1,159 research outputs found
Totally ordered commutative monoids
A totally ordered monoid - or tomonoid, for short - is a commutative semigroup with identity S equipped with a total order ≤s that is translation invariant, i.e., that satisfies: ∀x, y, z ∈, x ≤s y ⇒ x + z ≤s y + z. We call a tomonoid that is a quotient of some totally ordered free commutative monoid formally integral. Our most significant results concern characterizations of this condition by means of constructions in the lattice Zn that are reminiscent of the geometric interpretation of the Buchberger algorithm that occurs in integer programming. In particular, we show that every two-generator tomonoid is formally integral. In addition, we give several (new) examples of tomonoids that are not formally integral, we present results on the structure of nil tomonoids and we show how a valuation-theoretic construction due to Hion reveals relationships between formally integral tomonoids and ordered commutative rings satisfying a condition introduced by Henriksen and Isbell
Evaluation of tantalum for mercury containment in the SNAP-8 boiler
Corrosion testing of tantalum for mercury containment in SNAP 8 boile
Constraints on the tectonic and landscape evolution of the Bhutan Himalaya from thermochronometry
The observed geomorphology and calculated thermal histories of the Bhutan Himalaya provide
an excellent platform to test ideas regarding the influence of tectonics and climate on the evolution of a
convergentmountain range. However, little consensus has been reached regarding the late Cenozoic history of
the Bhutan Himalaya. Some researchers have argued that observed geologic relationships show slowing
deformation rates, such that the range is decaying from a geomorphic perspective, while others see the range
as growing and steepening. We suggest that a better understanding is possible through the integrated
interpretation of geomorphic and thermochronometric data from the comparison of predictions from models
of landscape evolution and thermal-kinematic models of orogenic systems. New thermochronometric data
throughout Bhutan aremost consistent with a significant decrease in erosion rates, from2 to 3 km/Ma down to
0.1–0.3 km/Ma, around 6–4Ma. We interpret this pattern as a decrease in rock uplift rates due to the activation
of contractional structures of the Shillong Plateau, an uplifted region approximately 100 km south of Bhutan.
However, low-relief, fluvial landscapes throughout the Bhutanese hinterland record a late pulse of surface uplift
likely due to a recent increase in rock uplift rates. Constraints from our youngest thermochronometers suggest
that this increase in rock uplift and surface uplift occurred within the last 1.75Ma. These results imply that
the dynamics of the Bhutan Himalaya and Shillong Plateau have been linked during the late Cenozoic, with
structural elements of both regions active in variable ways and times over that interval
Meta-analysis & Review of Learner Performance & Preference: Virtual vs. Optical Microscopy
Background & Purpose: For nearly two decades, a wealth of literature has been published describing the various capabilities, uses, and adaptations of virtual microscopy (VM). Many studies have investigated the effects and benefits of VM on student learning compared to optical microscopy (OM). As such, this study statistically aggregated the findings of multiple comparative studies through a meta-analysis to summarize and substantiate the pedagogical efficacy of teaching with VM.
Methods Using predefined eligibility criteria, teams of paired researchers screened the titles and abstracts of VM studies retrieved from seven different databases. After two rounds of screening, numerical and thematic data were extracted from the eligible studies for analysis. A summary effect size and estimate of heterogeneity were calculated to determine the effects of VM on learner performance and the amount of variance between studies, respectively. Trends in student perceptions were also analyzed and reported.
Results: Of the 725 records screened, 72 studies underwent full-text review. In total, 12 studies were viable for meta-analysis and additional studies were reviewed to extract themes relating to learners’ perceptions of VM. The meta-analysis detected a small yet significant positive effect on learner performance (SMD=0.28, [CI=0.09, 0.47], p=0.003), indicating that learners experience marked knowledge gains when exposed to VM over OM. Variation among studies was evident as high heterogeneity was reported. An analysis of trends in learner perceptions noted that respondents favored VM over OM by a large margin.
Conclusions: Despite many individual studies reporting non-significant findings when comparing VM to OM, the enhanced power afforded by meta-analysis revealed that the pedagogical approach of VM is modestly superior to OM and is preferred by learners
Surface temperature measurements using a thin film thermal array
A thin film device was designed and fabricated to measure surface temperatures. An array of eight integrated thermal sensors are mounted on a 0.002 inch (0.05 mm) Kapton film and multiplexed to obtain an area thermal measurement. The device was tested on a flat plate airfoil and demonstrated a temperature variation of 0.55 C maximum and 0.05 C minimum compared to embedded thermocouples. Future improvements are also discussed
Shrinking and Splitting of drainage basins in orogenic landscapes from the migration of the main drainage divide
International audienceClimate, and in particular **the spatial pattern of precipitation, is thought to affect* *the topographic and tectonic evolution of mountain belts through erosion. Numerical model simulations of landscape erosion controlled **by horizontal tectonic motion or orographic precipitation result in the asymmetric topography that characterizes most natural mountain belts, and in a continuous migration of the main drainage divide. The effects of such a migration have, however, been challenging to observe in natural settings. Here I document the effects of a lateral precipitation gradient on a landscape undergoing constant uplift in a laboratory modelling experiment. In the experiment, the drainage divide migrates towards the drier, leeward side of the mountain range, causing the drainage basins on the leeward side to shrink and split into* *smaller basins. This mechanism results in a progressively increasing number of drainage basins on the leeward side of the mountain range as the divide migrates, such that the expected relationship between the spacing of drainage basins and the location of the main drainage divide is maintained. I propose that this mechanism could clarify the drainage divide migration and topographic asymmetry found in active orogenic mountain ranges, as exemplified by the Aconquija Range of Argentin
Dust filtration at gap edges: Implications for the spectral energy distributions of discs with embedded planets
The spectral energy distributions (SEDs) of some T Tauri stars display a
deficit of near-IR flux that could be a consequence of an embedded Jupiter-mass
planet partially clearing an inner hole in the circumstellar disc. Here, we use
two-dimensional numerical simulations of the planet-disc interaction, in
concert with simple models for the dust dynamics, to quantify how a planet
influences the dust at different radii within the disc. We show that pressure
gradients at the outer edge of the gap cleared by the planet act as a filter -
letting particles smaller than a critical size through to the inner disc while
holding back larger particles in the outer disc. The critical particle size
depends upon the disc properties, but is typically of the order of 10 microns.
This filtration process will lead to discontinuous grain populations across the
planet's orbital radius, with small grains in the inner disc and an outer
population of larger grains. We show that this type of dust population is
qualitatively consistent with SED modelling of systems that have optically thin
inner holes in their circumstellar discs. This process can also produce a very
large gas-to-dust ratio in the inner disc, potentially explaining those systems
with optically thin inner cavities that still have relatively high accretion
rates.Comment: 9 pages, 7 figures, Accepted fir publication in MNRA
Astrometry with Hubble Space Telescope: A Parallax of the Fundamental Distance Calibrator RR Lyrae
We present an absolute parallax and relative proper motion for the
fundamental distance scale calibrator, RR Lyr. We obtain these with astrometric
data from FGS 3, a white-light interferometer on HST. We find mas. Spectral classifications and VRIJHKTM and DDO51 photometry of
the astrometric reference frame surrounding RR Lyr indicate that field
extinction is low along this line of sight. We estimate =0.07\pm0.03 for
these reference stars. The extinction suffered by RR Lyr becomes one of the
dominant contributors to the uncertainty in its absolute magnitude. Adopting
the average field absorption, =0.07 \pm 0.03, we obtain M_V^{RR} = 0.61
^{-0.11}_{+0.10}. This provides a distance modulus for the LMC, m-M = 18.38 -
18.53^{-0.11}_{+0.10} with the average extinction-corrected magnitude of RR Lyr
variables in the LMC, , remaining a significant uncertainty. We compare
this result to more than 80 other determinations of the distance modulus of the
LMC.Comment: Several typos corrected. To appear in The Astronomical Journal,
January 200
THE SPIRAL WAVE INSTABILITY INDUCED BY A GIANT PLANET. I. PARTICLE STIRRING IN THE INNER REGIONS OF PROTOPLANETARY DISKS
We have recently shown that spiral density waves propagating in accretion
disks can undergo a parametric instability by resonantly coupling with and
transferring energy into pairs of inertial waves (or inertial-gravity waves
when buoyancy is important). In this paper, we perform inviscid
three-dimensional global hydrodynamic simulations to examine the growth and
consequence of this instability operating on the spiral waves driven by a
Jupiter-mass planet in a protoplanetary disk. We find that the spiral waves are
destabilized via the spiral wave instability (SWI), generating hydrodynamic
turbulence and sustained radially-alternating vertical flows that appear to be
associated with long wavelength inertial modes. In the interval , where denotes the semi-major axis of the planetary orbit
(assumed to be 5~au), the estimated vertical diffusion rate associated with the
turbulence is characterized by . For the disk model considered here, the diffusion rate is such that
particles with sizes up to several centimeters are vertically mixed within the
first pressure scale height. This suggests that the instability of spiral waves
launched by a giant planet can significantly disperse solid particles and trace
chemical species from the midplane. In planet formation models where the
continuous local production of chondrules/pebbles occurs over Myr time scales
to provide a feedstock for pebble accretion onto these bodies, this stirring of
solid particles may add a time constraint: planetary embryos and large
asteroids have to form before a gas giant forms in the outer disk, otherwise
the SWI will significantly decrease the chondrule/pebble accretion efficiency.Comment: Accepted for publication in the The Astrophysical Journal, 19 pages,
12 figures, 1 tabl
The Wide-field Infrared Survey Explorer (WISE): Mission Description and Initial On-orbit Performance
The all sky surveys done by the Palomar Observatory Schmidt, the European
Southern Observatory Schmidt, and the United Kingdom Schmidt, the InfraRed
Astronomical Satellite and the 2 Micron All Sky Survey have proven to be
extremely useful tools for astronomy with value that lasts for decades. The
Wide-field Infrared Survey Explorer is mapping the whole sky following its
launch on 14 December 2009. WISE began surveying the sky on 14 Jan 2010 and
completed its first full coverage of the sky on July 17. The survey will
continue to cover the sky a second time until the cryogen is exhausted
(anticipated in November 2010). WISE is achieving 5 sigma point source
sensitivities better than 0.08, 0.11, 1 and 6 mJy in unconfused regions on the
ecliptic in bands centered at wavelengths of 3.4, 4.6, 12 and 22 microns.
Sensitivity improves toward the ecliptic poles due to denser coverage and lower
zodiacal background. The angular resolution is 6.1, 6.4, 6.5 and 12.0
arc-seconds at 3.4, 4.6, 12 and 22 microns, and the astrometric precision for
high SNR sources is better than 0.15 arc-seconds.Comment: 22 pages with 19 included figures. Updated to better match the
accepted version in the A
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