain the X-ray emission but a spectral bend is required
near the optical band. For knots A1 and B1, the X-ray flux dominates the
emitted energy. For the remaining optical knots (C through H), localized X-ray
enhancements that might correspond to the optical features are not clearly
resolved. The position angle of the jet ridge line follows the optical shape
with distinct, aperiodic excursions of +/-1 deg from a median value of
-138.0deg. Finally, we find X-ray emission from the ``inner jet'' between 5 and
10" from the core.Comment: 10 pages, 5 figures; accepted for publication in the Astrophysical
Journal Letters. For the color image, see fig1.ps or
http://space.mit.edu/~hermanm/papers/3c273/fig1.jp
Use of Laboratory Data to Model Interstellar Chemistry
Our laboratory research program is about the formation of molecules on dust grains analogues in conditions mimicking interstellar medium environments. Using surface science techniques, in the last ten years we have investigated the formation of molecular hydrogen and other molecules on different types of dust grain analogues. We analyzed the results to extract quantitative information on the processes of molecule formation on and ejection from dust grain analogues. The usefulness of these data lies in the fact that these results have been employed by theoreticians in models of the chemical evolution of ISM environments
Formation of Molecular Hydrogen on Amorphous Water Ice: Influence of Morphology and Ultraviolet Exposure
In this paper, we report on the formation of molecular hydrogen on different types of amorphous water ice. We show that mass spectra of desorbing molecules upon formation are sensitive to the way in which ice is deposited on a cold substrate, to its thermal history, and to the action of UV photons. Implications that these results bear on H2 formation in dense quiescent clouds are presented and discussed
New Chandra observations of the jet in 3C273. I. Softer X-ray than radio spectra and the X-ray emission mechanism
The jet in 3C273 is a high-power quasar jet with radio, optical and X-ray
emission whose size and brightness allow a detailed study of the emission
processes acting in it. We present deep Chandra observations of this jet and
analyse the spectral properties of the jet emission from radio through X-rays.
We find that the X-ray spectra are significantly softer than the radio spectra
in all regions of the bright part of the jet except for the first bright "knot
A", ruling out a model in which the X-ray emission from the entire jet arises
from beamed inverse-Compton scattering of cosmic microwave background photons
in a single-zone jet flow. Within two-zone jet models, we find that a
synchrotron origin for the jet's X-rays requires fewer additional assumptions
than an inverse-Compton model, especially if velocity shear leads to efficient
particle acceleration in jet flows.Comment: 11 pages, 5 figures, emulateapj. Accepted by Ap
Measurement of the Kinetic Energy of Hydrogen Molecules Desorbing from Amorphous Water Ice
A hydrogen molecule that is formed on an interstellar grain might retain some of the 4.48 eV of energy that is released in the recombination reaction of two hydrogen atoms. We set up an experiment to measure the translational (kinetic) energy of hydrogen molecules after they are formed on and are ejected from the surface of an interstellar dust grain analog. Here we report the first measurements of the kinetic energy of molecular deuterium as it leaves the surface of an amorphous water sample. The astrophysical implications of such measurements are discussed
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