4,140 research outputs found
Nuclear composition and heating in accreting neutron-star crusts
Nuclear reactions in accreting neutron-star crusts and the heat release
accompanying them are studied, under different assumptions concerning the
composition of the outermost layer formed of the ashes of X-ray bursts.
Particular examples of ashes containing nuclides with A ~ 90-110 are considered
and compared with a standard A=56 case. In all cases, evolution of a crust
shell is followed from 10^8 g/cm^3 to a few times 10^{13} g/cm^3. The total
crustal heating produced in the non-equilibrium processes in the accreting
crust is 1.1-1.5 MeV per one accreted nucleon. The composition of the accreted
crust at densities exceeding the threshold for the pycnonuclear fusion is
essentially independent of the assumed initial composition of the X-ray burst
ashes.Comment: 5 pages, 2 figures, accepted for publication in A&A Letter
Wirespeed: Extending the AFF4 forensic container format for scalable acquisition and live analysis
AbstractCurrent approaches to forensic acquisition are failing to scale to large devices and fast storage interfaces. The research described in this paper identifies limitations in current widely deployed forensic image formats which limit both the ability to acquire evidence at maximal rates, and to undertake live analysis in today's environment. Extensions to the AFF4 forensic file format are proposed which address these limitations. The proposals have been implemented and proof of concept demonstrated by demonstrating that non-linear partial images may be taken at rates that exceed current physical acquisition approaches, and by demonstrating linear acquisition at rates significantly exceeding current approaches: in the range of 400 MB/s–500 MB/s (24–30 GB/min)
Models of crustal heating in accreting neutron stars
Heating associated with non-equilibrium nuclear reactions in accreting
neutron-star crusts is reconsidered, taking into account suppression of
neutrino losses demonstrated recently by Gupta et al. Two initial compositions
of the nuclear burning ashes, A=56 and A=106, are considered. Dependence of the
integrated crustal heating on uncertainties plaguing pycnonuclear reaction
models is studied.
One-component plasma approximation is used, with compressible liquid-drop
model of Mackie and Baym to describe nuclei. Evolution of a crust shell is
followed from 10^8 g/cm^3 to 10^(13.6) g/cm^3
The integrated heating in the outer crust agrees nicely with results of
self-considtent multicomponent plasma simulations of Gupta et al.; their
results fall between our curves obtained for A=56 and A=106. Total crustal heat
per one accreted nucleon ranges between 1.5 MeV to 1.9 MeV for A=106 and A=56,
respectively. The value of total crustal heat per nucleon depends weakly on the
presence of pycnonuclear reactions at densities 10^(12)-10^(13) g/cm^3.
Remarkable insensitivity of the total crustal heat on the details of the
distribution of nuclear processes in accreted crust is explained.Comment: 8 pages, 5 figures, Submitted to A&
AFM pulling and the folding of donor-acceptor oligorotaxanes: phenomenology and interpretation
The thermodynamic driving force in the self-assembly of the secondary
structure of a class of donor-acceptor oligorotaxanes is elucidated by means of
molecular dynamics simulations of equilibrium isometric single-molecule force
spectroscopy AFM experiments. The oligorotaxanes consist of
cyclobis(paraquat-\emph{p}-phenylene) rings threaded onto an oligomer of
1,5-dioxynaphthalenes linked by polyethers. The simulations are performed in a
high dielectric medium using MM3 as the force field. The resulting force vs.
extension isotherms show a mechanically unstable region in which the molecule
unfolds and, for selected extensions, blinks in the force measurements between
a high-force and a low-force regime. From the force vs. extension data the
molecular potential of mean force is reconstructed using the weighted histogram
analysis method and decomposed into energetic and entropic contributions. The
simulations indicate that the folding of the oligorotaxanes is energetically
favored but entropically penalized, with the energetic contributions overcoming
the entropy penalty and effectively driving the self-assembly. In addition, an
analogy between the single-molecule folding/unfolding events driven by the AFM
tip and the thermodynamic theory of first-order phase transitions is discussed
and general conditions, on the molecule and the cantilever, for the emergence
of mechanical instabilities and blinks in the force measurements in equilibrium
isometric pulling experiments are presented. In particular, it is shown that
the mechanical stability properties observed during the extension are
intimately related to the fluctuations in the force measurements.Comment: 42 pages, 17 figures, accepted to the Journal of Chemical Physic
Dependence of X-Ray Burst Models on Nuclear Reaction Rates
X-ray bursts are thermonuclear flashes on the surface of accreting neutron
stars and reliable burst models are needed to interpret observations in terms
of properties of the neutron star and the binary system. We investigate the
dependence of X-ray burst models on uncertainties in (p,),
(,), and (,p) nuclear reaction rates using fully
self-consistent burst models that account for the feedbacks between changes in
nuclear energy generation and changes in astrophysical conditions. A two-step
approach first identified sensitive nuclear reaction rates in a single-zone
model with ignition conditions chosen to match calculations with a
state-of-the-art 1D multi-zone model based on the {\Kepler} stellar evolution
code. All relevant reaction rates on neutron deficient isotopes up to mass 106
were individually varied by a factor of 100 up and down. Calculations of the 84
highest impact reaction rate changes were then repeated in the 1D multi-zone
model. We find a number of uncertain reaction rates that affect predictions of
light curves and burst ashes significantly. The results provide insights into
the nuclear processes that shape X-ray burst observables and guidance for
future nuclear physics work to reduce nuclear uncertainties in X-ray burst
models.Comment: 24 pages, 13 figures, 4 tables, submitte
Long-Wavelength Instability in Surface-Tension-Driven Benard Convection
Laboratory studies reveal a deformational instability that leads to a drained
region (dry spot) in an initially flat liquid layer (with a free upper surface)
heated uniformly from below. This long-wavelength instability supplants
hexagonal convection cells as the primary instability in viscous liquid layers
that are sufficiently thin or are in microgravity. The instability occurs at a
temperature gradient 34% smaller than predicted by linear stability theory.
Numerical simulations show a drained region qualitatively similar to that seen
in the experiment.Comment: 4 pages. The RevTeX file has a macro allowing various styles. The
appropriate style is "mypprint" which is the defaul
16GT: A fast and sensitive variant caller using a 16-genotype probabilistic model
© The Author 2017. Published by Oxford University Press. 16GT is a variant caller for Illumina whole-genome and whole-exome sequencing data. It uses a new 16-genotype probabilistic model to unify single nucleotide polymorphism and insertion and deletion calling in a single variant calling algorithm. In benchmark comparisons with 5 other widely used variant callers on a modern 36-core server, 16GT demonstrated improved sensitivity in calling single nucleotide polymorphisms, and it provided comparable sensitivity and accuracy for calling insertions and deletions as compared to the GATK HaplotypeCaller. 16GT is available at https://github.com/aquaskyline/16GT.Link_to_subscribed_fulltex
Models for Type I X-Ray Bursts with Improved Nuclear Physics
Multi-zone models of Type I X-ray bursts are presented that use an adaptive
nuclear reaction network of unprecedented size, up to 1300 isotopes. Sequences
of up to 15 bursts are followed for two choices of accretion rate and
metallicity. At 0.1 Eddington (and 0.02 Eddington for low metallicity),
combined hydrogen-helium flashes occur. The rise times, shapes, and tails of
these light curves are sensitive to the efficiency of nuclear burning at
various waiting points along the rp-process path and these sensitivities are
explored. The bursts show "compositional inertia", in that their properties
depend on the fact that accretion occurs onto the ashes of previous bursts
which contain left-over hydrogen, helium and CNO nuclei. This acts to reduce
the sensitivity of burst properties to metallicity. For the accretion rates
studied, only the first anomalous burst in one model produces nuclei as heavy
as A=100, other bursts make chiefly nuclei with A~64. The amount of carbon
remaining after hydrogen-helium bursts is typically <1% by mass, and decreases
further as the ashes are periodically heated by subsequent bursts. At the lower
accretion rate of 0.02 Eddington and solar metallicity, the bursts ignite in a
hydrogen-free helium layer. At the base of this layer, up to 90% of the helium
has already burned to carbon prior to the unstable ignition. These
helium-ignited bursts have briefer, brighter light curves with shorter tails,
very rapid rise times (<0.1 s), and ashes lighter than the iron group.Comment: Submitted to the Astrophysical Journal (42 pages; 27 figures
On the linear response and scattering of an interacting molecule-metal system
A many-body Green's function approach to the microscopic theory of
plasmon-enhanced spectroscopy is presented within the context of localized
surface-plasmon resonance spectroscopy and applied to investigate the coupling
between quantum-molecular and classical-plasmonic resonances in
monolayer-coated silver nanoparticles. Electronic propagators or Green's
functions, accounting for the repeated polarization interaction between a
single molecule and its image in a nearby nanoscale metal, are explicitly
computed and used to construct the linear-response properties of the combined
molecule-metal system to an external electromagnetic perturbation. Shifting and
finite lifetime of states appear rigorously and automatically within our
approach and reveal an intricate coupling between molecule and metal not fully
described by previous theories. Self-consistent incorporation of this
quantum-molecular response into the continuum-electromagnetic scattering of the
molecule-metal target is exploited to compute the localized surface-plasmon
resonance wavelength shift with respect to the bare metal from first
principles.Comment: under review at Journal of Chemical Physic
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