3,367 research outputs found
HOMELESS HIGH SCHOOL STUDENTS IN AMERICA: WHO COUNTS?
After interviewing homeless high school students, the research team in a Colorado school district discovered that many students had not revealed their true living conditions (homelessness) to anyone in the school district. This research team developed an anonymous survey written around the homeless categories identified in the McKinney-Vento federal legislation. Results revealed students who identified as homeless for a portion of their high school years in numbers and percentages alarmingly higher than the district had on file. In fact, over 25 times as many homeless students were identified by this process than by the previously-used district system for identifying homelessness. An equally alarming finding is that very few students identified their homeless status to a teacher, counselor, or school administrator. This article identifies statistical patterns to predict homelessness and provides recommendations for administrative practices
Low Frequency Observations of Millisecond Pulsars with the WSRT
With LOFAR beginning operation in 2008 there is huge potential for studying
pulsars with high signal to noise at low frequencies. We present results of
observations made with the Westerbork Synthesis Radio Telescope to revisit,
with modern technology, this frequency range. Coherently dedispersed profiles
of millisecond pulsars obtained simultaneously between 115-175 MHz are
presented. We consider the detections and non-detections of 14 MSPs in light of
previous observations and the fluxes, dispersion measures and spectral indices
of these pulsars. The excellent prospects for LOFAR finding new MSPs and
studying the existing systems are then discussed in light of these results.Comment: 5 pages, 5 figures, To appear in the proceedings of "40 Years of
Pulsars: Millisecond Pulsars, Magnetars, and More", August 12-17, 2007,
McGill University, Montreal, Canad
A Search for Single Radio Pulses and Bursts from Southern AXPs
We observed four southern AXPs in 1999 near 1400 MHz with the Parkes 64-m
radio telescope to search for periodic radio emission. No Fourier candidates
were discovered in the initial analysis, but the recent radio activity observed
for the AXP XTE J1810-197 has prompted us to revisit these data to search for
single radio pulses and bursts. The data were searched for both persistent and
bursting radio emission at a wide range of dispersion measures, but no
detections of either kind were made. These results further weaken the proposed
link between rotating radio transient sources and magnetars. However, continued
radio searches of these and other AXPs at different epochs are warranted given
the transient nature of the radio emission seen from XTE J1810-197, which until
very recently was the only known radio-emitting AXP.Comment: 3 pages, including 1 table. To appear in the proceedings of "40 Years
of Pulsars: Millisecond Pulsars, Magnetars, and More", August 12-17, 2007,
McGill University, Montreal, Canad
The GBT350 Survey of the Northern Galactic Plane for Radio Pulsars and Transients
Using the Green Bank Telescope (GBT) and Pulsar Spigot at 350MHz, we have
surveyed the Northern Galactic Plane for pulsars and radio transients. This
survey covers roughly 1000 square degrees of sky within 75 deg < l < 165 deg
and |b| < 5.5 deg, a region of the Galactic Plane inaccessible to both the
Parkes and Arecibo multibeam surveys. The large gain of the GBT along with the
high time and frequency resolution provided by the Spigot make this survey more
sensitive by factors of about 4 to slow pulsars and more than 10 to millisecond
pulsars (MSPs), compared with previous surveys of this area. In a preliminary,
reduced-resolution search of all the survey data, we have discovered 33 new
pulsars, almost doubling the number of known pulsars in this part of the
Galaxy. While most of these sources were discovered by normal periodicity
searches, 5 of these sources were first identified through single, dispersed
bursts. We discuss the interesting properties of some of these new sources.
Data processing using the data's full-resolution is ongoing, with the goal of
uncovering MSPs missed by our first, coarse round of processing.Comment: To appear in the proceedings of "Forty Years of Pulsars: Millisecond
Pulsars, Magnetars and More" held in Montreal, Canada, August 12-17, 2007. 3
pages, 2 figure
Photometry of SN 2002ic and Implications for the Progenitor Mass-Loss History
We present new pre-maximum and late-time optical photometry of the Type
Ia/IIn supernova 2002ic. These observations are combined with the published
V-band magnitudes of Hamuy et al. (2003) and the VLT spectrophotometry of Wang
et al. (2004) to construct the most extensive light curve to date of this
unusual supernova. The observed flux at late time is significantly higher
relative to the flux at maximum than that of any other observed Type Ia
supernova and continues to fade very slowly a year after explosion. Our
analysis of the light curve suggests that a non-Type Ia supernova component
becomes prominent days after explosion. Modeling of the non-Type Ia
supernova component as heating from the shock interaction of the supernova
ejecta with pre-existing circumstellar material suggests the presence of a
cm gap or trough between the progenitor system and the
surrounding circumstellar material. This gap could be due to significantly
lower mass-loss years prior to explosion or
evacuation of the circumstellar material by a low-density fast wind. The latter
is consistent with observed properties of proto-planetary nebulae and with
models of white-dwarf + asymptotic giant branch star progenitor systems with
the asymptotic giant branch star in the proto-planetary nebula phase.Comment: accepted for publication in Ap
Exploring the environmental drivers of waterfowl movement in arid landscapes using first-passage time analysis
BackgroundThe movement patterns of many southern African waterfowl are typified by nomadism, which is thought to be a response to unpredictable changes in resource distributions. Nomadism and the related movement choices that waterfowl make in arid environments are, however, poorly understood. Tracking multiple individuals across wide spatiotemporal gradients offers one approach to elucidating the cues and mechanisms underpinning movement decisions. We used first-passage time (FPT) to analyse high spatial and temporal resolution telemetry data for Red-billed Teal and Egyptian Geese across a 1500km geographical gradient between 2008 and 2014. We tested the importance of several environmental variables in structuring movement patterns, focusing on two competing hypotheses: (1) whether movements are driven by resource conditions during the current period of habitat occupation (reactive movement hypothesis), or (2) whether movements are structured by shifts in the magnitude and direction of environmental variables at locations prior to occupation (prescient movement hypothesis).ResultsAn increase in rainfall at a 32day lag (i.e., prior to wetland occupancy), along with tagging site, were significant predictors of FPT in both waterfowl species. There was a positive relationship between NDVI and FPT for Egyptian Geese during this 32day period; the relationship was negative for Red-billed Teal. Consistent with findings for migratory grazing geese, Egyptian Geese prioritised food quality over food biomass. Red-billed Teal showed few immediate responses to wetland filling, contrary to what one would predict for a dabbling duck, suggesting high dietary flexibility. Our results were consistent with the prescient movement hypothesis.ConclusionsUsing FPT analysis we showed that the proximate drivers of southern African waterfowl movement are the dynamics of rainfall and primary productivity. Waterfowl appeared to be able to perceive and respond to temporal shifts in resource conditions prior to habitat patch occupation. This in turn suggests that their movements in semi-arid landscapes may be underpinned by intimate knowledge of the local environment; waterfowl pursue a complex behavioural strategy, locating suitable habitat patches proactively, rather than acting as passive respondents
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Mitigation of Moral Hazard and Adverse Selection in Venture Capital Financing: The Influence of the Countryâs Institutional Setting
A venture capitalist (VC) needs to trade off benefits and costs when attempting to mitigate agency problems in their investor-investee relationship. We argue that signals of ventures complement the VCâs capacity to screen and conduct a due diligence during the pre-investment phase, but its attractiveness may diminish in institutional settings supporting greater transparency. Similarly, whereas a VC may opt for contractual covenants to curb potential opportunism by ventures in the post-investment phase, this may only be effective in settings supportive of shareholder rights enforcement. Using an international sample of VC contracts, our study finds broad support for these conjectures. It delineates theoretical and practical implications for how investors can best deploy their capital in different institutional settings whilst nurturing their relationships with entrepreneurs
Destabilisation of a homogeneous bubbly flow in an annular gap bubble column
Experimental results are presented to show that there are very significant differences in the
mean gas void fractions measured in an open tube and a annular gap bubble column, when
operated at the same gas superficial velocity, using a porous sparger. The mean gas void
fraction decreases with increasing ratio of the inner to outer diameter of the annular gap
column and the transition to heterogeneous flow occurs at lower gas superficial velocities and
lower void fractions. Two reasons are proposed and validated by experimental investigations:
(1) the presence of the inner tube causes large bubbles to form near the sparger, which
destabilize the homogeneous bubbly flow and reduce the mean void fraction; this was
confirmed by deliberately injecting large bubbles into a homogeneous dispersion of smaller
bubbles and (2) the shape of the void fraction profiles changes with gap geometry and this
affects the distribution parameter in the drift flux model
Destabilisation of a homogeneous bubbly flow in an annular gap bubble column
Destabilisation of a homogeneous bubbly flow in an annular gap bubble colum
Experimental study of void fraction behaviour in vertical bubbly gas-liquid flow using conductivity and measurements
The void fraction is an important variable in describing gas-liquid two-phase flows,
since it is required to predict the heat and mass transfer coefficients and the pressure
drop and is an indicator of the flow regime. The contrast in conductivity between
water and air is one way to measure the void fraction in gas-liquid flow. This project
has examined use of the ring conductivity electrodes to measuring the void fraction in
an up-flow bubble column. The conductivity method has potential to be a low cost,
safe and accurate method of measuring local void fractions in pipes and other process
engineering mass transfer devices. In this project, the void fraction was measured in an
air-water system by using conductivity in a 2" pipe equipped with two ring electrodes.
Further gas hold-up experiments were conducted in the annular channel formed
between 2" and a 4" pipe, using a system of four pairs of electrodes. The data obtained
from the experiments agreed fairly well with the Maxwell and Burggeman theories
which relate the dimensionless conductance to the void fraction. The measured void
fractions were correlated using the drift-flux model, as proposed by Zuber and
Findlay. Significant differences were observed between the void fraction
measurements obtained for the annular channel and for an empty pipe, when operated
at the same gas superficial velocity
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