1,789 research outputs found
Current Results from the RRFID Kinematic Survey: Apparent Speeds from the First Five Years of Data
We present current results from our ongoing project to study the parsec-scale
relativistic jet kinematics of sources in the U.S. Naval Observatory's Radio
Reference Frame Image Database (RRFID). The RRFID consists of snapshot
observations using the VLBA plus up to 9 additional antennas at 8 and 2 GHz.
The Image Database currently contains about 3000 images of 450 sources from
1994 to 2004, with some sources having images at 20 epochs or more. We have now
completed analysis of the 8 GHz images for all sources observed at 3 or more
epochs from 1994 to 1998. The completed analysis comprises 966 images of 87
sources, or an average of 11 epochs per source. Apparent jet speeds have been
measured for these sources, and the resulting speed distribution has been
compared with results obtained by other large VLBI surveys. The measured
apparent speed distribution agrees with those found by the 2 cm survey and
Caltech-Jodrell Bank (CJ) survey; however, when a source-by-source comparison
is done with the 2 cm survey results, significant disagreement is found in the
apparent speed measurements for a number of sources. This disagreement can be
traced in most cases to either an insufficient time baseline for the current
RRFID results, or to apparent component mis-identification in the 2 cm survey
results caused by insufficient time sampling. These results emphasize the need
for long time baselines and dense time sampling for multi-epoch monitoring of
relativistic jets.Comment: 4 pages, To be published in the Proceedings of the 7th European VLBI
Network Symposiu
A dynamic high-resolution simulation of 2-D elastic-plastic crack problems
In this paper we present a numerical method for performing higher-order simulations of elastic-plastic waves in solids and we illustrate the accuracy of our approach by various numerical simulations of wave phenomena around cracks. The simulation of crack problems in solids is especially challenging, since the singularities in physical variables occurring at the crack are particularly difficult for numerical schemes to simulate, but they create many interesting physical effect
Low cost solar array project silicon materials task. Development of a process for high capacity arc heater production of silicon for solar arrays
The experimental verification system for the production of silicon via the arc heater-sodium reduction of SiCl4 was designed, fabricated, installed, and operated. Each of the attendant subsystems was checked out and operated to insure performance requirements. These subsystems included: the arc heaters/reactor, cooling water system, gas system, power system, Control & Instrumentation system, Na injection system, SiCl4 injection system, effluent disposal system and gas burnoff system. Prior to introducing the reactants (Na and SiCl4) to the arc heater/reactor, a series of gas only-power tests was conducted to establish the operating parameters of the three arc heaters of the system. Following the successful completion of the gas only-power tests and the readiness tests of the sodium and SiCl4 injection systems, a shakedown test of the complete experimental verification system was conducted
The Celestial Reference Frame at 24 and 43 GHz. II. Imaging
We have measured the sub-milli-arcsecond structure of 274 extragalactic
sources at 24 and 43 GHz in order to assess their astrometric suitability for
use in a high frequency celestial reference frame (CRF). Ten sessions of
observations with the Very Long Baseline Array have been conducted over the
course of 5 years, with a total of 1339 images produced for the 274
sources. There are several quantities that can be used to characterize the
impact of intrinsic source structure on astrometric observations including the
source flux density, the flux density variability, the source structure index,
the source compactness, and the compactness variability. A detailed analysis of
these imaging quantities shows that (1) our selection of compact sources from
8.4 GHz catalogs yielded sources with flux densities, averaged over the
sessions in which each source was observed, of about 1 Jy at both 24 and 43
GHz, (2) on average the source flux densities at 24 GHz varied by 20%-25%
relative to their mean values, with variations in the session-to-session flux
density scale being less than 10%, (3) sources were found to be more compact
with less intrinsic structure at higher frequencies, and (4) variations of the
core radio emission relative to the total flux density of the source are less
than 8% on average at 24 GHz. We conclude that the reduction in the effects due
to source structure gained by observing at higher frequencies will result in an
improved CRF and a pool of high-quality fiducial reference points for use in
spacecraft navigation over the next decade.Comment: 63 pages, 18 figures, 6 tables, accepted by the Astronomical Journa
Astrometric suitability of optically-bright ICRF sources for the alignment with the future Gaia celestial reference frame
The ICRF, currently based on the position of 717 extragalactic radio sources
observed by VLBI, is the fundamental celestial reference frame adopted by the
IAU in 1997. Within the next 10 years, the European space astrometry mission
Gaia, to be launched by 2011, will permit determination of the extragalactic
reference frame directly in the visible for the first time. Aligning these two
frames with the highest accuracy will therefore be very important in the future
for ensuring consistency between the measured radio and optical positions. This
paper is aimed at evaluating the current astrometric suitability of the
individual ICRF radio sources which are considered appropriate for the
alignment with the future Gaia frame. To this purpose, we cross-identified the
ICRF and the optical catalog V\'eron-Cetty and V\'eron (2006), to identify the
optically-bright ICRF sources that will be positioned with the highest accuracy
with Gaia. Then we investigated the astrometric suitability of these sources by
examining their VLBI brightness distribution. We identified 243 candidate ICRF
sources for the alignment with the Gaia frame (with an optical counterpart
brighter than the apparent magnitude 18), but only 70 of these (10% of the ICRF
sources) are found to have the necessary high astrometric quality (i.e. a
brightness distribution that is compact enough) for this link. Additionally, it
was found that the QSOs that will have the most accurate positions in the Gaia
frame tend to have less-accurate VLBI positions, most probably because of their
physical structures. Altogether, this indicates that identifying other
high-quality VLBI radio sources suitable for the alignment with the future Gaia
frame is mandatory.Comment: To be published in A&A (2008)
Relativistic Jets in the Radio Reference Frame Image Database. I. Apparent Speeds from the First 5 Years of Data
We present the results of an analysis of relativistic jet apparent speeds from VLBI images in the Radio Reference Frame Image Database (RRFID). The images are snapshot VLBI images at 8 and 2 GHz using the VLBA, plus up to 10 additional antennas that provide global VLBI coverage. We have analyzed the 8 GHz images from the first 5 years of the database (1994-1998), for all sources observed at three or more epochs during this time range. This subset comprises 966 images of 87 sources. The sources in this subset have an average of 11 epochs of observation over the years 1994-1998, with the best-observed sources having 19 epochs. About half of the sources in this RRFID kinematic survey have not been previously studied with multiepoch VLBI observations. We have measured apparent speeds for a total of 184 jet components in 77 sources, of which the best-measured 94 component speeds in 54 sources are used in the final analysis. The apparent speed distribution shows a peak at low apparent speeds (consistent with stationary components), a tail extending out to apparent speeds of about 30c, and a mean apparent speed of 3.6c. A total of 36 of the sources in this paper are also included in the 2 cm VLBA survey by Kellermann et al., with similar angular resolution, sensitivity, and time range. For those sources, we present a detailed component-by-component comparison of the apparent speeds measured by the 2 cm survey and those measured in this paper. Many of the independent apparent speed measurements agree very well, but for approximately 25% of the components we find significant differences in the apparent speeds measured by the two surveys. The leading cause of these discrepancies is differences in how the two surveys have identified jet components from epoch to epoch
Expansion of the ligand knowledge base for chelating P,P-donor ligands (LKB-PP)
[Image: see text] We have expanded the ligand knowledge base for bidentate P,P- and P,N-donor ligands (LKB-PP, Organometallics2008, 27, 1372–1383) by 208 ligands and introduced an additional steric descriptor (nHe(8)). This expanded knowledge base now captures information on 334 bidentate ligands and has been processed with principal component analysis (PCA) of the descriptors to produce a detailed map of bidentate ligand space, which better captures ligand variation and has been used for the analysis of ligand properties
Strong Spherical Asymptotics for Rotor-Router Aggregation and the Divisible Sandpile
The rotor-router model is a deterministic analogue of random walk. It can be
used to define a deterministic growth model analogous to internal DLA. We prove
that the asymptotic shape of this model is a Euclidean ball, in a sense which
is stronger than our earlier work. For the shape consisting of
sites, where is the volume of the unit ball in , we show that
the inradius of the set of occupied sites is at least , while the
outradius is at most for any . For a related
model, the divisible sandpile, we show that the domain of occupied sites is a
Euclidean ball with error in the radius a constant independent of the total
mass. For the classical abelian sandpile model in two dimensions, with particles, we show that the inradius is at least , and the
outradius is at most . This improves on bounds of Le Borgne
and Rossin. Similar bounds apply in higher dimensions.Comment: [v3] Added Theorem 4.1, which generalizes Theorem 1.4 for the abelian
sandpile. [v4] Added references and improved exposition in sections 2 and 4.
[v5] Final version, to appear in Potential Analysi
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