483 research outputs found
Gaia Focused Product Release: Asteroid orbital solution. Properties and assessment
Context. We report the exploitation of a sample of epoch astrometry for 157
000 asteroids, the same object in the Gaia Data Release 3, extended over the
time coverage planned for the Gaia DR4, which is not expected before the end of
2025. This data set covers more than one full orbital period for the vast
majority of these asteroids. The orbital solutions are derived from the Gaia
data alone over a relatively short arc compared to the observation history of
many of these asteroids. Aims. The work aims to produce orbital elements for a
large set of asteroids based on 66 months of accurate astrometry provided by
Gaia and to assess the accuracy of these orbital solutions with a comparison to
the best available orbits derived from independent observations. A second
validation is performed with accurate occultation timings. Methods. We
processed the raw astrometric measurements of Gaia to obtain astrometric
positions of moving objects with 1D sub-mas accuracy at the bright end. For
each asteroid that we matched to the data, an orbit fitting was attempted in
the form of the best fit of the initial conditions at the median epoch.
Results. Orbits are provided in the form of state vectors in the International
Celestial Reference Frame for 156 764 asteroids, including near-Earth objects,
main-belt asteroids, and Trojans. For the asteroids with the best observations,
the (formal) relative uncertainty is better than 1E10. Results are compared to
orbits available from the Jet Propulsion Laboratory and MPC. Their orbits are
based on much longer data arcs, but from positions of lower quality. The
relative differences in semi-major axes have a mean of 5E10 and a scatter of
5E9
Gaia: Organisation and challenges for the data processing
Gaia is an ambitious space astrometry mission of ESA with a main objective to
map the sky in astrometry and photometry down to a magnitude 20 by the end of
the next decade. While the mission is built and operated by ESA and an
industrial consortium, the data processing is entrusted to a consortium formed
by the scientific community, which was formed in 2006 and formally selected by
ESA one year later. The satellite will downlink around 100 TB of raw telemetry
data over a mission duration of 5 years from which a very complex iterative
processing will lead to the final science output: astrometry with a final
accuracy of a few tens of microarcseconds, epoch photometry in wide and narrow
bands, radial velocity and spectra for the stars brighter than 17 mag. We
discuss the general principles and main difficulties of this very large data
processing and present the organisation of the European Consortium responsible
for its design and implementation.Comment: 7 pages, 2 figures, Proceedings of IAU Symp. 24
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
Gaia Data Processing Architecture
Gaia is ESA's ambitious space astrometry mission the main objective of which
is to astrometrically and spectro-photometrically map 1000 Million celestial
objects (mostly in our galaxy) with unprecedented accuracy. The announcement of
opportunity for the data processing will be issued by ESA late in 2006. The
Gaia Data Processing and Analysis Consortium (DPAC) has been formed recently
and is preparing an answer. The satellite will downlink close to 100 TB of raw
telemetry data over 5 years. To achieve its required accuracy of a few 10s of
Microarcsecond astrometry, a highly involved processing of this data is
required.
In addition to the main astrometric instrument Gaia will host a Radial
Velocity instrument, two low-resolution dispersers for multi-color photometry
and two Star Mappers. Gaia is a flying Giga Pixel camera. The various
instruments each require relatively complex processing while at the same time
being interdependent. We describe the overall composition of the DPAC and the
envisaged overall architecture of the Gaia data processing system. We shall
delve further into the core processing - one of the nine, so-called,
coordination units comprising the Gaia processing system.Comment: 10 Pages, 2 figures. To appear in ADASS XVI Proceeding
Detectability of Weakly Interacting Massive Particles in the Sagittarius Dwarf Tidal Stream
Tidal streams of the Sagittarius dwarf spheroidal galaxy (Sgr) may be
showering dark matter onto the solar system and contributing approx (0.3--23)%
of the local density of our Galactic Halo. If the Sagittarius galaxy contains
WIMP dark matter, the extra contribution from the stream gives rise to a
step-like feature in the energy recoil spectrum in direct dark matter
detection. For our best estimate of stream velocity (300 km/sec) and direction
(the plane containing the Sgr dwarf and its debris), the count rate is maximum
on June 28 and minimum on December 27 (for most recoil energies), and the
location of the step oscillates yearly with a phase opposite to that of the
count rate. In the CDMS experiment, for 60 GeV WIMPs, the location of the step
oscillates between 35 and 42 keV, and for the most favorable stream density,
the stream should be detectable at the 11 sigma level in four years of data
with 10 keV energy bins. Planned large detectors like XENON, CryoArray and the
directional detector DRIFT may also be able to identify the Sgr stream.Comment: 26 pages, 4 figure
The effect of the motion of the Sun on the light-time in interplanetary relativistic experiments
In 2002 a measurement of the effect of solar gravity upon the phase of
coherent microwave beams passing near the Sun has been carried out with the
Cassini mission, allowing a very accurate measurement of the PPN parameter
. The data have been analyzed with NASA's Orbit Determination Program
(ODP) in the Barycentric Celestial Reference System, in which the Sun moves
around the centre of mass of the solar system with a velocity of
about 10 m/sec; the question arises, what correction this implies for the
predicted phase shift. After a review of the way the ODP works, we set the
problem in the framework of Lorentz (and Galilean) transformations and evaluate
the correction; it is several orders of magnitude below our experimental
accuracy. We also discuss a recent paper \cite{kopeikin07}, which claims wrong
and much larger corrections, and clarify the reasons for the discrepancy.Comment: Final version accepted by Classical and Quantum Gravity (8 Jan. 2008
Gaia: organisation and challenges for the data processing
Gaia is an ambitious space astrometry mission of ESA with a main objective to map the sky in astrometry and photometry down to a magnitude 20 by the end of the next decade. While the mission is built and operated by ESA and an industrial consortium, the data processing is entrusted to a consortium formed by the scientific community, which was formed in 2006 and formally selected by ESA one year later. The satellite will downlink around 100 TB of raw telemetry data over a mission duration of 5 years from which a very complex iterative processing will lead to the final science output: astrometry with a final accuracy of a few tens of microarcseconds, epoch photometry in wide and narrow bands, radial velocity and spectra for the stars brighter than 17 mag. We discuss the general principles and main difficulties of this very large data processing and present the organization of the European Consortium responsible for its design and implementatio
Using Galactic Cepheids to verify Gaia parallaxes
Context. The Gaia satellite will measure highly accurate absolute parallaxes
of hundreds of millions of stars by comparing the parallactic displacements in
the two fields of view of the optical instrument. The requirements on the
stability of the 'basic angle' between the two fields are correspondingly
strict, and possible variations (on the microarcsec level) are therefore
monitored by an on-board metrology system. Nevertheless, since even very small
periodic variations of the basic angle might cause a global offset of the
measured parallaxes, it is important to find independent verification methods.
Aims. We investigate the potential use of Galactic Cepheids as standard candles
for verifying the Gaia parallax zero point. Methods. We simulate the complete
population of Galactic Cepheids and their observations by Gaia. Using the
simulated data, simultaneous fits are made of the parameters of the
period-luminosity relation and a global parallax zero point. Results. The total
number of Galactic Cepheids is estimated at about 20 000, of which nearly half
could be observed by Gaia. In the most favourable circumstances, including
negligible intrinsic scatter and extinction errors, the determined parallax
zero point has an uncertainty of 0.2 microarcsec. With more realistic
assumptions the uncertainty is several times larger, and the result is very
sensitive to errors in the applied extinction corrections. Conclusions. The use
of Galactic Cepheids alone will not be sufficient to determine a possible
parallax zero-point error to the full potential systematic accuracy of Gaia.
The global verification of Gaia parallaxes will most likely depend on a
combination of many different methods, including this one.Comment: 7 pages, 6 figures. Accepted for publication in Astronomy and
Astrophysic
Tidal evolution of exo-planetary systems: WASP-50, GJ 1214 and CoRoT-7
We perform numerical simulations to investigate tidal evolution of two
single-planet systems, that is, WASP-50 and GJ 1214 and a two-planet system
CoRoT-7. The results of orbital evolution show that tidal decay and
circularization may play a significant role in shaping their final orbits,
which is related to the initial orbital data in the simulations. For GJ 1214
system, different cases of initial eccentricity are also considered as only an
upper limit of its eccentricity (0.27) is shown, and the outcome suggests a
possible maximum initial eccentricity (0.4) in the adopted dynamical model.
Moreover, additional runs with alternative values of dissipation factor
are carried out to explore tidal evolution for GJ 1214b, and these
results further indicate that the real of GJ 1214b may be much
larger than its typical value, which may reasonably suggest that GJ 1214b bears
a present-day larger eccentricity, undergoing tidal circularization at a slow
rate. For the CoRoT-7 system, tidal forces make two planets migrating towards
their host star as well as producing tidal circularization, and in this process
tidal effects and mutual gravitational interactions are coupled with each
other. Various scenarios of the initial eccentricity of the outer planet have
also been done to investigate final planetary configuration. Tidal decay
arising from stellar tides may still work for each system as the eccentricity
decreases to zero, and this is in association with the remaining lifetime of
each planet used to predict its future.Comment: 9 pages, 12 figures, accepted for publication in "SCIENCE CHINA
Physics,Mechanics & Astronomy
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