11,720 research outputs found
Supplanting crystallography or supplementing microscopy? A combined approach to the study of an enveloped virus
The recent advances in the resolution obtained by single-particle reconstructions from cryo-electron microscopy (cryo-EM) have led to an increase in studies that combine X-ray crystallographic results with those of electron microscopy (EM). Here, such a combination is described in the determination of the structure of an enveloped animal virus, Semliki Forest virus, at 9 Å resolution. The issues of model bias in determination of the structure, the definition of resolution in a single-particle reconstruction, the effect of the correction of the contrast-transfer function on the structure determined and the use of a high-resolution structure of a subunit in the interpretation of the structure of the complex are addressed
The genealogy of judgement: towards a deep history of academic freedom
The classical conception of academic freedom associated with Wilhelm von Humboldt and the rise of the modern university has a quite specific cultural foundation that centres on the controversial mental faculty of 'judgement'. This article traces the roots of 'judgement' back to the Protestant Reformation, through its heyday as the signature feature of German idealism, and to its gradual loss of salience as both a philosophical and a psychological concept. This trajectory has been accompanied by a general shrinking in the scope of academic freedom from the promulgation of world-views to the offering of expert opinion
SNAP 8 refractory boiler development program - Shell side hydraulic characteristics of a full scale SNAP 8 multiple tube model boiler Topical report no. 5
Shell side hydraulic characteristics of full-scale SNAP 8 multiple-tube model boiler over turbulent Reynolds number from 18,000 to 38,00
The initial conditions of stellar protocluster formation. II. A catalogue of starless and protostellar clumps embedded in IRDCs in the Galactic longitude range 15<l<55
We present a catalogue of starless and protostellar clumps associated with
infrared dark clouds (IRDCs) in a 40 degrees wide region of the inner Galactic
Plane (b<1). We have extracted the far-infrared (FIR) counterparts of 3493
IRDCs with known distance in the Galactic longitude range 15<l<55 and searched
for the young clumps using Hi-GAL, the survey of the Galactic Plane carried out
with the Herschel satellite. Each clump is identified as a compact source
detected at 160, 250 and 350 mum. The clumps have been classified as
protostellar or starless, based on their emission (or lack of emission) at 70
mum. We identify 1723 clumps, 1056 (61%) of which are protostellar and 667
(39%) starless. These clumps are found within 764 different IRDCs, 375 (49%) of
which are only associated with protostellar clumps, 178 (23%) only with
starless clumps, and 211 (28%) with both categories of clumps. The clumps have
a median mass of 250 M_sun and range up to >10^4$ M_sun in mass and up to 10^5
L_sun in luminosity. The mass-radius distribution shows that almost 30% of the
starless clumps identified in this survey could form high-mass stars, however
these massive clumps are confined in only ~4% of the IRDCs. Assuming a minimum
mass surface density threshold for the formation of high-mass stars, the
comparison of the numbers of massive starless clumps and those already
containing embedded sources suggests an upper limit lifetime for the starless
phase of 10^5 years for clumps with a mass M>500 M_sun.Comment: accepted for publication in MNRAS. Online catalogues available soon,
please contact the authors if intereste
Presupernova collapse models with improved weak-interaction rates
Improved values for stellar weak interaction rates have been recently
calculated based upon a large shell model diagonalization. Using these new
rates (for both beta decay and electron capture), we have examined the
presupernova evolution of massive stars in the range 15-40 Msun. Comparing our
new models with a standard set of presupernova models by Woosley and Weaver, we
find significantly larger values for the electron-to-baryon ratio Ye at the
onset of collapse and iron core masses reduced by approximately 0.1 Msun. The
inclusion of beta-decay accounts for roughly half of the revisions, while the
other half is a consequence of the improved nuclear physics. These changes will
have important consequences for nucleosynthesis and the supernova explosion
mechanism.Comment: 4 pages, 2 figure
A Survey of Coronal Cavity Density Profiles
Coronal cavities are common features of the solar corona that appear as darkened regions at the base of coronal helmet streamers in coronagraph images. Their darkened appearance indicates that they are regions of lowered density embedded within the comparatively higher density helmet streamer. Despite interfering projection effects of the surrounding helmet streamer (which we refer to as the cavity rim), Fuller et al. have shown that under certain conditions it is possible to use a Van de Hulst inversion of white-light polarized brightness (pB) data to calculate the electron density of both the cavity and cavity rim plasma. In this article, we apply minor modifications to the methods of Fuller et al. in order to improve the accuracy and versatility of the inversion process, and use the new methods to calculate density profiles for both the cavity and cavity rim in 24 cavity systems. We also examine trends in cavity morphology and how departures from the model geometry affect our density calculations. The density calculations reveal that in all 24 cases the cavity plasma has a flatter density profile than the plasma of the cavity rim, meaning that the cavity has a larger density depletion at low altitudes than it does at high altitudes. We find that the mean cavity density is over four times greater than that of a coronal hole at an altitude of 1.2 R_☉ and that every cavity in the sample is over twice as dense as a coronal hole at this altitude. Furthermore, we find that different cavity systems near solar maximum span a greater range in density at 1.2 R_☉ than do cavity systems near solar minimum, with a slight trend toward higher densities for systems nearer to solar maximum. Finally, we found no significant correlation of cavity density properties with cavity height—indeed, cavities show remarkably similar density depletions—except for the two smallest cavities that show significantly greater depletion
Neutrino-Neutrino Scattering and Matter-Enhanced Neutrino Flavor Transformation in Supernovae
We examine matter-enhanced neutrino flavor transformation
() in the region above the neutrino
sphere in Type II supernovae. Our treatment explicitly includes contributions
to the neutrino-propagation Hamiltonian from neutrino-neutrino forward
scattering. A proper inclusion of these contributions shows that they have a
completely negligible effect on the range of - vacuum
mass-squared difference, , and vacuum mixing angle, , or
equivalently , required for enhanced supernova shock re-heating.
When neutrino background effects are included, we find that -process
nucleosynthesis from neutrino-heated supernova ejecta remains a sensitive probe
of the mixing between a light and a with a
cosmologically significant mass. Neutrino-neutrino scattering contributions are
found to have a generally small effect on the
parameter region probed by -process nucleosynthesis. We point out that the
nonlinear effects of the neutrino background extend the range of sensitivity of
-process nucleosynthesis to smaller values of .Comment: 38 pages, tex, DOE/ER/40561-150-INT94-00-6
On Characterizing the Data Access Complexity of Programs
Technology trends will cause data movement to account for the majority of
energy expenditure and execution time on emerging computers. Therefore,
computational complexity will no longer be a sufficient metric for comparing
algorithms, and a fundamental characterization of data access complexity will
be increasingly important. The problem of developing lower bounds for data
access complexity has been modeled using the formalism of Hong & Kung's
red/blue pebble game for computational directed acyclic graphs (CDAGs).
However, previously developed approaches to lower bounds analysis for the
red/blue pebble game are very limited in effectiveness when applied to CDAGs of
real programs, with computations comprised of multiple sub-computations with
differing DAG structure. We address this problem by developing an approach for
effectively composing lower bounds based on graph decomposition. We also
develop a static analysis algorithm to derive the asymptotic data-access lower
bounds of programs, as a function of the problem size and cache size
Resonant Neutrino Spin-Flavor Precession and Supernova Nucleosynthesis and Dynamics
We discuss the effects of resonant spin-flavor precession (RSFP) of Majorana
neutrinos on heavy element nucleosynthesis in neutrino-heated supernova ejecta
and the dynamics of supernovae. In assessing the effects of RSFP, we explicitly
include matter-enhanced (MSW) resonant neutrino flavor conversion effects where
appropriate. We point out that for plausible ranges of neutrino magnetic
moments and proto-neutron star magnetic fields, spin-flavor conversion of
(or ) with a cosmologically significant mass (1--100 eV)
into a light could lead to an enhanced neutron excess in
neutrino-heated supernova ejecta. This could be beneficial for models of
-process nucleosynthesis associated with late-time neutrino-heated ejecta
from supernovae. Similar spin-flavor conversion of neutrinos at earlier epochs
could lead to an increased shock reheating rate and, concomitantly, a larger
supernova explosion energy. We show, however, that such increased neutrino
heating likely will be accompanied by an enhanced neutron excess which could
exacerbate the problem of the overproduction of the neutron number
nuclei in the supernova ejecta from this stage. In all of these scenarios, the
average energy will be increased over those predicted by supernova
models with no neutrino mixings. This may allow the SN1987a data to constrain
RSFP-based schemes.Comment: Latex file, 33 pages including 11 figures, uses psfig.sty, minor
changes about wording and clarification of the text, to be published in Phys.
Rev.
Weak Charge-Changing Flow in Expanding r-Process Environments
We assess the prospects for attaining steady nuclear flow equilibrium in
expanding r-process environments where beta decay and/or neutrino capture
determine the nuclear charge-changing rates. For very rapid expansions, we find
that weak steady flow equilibrium normally cannot be attained. However, even
when neutron capture processes freeze out in such nonequilibrium conditions,
abundance ratios of nuclear species in the r-process peaks might still mimic
those attained in weak steady flow. This result suggests that the r-process
yield in a regime of rapid expansion can be calculated reliably only when all
neutron capture, photodisintegration, and weak interaction processes are fully
coupled in a dynamical calculation. We discuss the implications of these
results for models of the r-process sited in rapidly expanding neutrino-heated
ejecta.Comment: 21 pages, AAS LaTex, 2 postscript figure
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