15,696 research outputs found
A density functional perspective for one-particle systems
Density functional theory is discussed in the context of one-particle
systems. We show that the ground state density and energy are
simply related to a family of external potential energy functions with ground
state wave functions and energies
for certain integer values of .Comment: 7 pages, ReVTeX4, submitted to Am. J. Phy
Vibration modes of an orbiting radio telescope
Vibration mode shape and frequencies of orbiting radio telescop
The flutter of towed rigid decelerators
Flutter of towed rigid decelerators, and relationships between geometric and aerodynamic parameters defining unconditional stability boundar
Photoionization, Numerical Resolution, and Galaxy Formation
Using cosmological simulations that incorporate gas dynamics and
gravitational forces, we investigate the influence of photoionization by a UV
radiation background on the formation of galaxies. In our highest resolution
simulations, we find that photoionization has essentially no effect on the
baryonic mass function of galaxies at , down to our resolution limit of
5e9 M_\sun. We do, however, find a strong interplay between the mass
resolution of a simulation and the microphysics included in the computation of
heating and cooling rates. At low resolution, a photoionizing background can
appear to suppress the formation of even relatively massive galaxies. However,
when the same initial conditions are evolved with a factor of eight better mass
resolution, this effect disappears. Our results demonstrate the need for care
in interpreting the results of cosmological simulations that incorporate
hydrodynamics and radiation physics. For example, we conclude that a simulation
with limited resolution may yield more realistic results if it ignores some
relevant physical processes, such as photoionization. At higher resolution, the
simulated population of massive galaxies is insensitive to the treatment of
photoionization and star formation, but it does depend significantly on the
amplitude of the initial density fluctuations. By , an cold
dark matter model normalized to produce the observed masses of present-day
clusters has already formed galaxies with baryon masses exceeding 1e11
M_\sun.Comment: 25 pages, w/ embedded figures. Submitted to ApJ. Also available at
http://www-astronomy.mps.ohio-state.edu/~dhw/Docs/preprints.htm
Documenting Food Safety Claims and Their Influence on Product Prices
In response to increasing customer attention to food attributes, agribusinesses are employing novel product differentiation strategies. As an example, we investigate the use of food safety claims on new packaged food products from the food manufacturers’ perspective. First, using two product innovation databases, we investigate claim use on labels in seven English-speaking countries over the period from 1980 to 2008. Then, based on manufacturer recommended selling prices and using U.S. data (from 2002 to 2008), we apply parametric and nonparametric hedonic methods to identify supply-side (agribusiness) valuations of chemical and microbiological claims in two food categories. We identify a significant 5 cent premium per ounce for a “preservative free†claim in spoonable yogurts. We do not find a statistically significant impact for “E. coli free†messages on meat and poultry products but find a significant price premium (19.3 cents and 25.7 cents per ounce in the two models) for “antibiotic free†claims in this category.food safety claims, parametric and nonparametric hedonic models, price premium, Marketing,
Cosmological Simulations with TreeSPH
We describe numerical methods for incorporating gas dynamics into
cosmological simulations and present illustrative applications to the cold dark
matter (CDM) scenario. Our evolution code, a version of TreeSPH (Hernquist \&
Katz 1989) generalized to handle comoving coordinates and periodic boundary
conditions, combines smoothed--particle hydrodynamics (SPH) with the
hierarchical tree method for computing gravitational forces. The Lagrangian
hydrodynamics approach and individual time steps for gas particles give the
algorithm a large dynamic range, which is essential for studies of galaxy
formation in a cosmological context. The code incorporates radiative cooling
for an optically thin, primordial composition gas in ionization equilibrium
with a user-specified ultraviolet background. We adopt a phenomenological
prescription for star formation that gradually turns cold, dense,
Jeans-unstable gas into collisionless stars, returning supernova feedback
energy to the surrounding medium. In CDM simulations, some of the baryons that
fall into dark matter potential wells dissipate their acquired thermal energy
and condense into clumps with roughly galactic masses. The resulting galaxy
population is insensitive to assumptions about star formation; we obtain
similar baryonic mass functions and galaxy correlation functions from
simulations with star formation and from simulations without star formation in
which we identify galaxies directly from the cold, dense gas.Comment: compressed postscript, 38 pages including 6 out of 7 embedded
figures. Submitted to ApJ Supplements. Version with all 7 figures available
from ftp://bessel.mps.ohio-state.edu/pub/dhw/Preprint
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