2,301 research outputs found
Refined and microlocal Kakeya-Nikodym bounds for eigenfunctions in two dimensions
We obtain some improved essentially sharp Kakeya-Nikodym estimates for
eigenfunctions in two-dimensions. We obtain these by proving stronger related
microlocal estimates involving a natural decomposition of phase space that is
adapted to the geodesic flow.Comment: 17 pages, 2 figure
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
Polymer matrix composites on LDEF experiments M0003-9 and M0003-10
Over 250 polymer matrix composites were exposed to the natural space environment on Long Duration Exposure Facility (LDEF) experiments M0003-9 and 10. The experiments included a wide variety of epoxy, thermoplastic, polyimide, and bismalimide matrix composites reinforced with graphite, glass, or organic fibers. A review of the significant observations and test results obtained to date is presented. Estimated recession depths from atomic oxygen exposure are reported and the resulting surface morphologies are discussed. The effects of the LDEF exposure on the flexural strength and modulus, short beam shear strength, and coefficient of thermal expansion of several classes of bare and coated composites are reviewed. Lap shear data are presented for composite-to-composite and composite-to-aluminum alloy samples that were prepared using different bonding techniques and subsequently flown on LDEF
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
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