1,601 research outputs found
Molecular Realism in Default Models for Information Theories of Hydrophobic Effects
This letter considers several physical arguments about contributions to
hydrophobic hydration of inert gases, constructs default models to test them
within information theories, and gives information theory predictions using
those default models with moment information drawn from simulation of liquid
water. Tested physical features include: packing or steric effects, the role of
attractive forces that lower the solvent pressure, and the roughly tetrahedral
coordination of water molecules in liquid water. Packing effects (hard sphere
default model) and packing effects plus attractive forces (Lennard-Jones
default model) are ineffective in improving the prediction of hydrophobic
hydration free energies of inert gases over the previously used Gibbs and flat
default models. However, a conceptually simple cluster Poisson model that
incorporates tetrahedral coordination structure in the default model is one of
the better performers for these predictions. These results provide a partial
rationalization of the remarkable performance of the flat default model with
two moments in previous applications. The cluster Poisson default model thus
will be the subject of further refinement.Comment: 5 pages including 3 figure
Kinetics and mechanism of proton transport across membrane nanopores
We use computer simulations to study the kinetics and mechanism of proton
passage through a narrow-pore carbon-nanotube membrane separating reservoirs of
liquid water. Free energy and rate constant calculations show that protons move
across the membrane diffusively in single-file chains of hydrogen-bonded water
molecules. Proton passage through the membrane is opposed by a high barrier
along the effective potential, reflecting the large electrostatic penalty for
desolvation and reminiscent of charge exclusion in biological water channels.
At neutral pH, we estimate a translocation rate of about 1 proton per hour and
tube.Comment: 4 pages, 4 figure
Optical properties monitor: Experiment definition phase
The stability of materials used in the space environment will continue to be a limiting technology for space missions. The Optical Properties Monitor (OPM) Experiment provides a comprehensive space research program to study the effects of the space environment-both natural and induced-on optical, thermal and space power materials. The OPM Experiment was selected for definition under the NASA/OAST In-Space Technology Experiment Program. The results of the OPM Definition Phase are presented. The OPM Experiment will expose selected materials to the space environment and measure the effects with in-space optical measurements. In-space measurements include total hemispherical reflectance total integrated scatter and VUV reflectance/transmittance. The in-space measurements will be augmented with extensive pre- and post-flight sample measurements to determine other optical, mechanical, electrical, chemical or surface effects of space exposure. Environmental monitors will provide the amount and time history of the sample exposure to solar irradiation, atomic oxygen and molecular contamination
The first detection of near-infrared CN bands in active galactic nuclei: signature of star formation
We present the first detection of the near-infrared CN absorption band in the
nuclear spectra of active galactic nuclei (AGN). This feature is a recent star
formation tracer, being particularly strong in carbon stars. The equivalent
width of the CN line correlates with that of the CO at 2.3 microns, as expected
in stellar populations (SP) with ages between ~ 0.2 and ~ 2 Gyr. The presence
of the 1.1 microns CN band in the spectra of the sources is taken as an
unambiguous evidence of the presence of young/intermediate SP close to the
central source of the AGN. Near-infrared bands can be powerful age indicators
for star formation connected to AGN, the understanding of which is crucial in
the context of galaxy formation and AGN feedback.Comment: Accepted for publication in The Astrophysical Journal Letters. 4
pages, 3 figure
Time-Dependent Corrections to the Ly-alpha Escape Probability During Cosmological Hydrogen Recombination
We consider the effects connected with the detailed radiative transfer during
the epoch of cosmological recombination on the ionization history of our
Universe. We focus on the escape of photons from the hydrogen Lyman-alpha
resonance at redshifts 600<~ z <~ 2000, one of two key mechanisms defining the
rate of cosmological recombination. We approach this problem within the
standard formulation, and corrections due to two-photon interactions are
deferred to another paper. As a main result we show here that within a
non-stationary approach to the escape problem, the resulting correction in the
free electron fraction, N_e, is about ~1.6-1.8% in the redshift range
800<~z<~1200. Therefore the discussed process results in one of the largest
modifications to the ionization history close to the maximum of
Thomson-visibility function at z~1100 considered so far. We prove our results
both numerically and analytically, deriving the escape probability, and
considering both Lyman-alpha line emission and line absorption in a way
different from the Sobolev approximation. In particular, we give a detailed
derivation of the Sobolev escape probability during hydrogen recombination, and
explain the underlying assumptions. We then discuss the escape of photons for
the case of coherent scattering in the lab frame, solving this problem
analytically in the quasi-stationary approximation and also in the
time-dependent case. We show here that during hydrogen recombination the
Sobolev approximation for the escape probability is not valid at the level of
DP/P~5-10%. This is because during recombination the ionization degree changes
significantly over a characteristic time Dz/z~10%, so that at percent level
accuracy the photon distribution is not evolving (abridged)Comment: 18 pages, 12 figures, accepted versio
Electron-Ion Recombination Rate Coefficients and Photoionization Cross Sections for Astrophysically Abundant Elements. V. Relativistic calculations for Fe XXIV and Fe XXV for X-ray modeling
Photoionization and recombination cross sections and rate coefficients are
calculated for Li-like Fe XXIV and He-like Fe XXV using the Breit-Pauli
R-matrix (BPRM) method. A complete set of total and level-specific parameters
is obtained to enable X-ray photoionization and spectral modeling. The ab
initio calculations for the unified (e + ion) recombination rate coefficients
include both the non-resonant and the resonant recombination (radiative and
di-electronic recombination, RR and DR, respectively) for (e + Fe XXV) -> Fe
XXIV and (e + Fe XXVI) -> Fe XXV. The level specific rates are computed for all
fine structure levels up to n = 10, enabling accurate computation of
recombination-cascade matrices and effective rates for the X-ray lines. The
total recombination rate coefficients for both Fe XXIV and Fe XXV differ
considerably, by several factors, from the sum of RR and DR rates currently
used to compute ionization fractions in astrophysical models. As the
photoionization/recombination calculations are carried out using an identical
eigenfunction expansion, the cross sections for both processes are
theoretically self-consistent; the overall uncertainty is estimated to be about
10-20%. All data for Fe XXIV and Fe XXV (and also for H-like Fe XXVI, included
for completeness) are available electronically.Comment: 31 pages, 10fug
Examining the Relationships Between Acculturation Orientations, Perceived and Actual Norms, and Drinking Behaviors of Short-Term American Sojourners in Foreign Environments
As little research has examined factors influencing increased and heavy drinking behavior among American sojourners abroad, this study was designed to examine how acculturation orientations (i.e., separation versus assimilation), host country per capita drinking rates, and perceptions about the drinking behavior among other sojourners and natives in the host country predicted alcohol risk abroad. A sample of 216 American college students completing study abroad programs completed a pre-abroad questionnaire to document their pre-abroad drinking levels, followed by a post-return questionnaire to assess drinking while abroad, acculturation orientations and perceived norms of drinking behavior within the foreign environment. A dichotomous variable was created to compare United States (U.S.) per capita drinking rates with those of the host country. Hierarchical repeated-measures ANOVAs examined the changes in drinking from pre-abroad to abroad levels. Participants studying in countries with higher drinking rates than the U.S. and those with higher perceptions about the drinking behavior in the country increased their drinking to a greater extent. Those with higher separation acculturation orientations and greater perceptions drank at heavier levels while abroad. Participants with a greater assimilation orientation and higher perceptions about native drinking, as well as those with a greater separation orientation and higher perceptions about other students’ alcohol use drank the heaviest while abroad. These findings have implications for future preventive work with American students and other sojourning groups to promote pre-abroad knowledge of more accurate drinking norms and greater engagement in the culture to potentially prevent increased and heavier drinking
High Resolution Infrared Imaging and Spectroscopy of the Pistol Nebula: Evidence for Ejection
We present new NICMOS/HST infrared images and CGS4/UKIRT Br-alpha (4.05 um)
spectroscopy of the Pistol Star and its associated nebula, finding strong
evidence to support the hypothesis that the Pistol Nebula was ejected from the
Pistol Star. The Pa-alpha NICMOS image shows that the nebula completely
surrounds the Pistol Star, although the line intensity is much stronger on its
northern and western edges. The Br-alpha spectra show the classical ring-like
signature of quasi-spherical expansion, with weak blueshifted emission (V_max
approx -60 km/s) and strong redshifted emission (V_max approx +10 km/s), where
the velocities are with respect to the velocity of the Pistol Star; further,
the redshifted emission appears to be "flattened" in the position-velocity
diagram. These data suggest that the nebula was ejected from the star several
thousand years ago, with a velocity between the current terminal velocity of
the stellar wind (95 km/s) and the present expansion velocity of gas in the
outer shell of the nebula (60 km/s). The Pa-alpha image reveals several
emission-line stars in the region, including two newly-identified emission-line
stars north of the Pistol Star with spectral types earlier than WC8 (T_eff >
50,000 K). The presence of these stars, the morphology of the Pa-alpha
emission, and the velocity field in the gas suggest that the side of the nebula
furthest from us is approaching, and being ionized by, the hot stars of the
Quintuplet, and that the highest velocity redshifted gas has been decelerated
by winds from the Quintuplet stars. We also discuss the possibility that the
nebular gas might be magnetically confined by the ambient magnetic field
delineated by the nearby nonthermal filaments.Comment: Figure 1 is included as a JPG file. Figure 1 and 2 also available at
ftp://quintup.astro.ucla.edu/pistol2
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