47 research outputs found
Determination of Ceres mass based on the most gravitationally efficient close encounters
Here is presented recalculated value of the mass of Ceres, based on explicit
tracking of its gravitational influence on orbits evolution of 21 selected
asteroids during their mutual close encounters (CE). It was applied a new
modified method (MM) for mass determination, based on the connecting of
pre-encounter observations to the orbit determined from post-encounter ones.
The calculated weighted mean value of Ceres mass, based on modified method, is
while standard procedure (SM) provided
result of . We found that correlation between
individual estimated masses based on modified and standard method is 0.78,
which confirms reliability of using modified method.Comment: MNRAS:Accepted 2011 September 28. Received 2011 September 28; in
original form 2011 January 2
M2000 : an astrometric catalog in the Bordeaux Carte du Ciel zone +11 degrees < {delta} < +18 degrees
During four years, systematic observations have been conducted in drift scan
mode with the Bordeaux automated meridian circle in the declination band [+11 ;
+18]. The resulting astrometric catalog includes about 2 300 000 stars down to
the magnitude limit V_M=16.3. Nearly all stars (96%) have been observed at
least 6 times, the catalog being complete down to V_M=15.4. The median internal
standard error in position is about 35 mas in the V_M magnitude range [11 ;
15], which degrades to about 50 mas when the faintest stars are considered.
M2000 provides also one band photometry with a median internal standard error
of 0.04 mag. Comparisons with the Hipparcos and bright part of Tycho-2 catalogs
have enabled to estimate external errors in position to be lower than 40 mas.
In this zone and at epoch 1998, the faint part of Tycho-2 is found to have an
accuracy of 116 mas in alpha instead of 82 mas deduced from the model-based
standard errors given in the catalog.Comment: The catalogue can be fetched directly from:
ftp://cdsarc.u-strasbg.fr/cats/I/272 or queried from:
http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=I/272 More information at :
http://www.observ.u-bordeaux.fr/~soubiran/m2000.ht
Time-Resolved Photometry of Kuiper Belt Objects: Rotations, Shapes and Phase Functions
We present a systematic investigation of the rotational lightcurves of
trans-Neptunian objects based on extensive optical data from Mauna Kea. Four of
13 objects (corresponding to 31%) in our sample ((33128) 1998 BU48, 2000 GN171,
(20000) Varuna and 1999 KR16) were found to exhibit lightcurves with
peak-to-peak range > 0.15 magnitude. In a larger sample obtained by combining
our data with reliably determined lightcurves from the literature, 7 of 22
objects (32%) display significant (> 0.15 magnitude range) lightcurves. About
23% of the sampled objects have lightcurve ranges > 0.4 magnitudes. Curiously,
the objects are very large (> 250 km diameter, assuming an albedo of 0.04) and,
in the absence of rotation, should be near spherical due to self compression.
We propose that the large amplitude, short period objects are rotationally
distorted, low density rubble piles. Statistically, the trans-Neptunian objects
are less spherical than their main-belt asteroid counterparts, indicating a
higher specific angular momentum perhaps resulting from the formation epoch. In
addition to the rotational lightcurves, we measured phase darkening for 7
Kuiper Belt objects in the 0 to 2 degree phase angle range. Unlike Pluto, the
measured values show steep slopes and moderate opposition surge indicating
backscatter from low albedo porous surface materials.Comment: 22 pages, 33 figures, 12 tables, To appear in AJ sept. 200
Near-Infrared Mapping and Physical Properties of the Dwarf-Planet Ceres
We study the physical characteristics (shape, dimensions, spin axis
direction, albedo maps, mineralogy) of the dwarf-planet Ceres based on
high-angular resolution near-infrared observations. We analyze adaptive optics
J/H/K imaging observations of Ceres performed at Keck II Observatory in
September 2002 with an equivalent spatial resolution of ~50 km. The spectral
behavior of the main geological features present on Ceres is compared with
laboratory samples. Ceres' shape can be described by an oblate spheroid (a = b
= 479.7 +/- 2.3 km, c = 444.4 +/- 2.1 km) with EQJ2000.0 spin vector
coordinates RA = 288 +/- 5 deg. and DEC = +66 +/- 5 deg. Ceres sidereal period
is measured to be 9.0741 +/- 0.0001 h. We image surface features with diameters
in the 50-180 km range and an albedo contrast of ~6% with respect to the
average Ceres albedo. The spectral behavior of the brightest regions on Ceres
is consistent with phyllosilicates and carbonate compounds. Darker isolated
regions could be related to the presence of frost.Comment: 11 pages, 8 Postscript figures, Accepted for publication in A&
Long-term optical variability of the blazars PKS 2005-489 and PKS 2155-304
We present optical light curves for the period 1996-2000, of two of the
brightest known EGRET BL Lac objects: PKS 2005-489 and PKS 2155-304, the latter
also one of the few known TeV sources. For both objects, quiescent epochs of
low level of variability were followed by active periods, without any
indication of periodicity. In PKS 2005-489, several variability events with
duration of about 20 days were observed. In PKS 2155-304 fast drops and
subsequent rises in luminosity occurred in time scales of days. We proposed an
explanation in which a region moving along the relativistic jet is eclipsed by
broad line region clouds or star clusters in the host galaxy. We compare our
light curves with contemporaneous X-ray observations from All-Sky Monitor/RXTE.
Correlations between optical and X-ray activity were not found in any of the
sources at long time scales. However in PKS 2005-489 possible correlations were
observed in 1997 and 1998 at short time scales, with optical variability
preceding X-rays by 30 days in 1997 and succeeding them by about 10 days in
1998. The analysis of the SED, using the optical data presented here and
BeppoSAX contemporaneous observations obtained from the literature, shows only
small shifts in the synchrotron peak as the X-ray flux density changes.Comment: 23 pages,12 figures, accepted for publication in A
Double-plating of ovine critical sized defects of the tibia: a low morbidity model enabling continuous in vivo monitoring of bone healing
<p>Abstract</p> <p>Background</p> <p>Recent studies using sheep critical sized defect models to test tissue engineered products report high morbidity and complications rates. This study evaluates a large bone defect model in the sheep tibia, stabilized with two, a novel Carbon fibre Poly-ether-ether-ketone (CF-PEEK) and a locking compression plate (LCP) which could sustain duration for up to 6 month with an acceptable low complication rate.</p> <p>Methods</p> <p>A large bone defect of 3 cm was performed in the mid diaphysis of the right tibia in 33 sheep. The defect was stabilised with the CF - PEEK plate and an LCP. All sheep were supported with slings for 8 weeks after surgery. The study was carried out for 3 months in 6 and for 6 months in 27 animals.</p> <p>Results</p> <p>The surgical procedure could easily be performed in all sheep and continuous in vivo radiographic evaluation of the defect was possible. This long bone critical sized defect model shows with 6.1% a low rate of complications compared with numbers mentioned in the literature.</p> <p>Conclusions</p> <p>This experimental animal model could serve as a standard model in comparative research. A well defined standard model would reduce the number of experimental animals needed in future studies and would therefore add to ethical considerations.</p
Improvement Of The Orbits Of Asteroids And The Mass Of (1) Ceres
We present the results of work concerning the exploitation of the observations of asteroids made by Hipparcos, in addition to ground-based observations, to determine the mass of (1) Ceres from its perturbations on the orbits of 4 asteroids. The value obtained, (4:785 \Sigma 0:039) 10 \Gamma10 M fi , is in good agreement with most recent results obtained by other authors and shows that the value of the mass of Ceres recommended by the IAU should be decreased. A second work in progress, concerning the improvement of the orbital data of all the asteroids observed by Hipparcos, is also presented. We show that the addition of the Hipparcos data to the ground-based observations leads to a high improvement of the accuracy of the orbital elements of these asteroids, mostly for the inclination and the longitude of the node. Key words: minor planets: (1) Ceres; masses; ephemerides. 1. INTRODUCTION 48 minor planets were observed by Hipparcos from 1989 to 1993. The data obtained have very good a..