Aims. Astrometric observations performed by the Gaia Follow-Up Network for Solar System Objects (Gaia-FUN-SSO) play a key role in ensuring that moving objects first detected by ESA’s Gaia mission ...remain recoverable after their discovery. An observation campaign on the potentially hazardous asteroid (99 942) Apophis was conducted during the asteroid’s latest period of visibility, from 12/21/2012 to 5/2/2013, to test the coordination and evaluate the overall performance of the Gaia-FUN-SSO . Methods. The 2732 high quality astrometric observations acquired during the Gaia-FUN-SSO campaign were reduced with the Platform for Reduction of Astronomical Images Automatically (PRAIA), using the USNO CCD Astrograph Catalogue 4 (UCAC4) as a reference. The astrometric reduction process and the precision of the newly obtained measurements are discussed. We compare the residuals of astrometric observations that we obtained using this reduction process to data sets that were individually reduced by observers and accepted by the Minor Planet Center. Results. We obtained 2103 previously unpublished astrometric positions and provide these to the scientific community. Using these data we show that our reduction of this astrometric campaign with a reliable stellar catalog substantially improves the quality of the astrometric results. We present evidence that the new data will help to reduce the orbit uncertainty of Apophis during its close approach in 2029. We show that uncertainties due to geolocations of observing stations, as well as rounding of astrometric data can introduce an unnecessary degradation in the quality of the resulting astrometric positions. Finally, we discuss the impact of our campaign reduction on the recovery process of newly discovered asteroids.
We obtained improved optical positions for 300 ICRF2 sources - the Rio survey. We compared the Rio survey with 10 other selected optical astrometric surveys and studied the link between the Hipparcos ...Catalogue Reference Frame (HCRF) and the International Celestial Reference Frame, Second Realization (ICRF2). We investigated the possible causes for the observed non-coincidence between the optical and ICRF2 positions. The Rio survey positions were referred to the second version of the United States Naval Observatory CCD Astrograph Catalog (UCAC2), currently the best tested HCRF densification. The sources are between −90° < δ < +30°. We used two telescopes with suitable diameters and focal lengths to properly link the observed ICRF2 sources with the UCAC2, using intermediate brightness stars. We certified the astrometry done with many statistical tests. The average 'optical minus ICRF2' offsets and respective standard deviations in (α, δ) were −3 mas (41 mas) and +4 (45 mas). The Rio survey represents well the zero-point offset of the other surveys. The standard error of 3.5 mas found for the HCRF/ICRF2 link indicates an error excess that can be originated by a non-coincidence between the observed optical/VLBI positions. We thus discussed the influence of the errors from the UCAC2. Then, we searched for correlations with the source morphology, represented by structure indices defined in the radio and in the optical domain. Finally, we studied how the position offsets could originate from the perturbation of the optical point spread function (PSF) of the source's core, by a second source of flux. We found an analytical relation that describes the resulting centroid shift, as a function of the atmospheric seeing, the brightness ratio and the relative distance between the two contributing flux sources. Two scenarios, modelled by this relation, are discussed: an extinction window in the dust torus nearby the core, and a Galactic star near the line of sight.
Asteroid (234) Barbara is the prototype of a category of asteroids that has been shown to be extremely rich in refractory inclusions, the oldest material ever found in the Solar system. It exhibits ...several peculiar features, most notably its polarimetric behaviour. In recent years other objects sharing the same property (collectively known as ‘Barbarians’) have been discovered. Interferometric observations in the mid-infrared with the ESO VLTI (Very Large Telescope Interferometer) suggested that (234) Barbara might have a bi-lobated shape or even a large companion satellite. We use a large set of 57 optical light curves acquired between 1979 and 2014, together with the timings of two stellar occultations in 2009, to determine the rotation period, spin-vector coordinates, and 3-D shape of (234) Barbara, using two different shape reconstruction algorithms. By using the light curves combined to the results obtained from stellar occultations, we are able to show that the shape of (234) Barbara exhibits large concave areas. Possible links of the shape to the polarimetric properties and the object evolution are discussed. We also show that VLTI data can be modelled without the presence of a satellite.
We report the results of two multichord stellar occultations by the dwarf planet (1) Ceres that were observed from Brazil on 2010 August 17, and from the USA on 2013 October 25. Four positive ...detections were obtained for the 2010 occultation, and nine for the 2013 occultation. Elliptical models were adjusted to the observed chords to obtain Ceres’ size and shape. Two limb-fitting solutions were studied for each event. The first one is a nominal solution with an indeterminate polar aspect angle. The second one was constrained by the pole coordinates as given by Drummond et al. Assuming a Maclaurin spheroid, we determine an equatorial diameter of 972 ± 6 km and an apparent oblateness of 0.08 ± 0.03 as our best solution. These results are compared to all available size and shape determinations for Ceres made so far, and shall be confirmed by the NASA's Dawn space mission.
Every 42 years, the Earth and the Sun pass through the plane of the orbits of the main satellites of Uranus. In these occasions, mutual occultations and eclipses between these bodies can be seen from ...the Earth. The current Uranus equinox from 2007 to 2009 offers a precious opportunity to observe these events. Here, we present the analysis of five occultations and two eclipses observed from Brazil during 2007. For the reduction of the CCD images, we developed a digital coronagraphic method that removed the planet's scattered light around the satellites. A simple geometric model of the occultation/eclipse was used to fit the observed light curves. Dynamical quantities such as the impact parameter, the relative speed, and the central time of the event were then obtained with precisions of 7.6 km, 0.18 km s-1, and 2.9 s, respectively. These results can be further used to improve the parameters of the dynamical theories of the main Uranus satellites.
The light curves of mutual eclipses and occultations between the natural satellites of a planet allow us to obtain high-precision position and relative motion from differential photometry, enough to ...detect weak orbital disturbing forces, such as tidal forces. The observations are made during the equinoxes of the planet.
We studied 25 light curves observed in Brazil during the 2009 campaign of the Galilean satellites' mutual phenomena. A narrow-band filter centred at 890 nm was used, strongly attenuating the Jupiter's scattered light. We fitted the occultation and eclipse light curves using semi-analytical models that take into account the gradual decrease of light over the shadow, the solar limb darkening and the solar phase angle. The Oren-Nayar reflexive model was used to map the inhomogeneous light scattering on the surface of the satellites. For the first time it is used in a work about mutual events. Here, we include the study that made us decide for this model.
We measured the impact parameter, relative velocity and central instant with average precisions of 7.46 km (2.2 mas), 0.08 km s−1 (0.02 mas s−1) and 0.42 s (6.13 km), respectively. The fit precision of the normalized light-curve fluxes ranged between 0.4 and 4.4 per cent.
A long-term adaptive optics (AO) campaign of observing the double Asteroid (90) Antiope has been carried out in 2003–2005 using 8–10-m class telescopes, allowing prediction of the circumstances of ...mutual events occurring during the July 2005 opposition Marchis, F., Descamps, P., Hestroffer, D., Berthier, J., de Pater, I., 2004. Bull. Am. Astron. Soc. 36, 1180. This is the first opportunity to use complementary lightcurve and AO observations to extensively study the (90) Antiope system, an interesting visualized binary doublet system located in the main belt. The orbital parameters derived from the AO observations have served as input quantities for the derivation of a whole set of other physical parameters (namely shapes, surface scattering, bulk density, and internal properties) from analysis of collected lightcurves. To completely model the observed lightcurves, we employed Roche figures to construct an overall shape solution. The combination of these complementary observations has enabled us to derive a reliable physical and orbital solution for the system. Our model is consistent with a system of slightly non-spherical components, having a size ratio of 0.95 (with
R
avg
=
42.9
±
0.5
km
, separation
=
171
±
1
km
), and exhibiting equilibrium figures for homogeneous rotating bodies. A comparison with grazing occultation event lightcurves suggests that the real shapes of the components do not depart from Roche equilibrium figures by more than 10%. The J2000 ecliptic coordinates of the pole of the system are
λ
n
=
200
°
±
2
°
and
α
n
=
38
°
±
2
°
. The orbital period was refined to
P
=
16.5051
±
0.0001
h
, and the density is found to be slightly lower than previous determinations, with a value of
1.25
±
0.05
g
/
cm
3
, leading to a significant macro-porosity of 30%. Possible scenarios for the origin of the system are also discussed.
Context. The photometry of mutual occultations and eclipses of natural planetary satellites can be used to infer very accurate astrometric data. This can be achieved by processing the light curves of ...the satellites observed during international campaigns of photometric observations of these mutual events. Aims. This work focuses on processing the complete database of photometric observations of the mutual occultations and eclipses of the Uranian satellites made during the international campaign in 2007. The final goal is to derive new accurate astrometric data. Methods. We used an accurate photometric model of mutual events that explicitly depends on parameters that these accurate observations should be sensitive to, including the albedos of the satellites. Our original method is applied to derive astrometric data in relative positions from photometric observations of mutual occultations and eclipses of the Uranian satellites. Results. We process the 41 light-curves obtained during the international campaign of photometric observations of the Uranian satellites in 2007. The root-mean-square (rms) of the residuals “observations minus calculations” (O–C) with respect to theory for the best 34 observations are equal to 10.3 and 17.7 mas in right ascension and declination, respectively. For five observations only the position angle was derived. Topocentric or heliocentric angular differences for satellites pairs were obtained from 25 central instant offsets between observation and theory during the time period from May 4, 2007 to January 4, 2008. Conclusions. The rms of the residuals is from 10 to 20 mas that corresponds in situ to 10 to 20 km. These mutual event observations appear to be the most accurate astrometric ground-based observations of the major Uranian satellites to-date and should be used for dynamical purposes.
Aims. Occultations of bright stars by planets provide information on the state of their atmospheres. An occultation of the bright star 45 Capricornii (HIP 107302) by Jupiter occurred on the night of ...3/4 August 2009. Methods. The event was observed at multiple sites in Europe, Africa and South America and with instruments ranging in aperture from 0.4 m to 2.2 m. All observations, except one, were carried out in methane absorption bands centred at 0.89 μm and 2.2 μm to minimise the planetary contribution to the measured stellar flux. Following the application of special post-processing techniques, differential photometry was performed. Nearby bright satellites were used as reference sources. Results. Fifteen lightcurves were obtained. The photometric time series for fourteen of these were fitted to a model atmosphere of constant scale height (H). Estimates of H for most lightcurves lie within the range 20−30 km with an inverse-variance weighted mean of 23.6 ± 0.4 km, in good agreement with previous works. A comparison between half-light times at ingress and at egress implies an astrometric offset of 10−15 mas in Jupiter’s position relative to the star. Five lightcurves – two for ingress and three for egress – were numerically inverted into profiles of pressure versus temperature. Isothermal, mutually consistent behaviour is observed within the pressure range 3−10 μbar. The inferred temperature of 165 ± 5 K is consistent with, but slightly higher than, that measured by the Galileo Probe at 5° S latitude in 1995 at the same pressure level. Subtraction of isothermal models for nine cases show the presence of at least one, and possibly two, non-isothermal layers a few tens of km below the half-light datum. Their altitudes are similar to those of features previously reported during the occultation of HIP 9369 in 1999. Our temperature estimates are consistent with the expected small magnitude of the perturbation of the atmosphere following the impact event on Jupiter in July 2009.
Context. We investigate the differences between positions as determined by optical (direct imaging) and Very Long Baseline Interferometry (VLBI) techniques of extragalactic sources listed in the ...second realization of the International Celestial Reference Frame (ICRF2). Aims. We aim to verify the influence of the source’s intrinsic structure on these differences. Methods. Instruments with mosaics of CCDs were used to acquire the optical positions presented here, which lead us to opt for overlapping techniques to build a virtual, continuous CCD over the whole angular size of the respective fields of view, whose translation of the resulting intrumental positions into positions that are consistent with those in the ICRF2 was obtained with the help of the UCAC2. Results. The differences obtained between the optical and VLBI positions of the observed sources can reach more than 80 milliarsec for some measurements and, considering that they can hardly be explained by statistical fluctuations or systematic errors in the optical reference frame used here only, we argue that these differences can be related to the sources’ X-band structure index (8.4 GHz). Conclusions. In this context, the presence of the intrinsic structure should be considered when comparing the optical and VLBI positions of ICRF2 sources in the future.
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