739 research outputs found
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Sound propagation over mixed impedances
Outdoor propagation of surface transportation noise is strongly influenced by reflections from the ground surface due to the creation of an intereference pattern. The result is an attenuation with distance over and above that expected from simple spherical divergence and atmospheric absorption.
A review of literature shows that ground effects can be characterised by a spherical wave reflection coefficient and a normal surface impedance. Theoretical solutions of point to point propagation over three different ground surface models (local reaction, extended reaction and a rigid backed layer) are described and the first two are shown to give good estimates of the excess attenuation as a function of distance.
Methods for measuring the normal surface impedance are discussed and experimental measurements reported. It is concluded that an indirect method employing the local reaction theoretical solution gives reasonable estimates over the frequency range 1-70 kHz.
For practical applications the homogeneous ground surface is modified to include the effects of impedance discontinuities. The theoretical solutions are compared with the results obtained from experiments on laboratory models built to a notional scale of 1:100. In the case of the source over a rigid surface and receiver over an acoustically soft ground surface, it is shown that (a) for a low noise source (e.g. cars) the impedance discontinuity has negligible effect and (b) that for elevated sources (e.g. lorries) the excess attenuation is increased relative to that of a homogeneous surface.
The double discontinuity introduced by a rigid strip in an acoustically soft ground surface is also examined. Experimental results indicate the effect of the strip is insignificant for strip widths less than 10% of the source receiver separation distance. For wider strips it is found that prediction can be improved by postulating an increased average surface impedance
All-Sky spectrally matched UBVRI-ZY and u'g'r'i'z' magnitudes for stars in the Tycho2 catalog
We present fitted UBVRI-ZY and u'g'r'i'z' magnitudes, spectral types and
distances for 2.4M stars, derived from synthetic photometry of a library
spectrum that best matches the Tycho2 BtVt, NOMAD Rn and 2MASS JHK_{2/S}
catalog magnitudes. We present similarly synthesized multi-filter magnitudes,
types and distances for 4.8M stars with 2MASS and SDSS photometry to g<16
within the Sloan survey region, for Landolt and Sloan primary standards, and
for Sloan Northern (PT) and Southern secondary standards.
The synthetic magnitude zeropoints for BtVt, UBVRI, ZvYv, JHK_{2/S},
JHK_{MKO}, Stromgren uvby, Sloan u'g'r'i'z' and ugriz are calibrated on 20
calspec spectrophotometric standards. The UBVRI and ugriz zeropoints have
dispersions of 1--3%, for standards covering a range of color from -0.3 < V-I <
4.6; those for other filters are in the range 2--5%.
The spectrally matched fits to Tycho2 stars provide estimated 1-sigma errors
per star of ~0.2, 0.15, 0.12, 0.10 and 0.08 mags respectively in either UBVRI
or u'g'r'i'z'; those for at least 70% of the SDSS survey region to g<16 have
estimated 1-sigma errors per star of ~0.2, 0.06, 0.04, 0.04, 0.05 in u'g'r'i'z'
or UBVRI.
The density of Tycho2 stars, averaging about 60 stars per square degree,
provides sufficient stars to enable automatic flux calibrations for most
digital images with fields of view of 0.5 degree or more. Using several such
standards per field, automatic flux calibration can be achieved to a few
percent in any filter, at any airmass, in most workable observing conditions,
to facilitate inter-comparison of data from different sites, telescopes and
instruments.Comment: 36 pages, 30 figures, 3 printed tables, several electronic tables,
accepted PASP Dec 201
Fundamental Properties of O-Type Stars
We present a comprehensive analysis of high-resolution, far-UV HST/STIS,
FUSE, and optical spectra of 17 O stars in the SMC. Our analysis is based on
NLTE metal line-blanketed model atmospheres calculated with our NLTE code
TLUSTY. We systematically explore the sensitivity of various UV and optical
lines to different stellar parameters. We have obtained consistent fits of the
UV and the optical spectrum to derive the effective temperature, surface
gravity, surface composition, and microturbulent velocity of each star. Stellar
radii, masses, luminosities and ages then follow. Similarly to more limited
recent studies, we derive cooler temperatures than the standard Teff
calibration of O stars. We propose a new calibration between the spectral type
and effective temperature based on our results from UV metal lines as well as
optical hydrogen and helium lines. For stars of the same spectral subtype, we
find a general good agreement between Teff determinations obtained with TLUSTY,
CMFGEN, and FASTWIND models. We derive ionizing luminosities that are smaller
by a factor of 3 compared to luminosities inferred from previous standard
calibrations. The chemical composition analysis reveals that the surface of
about 3/4 of the program stars is moderately to strongly enriched in nitrogen,
while showing the original helium, carbon, and oxygen abundances. Our results
support the new stellar evolution models that predict that the surface of fast
rotating stars becomes N-rich during the main sequence phase because of
rotationally-induced mixing. Most stars exhibit the ``mass discrepancy''
problem. This discrepancy too is a result of fast rotation which lowers the
measured effective gravity. Our study thus emphasizes the importance of
rotation in our understanding of the properties of massive stars. (abridged)Comment: Submitted to Astrophysical Journal; 69 page
Candidate Coronagraphic Detections of Protoplanetary Disks around Four Young Stars
We present potential detections of H-band scattered light emission around
four young star, selected from a total sample of 45 young stars observed with
the CIAO coronagraph of the Subaru telescope. Two CTTS, CI Tau and DI Cep, and
two WTTS, LkCa 14 and RXJ 0338.3+1020 were detected. In all four cases, the
extended emission is within the area of the residual PSF halo, and is revealed
only through careful data reduction. We compare the observed extended emission
with simulations of the scattered light emission, to evaluate the plausibility
and nature of the detected emission.Comment: 9 Figures, 40 page
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Deaths in people from Black, Asian and minority ethnic communities from both COVID-19 and non-COVID causes in the first weeks of the pandemic in London: a hospital case note review
Objective To undertake a case review of deaths in a 6-week period during the COVID-19 pandemic commencing with the first death in the hospital from COVID-19 on 12th of March 2020 and contrast this with the same period in 2019.
Setting A large London teaching hospital.
Participants Three groups were compared: group 1âCOVID-19-associated deaths in the 6-week period (n=243), group 2ânon-COVID deaths in the same period (n=136) and group 3âall deaths in a comparison period of the same 6 weeks in 2019 (n=194).
Primary and secondary outcome measures This was a descriptive analysis of death case series review and as such no primary or secondary outcomes were pre-stipulated.
Results Deaths in patients from the Black, Asian and minority ethnic (BAME) communities in the pandemic period significantly increased both in the COVID-19 group (OR=2.43, 95% CI=1.60â3.68, p<0.001) and the non-COVID group (OR=1.76, 95% CI=1.09â2.83, p=0.02) during this time period and the increase was independent of differences in comorbidities, sex, age or deprivation. While the absolute number of deaths increased in 2020 compared with 2019, across all three groups the distribution of deaths by age was very similar. Our analyses confirm major risk factors for COVID-19 mortality including male sex, diabetes, having multiple comorbidities and background from the BAME communities.
Conclusions There was no evidence of COVID-19 deaths occurring disproportionately in the elderly compared with non-COVID deaths in this period in 2020 and 2019. Deaths in the BAME communities were over-represented in both COVID-19 and non-COVID groups, highlighting the need for detailed research in order to fully understand the influence of ethnicity on susceptibility to illness, mortality and health-seeking behaviour during the pandemic
Optical Structure and Proper-Motion Age of the Oxygen-rich Supernova Remnant 1E 0102-7219 in the Small Magellanic Cloud
We present new optical emission-line images of the young SNR 1E 0102-7219
(E0102) in the SMC obtained with the HST Advanced Camera for Surveys (ACS).
E0102 is a member of the oxygen-rich class of SNRs showing strong oxygen, neon
, and other metal-line emissions in its optical and X-ray spectra, and an
absence of H and He. The progenitor of E0102 may have been a Wolf-Rayet star
that underwent considerable mass loss prior to exploding as a Type Ib/c or
IIL/b SN. The ejecta in this SNR are fast-moving (V > 1000 km/s) and emit as
they are compressed and heated in the reverse shock. In 2003, we obtained
optical [O III], H-alpha, and continuum images with the ACS Wide Field Camera.
The [O III] image captures the full velocity range of the ejecta, and shows
considerable high-velocity emission projected in the middle of the SNR that was
Doppler-shifted out of the narrow F502N bandpass of a previous Wide Field and
Planetary Camera 2 image from 1995. Using these two epochs separated by ~8.5
years, we measure the transverse expansion of the ejecta around the outer rim
in this SNR for the first time at visible wavelengths. From proper-motion
measurements of 12 ejecta filaments, we estimate a mean expansion velocity for
the bright ejecta of ~2000 km/s and an inferred kinematic age for the SNR of
\~2050 +/- 600 years. The age we derive from HST data is about twice that
inferred by Hughes et al.(2000) from X-ray data, though our 1-sigma error bars
overlap. Our proper-motion age is consistent with an independent optical
kinematic age derived by Eriksen et al.(2003) using spatially resolved [O III]
radial-velocity data. We derive an expansion center that lies very close to
X-ray and radio hotspots, which could indicate the presence of a compact
remnant (neutron star or black hole).Comment: 28 pages, 8 figures. Accepted to the Astrophysical Journal, to appear
in 20 April 2006 issue. Full resolution figures are posted at:
http://stevenf.asu.edu/figure
Paraphrases and summaries: A means of clarification or a vehicle for articulating a preferred version of student accounts?
The use of group discussions as a means to facilitate learning from experiences is well documented in adventure education literature. Priest and Naismith (1993) assert that the use of the circular discussion method, where the leader poses questions to the participants, is the most common form of facilitation in adventure education. This paper draws on transcripts of facilitation sessions to argue that the widely advocated practice of leader summaries or paraphrases of student responses in these sessions functions as a potential mechanism to control and sponsor particular knowledge(s). Using transcripts from recorded facilitation sessions the analysis focuses on how the leader paraphrases the studentsâ responses and how these paraphrases or âformulationsâ function to modify or exclude particular aspects of the studentsâ responses. I assert that paraphrasing is not simply a neutral activity that merely functions to clarify a student response, it is a subtle means by which the leader of the session can, often inadvertently or unknowingly, alter the studentâs reply with the consequence of favouring particular knowledge(s). Revealing the subtle work that leader paraphrases perform is of importance for educators who claim to provide genuine opportunities for students to learn from their experience
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