269 research outputs found
Fast algorithms for matching CCD images to a stellar catalogue
Two new algorithms are described for matching two dimensional coordinate
lists of point sources that are signifcantly faster than previous methods. By
matching rarely occurring triangles (or more complex shapes) in the two lists,
and by ordering searches by decreasing probability of success, it is
demonstrated that very few candidates need be considered to find a successful
match. Moreover, by immediately testing the suitability of a potential match
using an efficient mechanism, the need to process the entire candidate set is
avoided, yielding considerable performance improvements. Triangles are
described by a cosine metric that reduces the density of triangle space,
permitting efficient searches. An alternative shape characterization method
that reduces computational overhead in the construction phase is discussed. The
algorithms are tested on a set of 10 063 wide-field survey images, with
fields-of-view up to 4.8 x 3.6 deg, successfully matching 100% of the images in
a mean elapsed time of 6 ms (2.4 GHz Athlon CPU). The elapsed time of the
searching phase is shown to vary by less than 1 ms for list sizes between 10
and 200 points, demonstrating that fast, robust searches may be completed in
nearly constant time, independent of list size.Comment: Accepted for publication in Publications of the Astronomical Society
of Australi
Statistical and Numerical Study of Asteroid Orbital Uncertainty
The knowledge of the orbit or the ephemeris uncertainty of asteroid presents
a particular interest for various purposes. These quantities are for instance
useful for recovering asteroids, for identifying lost asteroids or for planning
stellar occultation campaigns. They are also needed to estimate the close
approach of Near-Earth asteroids, and subsequent risk of collision. Ephemeris
accuracy can also be used for instrument calibration purposes or for scientific
applications. Asteroid databases provide information about the uncertainty of
the orbits allowing the measure of the quality of an orbit. The aims of this
paper is to analyse these different uncertainty parameters and to estimate the
impact of the different measurements on the uncertainty of orbits. We
particularly deal with two main databases ASTORB and MPCORB providing
uncertainty parameters for asteroid orbits. Statistical methods are used in
order to estimate orbital uncertainty and compare with parameters from
databases. Simulations are also generated to deal with specific measurements
such as future Gaia or present radar measurements. Relations between the
uncertainty parameter and the characteristics of the asteroid (orbital arc,
absolute magnitude, ...) are highlighted. Moreover, a review of the different
measuments are compiled and the impact of these measures on the accuracy of the
orbit is also estimated.Comment: 11 pages, 10 figures, accepted by A&
ESO Imaging Survey. The Stellar Catalogue in the Chandra Deep Field South
(abridged) Stellar catalogues in five passbands (UBVRI) over an area of
approximately 0.3 deg^2, comprising about 1200 objects, and in seven passbands
(UBVRIJK) over approximately 0.1 deg^2, comprising about 400 objects, in the
direction of the Chandra Deep Field South are presented.
The 90% completeness level of the number counts is reached at approximately U
= 23.8, B = 24.0, V = 23.5, R = 23.0, I = 21.0, J = 20.5, K = 19.0.
A scheme is presented to select point sources from these catalogues, by
combining the SExtractor parameter CLASS_STAR from all available passbands.
Probable QSOs and unresolved galaxies are identified by using the previously
developed \chi^2-technique (Hatziminaoglou et al 2002), that fits the overall
spectral energy distributions to template spectra and determines the best
fitting template.
The observed number counts, colour-magnitude diagrams, colour-colour diagrams
and colour distributions are presented and, to judge the quality of the data,
compared to simulations based on the predictions of a Galactic Model convolved
with the estimated completeness functions and the error model used to describe
the photometric errors of the data.
The resulting stellar catalogues and the objects identified as likely QSOs
and unresolved galaxies with coordinates, observed magnitudes with errors and
assigned spectral types by the -technique are presented and are
publicly available.Comment: Paper as it will appear in print. Complete figures and tables can be
obtained from: http://www.eso.org/science/eis/eis_pub/eis_pub.html. Astronomy
& Astrophysics, accepted for publicatio
Detection of Yarkovsky acceleration in the context of precovery observations and the future Gaia catalogue
The Yarkovsky effect is a weak non-gravitational force leading to a small
variation of the semi-major axis of an asteroid. Using radar measurements and
astrometric observations, it is possible to measure a drift in semi-major axis
through orbit determination. This paper aims to detect a reliable drift in
semi-major axis of near-Earth asteroids (NEAs) from ground-based observations
and to investigate the impact of precovery observations and the future Gaia
catalogue in the detection of a secular drift in semi-major axis. We have
developed a precise dynamical model of an asteroid's motion taking the
Yarkovsky acceleration into account and allowing the fitting of the drift in
semi-major axis. Using statistical methods, we investigate the quality and the
robustness of the detection. By filtering spurious detections with an estimated
maximum drift depending on the asteroid's size, we found 46 NEAs with a
reliable drift in semi-major axis in good agreement with the previous studies.
The measure of the drift leads to a better orbit determination and constrains
some physical parameters of these objects. Our results are in good agreement
with the 1/D dependence of the drift and with the expected ratio of prograde
and retrograde NEAs. We show that the uncertainty of the drift mainly depends
on the length of orbital arc and in this way we highlight the importance of the
precovery observations and data mining in the detection of consistent drift.
Finally, we discuss the impact of Gaia catalogue in the determination of drift
in semi-major axis.Comment: 11 pages, 5 figures, accepted in A&
Phoebe's orbit from ground-based and space-based observations
The ephemeris of Phoebe, the ninth satellite of Saturn, is not very accurate.
Previous dynamical models were usually too simplified, the astrometry is
heterogeneous and, the Saturn's ephemeris itself is an additionnal source of
error. The aim is to improve Phoebe's ephemeris by using a large set of
observations, correcting some systematic errors and updating the dynamical
model. The dynamical model makes use of the most recent ephemeris of planets
and Saturnian satellites. The astrometry of Phoebe is improved by using a
compilation of ground-based and space-based observations and by correcting the
bias in stellar catalogues used for the reduction. We present an accurate
ephemeris of Phoebe with residuals of 0.45 arcsec and with an estimated
accuracy of Phoebe's position of less that 100 km on 1990-2020 period.Comment: 10 pages, 9 figures, accepted by A&
Star complexes and stellar populations in NGC 6822 - Comparison with the Magellanic Clouds
The star complexes (large scale star forming regions) of NGC 6822 were traced
and mapped and their size distribution was compared with the size distribution
of star complexes in the Magellanic Clouds (MCs). Furthermore, the spatial
distributions of different age stellar populations were compared with each
other. The star complexes of NGC 6822 were determined by using the isopleths,
based on star counts, of the young stars of the galaxy, using a statistical
cutoff limit in density. In order to map them and determine their geometrical
properties, an ellipse was fitted to every distinct region satisfying this
minimum limit. The Kolmogorov-Smirnov statistical test was used to study
possible patterns in their size distribution. Isopleths were also used to study
the stellar populations of NGC 6822. The star complexes of NGC 6822 were
detected and a list of their positions and sizes was produced. Indications of
hierarchical star formation, in terms of spatial distribution, time evolution
and preferable sizes were found in NGC 6822 and the MCs. The spatial
distribution of the various age stellar populations has indicated traces of an
interaction in NGC 6822, dated before 350 +/- 50 Myr.Comment: 10 pages, 7 figures, accepted by A&A; minor typeface correction
The stellar kinematics of co-rotating spiral arms in Gaia mock observations
We have observed an N-body/Smoothed Particle Hydrodynamics simulation of a
Milky Way like barred spiral galaxy. We present a simple method that samples
N-body model particles into mock Gaia stellar observations and takes into
account stellar populations, dust extinction and Gaia's science performance
estimates. We examine the kinematics around a nearby spiral arm at a similar
position to the Perseus arm at three lines of sight in the disc plane;
(l,b)=(90,0), (120,0) and (150,0) degrees. We find that the structure of the
peculiar kinematics around the co-rotating spiral arm, which is found in Kawata
et al. (2014b), is still visible in the observational data expected to be
produced by Gaia despite the dust extinction and expected observational errors
of Gaia. These observable kinematic signatures will enable testing whether the
Perseus arm of the Milky Way is similar to the co-rotating spiral arms commonly
seen in N-body simulations.Comment: 9 pages 4 Figures, submitted to MNRAS 22nd Dec 201
Mining the VVV: star formation and embedded clusters
The aim of this study is to locate previously unknown stellar clusters from
the VISTA variables in the V\'ia L\'actea Survey (VVV) catalogue data. The
method, fitting a mixture model of Gaussian densities and background noise
using the expectation maximization algorithm to a pre-filtered NIR survey
stellar catalogue data, was developed by the authors for the UKIDSS Galactic
Plane Survey (GPS). The search located 88 previously unknown mainly embedded
stellar cluster candidates and 39 previously unknown sites of star formation in
the 562 deg2 covered by VVV in the Galactic bulge and the southern disk
Orbit determination of Transneptunian objects and Centaurs for the prediction of stellar occultations
The prediction of stellar occultations by Transneptunian objects and Centaurs
is a difficult challenge that requires accuracy both in the occulted star
position as for the object ephemeris. Until now, the most used method of
prediction involving tens of TNOs/Centaurs was to consider a constant offset
for the right ascension and for the declination with respect to a reference
ephemeris. This offset is determined as the difference between the most recent
observations of the TNO and the reference ephemeris. This method can be
successfully applied when the offset remains constant with time. This paper
presents an alternative method of prediction based on a new accurate orbit
determination procedure, which uses all the available positions of the TNO from
the Minor Planet Center database plus sets of new astrometric positions from
unpublished observations. The orbit determination is performed through a
numerical integration procedure (NIMA), in which we develop a specific
weighting scheme. The NIMA method was applied for 51 selected TNOs/Centaurs.
For this purpose, we have performed about 2900 new observations during
2007-2014. Using NIMA, we succeed in predicting the stellar occultations of 10
TNOs and 3 Centaurs between 2013 and 2015. By comparing the NIMA and JPL
ephemerides, we highlighted the variation of the offset between them with time.
Giving examples, we show that the constant offset method could not accurately
predict 6 out of the 13 observed positive occultations successfully predicted
by NIMA. The results indicate that NIMA is capable of efficiently refine the
orbits of these bodies. Finally, we show that the astrometric positions given
by positive occultations can help to further refine the orbit of the TNO and
consequently the future predictions. We also provide the unpublished
observations of the 51 selected TNOs and their ephemeris in a usable format by
the SPICE library.Comment: 12 pages, 9 figures, accepted in A&
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