19,788 research outputs found
Kolmogorov-Burgers Model for Star Forming Turbulence
The process of star formation in interstellar molecular clouds is believed to
be controlled by driven supersonic magnetohydrodynamic turbulence. We suggest
that in the inertial range such turbulence obeys the Kolmogorov law, while in
the dissipative range it behaves as Burgers turbulence developing shock
singularities. On the base of the She-Leveque analytical model we then predict
the velocity power spectrum in the inertial range to be E_k ~ k^{-1.74}. This
result reproduces the observational Larson law, ~ l^{0.74...0.76},
[Larson, MNRAS 194 (1981) 809] and agrees well with recent numerical findings
by Padoan and Nordlund [astro-ph/0011465]. The application of the model to more
general dissipative structures, with higher fractal dimensionality, leads to
better agreement with recent observational results.Comment: revised, new material added, 8 page
Evaluating some computer enhancement algorithms that improve the visibility of cometary morphology
The observed morphology of cometary comae is determined by ejection circumstances and the interaction of the ejected material with the local environment. Anisotropic emission can provide useful information on such things as orientation of the nucleus, location of active areas on the nucleus, and the formation of ion structure near the nucleus. However, discrete coma features are usually diffuse, of low amplitude, and superimposed on a steep intensity gradient radial to the nucleus. To improve the visibility of these features, a variety of digital enhancement algorithms were employed with varying degrees of success. They usually produce some degree of spatial filtering, and are chosen to optimize visibility of certain detail. Since information in the image is altered, it is important to understand the effects of parameter selection and processing artifacts can have on subsequent interpretation. Using the criteria that the ideal algorithm must enhance low contrast features while not introducing misleading artifacts (or features that cannot be seen in the stretched, unprocessed image), the suitability of various algorithms that aid cometary studies were assessed. The strong and weak points of each are identified in the context of maintaining positional integrity of features at the expense of photometric information
Research on metal solidification in zero-g state
The containerless solidification of several pure metals and metallic alloys was studied in a low gravity environment. The tests were performed in the MSFC 4.2 s drop tower using a rapid wire melting apparatus designed and built for this purpose. Pure iron and nickel, and alloys of iron-nickel, iron-carbon, nickel-aluminum and tungsten-rhenium were all melted and solidified at a gravity level of approximately 100.000/-4 g. Interpretation of the results has led to an appreciation of the factors controlling the successful execution of this drop test experiment and to a delineation of the limits of applicability of the apparatus. Preliminary metallurgical evaluations are presented of the overall shapes, lattice parameters, surface microstructure,, cross-sectional microstructures, solidification and transformation sequences, evaporative segregation, and localized solute redistribution observed in the low-gravity specimens. The effects of low gravity on metallic solidification are discussed with particular emphasis on observations of spontaneous undercooling and evaporative segregation in uncontained melts
Fast-ignition design transport studies: realistic electron source, integrated PIC-hydrodynamics, imposed magnetic fields
Transport modeling of idealized, cone-guided fast ignition targets indicates
the severe challenge posed by fast-electron source divergence. The hybrid
particle-in-cell [PIC] code Zuma is run in tandem with the
radiation-hydrodynamics code Hydra to model fast-electron propagation, fuel
heating, and thermonuclear burn. The fast electron source is based on a 3D
explicit-PIC laser-plasma simulation with the PSC code. This shows a quasi
two-temperature energy spectrum, and a divergent angle spectrum (average
velocity-space polar angle of 52 degrees). Transport simulations with the
PIC-based divergence do not ignite for > 1 MJ of fast-electron energy, for a
modest 70 micron standoff distance from fast-electron injection to the dense
fuel. However, artificially collimating the source gives an ignition energy of
132 kJ. To mitigate the divergence, we consider imposed axial magnetic fields.
Uniform fields ~50 MG are sufficient to recover the artificially collimated
ignition energy. Experiments at the Omega laser facility have generated fields
of this magnitude by imploding a capsule in seed fields of 50-100 kG. Such
imploded fields are however more compressed in the transport region than in the
laser absorption region. When fast electrons encounter increasing field
strength, magnetic mirroring can reflect a substantial fraction of them and
reduce coupling to the fuel. A hollow magnetic pipe, which peaks at a finite
radius, is presented as one field configuration which circumvents mirroring.Comment: 16 pages, 17 figures, submitted to Phys. Plasma
Validity of adiabaticity in Cavity QED
This paper deals with the concept of adiabaticity for fully quantum
mechanically cavity QED models. The physically interesting cases of Gaussian
and standing wave shapes of the cavity mode are considered. An analytical
approximate measure for adiabaticity is given and compared with numerical wave
packet simulations. Good agreement is obtained where the approximations are
expected to be valid. Usually for cavity QED systems, the large atom-field
detuning case is considered as the adiabatic limit. We, however, show that
adiabaticity is also valid, for the Gaussian mode shape, in the opposite limit.
Effective semiclassical time dependent models, which do not take into account
the shape of the wave packet, are derived. Corrections to such an effective
theory, which are purely quantum mechanical, are discussed. It is shown that
many of the results presented can be applied to time dependent two-level
systems.Comment: 10 pages, 9 figure
Research of metal solidification in zero-g state
An experiment test apparatus that allows metal melting and resolidification in the three seconds available during free fall in a drop tower was built and tested in the tower. Droplets (approximately 0.05 cm) of pure nickel and 1090 steel were prepared in this fashion. The apparatus, including instrumentation, is described. As part of the instrumentation, a method for measuring temperature-time histories of the free floating metal droplets was developed. Finally, a metallurgical analysis of the specimens prepared in the apparatus is presented
Constraints on Stirring and Dissipation of MHD Turbulence in Molecular Clouds
We discuss constraints on the rates of stirring and dissipation of MHD
turbulence in molecular clouds. Recent MHD simulations suggest that turbulence
in clouds decays rapidly, thus providing a significant source of energy input,
particularly if driven at small scales by, for example, bipolar outflows. We
quantify the heating rates by combining the linewidth-size relations, which
describe global cloud properties, with numerically determined dissipation
rates. We argue that, if cloud turbulence is driven on small internal scales,
the CO flux (enhanced by emission from weakly supersonic shocks) will be
much larger than observed; this, in turn, would imply excitation temperatures
significantly above observed values. We reach two conclusions: (1) small-scale
driving by bipolar outflows cannot possibly account for cloud support and yield
long-lived clouds, unless the published MHD dissipation rates are seriously
overestimated; (2) driving on large scales (comparable to the cloud size) is
much more viable from an energetic standpoint, and if the actual net
dissipation rate is only slightly lower than what current MHD simulations
estimate, then the observationally inferred lifetimes and apparent virial
equilibrium of molecular clouds can be explained.Comment: 5 pages, 1 figure. To appear in ApJ (2001 April 10
Thermomagnetic analysis of meteorites. 3: C3 and C4 chondrites
Thermomagnetic analysis on all of the C3 and C4 chondrites, conducted under conditions of controlled oxygen fugacity, indicates the presence of a thermally unstable component in at least 5 of the C3 chondrites which upon heating results in magnetite production. This unstable component is most likely troilite (FeS). The presence of the unstable substance may affect the estimation of paleointensities in meteorites which contain it. Our results indicate that Grosnaja, Ornans, Kainsaz, Felix, and Warrenton are likely to be less complicated for paleointensity determinations than the other C3 chondrites. Both C4 chondrites should lead to reliable results
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