13,956 research outputs found
Educational television in Bophutatswana: Documentation and social assessment of the 'Edutel' project
Path integral evaluation of equilibrium isotope effects
A general and rigorous methodology to compute the quantum equilibrium isotope
effect is described. Unlike standard approaches, ours does not assume
separability of rotational and vibrational motions and does not make the
harmonic approximation for vibrations or rigid rotor approximation for the
rotations. In particular, zero point energy and anharmonicity effects are
described correctly quantum mechanically. The approach is based on the
thermodynamic integration with respect to the mass of isotopes and on the
Feynman path integral representation of the partition function. An efficient
estimator for the derivative of free energy is used whose statistical error is
independent of the number of imaginary time slices in the path integral,
speeding up calculations by a factor of 60 at 500 K. We describe the
implementation of the methodology in the molecular dynamics package Amber 10.
The method is tested on three [1,5] sigmatropic hydrogen shift reactions.
Because of the computational expense, we use ab initio potentials to evaluate
the equilibrium isotope effects within the harmonic approximation, and then the
path integral method together with semiempirical potentials to evaluate the
anharmonicity corrections. Our calculations show that the anharmonicity effects
amount up to 30% of the symmetry reduced reaction free energy. The numerical
results are compared with recent experiments of Doering and coworkers,
confirming the accuracy of the most recent measurement on
2,4,6,7,9-pentamethyl-5-(5,5-H)methylene-11,11a-dihydro-12H-naphthacene
as well as concerns about compromised accuracy, due to side reactions, of
another measurement on
2-methyl-10-(10,10-H)methylenebicyclo[4.4.0]dec-1-ene.Comment: 14 pages, 8 figures, 6 table
Abdominal intercostal hernia: a rare complication after blunt trauma.
Abdominal intercostal hernia (AIH) is uncommonly reported in the literature with only 20 cases reported to date.1â3 We report a case of a delayed incarcerated AIH secondary to blunt trauma from a motor vehicle accident in which the colon and diaphragm herniated through an associated chest defect that was repaired successfully through a transabdominal approach using primary repair of the defect in combination with onlay porcine patch reinforcement
A Simple Model for r-Process Scatter and Halo Evolution
Recent observations of heavy elements produced by rapid neutron capture
(r-process) in the halo have shown a striking and unexpected behavior: within a
single star, the relative abundances of r-process elements heavier than Eu are
the same as the same as those of solar system matter, while across stars with
similar metallicity Fe/H, the r/Fe ratio varies over two orders of magnitude.
In this paper we present a simple analytic model which describes a star's
abundances in terms of its ``ancestry,'' i.e., the number of nucleosynthesis
events (e.g., supernova explosions) which contributed to the star's
composition. This model leads to a very simple analytic expression for the
abundance scatter versus Fe/H, which is in good agreement with the data and
with more sophisticated numerical models. We investigate two classes of
scenarios for r-process nucleosynthesis, one in which r-process synthesis
events occur in only \sim 4% of supernovae but iron synthesis is ubiquitous,
and one in which iron nucleosynthesis occurs in only about 9% of supernovae.
(the Wasserburg- Qian model). We find that the predictions in these scenarios
are similar for [Fe/H] \ga -2.5, but that these models can be readily
distinguished observationally by measuring the dispersion in r/Fe at [Fe/H] \la
-3.Comment: AASTeX, 21 pages, includes 4 figure
Chevalier Jackson, M.D. (1865-1958): Il ne se repose jamais.
In the final year of the American Civil War, 1865, Chevalier Jackson was born on the 4th of November just outside Pittsburgh, Pennsylvania. The eldest of three sons of a poor, livestock-raising family, Jackson was raised in a period of social and political unrest. He was perhaps an even more unrestful boy. The description of his childhood days from his fatherâs fatherâIl ne se repose jamais, ââHe never restsâââwould ultimately reflect the man, doctor, and evangelist Jackson would later become.1 Indeed, he never did rest, Jackson would tirelessly pave the way for modern bronchoscopy and endoscopy as a whole; bringing international renown not only to himself, but also to his specialty
Emil Zuckerkandl, M.D. (1849-1910): Bridging Anatomic Study and the Operating Room Table.
In the mid-19th century, the Vienna School of Anatomy was at the epicenter of the rapidly growing field of anatomy. One of the schoolâs most distinguished professors, Hungarian-born anatomist Emil Zuckerkandl was instrumental in transforming anatomy from a descriptive science to one of practical and clinical value. A prolific researcher interested in nearly all areas of morphology and most famously, the chromaffin system, Zuckerkandlâs discoveries from more than a century ago still provide a foundation for surgeons to this day
Near-UV Observations of CS29497-030: New Constraints on Neutron-Capture Nucleosynthesis Processes
Employing spectra obtained with the new Keck I HIRES near-UV sensitive
detector, we have performed a comprehensive chemical composition analysis of
the binary blue metal-poor star CS29497-030. Abundances for 29 elements and
upper limits for an additional seven have been derived, concentrating on
elements largely produced via neutron-capture nucleosynthesis. Included in our
analysis are the two elements that define the termination point of the slow
neutron-capture process, lead and bismuth. We determine an extremely high value
of [Pb/Fe] = +3.65 +/- 0.07 (sigma = 0.13) from three features, supporting the
single-feature result obtained in previous studies. We also detect Bi for the
first time in a metal-poor star. Our derived Bi/Pb ratio is in accord with
those predicted from the most recent FRANEC calculations of the slow
neutron-capture process in low-mass AGB stars. We find that the neutron-capture
elemental abundances of CS29497-030 are best explained by an AGB model that
also includes very significant amounts of pre-enrichment of rapid
neutron-capture process material in the protostellar cloud out of which the
CS29497-030 binary system formed. Thus, CS29497-030 is both an ``r+s'' and
``extrinsic AGB'' star. Furthermore, we find that the mass of the AGB model can
be further constrained by the abundance of the light odd-element [Na/Fe] which
is sensitive to the neutron excess.Comment: 7 pages = 4 + 2 colour encapsulated postscript figures + 1 table; to
appear in ApJ Letters; additional jpeg figure available at
ftp://www.astro.caltech.edu/users/iii/cs2949703
Non-resonant inelastic x-ray scattering involving excitonic excitations
In a recent publication Larson \textit{et al.} reported remarkably clear
- excitations for NiO and CoO measured with x-ray energies well below the
transition metal edge. In this letter we demonstrate that we can obtain an
accurate quantitative description based on a local many body approach. We find
that the magnitude of can be tuned for maximum sensitivity for
dipole, quadrupole, etc. excitations. We also find that the direction of
with respect to the crystal axes can be used as an equivalent to
polarization similar to electron energy loss spectroscopy, allowing for a
determination of the local symmetry of the initial and final state based on
selection rules. This method is more generally applicable and combined with the
high resolution available, could be a powerful tool for the study of local
distortions and symmetries in transition metal compounds including also buried
interfaces
- âŠ