2,573 research outputs found
The effects of a background potential in star cluster evolution: a delay in the relaxation time-scale and runaway collision processes
Runaway stellar collisions in dense star clusters are invoked to explain the
presence of very massive stars or blue stragglers in the center of those
systems. This process has also been explored for the first star clusters in the
Universe and shown to yield stars that may collapse at some points into an
intermediate mass black hole. Although the early evolution of star clusters
requires the explicit modeling of the gas out of which the stars form, these
calculations would be extremely time-consuming and often the effects of the gas
can be accurately treated by including a background potential to account for
the extra gravitational force. We apply this approximation to model the early
evolution of the first dense star clusters formed in the Universe by performing
-body simulations, our goal is to understand how the additional
gravitational force affects the growth of a very massive star through stellar
mergers in the central parts of the star cluster. Our results show that the
background potential increases the velocities of the stars, causing an overall
delay in the evolution of the clusters and in the runaway growth of a massive
star at the center. The population of binary stars is lower due to the
increased kinetic energy of the stars, initially reducing the number of stellar
collisions, and we show that relaxation processes are also affected. Despite
these effects, the external potential enhances the mass of the merger product
by a factor 2 if the collisions are maintained for long times.Comment: 16 pages. Accepted for publication in Astronomy and Astrophysic
Control of defect-mediated tunneling barrier heights in ultrathin MgO films
The impact of oxygen vacancies on local tunneling properties across
rf-sputtered MgO thin films was investigated by optical absorption spectroscopy
and conducting atomic force microscopy. Adding O to the Ar plasma during
MgO growth alters the oxygen defect populations, leading to improved local
tunneling characteristics such as a lower density of current hotspots and a
lower tunnel current amplitude. We discuss a defect-based potential landscape
across ultrathin MgO barriers.Comment: 4 pages, 4 figure
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Evaluation of US demo helium-cooled blanket options
A He-V-Li blanket design was developed as a candidate for the U.S. fusion demonstration power plant. This paper presents an 18 MPa helium-cooled, lithium breeder, V-alloy design that can be coupled to the Brayton cycle with a gross efficiency of 46%. The critical issue of designing to high gas pressure and the compatibility between helium impurities and V-alloy are addressed
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Modular High Temperature Gas-Cooled Reactor heat source for coal conversion
In the industrial nations, transportable fuels in the form of natural gas and petroleum derivatives constitute a primary energy source nearly equivalent to that consumed for generating electric power. Nations with large coal deposits have the option of coal conversion to meet their transportable fuel demands. But these processes themselves consume huge amounts of energy and produce undesirable combustion by-products. Therefore, this represents a major opportunity to apply nuclear energy for both the environmental and energy conservation reasons. Because the most desirable coal conversion processes take place at 800[degree]C or higher, only the High Temperature Gas-Cooled Reactors (HTGRs) have the potential to be adapted to coal conversion processes. This report provides a discussion of this utilization of HTGR reactors
Stellar collisions in flattened and rotating Pop. III star clusters
Fragmentation often occurs in disk-like structures, both in the early
Universe and in the context of present-day star formation. Supermassive black
holes (SMBHs) are astrophysical objects whose origin is not well understood;
they weigh millions of solar masses and reside in the centers of galaxies. An
important formation scenario for SMBHs is based on collisions and mergers of
stars in a massive cluster, in which the most massive star moves to the center
of the cluster due to dynamical friction. This increases the rate of collisions
and mergers since massive stars have larger collisional cross sections. This
can lead to runaway growth of a very massive star which may collapse to become
an intermediate-mass black hole. Here we investigate the dynamical evolution of
Miyamoto-Nagai models that allow us to describe dense stellar clusters,
including flattening and different degrees of rotation. We find that the
collisions in these clusters depend mostly on the number of stars and the
initial stellar radii for a given radial size of the cluster. By comparison,
rotation seems to affect the collision rate by at most . For flatness, we
compared spherical models with systems that have a scale height of about
of their radial extent, in this case finding a change in the collision rate of
less than . Overall, we conclude that the parameters only have a minor
effect on the number of collisions. Our results also suggest that rotation
helps to retain more stars in the system, reducing the number of escapers by a
factor of depending on the model and the specific realization. After two
million years, a typical lifetime of a very massive star, we find that about
collisions occur in typical models with ,
and a half-mass radius of , leading to a mass of about
for the most massive object.Comment: 10 pages, 7 figure
Increased glycation and oxidative damage to apolipoprotein B100 of LDL cholesterol in patients with type 2 diabetes and effect of metformin
OBJECTIVE The aim of this study was to investigate whether apolipoprotein B100 of LDL suffers increased damage by glycation, oxidation, and nitration in patients with type 2 diabetes, including patients receiving metformin therapy.
RESEARCH DESIGN AND METHODS For this study, 32 type 2 diabetic patients and 21 healthy control subjects were recruited; 13 diabetic patients were receiving metformin therapy (median dose: 1.50 g/day). LDL was isolated from venous plasma by ultracentrifugation, delipidated, digested, and analyzed for protein glycation, oxidation, and nitration adducts by stable isotopic dilution analysis tandem mass spectrometry.
RESULTS Advanced glycation end product (AGE) content of apolipoprotein B100 of LDL from type 2 diabetic patients was higher than from healthy subjects: arginine-derived AGE, 15.8 vs. 5.3 mol% (P < 0.001); and lysine-derived AGE, 2.5 vs. 1.5 mol% (P < 0.05). Oxidative damage, mainly methionine sulfoxide residues, was also increased: 2.5 vs. 1.1 molar equivalents (P < 0.001). 3-Nitrotyrosine content was decreased: 0.04 vs. 0.12 mol% (P < 0.05). In diabetic patients receiving metformin therapy, arginine-derived AGE and methionine sulfoxide were lower than in patients not receiving metformin: 19.3 vs. 8.9 mol% (P < 0.01) and 2.9 vs. 1.9 mol% (P < 0.05), respectively; 3-nitrotyrosine content was higher: 0.10 vs. 0.03 mol% (P < 0.05). Fructosyl-lysine residue content correlated positively with fasting plasma glucose. Arginine-derived AGE residue contents were intercorrelated and also correlated positively with methionine sulfoxide.
CONCLUSIONS Patients with type 2 diabetes had increased arginine-derived AGEs and oxidative damage in apolipoprotein B100 of LDL. This was lower in patients receiving metformin therapy, which may contribute to decreased oxidative damage, atherogenicity, and cardiovascular disease
Nonlinear closures for scale separation in supersonic magnetohydrodynamic turbulence
This is the final version. Available on open access from IOP Publishing via the DOI in this recordTurbulence in compressible plasma plays a key role in many areas of astrophysics and engineering. The extreme plasma parameters in these environments, e.g. high Reynolds numbers, supersonic and super-Alfvenic flows, however, make direct numerical simulations computationally intractable even for the simplest treatment - magnetohydrodynamics (MHD). To overcome this problem one can use subgrid-scale (SGS) closures - models for the influence of unresolved, subgrid-scales on the resolved ones. In this work we propose and validate a set of constant coefficient closures for the resolved, compressible, ideal MHD equations. The SGS energies are modeled by Smagorinsky-like equilibrium closures. The turbulent stresses and the electromotive force (EMF) are described by expressions that are nonlinear in terms of large scale velocity and magnetic field gradients. To verify the closures we conduct a priori tests over 137 simulation snapshots from two different codes with varying ratios of thermal to magnetic pressure () and sonic Mach numbers (). Furthermore, we make a comparison to traditional, phenomenological eddy-viscosity and closures. We find only mediocre performance of the kinetic eddy-viscosity and closures, and that the magnetic eddy-viscosity closure is poorly correlated with the simulation data. Moreover, three of five coefficients of the traditional closures exhibit a significant spread in values. In contrast, our new closures demonstrate consistently high correlations and constant coefficient values over time and over the wide range of parameters tested. Important aspects in compressible MHD turbulence such as the bi-directional energy cascade, turbulent magnetic pressure and proper alignment of the EMF are well described by our new closures.University of GottingenDeutsche Forschungsgemeinschaft (DFG)Australian Research Council (ARC
Cosmic constraints rule out s-wave annihilation of light dark matter
Light dark matter annihilating into electron-positron pairs emits a
significant amount of internal bremsstrahlung that may contribute to the cosmic
gamma-ray background. The amount of emitted gamma-rays depends on the dark
matter clumping factor. Recent calculations indicate that this value should be
of order . That allows us to calculate the expected gamma-ray
background contribution from dark matter annihilation. We find that the light
dark matter model can be ruled out if a constant thermally-averaged cross
section is assumed (s-wave annihilation). For more massive dark matter
candidates like neutralinos, however, cosmic constraints are weaker.Comment: 5 pages, 2 figures, accepted at PR
Detection of CN gas in Interstellar Object 2I/Borisov
The detection of Interstellar Objects passing through the Solar System offers
the promise of constraining the physical and chemical processes involved in
planetary formation in other extrasolar systems. While the effect of outgassing
by 1I/2017 U1 ('Oumuamua) was dynamically observed, no direct detection of the
ejected material was made. The discovery of the active interstellar comet
2I/Borisov means spectroscopic investigations of the sublimated ices is
possible for this object. We report the first detection of gas emitted by an
interstellar comet via the near-UV emission of CN from 2I/Borisov at a
heliocentric distance of = 2.7 au on 2019 September 20. The production rate
was found to be Q(CN) = s, using a simple
Haser model with an outflow velocity of 0.5 km s. No other emission was
detected, with an upper limit to the production rate of C of
s. The spectral reflectance slope of the dust coma over
\AA\ \AA \ is steeper than at longer wavelengths, as
found for other comets. Broad band photometry on 2019 September 19 gave a
dust production rate of cm. Modelling of the observed gas and
dust production rates constrains the nuclear radius to km assuming
reasonable nuclear properties. Overall, we find the gas, dust and nuclear
properties for the first active Interstellar Object are similar to normal Solar
System comets.Comment: 9 pages, 4 figures, accepted for publication in ApJ
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