21,755 research outputs found
Ion microprobe elemental analyses of impact features on interplanetary dust experiment sensor surfaces
Hypervelocity impact features from very small particles (less than 3 microns in diameter) on several of the electro-active dust sensors used in the Interplanetary Dust Experiment (IDE) were subjected to elemental analysis using an ion microscope. The same analytical techniques were applied to impact and containment features on a set of ultra-pure, highly polished single crystal germanium wafer witness plates that were mounted on tray B12. Very little unambiguously identifiable impactor debris was found in the central craters or shatter zones of small impacts in this crystalline surface. The surface contamination, ubiquitous on the surface of the Long Duration Exposure Facility, has greatly complicated data collection and interpretation from microparticle impacts on all surfaces
Critical Casimir interaction of ellipsoidal colloids with a planar wall
Based on renormalization group concepts and explicit mean field calculations
we study the universal contribution to the effective force and torque acting on
an ellipsoidal colloidal particle which is dissolved in a critical fluid and is
close to a homogeneous planar substrate. At the same closest distance between
the substrate and the surface of the particle, the ellipsoidal particle prefers
an orientation parallel to the substrate and the magnitude of the fluctuation
induced force is larger than if the orientation of the particle is
perpendicular to the substrate. The sign of the critical torque acting on the
ellipsoidal particle depends on the type of boundary conditions for the order
parameter at the particle and substrate surfaces, and on the pivot with respect
to which the particle rotates
Place matters: but does local leadership?
The arrival of New Labour into Government witnessed the prominent re-emergence of place onto the policy agenda. This heralded a range of area-based-initiatives designed to both tackle neighbourhood forms of deprivation and to re-establish a sense of identity and connection between individuals and their local community. In terms of place-making, effective and inclusive participation, representation and leadership were all identified as prerequisites for the creation of sustainable communities . But how important is local leadership and strategic vision within local public service organisations in achieving the desired place-making outcomes? This paper examines the extent to which local leadership and strategic vision represents a significant factor in promoting higher levels of satisfaction, belonging, cohesion and participation across single tier councils in England. The ensuing empirical evidence raises significant questions not only about the importance of local leadership in place-making, but also the environmental and organizational factors that shape local places
30 Doradus - a Template for "Real Starbursts"?
30 Doradus is the closest massive star forming region and the best studied
template of a starburst. In this conference paper we first summarize the
properties of 30 Doradus and its stellar core, R136. We discuss the effects of
insufficient spatial resolution and cluster density profiles on dynamical mass
estimates of super star clusters, and show that their masses can be easily
overestimated by a factor of ten or more. From a very simple model, with
R136-like clusters as representative building blocks, we estimate typical
luminosities of the order 10^11 L_o for starburst galaxies.Comment: To be published in "Starbursts: From 30 Doradus to Lyman Break
Galaxies", eds. R. de Grijs & R.M. Gonzalez Delgad
Validity and practical utility of accelerometry for the measurement of in-hand physical activity in horses
Background:
Accelerometers are valid, practical and reliable tools for the measurement of habitual physical activity (PA). Quantification of PA in horses is desirable for use in research and clinical settings. The objective of this study was to evaluate a triaxial accelerometer for objective measurement of PA in the horse by assessment of their practical utility and validity.
Horses were recruited to establish both the optimal site of accelerometer attachment and questionnaire designed to explore owner acceptance. Validity and cut-off values were obtained by assessing PA at various gaits. Validation study- 20 horses wore the accelerometer while being filmed for 10Â min each of rest, walking and trotting and 5 mins of canter work. Practical utility study- five horses wore accelerometers on polls and withers for 18Â h; compliance and relative data losses were quantified.
Results:
Accelerometry output differed significantly between the four PA levels (P <0•001) for both wither and poll placement. For withers placement, ROC analyses found optimal sensitivity and specificity at a cut-off of <47 counts per minute (cpm) for rest (sensitivity 99.5 %, specificity 100 %), 967–2424 cpm for trotting (sensitivity 96.7 %, specificity 100 %) and ≥2425 cpm for cantering (sensitivity 96.0 %, specificity 97.0 %). Attachment at the poll resulted in optimal sensitivity and specificity at a cut-off of <707 counts per minute (cpm) for rest (sensitivity 97.5 %, specificity 99.6 %), 1546–2609 cpm for trotting (sensitivity 90.33 %, specificity 79.25 %) and ≥2610 cpm for cantering (sensitivity 100 %, specificity 100 %) In terms of practical utility, accelerometry was well tolerated and owner acceptance high.
Conclusion:
Accelerometry data correlated well with varying levels of in-hand equine activity. The use of accelerometers is a valid method for objective measurement of controlled PA in the horse
Curvature and Acoustic Instabilities in Rotating Fluid Disks
The stability of a rotating fluid disk to the formation of spiral arms is
studied in the tightwinding approximation in the linear regime. The dispersion
relation for spirals that was derived by Bertin et al. is shown to contain a
new, acoustic instability beyond the Lindblad resonances that depends only on
pressure and rotation. In this regime, pressure and gravity exchange roles as
drivers and inhibitors of spiral wave structures. Other instabilities that are
enhanced by pressure are also found in the general dispersion relation by
including higher order terms in the small parameter 1/kr for wavenumber k and
radius r. These instabilities are present even for large values of Toomre's
parameter Q. Unstable growth rates are determined in four cases: a
self-gravitating disk with a flat rotation curve, a self-gravitating disk with
solid body rotation, a non-self-gravitating disk with solid body rotation, and
a non-self-gravitating disk with Keplerian rotation. The most important
application appears to be as a source of spiral structure, possibly leading to
accretion in non-self-gravitating disks, such as some galactic nuclear disks,
disks around black holes, and proto-planetary disks. All of these examples have
short orbital times so the unstable growth time can be small.Comment: 30 pages, 5 figures, scheduled for ApJ 520, August 1, 199
Contaminant Interferences with SIMS Analyses of Microparticle Impactor Residues on LDEF Surfaces
Elemental analyses of impactor residues on high purity surface exposed to the low earth orbit (LEO) environment for 5.8 years on Long Duration Exposure Facility (LDEF) has revealed several probable sources for microparticles at this altitude, including natural micrometeorites and manmade debris ranging from paint pigments to bits of stainless steel. A myriad of contamination interferences were identified and their effects on impactor debris identification mitigated during the course of this study. These interferences included pre-, post-, and in-flight deposited particulate surface contaminants, as well as indigenous heterogeneous material contaminants. Non-flight contaminants traced to human origins, including spittle and skin oils, contributed significant levels of alkali-rich carbonaceous interferences. A ubiquitous layer of in-flight deposited silicaceous contamination varied in thickness with location on LDEF and proximity to active electrical fields. In-flight deposited (low velocity) contaminants included urine droplets and bits of metal film from eroded thermal blankets
Thermal Instability-Induced Interstellar Turbulence
We study the dynamics of phase transitions in the interstellar medium by
means of three-dimensional hydrodynamic numerical simulations. We use a
realistic cooling function and generic nonequilibrium initial conditions to
follow the formation history of a multiphase medium in detail in the absence of
gravity. We outline a number of qualitatively distinct stages of this process,
including a linear isobaric evolution, transition to an isochoric regime,
formation of filaments and voids (also known as "thermal" pancakes), the
development and decay of supersonic turbulence, an approach to pressure
equilibrium, and final relaxation of the multiphase medium. We find that 1%-2%
of the initial thermal energy is converted into gas motions in one cooling
time. The velocity field then randomizes into turbulence that decays on a
dynamical timescale E_k ~ t^-n, 1 < n < 2. While not all initial conditions
yield a stable two-phase medium, we examine such a case in detail. We find that
the two phases are well mixed with the cold clouds possessing a fine-grained
structure near our numerical resolution limit. The amount of gas in the
intermediate unstable phase roughly tracks the rms turbulent Mach number,
peaking at 25% when M_rms ~ 8, decreasing to 11% when M_rms ~ 0.4.Comment: To appear in the ApJ Letters, April 2002; 5 pages, 3 color figures,
mpeg animations available at http://akpc.ucsd.edu/ThermalLetter/thermal.htm
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