1,596 research outputs found
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
Experimental simulation of long term weathering in alkaline bauxite residue tailings
Bauxite residue is an alkaline, saline tailings material generated as a byproduct of the Bayer process used for alumina refining. Developing effective plans for the long term management of potential environmental impacts associated with storage of these tailings is dependent on understanding how the chemical and mineralogical properties of the tailings will change during weathering and transformation into a soil-like material. Hydrothermal treatment of bauxite residue was used to compress geological weathering timescales and examine potential mineral transformations during weathering. Gibbsite was rapidly converted to boehmite; this transformation was examined with in situ synchrotron XRD. Goethite, hematite, and calcite all precipitated over longer weathering timeframes, while tricalcium aluminate dissolved. pH, total alkalinity, and salinity (electrical conductivity) all decreased during weathering despite these experiments being performed under “closed” conditions (i.e., no leaching). This indicates the potential for auto-attenuation of the high alkalinity and salinity that presents challenges for long term environmental management, and suggests that management requirements will decrease during weathering as a result of these mineral transformations
The First Very Long Baseline Interferometric SETI Experiment
The first Search for Extra-Terrestrial Intelligence (SETI) conducted with
Very Long Baseline Interferometry (VLBI) is presented. By consideration of the
basic principles of interferometry, we show that VLBI is efficient at
discriminating between SETI signals and human generated radio frequency
interference (RFI). The target for this study was the star Gliese 581, thought
to have two planets within its habitable zone. On 2007 June 19, Gliese 581 was
observed for 8 hours at 1230-1544 with the Australian Long Baseline Array. The
dataset was searched for signals appearing on all interferometer baselines
above five times the noise limit. A total of 222 potential SETI signals were
detected and by using automated data analysis techniques, were ruled out as
originating from the Gliese 581 system. From our results we place an upper
limit of 7 MW/Hz on the power output of any isotropic emitter located in the
Gliese 581 system, within this frequency range. This study shows that VLBI is
ideal for targeted SETI, including follow-up observations. The techniques
presented are equally applicable to next-generation interferometers, such as
the long baselines of the Square Kilometre Array (SKA).Comment: 34 pages, 6 figures, 2 tables. Accepted on 25/05/2012 for publication
in The Astronomical Journa
Astrometric Positions and Proper Motions of 19 Radio Stars
We have used the Very Large Array, linked with the Pie Town Very Long
Baseline Array antenna, to determine astrometric positions of 19 radio stars in
the International Celestial Reference Frame (ICRF). The positions of these
stars were directly linked to the positions of distant quasars through phase
referencing observations. The positions of the ICRF quasars are known to 0.25
mas, thus providing an absolute reference at the angular resolution of our
radio observations. Average values for the errors in our derived positions for
all sources were 13 mas and 16 mas in R.A. and declination respectively, with
accuracies approaching 1-2 mas for some of the stars observed. Differences
between the ICRF positions of the 38 quasars, and those measured from our
observations showed no systematic offsets, with mean values of -0.3 mas in R.A.
and -1.0 mas in declination. Standard deviations of the quasar position
differences of 17 mas and 11 mas in R.A. and declination respectively, are
consistent with the mean position errors determined for the stars. Our measured
positions were combined with previous Very Large Array measurements taken from
1978-1995 to determine the proper motions of 15 of the stars in our list. With
mean errors of approximately 1.6 mas/yr, the accuracies of our proper motions
approach those derived from Hipparcos, and for a few of the stars in our
program, are better than the Hipparcos values. Comparing the positions of our
radio stars with the Hipparcos catalog, we find that at the epoch of our
observations, the two frames are aligned to within formal errors of
approximately 3 mas. This result confirms that the Hipparcos frame is inertial
at the expected level.Comment: 20 pages, 9 figures Accepted by the Astronomical Journal, 2003 March
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Monitoring ethnic minorities in the Netherlands
Item does not contain fulltextThe article first summarises the history of ethnic minority policy in the Netherlands and the development of the ‘ethnic minority’ and ‘allochthonous’ categories, which are peculiar in comparative perspective in emphasising socio-economic disadvantage as a constitutive dimension of minority status and in setting the minority question within the broader Dutch political principle of ‘pillarisation’. The article then examines the use of statistics in public policy, in a context where the national census has been discontinued since 1971, focusing more specifically on the case of education, where major statistical efforts have been devoted to identifying patterns of disadvantage and integration. Finally, the article briefly examines current debates on the situation of ethnic minorities in the Netherlands in the context of growing questioning of established Dutch models of minority policy.13 p
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
Energy sensitive X-ray phase contrast imaging with a CdTe-Timepix3 detector
The Timepix3 is a photon counting semiconductor detector that enables to simultaneously measure the energy and time of arrival of each incident X- ray photon. These properties, along with the high spatial resolution and high efficiency, due to the CdTe sensor material, can be exploited for several imaging applications, such as X-ray phase contrast imaging (XPCI). XPCI relies on the phase shift suffered by X-rays when traversing the sample. This study focuses on the free-space propagation XPCI and single mask edge illumination XPCI methods, which are two approaches that are well suited for laboratory implementations. Since both techniques are highly sensitive to charge-sharing, the Timepix3 energy and time information for each photon are used to minimize this effect by using pixel clustering methods. In addition, the performance of both XPCI techniques across a 30kVp source spectrum is studied using the energy-resolving capabilities of the detector. In both cases, the phase contrast and signal-to-noise ratio (SNR) are assessed as a function of different energy. Finally, it is demonstrated that phase contrast enhancement is feasible with pixel clustering and energy-selection for both XPCI techniques
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Narrative skills in adolescents with a history of SLI in relation to non-verbal IQ scores
There is a debate about whether the language of children with primary language disorders and normal cognitive levels is qualitatively different from those with language impairments who have low or borderline non-verbal IQ (NVIQ). As children reach adolescence, this distinction may be even harder to ascertain, especially in naturalistic settings. Narrative may provide a useful, ecologically valid way in which to assess the language ability of adolescents with specific language impairment (SLI) who have intact or lowered NVIQ and to determine whether there is any discernable difference in every day language. Nineteen adolescents with a history of SLI completed two narrative tasks: a story telling condition and a conversational condition. Just under half the group (n = 8) had non-verbal IQs of 85. The remaining 11 had NVIQs in the normal range or above. Four areas of narrative (productivity, syntax, cohesion and performance) were assessed. There were no differences between the groups on standardized tests of language. However, the group with low NVIQ were poorer on most aspects of narrative, suggesting that cognitive level is important, even when language is the primary disorder. The groups showed similar patterns of differences between story telling and conversational narrative. It was concluded that adolescents with a history of SLI and poor cognitive levels have poorer narrative skills than those with normal range NVIQ even though these may not be detected by standardized assessment. Their difficulties present as qualitatively similar to those with normal range NVIQ and narratives appear impoverished rather than inaccurate
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