Attack of the Flying Snakes : Formation of Isolated HI Clouds By Fragmentation of Long Streams

The existence of long (> 100 kpc) HI streams and small (< 20 kpc) free-floating HI clouds is well-known. While the formation of the streams has been investigated extensively, and the isolated clouds are often purported to be interaction debris, little research has been done on the formation of optically dark HI clouds that are not part of a larger stream. One possibility is that such features result from the fragmentation of more extended streams, while another idea is that they are primordial, optically dark galaxies. We test the validity of the fragmentation scenario (via harassment) using numerical simulations. In order to compare our numerical models with observations, we present catalogues of both the known long HI streams (42 objects) and free-floating HI clouds suggested as dark galaxy candidates (51 objects). In particular, we investigate whether it is possible to form compact features with high velocity widths (> 100 km/s), similar to observed clouds which are otherwise intriguing dark galaxy candidates. We find that producing such features is possible but extremely unlikely, occurring no more than 0.2% of the time in our simulations. In contrast, we find that genuine dark galaxies could be extremely stable to harassment and remain detectable even after 5 Gyr in the cluster environment (with the important caveat that our simulations only explore harassment and do not yet include the intracluster medium, heating and cooling, or star formation). We also discuss the possibility that such objects could be the progenitors of recently discovered ultra diffuse galaxies.Comment: 46 pages, 27 figures, accepted for publication in MNRA

Environment Induced Entanglement in Markovian Dissipative Dynamics

We show that two, non interacting 2-level systems, immersed in a common bath, can become mutually entangled when evolving according to a Markovian, completely positive reduced dynamics.Comment: 4 pages, LaTex, no figures, added reference

Frustration Driven Stripe Domain Formation in Co/Pt Multilayer Films

We report microscopic mechanisms for an unusual magnetization reversal behavior in Co/Pt multilayers where some of the first-order reversal curves protrude outside of the major loop. Transmission x-ray microscopy reveals a fragmented stripe domain topography when the magnetic field is reversed prior to saturation, in contrast to an interconnected pattern when reversing from a saturated state. The different domain nucleation and propagation behaviors are due to unannihilated domains from the prior field sweep. These residual domains contribute to random dipole fields that impede the subsequent domain growth and prevent domains from growing as closely together as for the interconnected pattern.Comment: 13 pages, 3 figures, to appear in AP

Powder characteristics, microstructure and properties of graphite platelet reinforced Poly Ether Ether Ketone composites in High Temperature Laser Sintering (HT-LS)

This is the author's accepted manuscript. The final published article is avilable from the publisher via DOI: 10.1016/j.matdes.2015.09.094Copyright © 2015 Elsevier Ltd. All rights reserved.The properties of graphite platelet reinforced Poly Ether Ether Ketone (PEEK/GP) composites from powder to laser sintered parts were investigated in this study. The flowability, particle size and laser absorption characteristics of PEEK/GP powders with various graphite loadings were studied. It was found that the addition of graphite improved laser absorption; however, the flowability of powder was reduced. Micro-CT scanning was used to study the distribution, dispersion and the orientation of graphite platelets as well as the porosity and maximum pore size of laser sintered PEEK/GP composites. The graphite platelets were observed to be distributed evenly in the structure without significant agglomeration. Most of the graphite had their in-plane surface orientated in the X-Y plane of fabrication, which increased the tensile strength of the composites incorporating 5. wt.% graphite. The investigation also demonstrated that the porosity and maximum pore size increased with increasing amounts of graphite. A significant increase in porosity and pore size was found in PEEK/GP composites with 7.5. wt.% graphite, and it is believed to be responsible for the drop in tensile strength. DMA analysis showed no reduction of the damping properties in the composites incorporating up to 5. wt.% graphite, whereas the composites with 7.5. wt.% graphite showed increased stiffness

Polymer viscosity, particle coalescence and mechanical performance in high-temperature laser sintering

The final publication is available at Springer via http://dx.doi.org/10.1007/s10853-016-9761-6High-temperature laser sintering (HT-LS) is an additive manufacturing technology whose potential could be limited by the restricted number of materials optimised for the process. Poly ether ether ketone (PEEK) with different melt viscosity values, PEEK 150PF and PEEK 450PF, have been used in parallel with the commercial grade, poly ether ketone (PEK) HP3, to investigate the role of material viscosity on particle coalescence, structure and mechanical performance of components manufactured in HT-LS. The material with lower viscosity, PEEK 150PF, was found to exhibit faster coalescence and lower tensile strength than the grades with higher viscosities, PEEK 450PF and PEK HP3

Stochastic analysis of global traveltime data: mantle heterogeneity and random errors in the ISC data

Analysis of global traveltime data has been formulated in terms of the stochastic properties of the Earth's heterogeneity pattern and random errors in the data. The formalism relates the coherency of traveltime residuals within bundles of rays (summary rays) of varying size to the spherical harmonic power spectrum of the slowness field of the medium. It has been applied to mantle P-wave data from the ISC catalogue. The measure of coherency is the variance within summary rays. It is estimated within bins in source depth, epicentral distance and the scale size of the area defining a summary ray. The variance at infinitesimal scale length represents the incoherent component of the data (random errors). The variation of the variance with scale length contains information about the autocorrelation function or power spectrum of slowness perturbations within the Earth. The variation with epicentral distance reflects the depth variation of the spectrum. The formalism accounts for the uneven distribution (clustering) of stations and events. We find that estimates of random errors correlate well with complexities on the traveltime curve of P-waves. The variance peaks at 1.0–2.0 s^2 at Δ ≈ 20°, where triplications occur on the traveltime curve, drops to 0.15–0.8s^2 at teleseismic distances, and rises to 0.4–1.3 s^2 approaching the core shadow, where the traveltime curves of P-waves and PcP-waves merge. These estimates should be considered upper bounds for the random error variance of the data. The signal to random noise ratio in the teleseismic ISC P-wave data is about S/N ≈ 2. Inversion of the scale-dependent structural signal in the data yields models that concentrate heterogeneity strongly in the upper mantle. The product of correlation length and power drops by about two orders of magnitude from the surface of the Earth to the lower mantle. About half of this quantity in the upper mantle is due to small-scale features (<300km). The lower mantle is devoid of small-scale structure. It contains 0.1 per cent velocity variations at a characteristic scale of about 1000km. This corresponds to a spectral band-width of l ≈ 7. The D″ layer at the bottom 100–200 km of the mantle shows up as a distinct layer in our results. It has 0.3 per cent velocity variations at a characteristic scale of 350km. The top of the lower mantle contains 0.3 per cent velocity variations on a scale of 500km and also contains some small-scale power. These results are compatible with previous deterministic lower mantle studies, although some details differ. The strength of heterogeneity in the upper mantle may obscure attempts to model the Earth's deep interior