4,993 research outputs found
The Southern Proper Motion Program III. A Near-Complete Catalog to V=17.5
We present the third installment of the Yale/San Juan Southern Proper Motion
Catalog, SPM3. Absolute proper motions, positions, and photographic B,V
photometry are given for roughly 10.7 million objects, primarily stars, down to
a magnitude of V=17.5. The Catalog covers an irregular area of 3700 square
degrees, between the declinations of -20 and -45 degrees, excluding the
Galactic plane. The proper-motion precision, for well-measured stars, is
estimated to be 4.0 mas/yr. Unlike previous releases of the SPM Catalog, the
proper motions are on the International Celestial Reference System by way of
Hipparcos Catalog stars, and have an estimated systematic uncertainty of 0.4
mas/yr. The SPM3 Catalog is available via electronic
transfer,(http://www.astro.yale.edu/astrom/) As an example of the potential of
the SPM3 proper motions, we examine the Galactocentric velocities of a group of
metal-poor, main-sequence A stars. The majority of these exhibit thick-disk
kinematics, lending support to their interpretation as thick-disk blue
stragglers, as opposed to being an accreted component.Comment: 23 pages, 10 figures, accepted for publication in Astronomical
Journa
Neutron diffraction and gravimetric study of the manganese nitriding reaction under ammonia decomposition conditions
Manganese and its nitrides have recently been shown to co-catalyse the ammonia decomposition reaction. The nitriding reaction of manganese under ammonia decomposition conditions is studied in situ simultaneously by thermogravimetric analysis and neutron diffraction. Combining these complementary measurements has yielded information on the rate of manganese nitriding as well as the elucidation of a gamut of different manganese nitride phases. The neutron diffraction background was shown to be related to the extent of the ammonia decomposition and therefore the gas composition. From this and the sample mass, implications about the rate-limiting steps for nitriding by ammonia and nitriding by nitrogen are discussed
Dimer-mediated cation diffusion in the stoichiometric ionic conductor Li3N
Non-equilibrium molecular dynamics has been used to model cation diffusion in stoichiometric Li3N over the temperature range 50 < T/K < 800. The resulting diffusion coefficients are in excellent agreement with the available experimental data. We present a detailed atomistic account of the diffusion process. Contrary to the conclusions drawn from previous studies, our calculations show that it is unnecessary to invoke the presence of a small concentration of intrinsic defects in order to initiate diffusion. The structure can be considered to consist of alternating layers of composition Li2N and Li. As the temperature increases an increasing number of cations leave the Li2N layers and migrate either to the interlayer space or to the Li layer. Those that move into the interlayer space form Li2 dimers with cations in the Li2N layers and those that move into the neighboring layer form dimers with cations therein. The two types of dimer are aligned parallel and perpendicular to [001], respectively and have lifetimes of ∼3 ps. The vacancies so created facilitate rapid diffusion in the Li2N layers and the interlayer cation motion results in slower diffusion perpendicular to the layers
A Deep Proper-Motion Survey in Kapteyn Selected Areas: I. Survey Description and First Results for Stars in the Tidal Tail of Sagittarius and in the Monoceros Ring
We describe a high-precision, deep (to V ~ 19-21) absolute proper-motion
survey that samples ~50 lines of sight in the Kapteyn Selected Areas along
declination zones -15, 0 and 15 degrees. In many fields the astrometric
baseline reaches nearly a century. We demonstrate that these data provide
typical per star precisions between ~ 1 and 3 mas/yr to the above magnitude
limits, with the absolute reference frame established by numerous extragalactic
sources in each survey field. Combined with existing and ongoing photometric
and radial velocity surveys in these fields, these astrometric data will
enable, among other things, accurate, detailed dynamical modeling of satellite
interactions with our Galaxy. In this contribution we describe the astrometric
part of our survey and show preliminary results along the trailing tail of the
Sagittarius dwarf galaxy, and in the Monoceros ring region.Comment: Accepter for publication in the Astronomical Journa
Diffusion in Li<sub>2</sub>O studied by non-equilibrium molecular dynamics for 873 <T/K <1603
The use of non-equilibrium molecular dynamics facilitates the calculation of the cation diffusion constant of Li2O at temperatures too low to be accessible by other methods. Excellent agreement with experimental diffusion coefficients has been obtained over the temperature range 873 < T/K < 1603. Diffusion below 1200 K was shown to be dominated by a concerted nearest-neighbour hopping process, whereas in the high-temperature superionic region an additional mechanism involving a six-coordinate interstitial cation site in the anti-fluorite structure becomes increasingly dominant. Our model thus accounts for the transition from the superionic regime to the non-superionic regime.</p
Observations of Binary Stars with the Differential Speckle Survey Instrument. VII. Measures from 2010 September to 2012 February at the WIYN Telescope
We report on speckle observations of binary stars carried out at the WIYN
Telescope over the period from September 2010 through February 2012, providing
relative astrometry for 2521 observations of 883 objects, 856 of which are
double stars and 27 of which are triples. The separations measured span a range
of 0.01 to 1.75 arc seconds. Wavelengths of 562 nm, 692 nm, and 880 nm were
used, and differential photometry at one or more of these wavelengths is
presented in most cases. Sixty-six components were resolved for the first time.
We also estimate detection limits at 0.2 and 1.0 arc seconds for high-quality
observations in cases where no companion was seen, a total of 176 additional
objects. Detection limits vary based on observing conditions and
signal-to-noise ratio, but are approximately 4 magnitudes at 0.2 arc seconds
and 6 magnitudes at 1.0 arc seconds on average. Analyzing the measurement
precision of the data set, we find that the individual separations obtained
have linear measurement uncertainties of approximately 2 mas, and photometry is
uncertain to approximately 0.1 magnitudes in general. This work provides
fundamental, well-calibrated data for future orbit and mass determinations, and
we present three first orbits and total mass estimates of nearby K-dwarf
systems as examples of this potential
Ammonia decomposition catalysis using lithium–calcium imide
Lithium–calcium imide is explored as a catalyst for the decomposition of ammonia. It shows the highest ammonia decomposition activity yet reported for a pure light metal amide or imide, comparable to lithium imide–amide at high temperature, with superior conversion observed at lower temperatures. Importantly, the post-reaction mass recovery of lithium–calcium imide is almost complete, indicating that it may be easier to contain than the other amide–imide catalysts reported to date. The basis of this improved recovery is that the catalyst is, at least partially, solid across the temperature range studied under ammonia flow. However, lithium–calcium imide itself is only stable at low and high temperatures under ammonia, with in situ powder diffraction showing the decomposition of the catalyst to lithium amide–imide and calcium imide at intermediate temperatures of 200–460 °C.</p
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