15,942 research outputs found
Hypervelocity impact microfoil perforations in the LEO space environment (LDEF, MAP AO-023 experiment)
The Microabrasion Foil Experiment comprises arrays of frames, each supporting two layers of closely spaced metallic foils and a back-stop plate. The arrays, deploying aluminum and brass foil ranging from 1.5 to some 30 microns were exposed for 5.78 years on NASA's LDEF at a mean altitude of 458 km. They were deployed on the North, South, East, West, and Space pointing faces; results presented comprise the perforation rates for each location as a function of foil thickness. Initial results refer primarily to aluminum of 5 microns thickness or greater. This penetration distribution, comprising 2,342 perforations in total, shows significantly differing characteristics for each detector face. The anisotropy confirms, incorporating the dynamics of particulate orbital mechanics, the dominance of incorporating extraterrestrial particulates penetrating thicknesses greater than 20 microns in Al foil, yielding fluxes compatible with hyperbolic geocentric velocities. For thinner foils, a disproportionate increase in flux of particles on the East, North, and South faces shows the presence of orbital particulates which exceed the extraterrestrial component perforation rate at 5 micron foil thickness by a factor of approx. 4
Lingering grains of truth around comet 17P/Holmes
Comet 17P/Holmes underwent a massive outburst in 2007 Oct., brightening by a
factor of almost a million in under 48 hours. We used infrared images taken by
the Wide-Field Survey Explorer mission to characterize the comet as it appeared
at a heliocentric distance of 5.1 AU almost 3 years after the outburst. The
comet appeared to be active with a coma and dust trail along the orbital plane.
We constrained the diameter, albedo, and beaming parameter of the nucleus to
4.135 0.610 km, 0.03 0.01 and 1.03 0.21, respectively. The
properties of the nucleus are consistent with those of other Jupiter Family
comets. The best-fit temperature of the coma was 134 11 K, slightly
higher than the blackbody temperature at that heliocentric distance. Using
Finson-Probstein modeling we found that the morphology of the trail was
consistent with ejection during the 2007 outburst and was made up of dust
grains between 250 m and a few cm in radius. The trail mass was 1.2
- 5.3 10 kg.Comment: Accepted to ApJ. 2 tables, 4 figure
QCD Corrections to t anti-b H^- Associated Production in e^+ e^- Annihilation
We calculate the QCD corrections to the cross section of e^+ e^- -> t anti-b
H^- and its charge-conjugate counterpart within the minimal supersymmetric
extension of the Standard Model. This process is particularly important if m_t
b H^+ and e^+ e^- -> H^+ H^- are
not allowed kinematically. Large logarithmic corrections that arise in the
on-mass-shell scheme of quark mass renormalization, especially from the t
anti-b H^- Yukawa coupling for large values of tan(beta), are resummed by
adopting the modified minimal-subtraction scheme, so that the convergence
behavior of the perturbative expansion is improved. The inclusion of the QCD
corrections leads to a significant reduction of the theoretical uncertainties
due to scheme and scale dependences.Comment: 21 pages (Latex), 8 figures (Postscript); detailed discussion of
scheme and scale dependences adde
The unrestricted Skyrme-tensor time-dependent Hartree-Fock and its application to the nuclear response from spherical to triaxial nuclei
The nuclear time-dependent Hartree-Fock model formulated in the
three-dimensional space,based on the full Skyrme energy density functional and
complemented with the tensor force,is presented for the first time. Full
self-consistency is achieved by the model. The application to the isovector
giant dipole resonance is discussed in the linear limit, ranging from spherical
nuclei (16O, 120Sn) to systems displaying axial or triaxial deformation (24Mg,
28Si, 178Os, 190W, 238U).
Particular attention is paid to the spin-dependent terms from the central
sector of the functional, recently included together with the tensor. They turn
out to be capable of producing a qualitative change on the strength
distribution in this channel. The effect on the deformation properties is also
discussed. The quantitative effects on the linear response are small and,
overall, the giant dipole energy remains unaffected.
Calculations are compared to predictions from the (quasi)-particle random
phase approximation and experimental data where available, finding good
agreement
Asteroid family identification using the Hierarchical Clustering Method and WISE/NEOWISE physical properties
Using albedos from WISE/NEOWISE to separate distinct albedo groups within the
Main Belt asteroids, we apply the Hierarchical Clustering Method to these
subpopulations and identify dynamically associated clusters of asteroids. While
this survey is limited to the ~35% of known Main Belt asteroids that were
detected by NEOWISE, we present the families linked from these objects as
higher confidence associations than can be obtained from dynamical linking
alone. We find that over one-third of the observed population of the Main Belt
is represented in the high-confidence cores of dynamical families. The albedo
distribution of family members differs significantly from the albedo
distribution of background objects in the same region of the Main Belt, however
interpretation of this effect is complicated by the incomplete identification
of lower-confidence family members. In total we link 38,298 asteroids into 76
distinct families. This work represents a critical step necessary to debias the
albedo and size distributions of asteroids in the Main Belt and understand the
formation and history of small bodies in our Solar system.Comment: Accepted to ApJ. Full version of Table 3 to be published
electronically in Ap
A Mesolithic settlement site at Howick, Northumberland: a preliminary report
Excavations at a coastal site at Howick during 2000 and 2002 have revealed evidence for a substantial Mesolithic settlement and a Bronze Age cist cemetery. Twenty one radiocarbon determinations of the earlier eighth millennium BP (Cal.) indicate that the Mesolithic site is one of the earliest known in northern Britain. An 8m core of sediment was recovered from stream deposits adjacent to the archaeological site which provides information on local environmental conditions. Howick offers a unique opportunity to understand aspects of hunter-gatherer colonisation and settlement during a period of rapid palaeogeographical change around the margins of the North Sea basin, at a time when it was being progressively inundated by the final stages of the postglacial marine transgression. The cist cemetery will add to the picture of Bronze Age occupation of the coastal strip and again reveals a correlation between the location of Bronze Age and Mesolithic sites which has been observed elsewhere in the region
Optimization of the visibility of graphene on poly-Si film by thin-film optics engineering
A multilayer optical system containing poly-Si film, SiO 2 film, and Si substrate (poly-Si substrate) has been designed to enhance the visibility of graphene in contact with poly-Si. Film thicknesses of poly-Si and SiO 2 have been optimized by parametric study of the integral contrast of single layer graphene using transfer matrix theory. The multilayer poly-Si substrate and the most commonly used 285 nm SiO 2 /Si substrate (SiO 2 substrate) have been fabricated. Graphene grown by chemical vapor deposition on Ni catalyst has been transferred to the substrates and the visibility of the graphene on the different substrates has been compared. The samples have been characterized by optical microscope, illuminated with light from halogen lamp, and/or filtered with a 600 nm narrow band optical filter. The contrast of graphene on poly-Si substrate has been increased to near 8.7 under 600 nm narrow band illumination from nearly invisible under ordinary illumination, while the contrast of graphene on SiO 2 remains almost the same. Raman spectroscopy has been used to verify the presence of the single layer graphene on the poly-Si substrate.</p
ULTRACAM photometry of the eclipsing cataclysmic variable OU Vir
We present high-speed, three-colour photometry of the faint eclipsing
cataclysmic variable OU Vir. For the first time in OU Vir, separate eclipses of
the white dwarf and bright spot have been observed. We use timings of these
eclipses to derive a purely photometric model of the system, obtaining a mass
ratio of q = 0.175 +/- 0.025, an inclination of i = 79.2 +/- 0.7 degrees and a
disc radius of Rd/a = 0.2315 +/- 0.0150. We separate the white dwarf eclipse
from the lightcurve and, by fitting a blackbody spectrum to its flux in each
passband, obtain a white dwarf temperature of T = 21700 +/- 1200 K and a
distance of D = 650 +/- 210 pc. Assuming that the primary obeys the Nauenberg
(1972) mass-radius relation for white dwarfs and allowing for temperature
effects, we also find a primary mass Mw/Msun = 0.90 +/- 0.19, primary radius
Rw/Rsun = 0.0097 +/- 0.0031 and orbital separation a/Rsun = 0.75 +/- 0.05.Comment: 8 pages LaTeX, 6 figures. Accepted by MNRAS; erratum added at end.
Mon.Not.Roy.Astron.Soc. 347 (2004) 1173, erratum in pres
Main Belt Asteroids with WISE/NEOWISE: Near-Infrared Albedos
We present revised near-infrared albedo fits of 2835 Main Belt asteroids
observed by WISE/NEOWISE over the course of its fully cryogenic survey in 2010.
These fits are derived from reflected-light near-infrared images taken
simultaneously with thermal emission measurements, allowing for more accurate
measurements of the near-infrared albedos than is possible for visible albedo
measurements. As our sample requires reflected light measurements, it
undersamples small, low albedo asteroids, as well as those with blue spectral
slopes across the wavelengths investigated. We find that the Main Belt
separates into three distinct groups of 6%, 16%, and 40% reflectance at 3.4 um.
Conversely, the 4.6 um albedo distribution spans the full range of possible
values with no clear grouping. Asteroid families show a narrow distribution of
3.4 um albedos within each family that map to one of the three observed
groupings, with the (221) Eos family being the sole family associated with the
16% reflectance 3.4 um albedo group. We show that near-infrared albedos derived
from simultaneous thermal emission and reflected light measurements are an
important indicator of asteroid taxonomy and can identify interesting targets
for spectroscopic followup.Comment: Accepted for publication in ApJ; full version of Table1 to be
published electronically in the journa
- …