670 research outputs found
PYROMEMS IGNITER BASED ON A TEMPERATURE GRADIENT: CONCEPT, FABRICATION AND CHARACTERIZATION
A pyroMEMS igniter with increased combustion reliability is presented. The igniter consisted of a thin-film platinum Joule heater fabricated on a borosilicate glass substrate. Two different igniter layouts (meandering and annular) and three different binder mass fractions (5, 10 and 20 %) were evaluated. High-speed videos were used to judge the success or failure of the combustion events. Although the ignition success rate was 100 %, the combustion success rate was approximately 87.5 ± 7.1 % for the annular design versus 12.5 ± 7.1 % for the meandering layout. No effect on success rate was observed for the different binder contents tested. Rather, increasing the binder mass fraction increased the combustion duration
HE0107-5240, A Chemically Ancient Star.I. A Detailed Abundance Analysis
We report a detailed abundance analysis for HE0107-5240, a halo giant with
[Fe/H]_NLTE=-5.3. This star was discovered in the course of follow-up
medium-resolution spectroscopy of extremely metal-poor candidates selected from
the digitized Hamburg/ESO objective-prism survey. On the basis of
high-resolution VLT/UVES spectra, we derive abundances for 8 elements (C, N,
Na, Mg, Ca, Ti, Fe, and Ni), and upper limits for another 12 elements. A
plane-parallel LTE model atmosphere has been specifically tailored for the
chemical composition of {\he}. Scenarios for the origin of the abundance
pattern observed in the star are discussed. We argue that HE0107-5240 is most
likely not a post-AGB star, and that the extremely low abundances of the
iron-peak, and other elements, are not due to selective dust depletion. The
abundance pattern of HE0107-5240 can be explained by pre-enrichment from a
zero-metallicity type-II supernova of 20-25M_Sun, plus either self-enrichment
with C and N, or production of these elements in the AGB phase of a formerly
more massive companion, which is now a white dwarf. However, significant radial
velocity variations have not been detected within the 52 days covered by our
moderate-and high-resolution spectra. Alternatively, the abundance pattern can
be explained by enrichment of the gas cloud from which HE0107-5240 formed by a
25M_Sun first-generation star exploding as a subluminous SNII, as proposed by
Umeda & Nomoto (2003). We discuss consequences of the existence of HE0107-5240
for low-mass star formation in extremely metal-poor environments, and for
currently ongoing and future searches for the most metal-poor stars in the
Galaxy.Comment: 60 pages, 16 figures. Accepted for publication in Ap
Optical Excitations and Field Enhancement in Short Graphene Nanoribbons
The optical excitations of elongated graphene nanoflakes of finite length are
investigated theoretically through quantum chemistry semi-empirical approaches.
The spectra and the resulting dipole fields are analyzed, accounting in full
atomistic details for quantum confinement effects, which are crucial in the
nanoscale regime. We find that the optical spectra of these nanostructures are
dominated at low energy by excitations with strong intensity, comprised of
characteristic coherent combinations of a few single-particle transitions with
comparable weight. They give rise to stationary collective oscillations of the
photoexcited carrier density extending throughout the flake, and to a strong
dipole and field enhancement. This behavior is robust with respect to width and
length variations, thus ensuring tunability in a large frequency range. The
implications for nanoantennas and other nanoplasmonic applications are
discussed for realistic geometries
Transport Coefficients for Granular Media from Molecular Dynamics Simulations
Under many conditions, macroscopic grains flow like a fluid; kinetic theory
pred icts continuum equations of motion for this granular fluid. In order to
test the theory, we perform event driven molecular simulations of a
two-dimensional gas of inelastic hard disks, driven by contact with a heat
bath. Even for strong dissipation, high densities, and small numbers of
particles, we find that continuum theory describes the system well. With a bath
that heats the gas homogeneously, strong velocity correlations produce a
slightly smaller energy loss due to inelastic collisions than that predicted by
kinetic theory. With an inhomogeneous heat bath, thermal or velocity gradients
are induced. Determination of the resulting fluxes allows calculation of the
thermal conductivity and shear viscosity, which are compared to the predictions
of granular kinetic theory, and which can be used in continuum modeling of
granular flows. The shear viscosity is close to the prediction of kinetic
theory, while the thermal conductivity can be overestimated by a factor of 2;
in each case, transport is lowered with increasing inelasticity.Comment: 14 pages, 17 figures, 39 references, submitted to PRE feb 199
Soft systems methodology: a context within a 50-year retrospective of OR/MS
Soft systems methodology (SSM) has been used in the practice of operations research and management science OR/MS) since the early 1970s. In the 1990s, it emerged as a viable academic discipline. Unfortunately, its proponents consider SSM and traditional systems thinking to be mutually exclusive. Despite the differences claimed by SSM proponents between the two, they have been complementary. An extensive sampling of the OR/MS literature over its entire lifetime demonstrates the richness with which the non-SSM literature has been addressing the very same issues as does SSM
The modulation of auditory novelty processing by working memory load in school age children and adults: a combined behavioral and event-related potential study
<p>Abstract</p> <p>Background</p> <p>We investigated the processing of task-irrelevant and unexpected novel sounds and its modulation by working-memory load in children aged 9-10 and in adults. Environmental sounds (novels) were embedded amongst frequently presented standard sounds in an auditory-visual distraction paradigm. Each sound was followed by a visual target. In two conditions, participants evaluated the position of a visual stimulus (0-back, low load) or compared the position of the current stimulus with the one two trials before (2-back, high load). Processing of novel sounds were measured with reaction times, hit rates and the auditory event-related brain potentials (ERPs) Mismatch Negativity (MMN), P3a, Reorienting Negativity (RON) and visual P3b.</p> <p>Results</p> <p>In both memory load conditions novels impaired task performance in adults whereas they improved performance in children. Auditory ERPs reflect age-related differences in the time-window of the MMN as children showed a positive ERP deflection to novels whereas adults lack an MMN. The attention switch towards the task irrelevant novel (reflected by P3a) was comparable between the age groups. Adults showed more efficient reallocation of attention (reflected by RON) under load condition than children. Finally, the P3b elicited by the visual target stimuli was reduced in both age groups when the preceding sound was a novel.</p> <p>Conclusion</p> <p>Our results give new insights in the development of novelty processing as they (1) reveal that task-irrelevant novel sounds can result in contrary effects on the performance in a visual primary task in children and adults, (2) show a positive ERP deflection to novels rather than an MMN in children, and (3) reveal effects of auditory novels on visual target processing.</p
A spectroscopic and proper motion search of Sloan Digital Sky Survey : red subdwarfs in binary systems
Red subdwarfs in binary systems are crucial for both model calibration and spectral classification. We search for red subdwarfs in binary systems from a sample of high proper motion objects with Sloan Digital Sky Survey spectroscopy. We present here discoveries from this search, as well as highlight several additional objects of interest. We find 30 red subdwarfs in wide binary systems including: two with spectral type of esdM5.5, 6 companions to white dwarfs and 3 carbon-enhanced red subdwarfs with normal red subdwarf companions. 15 red subdwarfs in our sample are partially resolved close binary systems. With this binary sample, we estimate the low limit of the red subdwarf binary fraction of similar to 10 per cent. We find that the binary fraction goes down with decreasing masses and metallicities of red subdwarfs. A spectroscopic esdK7 subdwarf + white dwarf binary candidate is also reported. 30 new M subdwarfs have spectral type of >= M6 in our sample. We also derive relationships between spectral types and absolute magnitudes in the optical and near-infrared for M and L subdwarfs, and we present an M subdwarf sample with measured U, V, W space velocities.Peer reviewe
A stellar relic from the early Milky Way
The chemical composition of the most metal-deficient stars reflects the
composition of the gas from which they formed. These old stars provide crucial
clues to the star formation history and the synthesis of chemical elements in
the early Universe. They are the local relics of epochs otherwise observable
only at very high redshifts; if totally metal-free (``population III'') stars
could be found, this would allow the direct study of the pristine gas from the
Big Bang. Earlier searches for such stars found none with an iron abundance
less than 1/10,000 that of the Sun, leading to the suggestion that low-mass
stars could only form from clouds above a critical iron abundance. Here we
report the discovery of a low-mass star with an iron abundance as low as
1/200,000 of the solar value. This discovery suggests that population III stars
could still exist, that is, that the first generation of stars also contained
long-lived low-mass objects. The previous failure to find them may be an
observational selection effect.Comment: Offprint of Nature 419 (2002), 904-906 (issue 31 October 2002
- …