7,276 research outputs found
Neutron Star Mergers Are the Dominant Source of the r-process in the Early Evolution of Dwarf Galaxies
There are many candidate sites of the r-process: core-collapse supernovae
(including rare magnetorotational core-collapse supernovae), neutron star
mergers, and neutron star/black hole mergers. The chemical enrichment of
galaxies---specifically dwarf galaxies---helps distinguish between these
sources based on the continual build-up of r-process elements. This technique
can distinguish between the r-process candidate sites by the clearest
observational difference---how quickly these events occur after the stars are
created. The existence of several nearby dwarf galaxies allows us to measure
robust chemical abundances for galaxies with different star formation
histories. Dwarf galaxies are especially useful because simple chemical
evolution models can be used to determine the sources of r-process material. We
have measured the r-process element barium with Keck/DEIMOS medium-resolution
spectroscopy. We present the largest sample of barium abundances (almost 250
stars) in dwarf galaxies ever assembled. We measure [Ba/Fe] as a function of
[Fe/H] in this sample and compare with existing [alpha/Fe] measurements. We
have found that a large contribution of barium needs to occur at more delayed
timescales than core-collapse supernovae in order to explain our observed
abundances, namely the significantly more positive trend of the r-process
component of [Ba/Fe] vs. [Fe/H] seen for [Fe/H] <~ -1.6 when compared to the
[Mg/Fe] vs. [Fe/H] trend. We conclude that neutron star mergers are the most
likely source of r-process enrichment in dwarf galaxies at early times.Comment: Accepted to ApJ on 2018 October 2
Progress in the development of an 88-mm bore 10 Tn3Sn dipole magnet
A 10 T, 2-layer cos(&thetas;)-dipole model magnet with an 88 mm clear bore utilizing an advanced powder-in-tube Nb3Sn conductor is being developed for the LHC. A dedicated conductor development program has resulted in a well performing Rutherford cable containing strands that uniquely exhibit both an overall current density of 600 A/mm2 @ 11 T and filaments with a diameter of 20 ¿m. The resistance between crossing strands amounts to 30-70 ¿¿ by insertion of a stainless steel core. After being exposed to a transverse pressure of 200 MPa identical cables show negligible permanent degradation of the critical current. The mechanical support structure is further optimized in order to reduce the peak stress in the mid-plane to below 130 MPa at full excitation and to control the pre-stress build-up during system assembly. Prior to the manufacturing of the final coils a dummy 2-layer pole is wound, heat-treated at 675°C and vacuum resin impregnated. This paper presents the current status of the magnet development program and highlights in particular the successful conductor developmen
Evaluation of the optical switching characteristics of erbium-doped fibres for the development of a fibre Bragg grating sensor interrogator
A polling topology that employs optical switching based on the properties of erbium-doped fibres (EDFs) is used to interrogate an array of FBGs. The properties of the EDF are investigated in its pumped and un-pumped states and the EDFsâ switching properties are evaluated by comparing them with a high performance electronically controlled MEM optical switch. Potential advantages of the proposed technique are discussed. © (2014) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only
Attracted Diffusion-Limited Aggregation
In this paper, we present results of extensive Monte Carlo simulations of
diffusion-limited aggregation (DLA) with a seed placed on an attractive plane
as a simple model in connection with the electrical double layers. We compute
the fractal dimension of the aggregated patterns as a function of the
attraction strength \alpha. For the patterns grown in both two and three
dimensions, the fractal dimension shows a significant dependence on the
attraction strength for small values of \alpha, and approaches to that of the
ordinary two-dimensional (2D) DLA in the limit of large \alpha. For
non-attracting case with \alpha=1, our results in three dimensions reproduce
the patterns of 3D ordinary DLA, while in two dimensions our model leads to
formation of a compact cluster with dimension two. For intermediate \alpha, the
3D clusters have quasi-2D structure with a fractal dimension very close to that
of the ordinary 2D-DLA. This allows one to control morphology of a growing
cluster by tuning a single external parameter \alpha.Comment: 6 pages, 6 figures, to appear in Phys. Rev. E (2012
Linking dwarf galaxies to halo building blocks with the most metal-poor star in Sculptor
Current cosmological models indicate that the Milky Way's stellar halo was
assembled from many smaller systems. Based on the apparent absence of the most
metal-poor stars in present-day dwarf galaxies, recent studies claimed that the
true Galactic building blocks must have been vastly different from the
surviving dwarfs. The discovery of an extremely iron-poor star (S1020549) in
the Sculptor dwarf galaxy based on a medium-resolution spectrum cast some doubt
on this conclusion. However, verification of the iron-deficiency and
measurements of additional elements, such as the alpha-element Mg, are
mandatory for demonstrating that the same type of stars produced the metals
found in dwarf galaxies and the Galactic halo. Only then can dwarf galaxy stars
be conclusively linked to early stellar halo assembly. Here we report
high-resolution spectroscopic abundances for 11 elements in S1020549,
confirming the iron abundance of less than 1/4000th that of the Sun, and
showing that the overall abundance pattern mirrors that seen in low-metallicity
halo stars, including the alpha-elements. Such chemical similarity indicates
that the systems destroyed to form the halo billions of years ago were not
fundamentally different from the progenitors of present-day dwarfs, and
suggests that the early chemical enrichment of all galaxies may be nearly
identical.Comment: 16 pages, including 2 figures. Accepted for publication in Nature. It
is embargoed for discussion in the press until formal publication in Natur
Staying out of range: increasing attacking distance in fencing
To avoid being hit, fencers typically adopt an out of range position, which was hypothesized to be governed by body- and action-scaled affordances. This theory was measured in elite and national level junior (u20) fencers. Associations between âreachabilityâ of lunging and step-lunging attacks, was assessed against height, arm-span, leg-span, body mass and lower-body power, and then compared across level. Reachability was determined as the distance covered by fencers during these attacks and was reported as actual and estimated distances. Elite fencers are better at estimating their lunging and step-lunge distance compared to national ranked junior fencers (-0.9 vs. 7.3 % and 5.4 vs. 10.9 % respectively). Surprisingly, elite fencersâ actual and estimated distances for these was less than the junior fencersâ (222.6 vs. 251.5 cm and 299.3 vs. 360.2 cm respectively), and significantly so in the former. Finally only arm (r = .81) and leg span (r = .71) were significantly correlated to estimated lunging distance and this was only in elite fencers. Findings suggest that better fencers can accurately predict their attack range and that reachability appears to be positively influenced by arm and leg-span; these may feed in to talent identification. Given that distances were less in elite fencers, findings suggests that timing and distance estimation are key skills to master, and that the mastery of these in offensive actions can mitigate to a large extent, the physical benefits of an opponentâs greater height
Multi-Element Abundance Measurements from Medium-Resolution Spectra. III. Metallicity Distributions of Milky Way Dwarf Satellite Galaxies
We present metallicity distribution functions (MDFs) for the central regions
of eight dwarf satellite galaxies of the Milky Way: Fornax, Leo I and II,
Sculptor, Sextans, Draco, Canes Venatici I, and Ursa Minor. We use the
published catalog of abundance measurements from the previous paper in this
series. The measurements are based on spectral synthesis of iron absorption
lines. For each MDF, we determine maximum likelihood fits for Leaky Box,
Pre-Enriched, and Extra Gas (wherein the gas supply available for star
formation increases before it decreases to zero) analytic models of chemical
evolution. Although the models are too simplistic to describe any MDF in
detail, a Leaky Box starting from zero metallicity gas fits none of the
galaxies except Canes Venatici I well. The MDFs of some galaxies, particularly
the more luminous ones, strongly prefer the Extra Gas Model to the other
models. Only for Canes Venatici I does the Pre-Enriched Model fit significantly
better than the Extra Gas Model. The best-fit effective yields of the less
luminous half of our galaxy sample do not exceed 0.02 Z_sun, indicating that
gas outflow is important in the chemical evolution of the less luminous
galaxies. We surmise that the ratio of the importance of gas infall to gas
outflow increases with galaxy luminosity. Strong correlations of average [Fe/H]
and metallicity spread with luminosity support this hypothesis.Comment: 17 pages, 5 figures; accepted for publication in ApJ; minor
corrections in v3; corrected typographical errors in Tables 1 and 3 in v
Fast Ramping Superconducting Magnet Design Issues for Future Injector Upgrades at CERN
An upgrade of the LHC injection chain, and especially the sequence of PS and SPS, up to an extraction energy of 1ĂÂ TeV, is one of the steps considered to improve the performance of the whole accelerator complex. The magnets for this upgrade require central magnetic field from 2 T (for a PS upgrade) to 4.5 T (for an SPS upgrade), for which superconducting magnets are a candidate. Due to the fast field sweep rate of the magnets (from about 1.5 T/s to 2.5ĂÂ T/s), internal heating from eddy and persistent current effects (AC loss) must be minimized. In this paper we discuss a rationale for the design and optimization of fast ramped superconducting accelerator magnets, specifically aimed at the LHC injectors. We introduce a design parameter, the product of bore field and field ramp-rate, providing a measure of the magnet performance, and we apply it to choose the design range for a technology demonstration magnet. We finally discuss the dependence of key design parameters on the bore field and the bore diameter, to provide an approximate scaling and guidelines for critical R&D
- âŠ