4,472 research outputs found
Critical evaluation and compilation of viscosity and diffusivity data Semiannual status report, 1 Nov. 1967 - 30 Apr. 1968
Data logging and retrieval system for evaluating molecular diffusion coefficients of binary liquid system
Current Approaches to Improving the Value of Care: A Physician's Perspective
Evaluates the utility of judgment-based approaches to quality improvement -- pay-for-performance, public reporting, consumer-directed health plans, and tiering -- as ways to control costs. Recommends incentive- and accountability-based programs
Coming From Good Stock: Career Histories and New Venture Formation
We examine how the social structure of existing organizations influences entrepreneurship and suggest that resources accrue to entrepreneurs based on the structural position of their prior employers. We argue that information advantages allow individuals from entrepreneurially prominent prior firms to identify new opportunities. Entrepreneurial prominence also reduces the perceived uncertainty of a new venture. Using a sample of Silicon Valley start-ups, we demonstrate that entrepreneurial prominence is associated with initial strategy and the probability of attracting external financing. New ventures with high prominence are more likely to be innovators; furthermore, innovators with high prominence are more likely to obtain financing
Properties of Reactive Oxygen Species by Quantum Monte Carlo
The electronic properties of the oxygen molecule, in its singlet and triplet
states, and of many small oxygen-containing radicals and anions have important
roles in different fields of Chemistry, Biology and Atmospheric Science.
Nevertheless, the electronic structure of such species is a challenge for
ab-initio computational approaches because of the difficulties to correctly
describe the statical and dynamical correlation effects in presence of one or
more unpaired electrons. Only the highest-level quantum chemical approaches can
yield reliable characterizations of their molecular properties, such as binding
energies, equilibrium structures, molecular vibrations, charge distribution and
polarizabilities. In this work we use the variational Monte Carlo (VMC) and the
lattice regularized Monte Carlo (LRDMC) methods to investigate the equilibrium
geometries and molecular properties of oxygen and oxygen reactive species.
Quantum Monte Carlo methods are used in combination with the Jastrow
Antisymmetrized Geminal Power (JAGP) wave function ansatz, which has been
recently shown to effectively describe the statical and dynamical correlation
of different molecular systems. In particular we have studied the oxygen
molecule, the superoxide anion, the nitric oxide radical and anion, the
hydroxyl and hydroperoxyl radicals and their corresponding anions, and the
hydrotrioxyl radical. Overall, the methodology was able to correctly describe
the geometrical and electronic properties of these systems, through compact but
fully-optimised basis sets and with a computational cost which scales as
, where is the number of electrons. This work is therefore opening
the way to the accurate study of the energetics and of the reactivity of large
and complex oxygen species by first principles
An Empirical Relation Between The Large-Scale Magnetic Field And The Dynamical Mass In Galaxies
The origin and evolution of cosmic magnetic fields as well as the influence
of the magnetic fields on the evolution of galaxies are unknown. Though not
without challenges, the dynamo theory can explain the large-scale coherent
magnetic fields which govern galaxies, but observational evidence for the
theory is so far very scarce. Putting together the available data of
non-interacting, non-cluster galaxies with known large-scale magnetic fields,
we find a tight correlation between the integrated polarized flux density,
S(PI), and the rotation speed, v(rot), of galaxies. This leads to an almost
linear correlation between the large-scale magnetic field B and v(rot),
assuming that the number of cosmic ray electrons is proportional to the star
formation rate, and a super-linear correlation assuming equipartition between
magnetic fields and cosmic rays. This correlation cannot be attributed to an
active linear alpha-Omega dynamo, as no correlation holds with global shear or
angular speed. It indicates instead a coupling between the large-scale magnetic
field and the dynamical mass of the galaxies, B ~ M^(0.25-0.4). Hence, faster
rotating and/or more massive galaxies have stronger large-scale magnetic
fields. The observed B-v(rot) correlation shows that the anisotropic turbulent
magnetic field dominates B in fast rotating galaxies as the turbulent magnetic
field, coupled with gas, is enhanced and ordered due to the strong gas
compression and/or local shear in these systems. This study supports an
stationary condition for the large-scale magnetic field as long as the
dynamical mass of galaxies is constant.Comment: 23 pages, 4 figures, accepted for publication in the Astrophysical
Journal Letter
The Kinematics of the Ionized and Molecular Hydrogen in the Starburst Galaxy NGC 253
Near-infrared H_2 1-0 S(1) and Br_gamma velocity curves along the major axis
of NGC 253 have revealed a central velocity gradient that is seven times
steeper than that shown by the optical velocity curve. This is interpreted as
an optical depth effect due to dust. Approximately 35 mag of visual extinction
in the center is required to match the SW side of the optical velocity curve.
The spatial variation of the ratio of these lines to the CO (J=1-0) line is
compared among starburst galaxies NGC 253, M82, and NGC 4945 to investigate the
excitation mechanism responsible for the H_2 1-0 S(1) line.Comment: Uuencoded postscript file, 10 pages (4 tables included), 8 figures
available on request to [email protected], Ap.J. (in press
Recommended from our members
Observations of the J = 2→1 transitions of <sup>12</sup>C<sup>16</sup>O and <sup>12</sup>C<sup>18</sup>O towards galactic H II regions
Observations are reported of the J = 2→1 transitions of CO and 12C18O at 230 and 219 GHz respectively from a number of galactic sources. A map of the central 1/2° × 1/2° of the Orion A molecular cloud is presented. The spectra are interpreted to derive molecular densities and abundance ratios in the molecular clouds observed
On the Environmental Dependence of Cluster Galaxy Assembly Timescale
We present estimates of CN and Mg overabundances with respect to Fe for
early-type galaxies in 8 clusters over a range of richness and morphology.
Spectra were taken from the Sloan Digital Sky Survey (SDSS) DR1, and from WHT
and CAHA observations. Abundances were derived from absorption lines and single
burst population models, by comparing galaxy spectra with appropriately
broadened synthetic model spectra. We detect correlations between [Mg/CN] and
[CN/Fe] and cluster X-ray luminosity. No correlation is observed for [Mg/Fe].
We also see a clear trend with the richness and morphology of the clusters.
This is interpreted given varying formation timescales for CN, Mg and Fe, and a
varying star formation history in early-type galaxies as a function of their
environment: intermediate-mass early-type galaxies in more massive clusters are
assembled on shorter timescales than in less massive clusters, with an upper
limit of ~1 Gyr.Comment: Accepted for publication in ApJ Letter
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