17,697 research outputs found
Experimental rate coefficients for collisional excitation of lithium-like ions
Collisional excitation rates for lithium-like ions derived from diagnosed plasma produced in theta pinch device and line intensities emitted by these ion
Milky Way Mass and Potential Recovery Using Tidal Streams in a Realistic Halo
We present a new method for determining the Galactic gravitational potential
based on forward modeling of tidal stellar streams. We use this method to test
the performance of smooth and static analytic potentials in representing
realistic dark matter halos, which have substructure and are continually
evolving by accretion. Our FAST-FORWARD method uses a Markov Chain Monte Carlo
algorithm to compare, in 6D phase space, an "observed" stream to models created
in trial analytic potentials. We analyze a large sample of streams evolved in
the Via Lactea II (VL2) simulation, which represents a realistic Galactic halo
potential. The recovered potential parameters are in agreement with the best
fit to the global, present-day VL2 potential. However, merely assuming an
analytic potential limits the dark matter halo mass measurement to an accuracy
of 5 to 20%, depending on the choice of analytic parametrization. Collectively,
mass estimates using streams from our sample reach this fundamental limit, but
individually they can be highly biased. Individual streams can both under- and
overestimate the mass, and the bias is progressively worse for those with
smaller perigalacticons, motivating the search for tidal streams at
galactocentric distances larger than 70 kpc. We estimate that the assumption of
a static and smooth dark matter potential in modeling of the GD-1 and Pal5-like
streams introduces an error of up to 50% in the Milky Way mass estimates.Comment: 12 pages, 6 figures, submitted to ApJ; more information on our stream
sample and a movie of the potential recovery method used can be found at
http://www.astro.yale.edu/abonaca/research/potential_recovery.htm
Electrochemical deposition of silver crystals aboard Skylab 4
Silver crystals were grown aboard Skylab 4 by an electro-chemical reaction and subsequently returned to earth for comparison with crystals grown at 1- and 5-g. Both the Skylab and earth-grown crystals show a variety of structures. Certain tendencies in structure dependency on gravity level, however, can be discerned. In addition, downward growing dendrite streamers; upward growing chunky crystal streamers; growth along an air/liquid interface; and ribbon, film, and fiber crystal habits were observed in experiments conducted on the ground with solutions of varying concentrations. It was also observed that the crystal structures of space and ground electro-deposited silver crystals were very similar to the structures of germanium selenide and germanium telluride crystals grown in space and on the ground by a vapor transport technique. Consideration of the data leads to the conclusions that: (1) the rate of electrochemical displacement of silver ions from a 5 percent aqueous solution by copper is predominantly diffussion controlled in space and kinetically controlled in 1- and higher-g because of augmentation of mass transport by convection; (2) downward and upward crystal streamers are the result of gravity-driven convection, the flow patterns of which can be delineated. Lateral growths along an air/liquid interface are the result of surface-tension-driven convection, the pattern of which also can be delineated; (3) electrolysis in space or low-g environments can produce either dendritic crystals with more perfect microcrystalline structures or massive, single crystals with fewer defects than those grown on ground or at higher g-levels. Ribbons or films of space-grown silicon crystals would find a ready market for electronic substrate and photocell applications. Space-grown dendritic, metal crystals present the possibility of unique catalysts. Large perfect crystals of various materials are desired for a number of electronic and optical applications; and (4) vapor transport growth of germanium selenide and germanium telluride is affected by convection mechanisms similar to the mechanisms hypothesized for the electrochemical deposition of silver crystals. Evidence and considerations leading to the preceding summaries and conclusions are presented. The implications of the findings and conclusions for technological applications are discussed, and recommendations for further experiments are presented
Astronomy using basic Mark 2 very long baseline interferometry
Two experiments were performed in April and September 1976 to determine precise positions of radio sources using conventional Mark 2 VLBI techniques. Four stations in the continental United States observed at a wavelength of 18 cm. The recording bandwidth was 2 MHz. The preliminary results using analyses of fringe rate and delay are discussed and the source positions compared with the results of other measurements
Globular Cluster Streams as Galactic High-Precision Scales - The Poster Child Palomar 5
Using the example of the tidal stream of the Milky Way globular cluster
Palomar 5 (Pal 5), we demonstrate how observational data on streams can be
efficiently reduced in dimensionality and modeled in a Bayesian framework. Our
approach combines detection of stream overdensities by a
Difference-of-Gaussians process with fast streakline models, a continuous
likelihood function built from these models, and inference with MCMC. By
generating model streams, we show that the geometry of the Pal 5
debris yields powerful constraints on the solar position and motion, the Milky
Way and Pal 5 itself. All 10 model parameters were allowed to vary over large
ranges without additional prior information. Using only SDSS data and a few
radial velocities from the literature, we find that the distance of the Sun
from the Galactic Center is kpc, and the transverse velocity is
km/s. Both estimates are in excellent agreement with independent
measurements of these quantities. Assuming a standard disk and bulge model, we
determine the Galactic mass within Pal 5's apogalactic radius of 19 kpc to be
M. Moreover, we find the potential of the
dark halo with a flattening of to be essentially
spherical within the radial range that is effectively probed by Pal 5. We also
determine Pal 5's mass, distance and proper motion independently from other
methods, which enables us to perform vital cross-checks. We conclude that with
more observational data and by using additional prior information, the
precision of this method can be significantly increased.Comment: 28 pages, 14 figures, submitted to ApJ (revised version), comments
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