13,318 research outputs found
Extraplanar Dust in the Edge-On Spiral NGC 891
We present high-resolution (<0.65") optical broad-band images of the edge-on
Sb galaxy NGC 891 obtained with the WIYN 3.5-m telescope. These BVR images
reveal a complex network of hundreds of dust absorbing structures far from the
mid-plane of the galaxy. The dust structures have a wide range of morphologies
and are clearly visible to |z|<1.5 kpc from the mid-plane. In this paper we
discuss the general characteristics of the population of absorbing structures,
as well as physical properties of 12 individual features. These 12 structures
are characterised by N_H >10^21 cm^-2, with masses estimated to be more than
2x10^5 - 5x10^6 solar masses, assuming Galactic gas-to-dust relationships. The
gravitational potential energies of the individual dust structures, given their
observed heights and derived masses, lie in the range of 20-200x10^51 ergs.
Rough number counts of extraplanar dust features suggest the mass of high-z gas
associated with extraplanar dust in NGC 891 likely exceeds 2x10^8 solar masses,
or ~2% of the total neutral ISM mass of the galaxy.
We discuss several mechanisms which may produce high-z dust structures such
as those seen in the images presented here. It is not yet known which of these
mechanisms are primarily responsible for the extensive extraplanar dust
structures seen in our images. The data presented are part of a larger program
to search for and characterize off-plane dust structures in edge-on systems.
(Abstract Abridged)Comment: To appear in the Astronomical Journal: 37 pages, Latex; 9 separate
figures; the paper and high-resolution figures are also available at
http://www.astro.wisc.edu/~howk/Papers/papers.htm
The optimal design of standard gearsets
A design procedure for sizing standard involute spur gearsets is presented. The procedure is applied to find the optimal design for two examples - an external gear mesh with a ratio of 5:1 and an internal gear mesh with a ratio of 5:1. In the procedure, the gear mesh is designed to minimize the center distance for a given gear ratio, pressure angle, pinion torque, and allowable tooth strengths. From the methodology presented, a design space may be formulated for either external gear contact or for internal contact. The design space includes kinematics considerations of involute interference, tip fouling, and contact ratio. Also included are design constraints based on bending fatigue in the pinion fillet and Hertzian contact pressure in the full load region and at the gear tip where scoring is possible. This design space is two dimensional, giving the gear mesh center distance as a function of diametral pitch and the number of pinion teeth. The constraint equations were identified for kinematic interference, fillet bending fatigue, pitting fatigue, and scoring pressure, which define the optimal design space for a given gear design. The locus of equal size optimum designs was identified as the straight line through the origin which has the least slope in the design region
An update on the middle levels problem
The middle levels problem is to find a Hamilton cycle in the middle levels,
M_{2k+1}, of the Hasse diagram of B_{2k+1} (the partially ordered set of
subsets of a 2k+1-element set ordered by inclusion). Previously, the best
result was that M_{2k+1} is Hamiltonian for all positive k through k=15. In
this note we announce that M_{33} and M_{35} have Hamilton cycles. The result
was achieved by an algorithmic improvement that made it possible to find a
Hamilton path in a reduced graph of complementary necklace pairs having
129,644,790 vertices, using a 64-bit personal computer.Comment: 11 pages, 5 figure
The optimal design of involute gear teeth with unequal addenda
The design of a gear mesh is treated with the objective of minimizing the gear size for a given gear ratio, pinion torque, pressure angle, and allowable tooth lengths. Tooth strengths considered include scoring, pitting fatigue, and bending fatigue. Kinematic involute interference is avoided. The design variation on standard spur gear teeth called the long and short addendum system, is considered. In this system the mesh center distance and pressure angle are maintained as is the ability to manufacture the teeth with standard tooling. However, the pinion and gear tooth proportions are altered in order to obtain fewer teeth numbers for the same ratio as standard gears without kinematic involute interference. The effect of this nonstandard gearing geometry with on tooth strengths and gear mesh size are studied. For a 2:1 gearing ratio, the optimal nonstandard gear design is compared with the optimal standard gear design
A Method for Deriving Accurate Gas-Phase Abundances for the Multiphase Interstellar Galactic Halo
We describe a new method for determining total gas-phase abundances for the
Galactic ISM with minimal ionization uncertainties. For sight lines toward
globular clusters containing both UV-bright stars and radio pulsars, one can
measure column densities of HI and several metal ions using UV absorption
measurements and of H II using radio dispersion measurements, thereby
minimizing ionization uncertainties. We apply this method to the globular
cluster Messier 3 sight line using FUSE and HST ultraviolet spectroscopy of the
post-asymptotic giant branch star von Zeipel 1128 and radio observations by
Ransom et al. of millisecond pulsars. Ionized hydrogen is 45+/-5% of the total
along this sight line, the highest measured fraction along a high-latitude
pulsar sight line. We derive total gas-phase abundances log N(S)/N(H) =
-4.87+/-0.03 and log N(Fe)/N(H) = -5.27+/-0.05. Our derived sulfur abundance is
in excellent agreement with recent solar system determinations of Asplund,
Grevesse, & Sauval, but -0.14 dex below the solar system abundance typically
adopted in studies of the ISM. The iron abundance is ~-0.7 dex below the solar
system abundance, consistent with significant depletion. Abundance estimates
derived by simply comparing S II and Fe II to H I are +0.17 and +0.11 dex
higher, respectively, than our measurements. Ionization corrections to the
gas-phase abundances measured in the standard way are, therefore, significant
compared with the measurement uncertainties along this sight line. The
systematic uncertainties associated with the uncertain contribution to the
electron column density from ionized helium could raise these abundances by
<+0.03 dex (+7%). [Abridged]Comment: To appear in the ApJ. 25 pages, including figures and tex
Gear mesh compliance modeling
A computer model has been constructed to simulate the compliance and load sharing in a spur gear mesh. The model adds the effect of rim deflections to previously developed state-of-the-art gear tooth deflection models. The effects of deflections on mesh compliance and load sharing are examined. The model can treat gear meshes composed to two external gears or an external gear driving an internal gear. The model includes deflection contributions from the bending and shear in the teeth, the Hertzian contact deformations, and primary and secondary rotations of the gear rims. The model shows that rimmed gears increase mesh compliance and, in some cases, improve load sharing
A survey of interstellar HI from L alpha absorption measurements 2
The Copernicus satellite surveyed the spectral region near L alpha to obtain column densities of interstellar HI toward 100 stars. The distance to 10 stars exceeds 2 kpc and 34 stars lie beyond 1 kpc. Stars with color excess E(B-V) up to 0.5 mag are observed. The value of the mean ratio of total neutral hydrogen to color excess was found to equal 5.8 x 10 to the 21st power atoms per (sq cm x mag). For stars with accurate E(B-V), the deviations from this mean are generally less than a factor of 1.5. A notable exception is the dark cloud star, rho Oph. A reduction in visual reddening efficiency for the grains that are larger than normal in the rho Oph dark cloud probably explains this result. The conversion of atomic hydrogen into molecular form in dense clouds was observed in the gas to E(B-V) correlation plots. The best estimate for the mean total gas density for clouds and the intercloud medium, as a whole, in the solar neighborhood and in the plane of the galaxy is 1.15 atoms per cu. cm; those for the atomic gas and molecular gas alone are 0.86 atoms per cu cm and 0.143 molecules per cu cm respectively. For the intercloud medium, where molecular hydrogen is a negligible fraction of the total gas, atomic gas density was found to equal 0.16 atoms per cu cm with a Gaussian scale height perpendicular to the plane of about 350 pc, as derived from high latitude stars
A method to study complex systems of mesons in Lattice QCD
Finite density systems can be explored with Lattice QCD through the
calculation of multi-hadron correlation functions. Recently, systems with up to
12 's or 's have been studied to determine the 3- and
3- interactions, and the corresponding chemical potentials have been
determined as a function of density. We derive recursion relations between
correlation functions that allow this work to be extended to systems of
arbitrary numbers of mesons and to systems containing many different types of
mesons, such as 's, 's, 's and 's. These relations
allow for the study of finite-density systems in arbitrary volumes, and for the
study of high-density systems.Comment: JLAB-THY-10-1121, NT@UW-10-01, journal versio
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