3,040 research outputs found
An optimal Mars Trojan asteroid search strategy
Trojan asteroids are minor planets that share the orbit of a planet about the
Sun and librate around the L4 or L5 Lagrangian points of stability. Although
only three Mars Trojans have been discovered, models suggest that at least ten
times this number should exist with diameters >= 1 km. We derive a model that
constrains optimal sky search areas and present a strategy for the most
efficient use of telescope survey time that maximizes the probability of
detecting Mars Trojans. We show that the Gaia space mission could detect any
Mars Trojans larger than 1 km in diameter, provided the relative motion
perpendicular to Gaia's CCD array is less than 0.40 arcsec per second.Comment: 6 pages, 6 figures, 3 tables, accepted for publication in MNRAS.
arXiv admin note: substantial text overlap with arXiv:1111.112
Detection of inner Solar System Trojan Asteroids by Gaia
The Gaia satellite, planned for launch by the European Space Agency (ESA) in
2013, is the next generation astrometry mission following Hipparcos. While
mapping the whole sky, the Gaia space mission is expected to discover thousands
of Solar System Objects. These will include Near-Earth Asteroids and objects at
Solar elongations as low as 45 degrees, which are difficult to observe with
ground-based telescopes. We present the results of simulations for the
detection of Trojan asteroids in the orbits of Earth and Mars by Gaia.Comment: 4 pages, 3 figures, based on a talk presented at the Gaia-FUN-SSO-2
International Workshop, Paris Observatory, 19-21 September 2012. Part of the
proceedings of that worksho
Meridional heat transport across the Antarctic Circumpolar Current by the Antarctic Bottom Water overturning cell
The heat transported by the lower limb of the Southern Ocean meridional overturning circulation is commonly held to be negligible in comparison with that transported by eddies higher in the water column. We use output from one of the first global high resolution models to have a reasonably realistic export of Antarctic Bottom Water, the OCCAM one twelfth degree model. The heat fluxed southward by the deep overturning cell using the annual mean field for 1994 at 56S is 0.033 PW, but the 5-day mean fields give a larger heat flux (0.048 and 0.061 PW depending on calculation method). This is more than 30% of previous estimates of the total heat flux. Eddies and other transients add considerably to the heat flux. These results imply that this component of meridional heat flux may not be negligible as has been supposed
A small yeast RNA selectively inhibits internal initiation of translation programmed by poliovirus RNA: specific interaction with cellular proteins that bind to the viral 5'-untranslated region
We have purified, sequenced, and prepared a synthetic clone of a small (60-nucleotide) RNA molecule from the yeast Saccharomyces cerevisiae that had previously been isolated on the basis of its ability to selectively block the translation of poliovirus mRNA. RNA derived from the clone by transcription with T7 RNA polymerase appears to block translation initiation by internal ribosome entry (cap independent) but does not significantly affect cap-dependent translation. Deletion analysis of the poliovirus 5'-untranslated region (5'-UTR) has shown that yeast inhibitor RNA (I-RNA) requires internal ribosome entry site sequences to inhibit the translation of poliovirus RNA in vitro. Using a bicistronic RNA construct, we show that I-RNA preferentially inhibits translation by internal ribosome entry. Gel retardation and UV cross-linking studies demonstrate that I-RNA specifically binds proteins which interact with RNA secondary structures within the poliovirus 5'-UTR presumably involved in internal initiation. Specifically, purified I-RNA competes with virus RNA structures within the 5'-UTR which bind a cellular protein with an approximate molecular mass of 52 kDa. Finally, when transfected into HeLa cells, I-RNA efficiently inhibits the replication of poliovirus RNA presumably by inhibiting translation of the input virus RNA
Loss of maternal measles antibody in black South African infants in the first year of life implications for age of vaccination
In order to investigate the feasibility of measles vaccination before the age of 9 months the duration of passive immunity against measles was estimated by conducting a longitudinal study of measles antibody levels in 20 black neonates delivered at term. Measles serum antibody (lgG) was measured by enzyme-linked immunosorbent assay in the mother at childbirth and on consecutive samples taken from the infants from birth until 9 months of age. Protective measles antibody level was defined as > 200 mlU. Unprotective levels were found in 88% (95% confidence interval (Cl) 81 - 99%) of 6-month-old infants, while at 9 months all were susceptible. The mean antibody level was 192 mlU (Cl 104 - 348%) at 4 months; 34 mlU (Cl 15 - 73%) at 6 months and 13 mlU (Cl 6-24%) at 9 months of age. Our data support the recent World Health Organisation recommendation to immunise children in developing countries at 6 months with the 'high titre' Edmonston-Zagreb measles vaccine, since most infants in our study had lost passive immunity against measles by this age
Proposed gravitational wave background from black hole-torus systems
Cosmological gamma-ray bursts may be powered by rotating black holes with
contemporaneous emission of gravitational radiation from a surrounding torus.
We calculate the resulting stochastic background radiation assuming strong
cosmological evolution and a uniform black hole mass distribution of M=
(4--14)M_odot. The predicted spectral flux density corresponds to a peak
spectral closure density of (1--2) times 10^{-7}, and has comparable
contributions at 450 Hz times kappa and over 300--450 Hz times kappa from
nearby and distant sources, respectively, where kappa refers to an uncertainty
factor of order unity in the radius of the torus. For two optimized advanced
LIGO-type detectors the proposed gravitational wave background could be
detectable within a year of integration.Comment: To appear in Ap
Theory and astrophysical consequences of a magnetized torus around a rapidly rotating black hole
(Abbrev.) We analyze the topology, lifetime, and emissions of a torus around
a black hole formed in hypernovae and black hole-neutron star coalescence. The
torus is ab initio uniformly magnetized, represented by two counter oriented
current-rings, and develops a state of suspended accretion against a "magnetic
wall" around the black hole. Magnetic stability of the torus gives rise to a
new fundamental limit EB/Ek<0.1 for the ratio of poloidal magnetic field
energy-to-kinetic energy. The lifetime of rapid spin of the black hole is
effectively defined by the timescale of dissipation of black hole-spin energy
in the horizon, and satisfies T= 40s (MH/7MSun)(R/6MH)^4(0.03MH/MT) for a black
hole of mass MH surrounded by a torus of mass MT and radius R. The torus
converts a major fraction Egw/Erot=0.1 into gravitational radiation through a
finite number of multipole mass-moments, and a smaller fraction into MeV
neutrinos and baryon-rich winds. At a source distance of 100Mpc, these
emissions over N=2e4 periods give rise to a characteristic strain amplitude
\sqrt{N}hchar=6e-21. We argue that torus winds create an open magnetic
flux-tube on the black hole, which carries a minor and standard fraction
Ej/Erot=1e-3 in baryon-poor outflows to infinity. We identify this baryon poor
output of tens of seconds with GRBs with contemporaneous and strongly
correlated emissions in gravitational radiation, conceivably at multiple
frequencies. Ultimately, this leaves a black hole binary surrounded by a
supernova remnant.Comment: To appear in ApJ (44p
Australian participation in the Gaia follow-up network for solar system objects
The Gaia satellite, planned for launch by the European Space Agency (ESA) in 2013, is the next-generation astrometry mission following Hipparcos. Gaia’s primary science goal is to determine the kinematics, chemical structure, and evolution of the Milky Way Galaxy. In addition to this core science goal, the Gaia space mission is expected to discover thousands of Solar System objects. Because of orbital constraints, Gaia will only have a limited opportunity for astrometric follow-up of these discoveries. In 2010, the Gaia Data Processing and Analysis Consortium (DPAC) initiated a program to identify ground-based optical telescopes for a Gaia follow-up network for Solar System Objects to perform the following critical tasks: confirmation of discovery, identification of body, object tracking to constrain orbits. To date, this network comprises 37 observing sites (representing 53 instruments). The Zadko Telescope, located in Western Australia, was highlighted as an important network node because of its southern location, longitude, and automated scheduling system. We describe the first follow-up tests using the fast moving Potentially Hazardous Asteroid 2005 YU55 as the target
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