23,883 research outputs found
Experimental study of a free turbulent shear flow at Mach 19 with electron-beam and conventional probes
An experimental study of the initial development region of a hypersonic turbulent free mixing layer was made. Data were obtained at three stations downstream of a M = 19 nozzle over a Reynolds range of 1.3 million to 3.3 million per meter and at a total temperature of about 1670 K. In general, good agreement was obtained between electron-beam and conventional probe measurements of local mean flow parameters. Measurements of fluctuating density indicated that peak root-mean-square (rms) levels are higher in the turbulent free mixing layer than in boundary layers for Mach numbers less than 9. The intensity of rms density fluctuations in the free stream is similar in magnitude to pressure fluctuations in high Mach number flows. Spectrum analyses of the measured fluctuating density through the shear layer indicate significant fluctuation energy at the lower frequencies (0.2 to 5 kHZ) which correspond to large-scale disturbances in the high-velocity region of the shear layer
The Outer Edges of Dwarf Irregular Galaxies: Stars and Gas
We have in recent years come to view dwarf galaxy evolution in the broader
context of the cosmic evolution of large-scale structure. Dwarf galaxies, as
the putative building blocks of hierarchical galaxy formation, and also as the
most numerous galaxies in the Universe, play a central role in cosmic
evolution. In particular, the interplay of galactic and intergalactic material
around dwarf irregulars must be more extensive than in more massive disk
galaxies because of their lower gravitational potential and lower interstellar
pressures. The outer regions of dwarf irregular galaxies therefore yield vital
clues to the dominant processes in this interaction zone.
The Workshop addressed a number of questions related to the role of the outer
regions in the evolution of dwarf galaxies and broader consequences. On-line
Workshop Proceedings are at http://www.lowell.edu/Workshops/Lowell02/Comment: Summary of the 2002 Lowell Observatory Workshop, to appear in PASP
Conference Highlights; 6 pp, uses aaspp4.sty. On-line Proceedings at
http://www.lowell.edu/Workshops/Lowell02
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Facing up to the challenge of behavioural observation in infant hearing assessment
The ability to assess detection and discrimination of speech by infants has proved elusive. Dr Iain Jackson and colleagues discuss how new technologies and fresh approaches might offer valuable insight into young infants’ behavioural responses to sound
Rotational Mixing in Magellanic Clouds B Stars - Theory versus Observation
We have used VLT FLAMES data to constrain the uncertain physics of rotational
mixing in stellar evolution models. We have simulated a population of single
stars and find two groups of observed stars that cannot be explained: (1) a
group of fast rotating stars which do not show evidence for rotational mixing
and (2) a group of slow rotators with strong N enrichment. Binary effects and
fossil magnetic fields may be considered to explain those two groups. We
suggest that the element boron could be used to distinguish between rotational
mixing and the binary scenario. Our single star population simulations quantify
the expected amount of boron in fast and slow rotators and allow a comparison
with measured nitrogen and boron abundances in B-stars.Comment: to appear in Comm. in Astroseismology - Contribution to the
Proceedings of the 38th LIAC, 200
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
Hierarchical Models for Independence Structures of Networks
We introduce a new family of network models, called hierarchical network
models, that allow us to represent in an explicit manner the stochastic
dependence among the dyads (random ties) of the network. In particular, each
member of this family can be associated with a graphical model defining
conditional independence clauses among the dyads of the network, called the
dependency graph. Every network model with dyadic independence assumption can
be generalized to construct members of this new family. Using this new
framework, we generalize the Erd\"os-R\'enyi and beta-models to create
hierarchical Erd\"os-R\'enyi and beta-models. We describe various methods for
parameter estimation as well as simulation studies for models with sparse
dependency graphs.Comment: 19 pages, 7 figure
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SID-1 is a dsRNA-Selective dsRNA-gated Channel
Systemic RNAi in Caenorhabditis elegans requires the widely conserved transmembrane protein SID-1 to transport RNAi silencing signals between cells. When expressed in Drosophila S2 cells, C. elegans SID-1 enables passive dsRNA uptake from the culture medium, suggesting that SID-1 functions as a channel for the transport of double-stranded RNA (dsRNA). Here we show that nucleic acid transport by SID-1 is specific for dsRNA and that addition of dsRNA to SID-1 expressing cells results in changes in membrane conductance, which indicate that SID-1 is a dsRNA gated channel protein. Consistent with passive bidirectional transport, we find that the RNA induced silencing complex (RISC) is required to prevent the export of imported dsRNA and that retention of dsRNA by RISC does not seem to involve processing of retained dsRNA into siRNAs. Finally, we show that mimics of natural molecules that contain both single- and double-stranded dsRNA, such as hairpin RNA and pre-microRNA, can be transported by SID-1. These findings provide insight into the nature of potential endogenous RNA signaling molecules in animals.Molecular and Cellular Biolog
A census of massive stars in NGC 346. Stellar parameters and rotational velocities
Spectroscopy for 247 stars towards the young cluster NGC 346 in the Small
Magellanic Cloud has been combined with that for 116 targets from the
VLT-FLAMES Survey of Massive Stars. Spectral classification yields a sample of
47 O-type and 287 B-type spectra, while radial-velocity variations and/or
spectral multiplicity have been used to identify 45 candidate single-lined
systems, 17 double-lined systems, and one triple-lined system. Atmospheric
parameters (T and log) and projected rotational velocities
(sin) have been estimated using TLUSTY model atmospheres; independent
estimates of sin were also obtained using a Fourier Transform method.
Luminosities have been inferred from stellar apparent magnitudes and used in
conjunction with the T and sin estimates to constrain stellar
masses and ages using the BONNSAI package. We find that targets towards the
inner region of NGC 346 have higher median masses and projected rotational
velocities, together with smaller median ages than the rest of the sample.
There appears to be a population of very young targets with ages of less than 2
Myr, which have presumably all formed within the cluster. The more massive
targets are found to have lower sin consistent with previous studies.
No significant evidence is found for differences with metallicity in the
stellar rotational velocities of early-type stars, although the targets in the
SMC may rotate faster than those in young Galactic clusters. The rotational
velocity distribution for single non-supergiant B-type stars is inferred and
implies that a significant number have low rotational velocity (10\%
with <40 km/s), together with a peak in the probability distribution at
300 km/s. Larger projected rotational velocity estimates have been
found for our Be-type sample and imply that most have rotational velocities
between 200-450 km/s.Comment: Accepted by A&
Wolf-Rayets in IC10: Probing the Nearest Starburst
IC10 is the nearest starburst galaxy, as revealed both by its Halpha surface
brightness and the large number of Wolf-Rayet stars (WRs) per unit area. The
relative number of known WC- to WN-type WRs has been thought to be unusually
high (~2), unexpected for IC10's metallicity. In this Letter we report the
first results of a new and deeper survey for WRs in IC10. We sucessfully
detected all of the spectroscopically known WRs, and based upon comparisons
with a neighboring control field, estimate that the total number of WRs in IC10
is about 100. We present spectroscopic confirmation of two of our WR
candidates, both of which are of WN type. Our photometric survey predicts that
the actual WC/WN ratio is ~0.3. This makes the WC/WN ratio of IC 10 consistent
with that expected for its metallicity, but greatly increases the already
unusually high number of WRs, resulting in a surface density that is about 20
times higher than in the LMC. If the majority of these candidates are
spectroscopically confirmed, IC10 must have an exceptional population of high
mass stars.Comment: Accepted by ApJL; only minor correction in this versio
The Complexity of Admissibility in Omega-Regular Games
Iterated admissibility is a well-known and important concept in classical
game theory, e.g. to determine rational behaviors in multi-player matrix games.
As recently shown by Berwanger, this concept can be soundly extended to
infinite games played on graphs with omega-regular objectives. In this paper,
we study the algorithmic properties of this concept for such games. We settle
the exact complexity of natural decision problems on the set of strategies that
survive iterated elimination of dominated strategies. As a byproduct of our
construction, we obtain automata which recognize all the possible outcomes of
such strategies
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