2,599 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
Development and distribution of the non-indigenous Pacific oyster (Crassostrea gigas) in the Dutch Wadden Sea
Pacific oysters (Crassostrea gigas) were first observed in the Dutch Wadden Sea near Texel in 1983. The population increased slowly in the beginning but grew exponentially from the mid-1990s onwards, although now some stabilisation seems to be occurring. They occur on a variety of substrates such as mussel beds (Mytilus edulis), shell banks, dikes and poles. After initial settlement spat may fall on older individuals and congregate to dense clumps and subsequently form reefs. Individual Pacific oysters grow 3â4 cm long in their first year and 2â3 cm in their second year. Many mussel beds (Mytilus edulis) are slowly taken over by Pacific oysters, but there are also several reports of mussel spat settling on Pacific oyster reefs. This might in the end result in combined reefs. Successful Pacific oyster spat fall seems to be related to high summer temperatures, but also after mild summers much spat can be found on old (Pacific oyster) shells. Predation is of limited importance. Mortality factors are unknown, but every now and then unexplained mass mortality occurs. The gradual spread of the Pacific oyster in the Dutch Wadden Sea is documented in the first instance based on historical and anecdotal information. At the start of the more in-depth investigation in 2002, Pacific oysters of all size classes were already present near Texel. Near Ameland the development could be followed from the first observed settlement. On dense reefs each square metre may contain more than 500 adult Pacific oysters, weighing more than 100 kg per m² fresh weigh
The Future of Student Affairs is Dependent on Choosing Roads Less Traveled
The authors provide a critique of the report from the 2010 Task Force on the Future of Student Affairs and provide recommendations for future changes in the profession not found in the 2010 report. The authors also note that the 2005 study of ACPA memberâs interests in developing knowledge competencies and skills sets indicate professionals may be at a different place than what professional associations currently offer. The authors provide a critique of some of the professional development opportunities offered by professional associations and proposes some alternatives to how these programs are approached in the profession
The Future of Student Affairs is Dependent on Choosing Roads Less Traveled
The authors provide a critique of the report from the 2010 Task Force on the Future of Student Affairs and provide recommendations for future changes in the profession not found in the 2010 report. The authors also note that the 2005 study of ACPA memberâs interests in developing knowledge competencies and skills sets indicate professionals may be at a different place than what professional associations currently offer. The authors provide a critique of some of the professional development opportunities offered by professional associations and proposes some alternatives to how these programs are approached in the profession.Ă Reference citation: Tyrell, S., & Fey, C. (2011). The future of student affairs is dependent on choosing roads less traveled. Ă CSPA-NYS Journal of Student Affairs, 11(1), 17-36. Ă Ă
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
In-phase and anti-phase synchronization of oscillators with Huygens' coupling
In this experimental study, the synchronized motion observed in pairs of nonlinear oscillators coupled through a suspended rigid bar, is analyzed. In particular, the dynamics of two mass-spring-damper oscillators and the dynamics of two van der Pol oscillators are considered. It is shown that in both cases, the oscillators may exhibit in-phase and anti-phase synchronization. The experiments are executed in an experimental setup, consisting of two mass-spring-damperoscillators coupled through a suspended rigid bar. A relation between the obtained results and Huygensâ experiment of pendulum clocks is emphasized
Synchronization of weakly nonlinear oscillators with Huygens' coupling
In this paper, the occurrence of synchronization in pairs of weakly nonlinear selfsustained oscillators that interact via Huygensâ coupling, i.e. a suspended rigid bar, is treated. In the analysis, a generalized version of the classical Huygensâ experiment of synchronization of two coupled pendulum clocks is considered, in which the clocks are replaced by arbitrary self-sustained oscillators. Sufficient conditions for the existence and stability of synchronous solutions in the coupled system are derived by using the Poincar´e method. The obtained results are supported by computer simulations and experiments conducted on a dedicated experimental platform. It is demonstrated that the mass of the coupling bar is an important parameter with respect to the limit synchronous behaviour in the oscillators
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 âclosedâ 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
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