18,541 research outputs found
The "True" Column Density Distribution in Star-Forming Molecular Clouds
We use the COMPLETE Survey's observations of the Perseus star-forming region
to assess and intercompare three methods for measuring column density in
molecular clouds: extinction mapping (NIR); thermal emission mapping (FIR); and
mapping the intensity of CO isotopologues. The structures shown by all three
tracers are morphologically similar, but important differences exist.
Dust-based measures give similar, log-normal, distributions for the full
Perseus region, once careful calibration corrections are made. We also compare
dust- and gas-based column density distributions for physically-meaningful
sub-regions of Perseus, and we find significant variations in the distributions
for those regions. Even though we have used 12CO data to estimate excitation
temperatures, and we have corrected for opacity, the 13CO maps seem unable to
give column distributions that consistently resemble those from dust measures.
We have edited out the effects of the shell around the B-star HD 278942. In
that shell's interior and in the parts where it overlaps the molecular cloud,
there appears to be a dearth of 13CO, likely due either to 13CO not yet having
had time to form in this young structure, and/or destruction of 13CO in the
molecular cloud. We conclude that the use of either dust or gas measures of
column density without extreme attention to calibration and artifacts is more
perilous than even experts might normally admit. And, the use of 13CO to trace
total column density in detail, even after proper calibration, is unavoidably
limited in utility due to threshold, depletion, and opacity effects. If one's
main aim is to map column density, then dust extinction seems the best probe.
Linear fits amongst column density tracers are given, quantifying the inherent
uncertainties in using one tracer (when compared with others). [abridged]Comment: Accepted in ApJ. 13 pages, 6 color figures. It includes small changes
to improve clarity. For a version with high-resolution figures see
http://www.cfa.harvard.edu/COMPLETE/papers/Goodman_ColumnDensity.pd
Helical Magnetorotational Instability in Magnetized Taylor-Couette Flow
Hollerbach and Rudiger have reported a new type of magnetorotational
instability (MRI) in magnetized Taylor-Couette flow in the presence of combined
axial and azimuthal magnetic fields. The salient advantage of this "helical''
MRI (HMRI) is that marginal instability occurs at arbitrarily low magnetic
Reynolds and Lundquist numbers, suggesting that HMRI might be easier to realize
than standard MRI (axial field only). We confirm their results, calculate HMRI
growth rates, and show that in the resistive limit, HMRI is a weakly
destabilized inertial oscillation propagating in a unique direction along the
axis. But we report other features of HMRI that make it less attractive for
experiments and for resistive astrophysical disks. Growth rates are small and
require large axial currents. More fundamentally, instability of highly
resistive flow is peculiar to infinitely long or periodic cylinders: finite
cylinders with insulating endcaps are shown to be stable in this limit. Also,
keplerian rotation profiles are stable in the resistive limit regardless of
axial boundary conditions. Nevertheless, the addition of toroidal field lowers
thresholds for instability even in finite cylinders.Comment: 16 pages, 2 figures, 1 table, submitted to PR
Evaluation of effectiveness of various devices for attenuation of trailing vortices based on model tests in a large towing basin
The effectiveness of various candidate aircraft-wing devices for attenuation of trailing vortices generated by large aircraft is evaluated on basis of results of experiments conducted with a 0.03-scale model of a Boeing 747 transport aircraft using a technique developed at the HYDRONAUTICS Ship Model Basin. Emphasis is on the effects produced by these devices in the far-field (up to 8 kilometers downstream of full-scale generating aircraft) where the unaltered vortex-wakes could still be hazardous to small following aircraft. The evaluation is based primarily on quantitative measurements of the respective vortex velocity distributions made by means of hot-film probe traverses in a transverse plane at selected stations downstream. The effects of these altered wakes on rolling moment induced on a small following aircraft are also studied using a modified lifting-surface theory with a synthesized Gates Learjet as a typical example. Lift and drag measurements concurrently obtained in the model tests are used to appraise the effects of each device investigated on the performance characteristics of the generating aircraft
Matrix De Rham complex and quantum A-infinity algebras
I establish the relation of the non-commutative BV-formalism with
super-invariant matrix integration. In particular, the non-commutative
BV-equation, defining the quantum A-infinity-algebras, introduced in "Modular
operads and Batalin-Vilkovisky geometry" IMRN, Vol. 2007, doi:
10.1093/imrn/rnm075, is represented via de Rham differential acting on the
matrix spaces related with Bernstein-Leites simple associative algebras with
odd trace q(N), and with gl(N|N). I also show that the Lagrangians of the
matrix integrals from "Noncommmutative Batalin-Vilkovisky geometry and Matrix
integrals", Comptes Rendus Mathematique, vol 348 (2010), pp. 359-362,
arXiv:0912.5484, are equivariantly closed differential forms.Comment: published versio
Radiobiological studies with monoenergetic neutrons
The Radiological Research Accelerator Facility (RARAF) has the capability of
producing essentially monoenergetic neutron beams, ranging in energy from 16.4 MeV
down to 220 keV. In addition, two lower energy neutron beams are available which
consist of a wide spectrum of energies and are described as the 110 keV and 60 keV
spectra. Seedlings of Vicia faba have been used to measure the oxygen enhancement
ratio (OER) and the relative biological effectiveness (RBE) of each of these neutron
beams. The OER decreases as the neutron energy is reduced between 15.4 MeV and
220 keV, but does not appear to decrease further for lower energy neutrons. RBE increases
as the neutron energy is reduced from 15.4 AleV to 440 keV; the curve then
goes through a maximum at around 350 keV, and for lower energies the RBE falls again
Best-Worst Scaling: A simple method to determine drinks and wine style preferences
Wine marketers are continually involved with measuring consumer preferences usually by means of surveys or consumer purchase panel data. In this paper we provide initial results using a relatively new and very straightforward method for measuring consumer preferences. The best-worst scaling method (also called max-diffs) simply asks consumers to look at sets of products, attributes, or other factors to be compared and choose from each set the best/most favourable and the worst/least favourable. A simple count and manipulation results in a single preference scale, where the differences may be compared as distances rather than rank order. Managerial implications of the importance of wine attributes that influence consumer drinks purchasing and wine style selection are discussed as well as suggestions for future research. The goal of this paper is to demonstrate the practical and a scholarly usefulness of this approach and present a call for replication in other markets in an ongoing manner.Steven Goodman, Larry Lockshin and Eli Cohe
Using the best-worst method to examine market segments and identify different influences of consumer choice
Wine marketers use market segmentation to target different products to different segments in order to increase sales, often with little evidence about what influences choice within or between segments. In this paper we provide initial results using a relatively new and very straightforward method for measuring consumer preferences. The best-worst scaling method (also called max-diffs) simply asks consumers to look at sets of products, attributes, or other factors to be compared and choose from each set the best/most favourable and the worst/least favourable. A simple count and manipulation results in a single preference scale, where the differences may be compared as distances rather than rank order. This paper shows how segmenting the consumers using factors such as gender, frequency of consumption, wine involvement and age produce segments with similar preferences for different varietal wines. Two country examples are used, Israel and Australia, to show the ability of the Best-Worst method to develop ‘maps’ of segments across markets based on patterns of choice. The goal of this paper is to demonstrate the practical and a scholarly usefulness of this approach and to show the method for a larger cross-national study across major wine consuming markets.Steven Goodman, Larry Lockshin and Eli Cohenhttp://academyofwinebusiness.com/?page_id=33
Image Storage in Hot Vapors
We theoretically investigate image propagation and storage in hot atomic
vapor. A system is adopted for imaging and an atomic vapor cell is placed
over the transform plane. The Fraunhofer diffraction pattern of an object in
the object plane can thus be transformed into atomic Raman coherence according
to the idea of ``light storage''. We investigate how the stored diffraction
pattern evolves under diffusion. Our result indicates, under appropriate
conditions, that an image can be reconstructed with high fidelity. The main
reason for this procedure to work is the fact that diffusion of opposite-phase
components of the diffraction pattern interfere destructively.Comment: 11 pages, 3 figure
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