26,312 research outputs found
Development and testing of dry chemicals in advanced extinguishing systems for jet engine nacelle fires
The effectiveness of dry chemical in extinguishing and delaying reignition of fires resulting from hydrocarbon fuel leaking onto heated surfaces such as can occur in jet engine nacelles is studied. The commercial fire extinguishant dry chemical tried are sodium and potassium bicarbonate, carbonate, chloride, carbamate (Monnex), metal halogen, and metal hydroxycarbonate compounds. Synthetic and preparative procedures for new materials developed, a new concept of fire control by dry chemical agents, descriptions of experiment assemblages to test dry chemical fire extinguishant efficiencies in controlling fuel fires initiated by hot surfaces, comparative testing data for more than 25 chemical systems in a 'static' assemblage with no air flow across the heated surface, and similar comparative data for more than ten compounds in a dynamic system with air flows up to 350 ft/sec are presented
Growing massive black holes through super-critical accretion of stellar-mass seeds
The rapid assembly of the massive black holes that power the luminous quasars
observed at remains a puzzle. Various direct collapse models have
been proposed to head-start black hole growth from initial seeds with masses
, which can then reach a billion solar mass while
accreting at the Eddington limit. Here we propose an alternative scenario based
on radiatively inefficient super-critical accretion of stellar-mass holes
embedded in the gaseous circum-nuclear discs (CNDs) expected to exist in the
cores of high redshift galaxies. Our sub-pc resolution hydrodynamical
simulations show that stellar-mass holes orbiting within the central 100 pc of
the CND bind to very high density gas clumps that arise from the fragmentation
of the surrounding gas. Owing to the large reservoir of dense cold gas
available, a stellar-mass black hole allowed to grow at super-Eddington rates
according to the "slim disc" solution can increase its mass by 3 orders of
magnitudes within a few million years. These findings are supported by
simulations run with two different hydro codes, RAMSES based on the Adaptive
Mesh Refinement technique and GIZMO based on a new Lagrangian Godunov-type
method, and with similar, but not identical, sub-grid recipes for star
formation, supernova feedback, black hole accretion and feedback. The low
radiative efficiency of super-critical accretion flows are instrumental to the
rapid mass growth of our black holes, as they imply modest radiative heating of
the surrounding nuclear environment.Comment: 12 pages, 8 figures, 2 tables. Accepted for publication in MNRA
The Emotional Self-Efficacy Scale: Adaptation and Validation for Young Adolescents
Emotional self-efficacy (ESE) is an important aspect of emotional functioning, with current measures for children and adolescents focused on the measurement of self-beliefs in relation to the management of emotions. In the present study, we report the psychometric properties of the first adaptation of the Emotional Self-Efficacy Scale for youth (Youth-ESES) that measures additional aspects of ESE, such as perceiving and understanding emotions and helping others modulate their emotions. Participants were 192 young adolescents aged 11 to 13 years from a U.K. state school. They completed the Youth-ESES and measures of ability emotional intelligence (EI) and cognitive ability. Results support the same four-factor structure that has been previously documented using the adult version of the ESES, with the four subscales being largely independent from cognitive ability and only moderately related to ability EI. However, the four subscales were less differentiated in the present study compared with adult data previously published, suggesting that there is a strong general factor underlying young adolescents’ ESE scores. Overall, the results suggest that the adapted Youth-ESES can be reliably used with youth, and that confidence in how a young person feels about his or her emotional functioning remains distinct from emotional skill
Gas Giant Protoplanets Formed by Disk Instability in Binary Star Systems
We present a suite of three dimensional radiative gravitational hydrodynamics
models suggesting that binary stars may be quite capable of forming planetary
systems similar to our own. The new models with binary companions do not employ
any explicit artificial viscosity, and also include the third (vertical)
dimension in the hydrodynamic calculations, allowing for transient phases of
convective cooling. The calculations of the evolution of initially marginally
gravitationally stable disks show that the presence of a binary star companion
may actually help to trigger the formation of dense clumps that could become
giant planets. We also show that in models without binary companions, which
begin their evolution as gravitationally stable disks, the disks evolve to form
dense rings, which then break-up into self-gravitating clumps. These latter
models suggest that the evolution of any self-gravitating disk with sufficient
mass to form gas giant planets is likely to lead to a period of disk
instability, even in the absence of a trigger such as a binary star companion.Comment: 52 pages, 28 figure
Orbital Decay of Supermassive Black Hole Binaries in Clumpy Multiphase Merger Remnants
We simulate an equal-mass merger of two Milky Way-size galaxy discs with
moderate gas fractions at parsec-scale resolution including a new model for
radiative cooling and heating in a multi-phase medium, as well as star
formation and feedback from supernovae. The two discs initially have a
supermassive black hole (SMBH) embedded in
their centers. As the merger completes and the two galactic cores merge, the
SMBHs form a a pair with a separation of a few hundred pc that gradually
decays. Due to the stochastic nature of the system immediately following the
merger, the orbital plane of the binary is significantly perturbed.
Furthermore, owing to the strong starburst the gas from the central region is
completely evacuated, requiring ~Myr for a nuclear disc to rebuild.
Most importantly, the clumpy nature of the interstellar medium has a major
impact on the the dynamical evolution of the SMBH pair, which undergo
gravitational encounters with massive gas clouds and stochastic torquing by
both clouds and spiral modes in the disk. These effects combine to greatly
delay the decay of the two SMBHs to separations of a few parsecs by nearly two
orders of magnitude, yr, compared to previous work. In mergers of
more gas-rich, clumpier galaxies at high redshift stochastic torques will be
even more pronounced and potentially lead to stronger modulation of the orbital
decay. This suggests that SMBH pairs at separations of several tens of parsecs
should be relatively common at any redshift.Comment: submitted to MNRAS; Comments very welcom
Low power, compact charge coupled device signal processing system
A variety of charged coupled devices (CCDs) for performing programmable correlation for preprocessing environmental sensor data preparatory to its transmission to the ground were developed. A total of two separate ICs were developed and a third was evaluated. The first IC was a CCD chirp z transform IC capable of performing a 32 point DFT at frequencies to 1 MHz. All on chip circuitry operated as designed with the exception of the limited dynamic range caused by a fixed pattern noise due to interactions between the digital and analog circuits. The second IC developed was a 64 stage CCD analog/analog correlator for performing time domain correlation. Multiplier errors were found to be less than 1 percent at designed signal levels and less than 0.3 percent at the measured smaller levels. A prototype IC for performing time domain correlation was also evaluated
Time-Dependent Models for Dark Matter at the Galactic Center
The prospects of indirect detection of dark matter at the galactic center
depend sensitively on the mass profile within the inner parsec. We calculate
the distribution of dark matter on sub-parsec scales by integrating the
time-dependent Fokker-Planck equation, including the effects of
self-annihilations, scattering of dark matter particles by stars, and capture
in the supermassive black hole. We consider a variety of initial dark matter
distributions, including models with very high densities ("spikes") near the
black hole, and models with "adiabatic compression" of the baryons. The
annihilation signal after 10 Gyr is found to be substantially reduced from its
initial value, but in dark matter models with an initial spike,
order-of-magnitude enhancements can persist compared with the rate in
spike-free models, with important implications for indirect dark matter
searches with GLAST and Air Cherenkov Telescopes like HESS and CANGAROO.Comment: Four page
The Hall instability of weakly ionized, radially stratified, rotating disks
Cool weakly ionized gaseous rotating disk, are considered by many models as
the origin of the evolution of protoplanetary clouds. Instabilities against
perturbations in such disks play an important role in the theory of the
formation of stars and planets. Thus, a hierarchy of successive fragmentations
into smaller and smaller pieces as a part of the Kant-Laplace theory of
formation of the planetary system remains valid also for contemporary
cosmogony. Traditionally, axisymmetric magnetohydrodynamic (MHD), and recently
Hall-MHD instabilities have been thoroughly studied as providers of an
efficient mechanism for radial transfer of angular momentum, and of density
radial stratification. In the current work, the Hall instability against
nonaxisymmetric perturbations in compressible rotating fluids in external
magnetic field is proposed as a viable mechanism for the azimuthal
fragmentation of the protoplanetary disk and thus perhaps initiating the road
to planet formation. The Hall instability is excited due to the combined effect
of the radial stratification of the disk and the Hall electric field, and its
growth rate is of the order of the rotation period.Comment: 15 pages, 2 figure
Heterogeneous Dynamics of Coarsening Systems
We show by means of experiments, theory and simulations, that the slow
dynamics of coarsening systems displays dynamic heterogeneity similar to that
observed in glass-forming systems. We measure dynamic heterogeneity via novel
multi-point functions which quantify the emergence of dynamic, as opposed to
static, correlations of fluctuations. Experiments are performed on a coarsening
foam using Time Resolved Correlation, a recently introduced light scattering
method. Theoretically we study the Ising model, and present exact results in
one dimension, and numerical results in two dimensions. For all systems the
same dynamic scaling of fluctuations with domain size is observed.Comment: Minor changes; to be published in Phys. Rev. Let
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