8,913 research outputs found
A Radio--Optical Reference Frame VIII. CCD observations from KPNO and CTIO: internal calibration and first results
In this pilot investigation, precise optical positions in the FK5 system are
presented for a set of 16 compact extragalactic radio sources, which will be
part of the future radio--optical reference frame. The 0.9 m KPNO and CTIO
telescopes equipped with 2K CCD's have been used for this project. The
astrometric properties of these instruments are investigated in detail. New
techniques of using wide field CCD observations for astrometry in general are
developed. An internal precision of 5 to 31 mas in position per single exposure
is found, depending on the brightness of the object. The tie to the primary
optical reference system is established by photographic astrometry using
dedicated astrographs on both hemispheres. An accuracy of mas per
source is estimated for the multi--step reduction procedure when based on the
future Hipparcos catalog, while the FK5--based positions suffer from system
errors of 100 to 200 mas as compared to the radio positions. This work provides
a contribution to the international effort to link the Hipparcos instrumental
coordinate system to the quasi--inertial VLBI radio reference frame. Precise
radio and optical astrometry of a large sample of compact extragalactic sources
will also contribute to the astrophysics of these objects by comparing the
respective centers of emission at the optical and radio wavelengths.Comment: AAS v.4 LaTeX, 2 parts on 1 file (main text + deluxetable), accepted
by AJ, Dec.95, fig. with reprint
A Compound model for the origin of Earth's water
One of the most important subjects of debate in the formation of the solar
system is the origin of Earth's water. Comets have long been considered as the
most likely source of the delivery of water to Earth. However, elemental and
isotopic arguments suggest a very small contribution from these objects. Other
sources have also been proposed, among which, local adsorption of water vapor
onto dust grains in the primordial nebula and delivery through planetesimals
and planetary embryos have become more prominent. However, no sole source of
water provides a satisfactory explanation for Earth's water as a whole. In view
of that, using numerical simulations, we have developed a compound model
incorporating both the principal endogenous and exogenous theories, and
investigating their implications for terrestrial planet formation and
water-delivery. Comets are also considered in the final analysis, as it is
likely that at least some of Earth's water has cometary origin. We analyze our
results comparing two different water distribution models, and complement our
study using D/H ratio, finding possible relative contributions from each
source, focusing on planets formed in the habitable zone. We find that the
compound model play an important role by showing more advantage in the amount
and time of water-delivery in Earth-like planets.Comment: Accepted for publication in The Astrophysical Journa
The StarScan plate measuring machine: overview and calibrations
The StarScan machine at the U.S. Naval Observatory (USNO) completed measuring
photographic astrograph plates to allow determination of proper motions for the
USNO CCD Astrograph Catalog (UCAC) program. All applicable 1940 AGK2 plates,
about 2200 Hamburg Zone Astrograph plates, 900 Black Birch (USNO Twin
Astrograph) plates, and 300 Lick Astrograph plates have been measured. StarScan
comprises of a CCD camera, telecentric lens, air-bearing granite table, stepper
motor screws, and Heidenhain scales to operate in a step-stare mode. The
repeatability of StarScan measures is about 0.2 micrometer. The CCD mapping as
well as the global table coordinate system has been calibrated using a special
dot calibration plate and the overall accuracy of StarScan x,y data is derived
to be 0.5 micrometer. Application to real photographic plate data shows that
position information of at least 0.65 micrometer accuracy can be extracted from
course grain 103a-type emulsion astrometric plates. Transformations between
"direct" and "reverse" measures of fine grain emulsion plate measures are
obtained on the 0.3 micrometer level per well exposed stellar image and
coordinate, which is at the limit of the StarScan machine.Comment: 24 pages, 8 figures, accepted for PAS
Neuropilin-1 Controls Endothelial Homeostasis by Regulating Mitochondrial Function and Iron-Dependent Oxidative Stress.
The transmembrane protein neuropilin-1 (NRP1) promotes vascular endothelial growth factor (VEGF) and extracellular matrix signaling in endothelial cells (ECs). Although it is established that NRP1 is essential for angiogenesis, little is known about its role in EC homeostasis. Here, we report that NRP1 promotes mitochondrial function in ECs by preventing iron accumulation and iron-induced oxidative stress through a VEGF-independent mechanism in non-angiogenic ECs. Furthermore, NRP1-deficient ECs have reduced growth and show the hallmarks of cellular senescence. We show that a subcellular pool of NRP1 localizes in mitochondria and interacts with the mitochondrial transporter ATP-binding cassette B8 (ABCB8). NRP1 loss reduces ABCB8 levels, resulting in iron accumulation, iron-induced mitochondrial superoxide production, and iron-dependent EC senescence. Treatment of NRP1-deficient ECs with the mitochondria-targeted antioxidant compound mitoTEMPO or with the iron chelator deferoxamine restores mitochondrial activity, inhibits superoxide production, and protects from cellular senescence. This finding identifies an unexpected role of NRP1 in EC homeostasis
Archeologische evaluatie en waardering van de abdijsite van Sint-Truiden (Sint-Truiden, provincie Limburg)
In dit rapport presenteert archeologisch projectbureau Aron een evaluatie en waardering vanuit archeologisch oogpunt van de abdijsite te Sint-Truiden. Het onderzoek bestond enerzijds uit terreinwerk, waarbij de volledige site en de diepste vloerniveaus digitaal ingemeten werden, anderzijds uit bureauonderzoek. In hoofdstuk 1 van dit rapport worden de stad Sint-Truiden en de abdijsite eerst algemeen gesitueerd. Vervolgens worden in hoofdstuk 2 kort de gebruikte methodieken en dataverwerking toegelicht. Hoofdstuk 3 bespreekt uitgebreid de bouwhistoriek van de abdij vanaf haar ontstaan tot haar ondergang meer dan elf eeuwen later. Er wordt daarbij ook ingegaan op de ingrepen waaronder het complex te lijden heeft gehad in de negentiende en twintigste eeuw. Dan volgt een uitgebreid overzicht van de opgravingen die in het voormalige abdijcomplex hebben plaatsgehad en van de vaststellingen die daarbij werden gedaan. Al deze informatie wordt gebundeld en gevisualiseerd in reconstructietekeningen van de verschillende bouwfases van de abdij. In hoofdstuk 4 wordt de site vervolgens uitgebreid getoetst aan de beschermingscriteria voor archeologische monumenten en zones. Ten slotte worden in hoofdstuk 5 en 6 respectievelijk maatregelen naar het behoud en beheer van de site geformuleerd en worden enkele suggesties naar toekomstig archeologisch onderzoek gedaan. Uit dit onderzoeksproject kon globaal worden geconcludeerd dat het studiegebied een erg rijke, gevarieerde en complexe bouwgeschiedenis heeft gekend en dat het bodemarchief nog maar een fractie van zijn geheimen heeft prijsgegeven. De studie besluit dat er goede argumenten zijn om een archeologische bescherming van de abdijsite van Sint-Truiden te rechtvaardigen, zowel voor de criteria vorm en inhoud als wat betreft het criterium beleving. De site is uniek voor Vlaanderen en het is van belang dat ze voor verdere verwoestende ingrepen behoed wordt en er werk gemaakt wordt van de restauratie en ontsluiting van de site en van de bewustmaking van het belang van deze site
Optimizing sampling strategies in high-resolution paleoclimate records
The aim of paleoclimate studies resolving climate variability from noisy
proxy records can in essence be reduced to a statistical problem. The
challenge is to extract meaningful information about climate variability
from these records by reducing measurement uncertainty through combining
measurements for proxies while retaining the temporal resolution needed to
assess the timing and duration of variations in climate parameters. In this
study, we explore the limits of this compromise by testing different methods
for combining proxy data (smoothing, binning, and sample size optimization)
on a particularly challenging paleoclimate problem: resolving seasonal
variability in stable isotope records. We test and evaluate the effects of
changes in the seasonal temperature and the hydrological cycle as well as
changes in the accretion rate of the archive and parameters such as sampling
resolution and age model uncertainty in the reliability of seasonality
reconstructions based on clumped and oxygen isotope analyses in 33 real and
virtual datasets. Our results show that strategic combinations of clumped
isotope analyses can significantly improve the accuracy of seasonality
reconstructions compared to conventional stable oxygen isotope analyses,
especially in settings in which the isotopic composition of the water is poorly
constrained. Smoothing data using a moving average often leads to an
apparent dampening of the seasonal cycle, significantly reducing the
accuracy of reconstructions. A statistical sample size optimization protocol
yields more precise results than smoothing. However, the most accurate
results are obtained through monthly binning of proxy data, especially in
cases in which growth rate or water composition cycles obscure the seasonal
temperature cycle. Our analysis of a wide range of natural situations
reveals that the effect of temperature seasonality on oxygen isotope records
almost invariably exceeds that of changes in water composition. Thus, in
most cases, oxygen isotope records allow reliable identification of growth
seasonality as a basis for age modeling in the absence of independent
chronological markers in the record. These specific findings allow us to
formulate general recommendations for sampling and combining data in
paleoclimate research and have implications beyond the reconstruction of
seasonality. We briefly discuss the implications of our results for solving
common problems in paleoclimatology and stratigraphy.</p
Retrieval methods of effective cloud cover for the GOME instrument: an intercomparison
International audienceThe radiative scattering by clouds leads to errors in the retrieval of column densities and concentration profiles of atmospheric trace gas species from satellites. Moreover, the presence of clouds changes the UV actinic flux and the photo-dissociation rates of various species significantly. The Global Ozone Monitoring Experiment (GOME) instrument on the ERS-2 satellite, principally designed to retrieve trace gases in the atmosphere is also capable of detecting clouds. Four cloud fraction retrieval methods for GOME data that have been developed are discussed in this paper (the Initial Cloud Fitting Algorithm, the PMD Cloud Retrieval Algorithm, the Optical Cloud Recognition Algorithm and the Fast Retrieval Scheme for Cloud Observables). Their results of cloud fraction retrieval are compared to each-other and also to synoptic surface observations. It is shown that all studied retrieval methods calculate an effective cloud fraction that is related to a cloud with a high optical thickness. Generally, we found ICFA to produce the lowest cloud fractions, followed by OCRA, then FRESCO and PC2K along four processed tracks (+2%, +10% and +15% compared to ICFA respectively). Synoptical surface observations gave the highest absolute cloud fraction when compared with individual PMD sub-pixels of roughly the same size
The Stellar Composition of the Star Formation Region CMa R1. II. Spectroscopic and Photometric Observations of 9 Young Stars
We present new high and low resolution spectroscopic and photometric data of
nine members of the young association CMa R1. All the stars have circumstellar
dust at some distance as could be expected from their association with
reflection nebulosity. Four stars (HD 52721, HD 53367, LkHalpha 220 and
LkHalpha 218) show Halpha emission and we argue that they are Herbig Be stars
with discs. Our photometric and spectroscopic observations on these stars
reveal new characteristics of their variability. We present first
interpretations of the variability of HD 52721, HD 53367 and the two LkHalpha
stars in terms of a partially eclipsing binary, a magnetic activity cycle and
circumstellar dust variations, respectively. The remaining five stars show no
clear indications of Halpha emission in their spectra, although their spectral
types and ages are comparable with those of HD 52721 and HD 53367. This
indicates that the presence of a disc around a star in CMa R1 may depend on the
environment of the star. In particular we find that all Halpha emission stars
are located at or outside the arc-shaped border of the H II region, which
suggests that the stars inside the arc have lost their discs through
evaporation by UV photons from nearby O stars, or from the nearby (< 25 pc)
supernova, about 1 Myr ago.Comment: 17 pages, 13 figures, accepted by MNRA
Retrieval methods of effective cloud cover from the GOME instrument: an intercomparison
The radiative scattering by clouds leads to errors in the retrieval of column densities and concentration profiles of atmospheric trace gas species from satellites. Moreover, the presence of clouds changes the UV actinic flux and the photo-dissociation rates of various species significantly. The Global Ozone Monitoring Experiment (GOME) instrument on the ERS-2 satellite, principally designed to retrieve trace gases in the atmosphere, is also capable of detecting clouds. Four cloud fraction retrieval methods for GOME data that have been developed are discussed in this paper (the Initial Cloud Fitting Algorithm, the PMD Cloud Recognition Algorithm, the Optical Cloud Recognition Algorithm (an in-house version and the official implementation) and the Fast Retrieval Scheme for Clouds from the Oxygen A-band). Their results of cloud fraction retrieval are compared to each-other and also to synoptic surface observations. It is shown that all studied retrieval methods calculate an effective cloud fraction that is related to a cloud with a high optical thickness. Generally, we found ICFA to produce the lowest cloud fractions, followed by our in-house OCRA implementation, FRESCO, PC2K and finally the official OCRA implementation along four processed tracks (+2%, +10%, +15% and +25% compared to ICFA respectively). Synoptical surface observations gave the highest absolute cloud fraction when compared with individual PMD sub-pixels of roughly the same size
Archeologisch onderzoek in Bilzen uitgevoerd in het kader van de herinrichting van het Stadscentrum: het Camille Husmansplein, het Deken Paquayplein en de markt
Dit rapport werd ingediend bij het agentschap samen met een aantal afzonderlijke digitale bijlagen. Een aantal van deze bijlagen zijn niet inbegrepen in dit pdf document en zijn niet online beschikbaar. Sommige bijlagen (grondplannen, fotos, spoorbeschrijvingen, enz.) kunnen van belang zijn voor een betere lezing en interpretatie van dit rapport. Indien u deze bijlagen wenst te raadplegen kan u daarvoor contact opnemen met: [email protected]
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