2,507 research outputs found
Atmospheric correlation time measurements using coherent CO2 lidar
A pulsed TEA-CO2 lidar with coherent detection was used to measure the correlation time of backscatter from an ensemble of atmospheric aerosol particles which are illuminated by the pulsed radiation. The correlation time of the backscatter return signal is important in studies of atmospheric turbulence and its effects on optical propagation and backscatter. If the temporal coherence of the pulse is large enough, then the temporal coherence of the return signal is dominated by the turbulence and shear for a variety of interesting atmospheric conditions. Various techniques for correlation time measurement are discussed and evaluated
Epicardial adiposity in people with atrial fibrillation modified by physical exercise: an umbrella review
Getting science to the citizen : 'food addiction' at the British Science Festival as a case study of interactive public engagement with high profile scientific controversy
Non peer reviewedPublisher PD
Spitzer Space Telescope spectral observations of AGB stars in the Fornax dwarf spheroidal galaxy
We have observed five carbon-rich AGB stars in the Fornax dwarf spheroidal
(dSph) galaxy, using the Infrared Spectrometer on board the Spitzer Space
Telescope. The stars were selected from a near-infrared survey of Fornax and
include the three reddest stars, with presumably the highest mass-loss rates,
in that galaxy. Such carbon stars probably belong to the intermediate-age
population (2-8 Gyr old and metallicity of [Fe/H] -1) of Fornax. The primary
aim of this paper is to investigate mass-loss rate, as a function of luminosity
and metallicity, by comparing AGB stars in several galaxies with different
metallicities. The spectra of three stars are fitted with a radiative transfer
model. We find that mass-loss rates of these three stars are 4-7x10^-6 Msun
yr-1. The other two stars have mass-loss rates below 1.3x10^-6 Msun yr-1. We
find no evidence that these rates depend on metallicity, although we do suggest
that the gas-to-dust ratio could be higher than at solar metallicity, in the
range 240 to 800. The C2H2 bands are stronger at lower metallicity because of
the higher C/O ratio. In contrast, the SiC fraction is reduced at low
metallicity, due to low silicon abundance. The total mass-loss rate from all
known carbon-rich AGB stars into the interstellar medium of this galaxy is of
the order of 2x10^-5 Msun yr-1. This is much lower than that of the dwarf
irregular galaxy WLM, which has a similar visual luminosity and metallicity.
The difference is attributed to the younger stellar population of WLM. The
suppressed gas-return rate to the ISM accentuates the difference between the
relatively gas-rich dwarf irregular and the gas-poor dwarf spheroidal galaxies.
Our study will be useful to constrain gas and dust recycling processes in low
metallicity galaxies.Comment: MNRAS accepte
Convoluted intake distortion measurements using stereo particle image velocimetry
The unsteady distorted flowfields generated within convoluted aeroengine intakes can compromise the engine performance and operability. Therefore, there is a need for a better understanding of the complex characteristics of the distorted flow at the exit of S-shaped intakes. This work presents a detailed analysis of the unsteady swirl distortion based on synchronous, high-spatial-resolution measurements using stereoscopic particle image velocimetry. Two S-duct configurations with different centerline offsets are investigated. The high-offset duct shows greater levels of dynamic and steady swirl distortion and a notably greater tendency toward bulk swirl patterns associated with high swirl distortion. More discrete distortion patterns with locally high swirl levels and the potential to impact the engine operability are identified. The most energetic coherent structures of the flowfield are observed using proper orthogonal decomposition. A switching mode is identified that promotes the alternating swirl switching mechanism and is mostly associated with the occurrence of potent bulk swirl events. A vertical mode that characterizes a perturbation of the vertical velocity field promotes most of the twin swirl flow distortion topologies. It is postulated that it is associated with the unsteadiness of the centerline shear layer
Geochemical and isotopic insights into the assembly, evolution and disruption of a magmatic plumbing system before and after a cataclysmic caldera-collapse eruption at Ischia volcano (Italy)
New geochemical and isotopic data on volcanic rocks spanning the period ~75â50 ka BP on Ischia volcano, Italy, shed light on the evolution of the magmatic system before and after the catastrophic, caldera-forming Monte Epomeo Green Tuff (MEGT) eruption. Volcanic activity during this period was influenced by a large, composite and differentiating magmatic system, replenished several times with isotopically distinct magmas of deep provenance. Chemical and isotopic variations highlight that the pre-MEGT eruptions were fed by trachytic/phonolitic magmas from an isotopically zoned reservoir that were poorly enriched in radiogenic Sr and became progressively less radiogenic with time. Just prior to the MEGT eruption, the magmatic system was recharged by an isotopically distinct magma, relatively more enriched in radiogenic Sr with respect to the previously erupted magmas. This second magma initially fed several SubPlinian explosive eruptions and later supplied the climactic, phonolitic-to-trachytic MEGT eruption(s). Isotopic data, together with erupted volume estimations obtained for MEGT eruption(s), indicate that >5â10 km3 of this relatively enriched magma had accumulated in the Ischia plumbing system. Geochemical modelling indicates that it accumulated at shallow depths (4â6 km), over a period of ca. 20 ka. After the MEGT eruption, volcanic activity was fed by a new batch of less differentiated (trachyte-latite) magma that was slightly less enriched in radiogenic Sr. The geochemical and SrâNd-isotopic variations through time reflect the upward flux of isotopically distinct magma batches, variably contaminated by Hercynian crust at 8â12 km depth. The deep-sourced latitic to trachytic magmas stalled at shallow depths (4â6 km depth), differentiated to phonolite through crystal fractionation and assimilation of a feldspar-rich mush, or ascended directly to the surface and erupted
Limits on Stellar and Planetary Companions in Microlensing Event OGLE-1998-BUL-14
We present the PLANET photometric data set for \ob14, a high magnification
() event alerted by the OGLE collaboration toward the
Galactic bulge in 1998. The PLANET data set consists a total of 461 I-band and
139 band points, the majority of which was taken over a three month period.
The median sampling interval during this period is about 1 hour, and the
scatter over the peak of the event is 1.5%. The excellent data
quality and high maximum magnification of this event make it a prime candidate
to search for the short duration, low amplitude perturbations that are
signatures of a planetary companion orbiting the primary lens. The observed
light curve for \ob14 is consistent with a single lens (no companion) within
photometric uncertainties. We calculate the detection efficiency of the light
curve to lensing companions as a function of the mass ratio and angular
separation of the two components. We find that companions of mass ratio are ruled out at the 95% confidence level for projected separations
between 0.4-2.4 \re, where \re is the Einstein ring radius of the primary
lens. Assuming that the primary is a G-dwarf with \re\sim3 {\rm AU} our
detection efficiency for this event is for a companion with the mass
and separation of Jupiter and for a companion with the mass and
separation of Saturn. Our efficiencies for planets like those around Upsilon
And and 14 Her are > 75%.Comment: Data available at http://www.astro.rug.nl/~planet/planetpapers.html
20 pages, 10 figures. Minor changes. ApJ, accepte
Optical Photometry of the Type Ia SN 1999ee and the Type Ib/c SN 1999ex in IC 5179
We present UBVRIz lightcurves of the Type Ia SN 1999ee and the Type Ib/c SN
1999ex, both located in the galaxy IC 5179. SN 1999ee has an extremely well
sampled lightcurve spanning from 10 days before Bmax through 53 days after
peak. Near maximum we find systematic differences ~0.05 mag in photometry
measured with two different telescopes, even though the photometry is reduced
to the same local standards around the supernova using the specific color terms
for each instrumental system. We use models for our bandpasses and
spectrophotometry of SN 1999ee to derive magnitude corrections (S-corrections)
and remedy this problem. This exercise demonstrates the need of accurately
characterizing the instrumental system before great photometric accuracies of
Type Ia supernovae can be claimed. It also shows that this effect can have
important astrophysical consequences since a small systematic shift of 0.02 mag
in the B-V color can introduce a 0.08 mag error in the extinction corrected
peak B magnitudes of a supernova and thus lead to biased cosmological
parameters. The data for the Type Ib/c SN 1999ex present us with the first ever
observed shock breakout of a supernova of this class. These observations show
that shock breakout occurred 18 days before Bmax and support the idea that Type
Ib/c supernovae are due to core collapse of massive stars rather than
thermonuclear disruption of white dwarfs.Comment: 55 pages, 15 figures, accepted by the Astronomical Journa
Limb-Darkening of a K Giant in the Galactic Bulge: PLANET Photometry of MACHO 97-BLG-28
We present the PLANET photometric dataset for the binary-lens microlensing
event MACHO 97-BLG-28 consisting of 696 I and V-band measurements, and analyze
it to determine the radial surface brightness profile of the Galactic bulge
source star. The microlensed source, demonstrated to be a K giant by our
independent spectroscopy, crossed the central isolated cusp of the lensing
binary, generating a sharp peak in the light curve that was well-resolved by
dense (3 - 30 minute) and continuous monitoring from PLANET sites in Chile,
South Africa, and Australia. Our modeling of these data has produced stellar
profiles for the source star in the I and V bands that are in excellent
agreement with those predicted by stellar atmospheric models for K giants. The
limb-darkening coefficients presented here are the first derived from
microlensing, among the first for normal giants by any technique, and the first
for any star as distant as the Galactic bulge. Modeling indicates that the
lensing binary has a mass ratio q = 0.23 and an (instantaneous) separation in
units of the angular Einstein ring radius of d = 0.69 . For a lens in the
Galactic bulge, this corresponds to a typical stellar binary with a projected
separation between 1 and 2 AU. If the lens lies closer, the separation is
smaller, and one or both of the lens objects is in the brown dwarf regime.
Assuming that the source is a bulge K2 giant at 8 kpc, the relative lens-source
proper motion is mu = 19.4 +/- 2.6 km/s /kpc, consistent with a disk or bulge
lens. If the non-lensed blended light is due to a single star, it is likely to
be a young white dwarf in the bulge, consistent with the blended light coming
from the lens itself.Comment: 32 Pages, including 1 table and 9 postscript figures. (Revised
version has slightly modified text, corrected typo, and 1 new figure.)
Accepted for publication in 1999 Astrophysical Journal; data are now
available at http://www.astro.rug.nl/~plane
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