499 research outputs found
Massive Milky Way Satellites in Cold and Warm Dark Matter: Dependence on Cosmology
We investigate the claim that the largest subhaloes in high resolution
dissipationless cold dark matter (CDM) simulations of the Milky Way are
dynamically inconsistent with observations of its most luminous satellites. We
find that the inconsistency is largely attributable to the large values of
\sigma_8 and n_s adopted in the discrepant simulations producing satellites
that form too early and therefore are too dense. We find the tension between
observations and simulations adopting parameters consistent with WMAP9 is
greatly diminished making the satellites a sensitive test of CDM. We find the
Via Lactea II halo to be atypical for haloes in a WMAP3 cosmology, a
discrepancy that we attribute to its earlier formation epoch than the mean for
its mass. We also explore warm dark matter (WDM) cosmologies for 1--4 keV
thermal relics. In 1 keV cosmologies subhaloes have circular velocities at kpc
scales ~ 60% lower than their CDM counterparts, but are reduced by only 10% in
4 keV cosmologies. Since relic masses < 2-3 keV are ruled out by constraints
from the number of Milky Way satellites and Lyman-\alpha\ forest, WDM has a
minor effect in reducing the densities of massive satellites. Given the
uncertainties on the mass and formation epoch of the Milky Way, the need for
reducing the satellite densities with baryonic effects or WDM is alleviated.Comment: 11 pages, 7 figures, submitted to MNRA
Dependence of the Inner DM Profile on the Halo Mass
I compare the density profile of dark matter (DM) halos in cold dark matter
(CDM) N-body simulations with 1 Mpc, 32 Mpc, 256 Mpc and 1024 Mpc box sizes. In
dimensionless units the simulations differ only for the initial power spectrum
of density perturbations. I compare the profiles when the most massive halos
are composed of about 10^5 DM particles. The DM density profiles of the halos
in the 1 Mpc box show systematically shallower cores with respect to the
corresponding halos in the 32 Mpc simulation that have masses, M_{dm}, typical
of the Milky Way and are fitted by a NFW profile. The DM density profiles of
the halos in the 256 Mpc box are consistent with having steeper cores than the
corresponding halos in the 32 Mpc simulation, but higher mass resolution
simulations are needed to strengthen this result. Combined, these results
indicate that the density profile of DM halos is not universal, presenting
shallower cores in dwarf galaxies and steeper cores in clusters. Physically the
result sustains the hypothesis that the mass function of the accreting
satellites determines the inner slope of the DM profile. In comoving
coordinates, r, the profile \rho_{dm} \propto 1/(X^\alpha(1+X)^{3-\alpha}),
with X=c_\Delta r/r_\Delta, r_\Delta is the virial radius and \alpha
=\alpha(M_{dm}), provides a good fit to all the DM halos from dwarf galaxies to
clusters at any redshift with the same concentration parameter c_\Delta ~ 7.
The slope, \gamma, of the outer parts of the halo appears to depend on the
acceleration of the universe: when the scale parameter is a=(1+z)^{-1} < 1, the
slope is \gamma ~ 3 as in the NFW profile, but \gamma ~ 4 at a > 1 when
\Omega_\Lambda ~ 1 and the universe is inflating.[abridged]Comment: Accepted for publication in MNRAS. 13 pages, including 11 figures and
2 tables. The revised version has an additional discussion section and work
on the velocity dispersion anisotrop
Modeling and simulation of nuclear hybrid energy systems architectures
The transition toward a low-carbon energy system and the increasing penetration of variable renewable energy (VRE) sources translate into a pressing need for dispatchable and low-carbon power sources. Nuclear hybrid energy systems (NHES) exploit the synergies between nuclear power and other energy sources together with energy storage devices and a variety of electric and non-electric applications. The expected benefits range from a high flexibility being able to supporting an increasing penetration of the VRE while complying with the grid demand and constraints to an increased profitability brought by the production of commodities beyond electricity (e.g., hydrogen, heat, etc.). A dedicated framework must be developed to evaluate different NHES configurations, particularly with regard to the complex interconnections among the tightly coupled components. In this work, illustrative examples of NHES components were selected and modeled with the object-oriented modeling language Modelica and implemented in the Dymola simulation environment. The technologies considered in this study are a Small Modular Reactor (SMR) based on pressurized water technology, a thermal energy storage (TES) system, and an alkaline electrolyzer for hydrogen production. The dynamic models are then collected in a new Modelica library and assembled into a variety of NHES topologies using a plug-and-play approach. The time-dependent behavior of the NHES layout can be simulated under different operational contexts, enabling the monitoring of key process variables, supporting system design, exploring alternative control strategies, and analyzing different scenarios. The NHESs are investigated in two exemplary scenarios – one representing typical load conditions and the other featuring high VRE penetration – in order to demonstrate the viability of the proposed approach as an initial effort toward the development of a holistic framework for analyzing NHES. The dynamic models effectively met the analysis requirements, for instance, by tracking the production of commodities throughout each operational transient, which is an essential result for evaluating the performance of NHES. In this regard, efficiency is adopted as the figure of merit to compare the different NHES architectures, with simulation results indicating significant overall efficiency improvements in NHES incorporating TES and using nuclear heat to drive non-electric applications
The NorthStar Ambulatory Assessment in Duchenne muscular dystrophy: considerations for the design of clinical trials.
With the emergence of experimental therapies for Duchenne muscular dystrophy (DMD), it is fundamental to understand the natural history of this disorder to properly design clinical trials. The aims of this study were to assess the effects produced on motor function by different DMD genotypes and early initiation of glucocorticoids
Energy Dissipation in Interstellar Cloud Collisions
We present a study of the kinetic energy dissipation in interstellar cloud
collisions. The main aim is to understand the dependence of the elasticity
(defined as the ratio of the final to the initial kinetic energy of the clouds)
on the velocity and mass ratio of the colliding clouds, magnetic field
strength, and gas metallicity for head-on collisions. The problem has been
studied both analytically and via numerical simulations. We have derived handy
analytical relationships that well approximate the analogous numerical results.
The main findings of this work are: (i) the kinetic energy dissipation in cloud
collisions is minimum (i.e. the collision elasticity is maximum) for a cloud
relative velocity ; (ii) the above minimum value is
proportional , where is the metallicity and is the cloud
size: the larger is the more dissipative (i.e. inelastic) the
collision will be; (iii) in general, we find that the energy dissipation
decreases when the magnetic field strength, and mass ratio of the clouds are
increased and the metallicity is decreased, respectively. We briefly discuss
the relevance of this study to the global structure of the interstellar medium
and to galaxy formation and evolution.Comment: 16 pages, aasms LaTeX, 7 figures. ApJ, accepte
A gated oscillator clock and data recovery circuit for nanowatt wake-up and data receivers
This article presents a data-startable baseband logic featuring a gated oscillator clock and data recovery (GO-CDR) circuit for nanowatt wake-up and data receivers (WuRxs). At each data transition, the phase misalignment between the data coming from the analog front-end (AFE) and the clock is cleared by the GO-CDR circuit, thus allowing the reception of long data streams. Any free-running frequency mismatch between the GO and the bitrate does not limit the number of receivable bits, but only the maximum number of equal consecutive bits (Nm). To overcome this limitation, the proposed system includes a frequency calibration circuit, which reduces the frequency mismatch to ±0.5%, thus enabling the WuRx to be used with different encoding techniques up to Nm = 100. A full WuRx prototype, including an always-on clockless AFE operating in subthreshold, was fabricated with STMicroelectronics 90 nm BCD technology. The WuRx is supplied with 0.6 V, and the power consumption, excluding the calibration circuit, is 12.8 nW during the rest state and 17 nW at a 1 kbps data rate. With a 1 kbps On-Off Keying (OOK) modulated input and −35 dBm of input RF power after the input matching network (IMN), a 10^(−3) missed detection rate with a 0 bit error tolerance is measured, transmitting 63 bit packets with the Nm ranging from 1 to 63. The total sensitivity, including the estimated IMN gain at 100 MHz and 433 MHz, is −59.8 dBm and −52.3 dBm, respectively. In comparison with an ideal CDR, the degradation of the sensitivity due to the GO-CDR is 1.25 dBm. False alarm rate measurements lasting 24 h revealed zero overall false wake-ups
Neutronic Analysis of a Plutonium Burner PWR Partially Fed with Inert Matrix Fuel
The plutonium coming from dismantled warheads and that already stockpiled coming from spent fuel reprocessing have raised many concerns related to proliferation resistance, environmental safety and economy. The option of disposing of plutonium by fission has been discussed and many proposals for plutonium burning in a safe and economical manner have been put forward. The advantages of utilizing the pressurized water reactors (PWRs) for plutonium disposition are the well developed and reliable technology and their diffusion. The mixide-oxide (MOX) fuel form, that is used for plutonium recycling in power reactors, is well developed. Nevertheless, to eliminate the production of additional plutonium during irradiation, an improved design fuel was analysed, which replace the natural or depleted uranium with inert oxides.
This type of fuel offers the potential for annihilation of the major portion of the plutonium: commercial PWRs operating in a once-through cycle scheme could burn more than 98% of the loaded Pu-239 and more than 73% of the overall initially loaded reactor-grade plutonium. The plutonium still left in the spent fuel was quality-poor and then offered a better proliferation resistance. Power peaking problems could be faced with the adoption of burnable absorbers: zirconium diboride coating in the form of integral fuel burnable absorber (IFBA) appeared particularly suitable. In spite of a reduction of the overall plutonium loaded mass by a factor 3.7, there was not evidence of an increase of the Minor Actinides radiotoxicity after a time period of about 25 years
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