571 research outputs found
Alas, the dark matter structures were not that trivial
The radial density profile of dark matter structures has been observed to
have an almost universal behaviour in numerical simulations, however, the
physical reason for this behaviour remains unclear. It has previously been
shown that if the pseudo phase-space density, rho/sigma_d^epsilon, is a
beautifully simple power-law in radius, with the "golden values" epsilon=3 and
d=r (i.e., the phase-space density is only dependent on the radial component of
the velocity dispersion), then one can analytically derive the radial variation
of the mass profile, dispersion profile etc. That would imply, if correct, that
we just have to explain why rho/sigma^3_r ~r^{-alpha}, and then we would
understand everything about equilibrated DM structures. Here we use a set of
simulated galaxies and clusters of galaxies to demonstrate that there are no
such golden values, but that each structure instead has its own set of values.
Considering the same structure at different redshifts shows no evolution of the
phase-space parameters towards fixed points. There is also no clear connection
between the halo virialized mass and these parameters. This implies that we
still do not understand the origin of the profiles of dark matter structures.Comment: 4 pages, 3 figures, accepted for publication in ApJ
How Glutamate Receptor Subunits Mix and Match: Details Uncovered
Until now, the atomic details explaining why certain subunits prefer to coassemble has been lacking in our understanding of glutamate receptor biogenesis. In this issue, Kumar et al. describe the structural basis by which preferential subunit assembly occurs for homomeric and heteromeric kainate-type glutamate receptors
Atrial fibrillation, liver cirrhosis, thrombosis, and bleeding:AÂ Danish population-based cohort study
OBJECTIVES: We examined the impact of liver cirrhosis on the risk of thromboembolic events and bleeding complications in patients with atrial fibrillation or flutter (AFF). METHODS: This populationâbased cohort study used data from Danish health registries. We identified all patients with a firstâtime diagnosis of AFF during 1995 to 2015, and followed them from their AFF diagnosis until the end of 2016. Patients were categorized according to the presence or absence of liver cirrhosis. We computed incidence rates per 1000 personâyears and hazard ratios (HRs) with 95% confidence intervals (CIs) based on Cox regression analyses, adjusting for age, CHA(2)DS(2)VASc score, and Charlson Comorbidity Index score. RESULTS: We identified 273Â 225 patients with AFF. Of these, 1463 (0.54%) had liver cirrhosis. During 0 to 5Â years of followâup, compared to patients without liver cirrhosis, patients with liver cirrhosis had higher incidence rates and hazards of ischemic stroke (29.7 vs 21.6; HR, 1.3; 95% CI, 1.1â1.6), venous thromboembolism (9.2 vs 5.5; HR, 1.5; 95% CI, 1.2â2.3), but not myocardial infarction (10.2 vs 11.2; HR, 0.9; 95% CI, 0.7â1.2). Patients with liver cirrhosis also had higher rates of hemorrhagic stroke (5.8 vs 3.3; HR, 1.7; 95% CI, 1.1â2.6), subdural hemorrhage (5.3 vs 1.6; HR, 3.2; 95% CI, 2.1â4.9), hemorrhage of the lung or urinary tract (24.6 vs 15.2; HR, 1.6; 95% CI, 1.3â2.0), and gastrointestinal hemorrhage (34.5 vs 10.4; HR, 3.3; 95% CI, 2.7â3.9). CONCLUSION: In patients with AFF, liver cirrhosis was associated with an elevated risk of ischemic stroke, venous thromboembolism, and all evaluated bleeding complications
Dark Matter Angular Momentum Profile from the Jeans Equation
Cosmological simulations of dark matter structures have shown that the
equilibrated dark matter structures have a fairly small angular momentum. It
appears from these N-body simulations that the radial profile of the angular
momentum has an almost universal behavior, even if the different dark matter
structures have experienced very different formation and merger histories. We
suggest a perturbed Jeans equation, which includes a rotational term. This is
done under a reasonable assumed form of the change in the distribution
function. By conjecturing that the (new) subdominant rotation term must be
proportional to the (old) dominant mass term, we find a clear connection, which
is in rather good agreement with the results of recent high resolution
simulations. We also present a new connection between the radial profiles of
the angular momentum and the velocity anisotropy, which is also in fair
agreement with numerical findings. Finally we show how the spin parameter
increases as a function of radius.Comment: 9 pages, 10 figures, accepted for publication in ApJ, Added
reference
Feasibility and Performance of the Staged Z-Pinch: A One-dimensional Study with FLASH and MACH2
Z-pinch platforms constitute a promising pathway to fusion energy research.
Here, we present a one-dimensional numerical study of the staged Z-pinch (SZP)
concept using the FLASH and MACH2 codes. We discuss the verification of the
codes using two analytical benchmarks that include Z-pinch-relevant physics,
building confidence on the codes' ability to model such experiments. Then,
FLASH is used to simulate two different SZP configurations: a xenon gas-puff
liner (SZP1*) and a silver solid liner (SZP2). The SZP2 results are compared
against previously published MACH2 results, and a new code-to-code comparison
on SZP1* is presented. Using an ideal equation of state and analytical
transport coefficients, FLASH yields a fuel convergence ratio (CR) of
approximately 39 and a mass-averaged fuel ion temperature slightly below 1 keV
for the SZP2 scheme, significantly lower than the full-physics MACH2
prediction. For the new SZP1* configuration, full-physics FLASH simulations
furnish large and inherently unstable CRs (> 300), but achieve fuel ion
temperatures of many keV. While MACH2 also predicts high temperatures, the fuel
stagnates at a smaller CR. The integrated code-to-code comparison reveals how
magnetic insulation, heat conduction, and radiation transport affect platform
performance and the feasibility of the SZP concept
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