721 research outputs found
Classification of image distortions in terms of Petrov types
An observer surrounded by sufficiently small spherical light sources at a
fixed distance will see a pattern of elliptical images distributed over the
sky, owing to the distortion effect (shearing effect) of the spacetime geometry
upon light bundles. In lowest non-trivial order with respect to the distance,
this pattern is completely determined by the conformal curvature tensor (Weyl
tensor) at the observation event. In this paper we derive formulas that allow
to calculate these distortion patterns in terms of the Newman-Penrose
formalism. Then we represent the distortion patterns graphically for all Petrov
types, and we discuss their dependence on the velocity of the observer.Comment: 22 pages, 8 eps-figures; revised version, parts of Introduction and
Conclusions rewritte
Assessment of emergency medical services in the Ashanti region of Ghana
Background: We aimed to assess the structure, function and performance of Ashanti Region’s emergency medical services system in the context of the regional need for prehospital emergency care.Design: A mixed-methods approach was employed, using retrospective collection of quantitative data and prospectively gathered qualitative data. Setting – pertinent data were collected from Ghanaian and international sources; interviews and technical assessments were performed primarily in the Ashanti Region of Ghana.Participants: All stakeholders relevant to emergency medical services in the Ashanti Region of Ghana were assessed; there was a special focus on National Ambulance Service (NAS) and Ashanti Region healthcare personnel.Intervention: This was an observational study using qualitative and quantitative assessment techniques.Main outcome measures: The structure, function and performance of the Ashanti emergency medical services system, guided by a relevant technical assessment framework.Results: NAS is the premier and only true prehospital agency in the Ashanti Region. NAS has developed almost every essential aspect of an EMS system necessary to achieve its mission within a low-resource setting. NAS continues to increase its number of response units to address the overwhelming Ashanti region demand, especially primary calls. Deficient areas in need of development are governance, reliable revenue, public access, community integration, clinical care guidelines, research and quality assurance processes.Conclusions: The Ashanti Region has a growing and thriving emergency medical services system. Although many essential areas for development were identified, NAS is well poised to meet the regional demand for prehospital emergency care and transport.Keywords: EMS, prehospital, ambulance, emergency, Ghana, Afric
Hubble Space Telescope Imaging of the Circumstellar Nebulosity of T Tauri
Short-exposure Planetary Camera images of T Tauri have been obtained using broadband filters spanning the wavelength range 0.55-0.80 μm. The optically visible star lies very close to an arc of reflection nebulosity. The arc's northern arm extends approximately 5" from the star, while its southwestern arm appears brighter and extends only 2". The arc shows an approximate symmetry along an axis toward the west-northwest, the direction of Hind's Nebula and the blueshifted molecular outflow. The morphology of the reflected light is similar to models of scattered light within an illuminated, axisymmetric outflow cavity in a circumbinary envelope, viewed ≈ 45° from the outflow axis. However, our model images do not successfully account for the amount of limb brightening that is seen. No optical counterpart to the infrared companion is seen to a limiting magnitude of V = 19.6, which suggests A_V > 7 mag toward this source. There is no evidence for an optical tertiary, to a limiting ΔV = 5.1 mag fainter than the primary, at the position where such an object has been previously reported
K Corrections For Type Ia Supernovae and a Test for Spatial Variation of the Hubble Constant
Cross-filter K corrections for a sample of "normal" Type Ia supernovae (SNe)
have been calculated for a range of epochs. With appropriate filter choices,
the combined statistical and systematic K correction dispersion of the full
sample lies within 0.05 mag for redshifts z<0.7. This narrow dispersion of the
calculated K correction allows the Type Ia to be used as a cosmological probe.
We use the K corrections with observations of seven SNe at redshifts 0.3 < z
<0.5 to bound the possible difference between the locally measured Hubble
constant (H_L) and the true cosmological Hubble constant (H_0).Comment: 6 pages, 3 Postscript figures, uuencoded uses crckapb.sty and
psfig.sty. To appear in Thermonuclear Supernovae (NATO ASI), eds. R. Canal,
P. Ruiz-LaPuente, and J. Isern. Postscript version is also available at
http://www-supernova.lbl.gov
The second Palomar Sky Survey
We describe the main characteristics of the second Palomar Observatory Sky Survey, currently being taken using the Oschin telescope on Palomar Mountain. The limiting magnitudes of the POSS II plates are directly comparable with those of the SERC/ESO southern surveys, at Β_j = 22.5 mag, R_c = 20.8 mag, and I_c = 19.5 mag. We discuss the main modifications made to the telescope and the photographic methods employed in the course of the survey and compare the POSS I and POSS II plates
Hubble Space Telescope Observations of the Draco Dwarf Spheroidal
We present an F606W-F814W color-magnitude diagram for the Draco dwarf
spheroidal galaxy based on Hubble Space Telescope WFPC2 images. The luminosity
function is well-sampled to 3 magnitudes below the turn-off. We see no evidence
for multiple turnoffs and conclude that, at least over the field of the view of
the WFPC2, star formation was primarily single-epoch. If the observed number of
blue stragglers is due to extended star formation, then roughly 6% (upper
limit) of the stars could be half as old as the bulk of the galaxy. The color
difference between the red giant branch and the turnoff is consistent with an
old population and is very similar to that observed in the old, metal-poor
Galactic globular clusters M68 and M92. Despite its red horizontal branch,
Draco appears to be older than M68 and M92 by 1.6 +/- 2.5 Gyrs, lending support
to the argument that the ``second parameter'' which governs horizontal branch
morphology must be something other than age. Draco's observed luminosity
function is very similar to that of M68, and the derived initial mass function
is consistent with that of the solar neighborhood.Comment: 16 pages, AASTeX, 9 postscript figures, figures 1 and 2 available at
ftp://bb3.jpl.nasa.gov/pub/draco/. Accepted for publication in the
Astronomical Journa
Curvature of the Universe and Observed Gravitational Lens Image Separations Versus Redshift
In a flat, k=0 cosmology with galaxies that approximate singular isothermal
spheres, gravitational lens image separations should be uncorrelated with
source redshift. But in an open k=-1 cosmology such gravitational lens image
separations become smaller with increasing source redshift. The observed
separations do become smaller with increasing source redshift but the effect is
even stronger than that expected in an Omega=0 cosmology. The observations are
thus not compatible with the "standard" gravitational lensing statistics model
in a flat universe. We try various open and flat cosmologies, galaxy mass
profiles, galaxy merging and evolution models, and lensing aided by clusters to
explain the correlation. We find the data is not compatible with any of these
possibilities within the 95% confidence limit, leaving us with a puzzle. If we
regard the observed result as a statistical fluke, it is worth noting that we
are about twice as likely to observe it in an open universe (with 0<Omega<0.4)
as we are to observe it in a flat one. Finally, the existence of an observed
multiple image lens system with a source at z=4.5 places a lower limit on the
deceleration parameter: q_0 > -2.0.Comment: 21 pages, 4 figures, AASTeX
Observations and Implications of the Star Formation History of the LMC
We present derivations of star formation histories based on color-magnitude
diagrams of three fields in the LMC from HST/WFPC2 observations. A significant
component of stars older than 4 Gyr is required to match the observed
color-magnitude diagrams. Models with a dispersion-free age-metallicity
relation are unable to reproduce the width of the observed main sequence;
models with a range of metallicity at a given age provide a much better fit.
Such models allow us to construct complete ``population boxes'' for the LMC
based entirely on color-magnitude diagrams; remarkably, these qualitatively
reproduce the age-metallicity relation observed in LMC clusters. We discuss
some of the uncertainties in deriving star formation histories. We find,
independently of the models, that the LMC bar field has a larger relative
component of older stars than the outer fields. The main implications suggested
by this study are: 1) the star formation history of field stars appears to
differ from the age distribution of clusters, 2) there is no obvious evidence
for bursty star formation, but our ability to measure bursts shorter in
duration than 25% of any given age is limited by the statistics of the
observed number of stars, 3) there may be some correlation of the star
formation rate with the last close passage of the LMC/SMC/Milky Way, but there
is no dramatic effect, and 4) the derived star formation history is probably
consistent with observed abundances, based on recent chemical evolution models.Comment: Accepted by AJ, 36 pages including 12 figure
Constraints on Type Ia Supernova Progenitor Companions from Early Ultraviolet Observations with Swift
We compare early ultraviolet (UV) observations of Type Ia Supernovae (SNe Ia)
with theoretical predictions for the brightness of the shock associated with
the collision between SN ejecta and a companion star. Our simple method is
independent of the intrinsic flux from the SN and treats the flux observed with
the Swift/Ultra-Violet Optical Telescope (UVOT) as conservative upper limits on
the shock brightness. Comparing this limit with the predicted flux for various
shock models, we constrain the geometry of the SN progenitor-companion system.
We find the model of a 1 M_sun red supergiant companion in Roche lobe overflow
to be excluded at a 95% confidence level for most individual SNe for all but
the most unfavorable viewing angles. For the sample of 12 SNe taken together,
the upper limits on the viewing angle are inconsistent with the expected
distribution of viewing angles for RG stars as the majority of companions with
high confidence. The separation distance constraints do allow MS companions. A
better understanding of the UV flux arising from the SN itself as well as
continued UV observations of young SNe Ia will further constrain the possible
progenitors of SNe Ia.Comment: accepted versio
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