► Mid-IR spectra from 5.2 to 38μm using the Spitzer/IRS of 28 asteroids. ► Analyzed using STM and a thermophysical model (TPM) if 3D-shape is available. ► Large (Deq>130km) binary asteroids are ...covered with a thick layer of thermally insulating regolith. ► Smaller (Deq<17km) asteroids have their surfaces are coarser, and therefore younger. ► Their average densities vary from 0.7–1.7g/cm3 (P-, C-type) to ∼2g/cm3 (S-type) and 3.2g/cm3 (M-type).
We collected mid-IR spectra from 5.2 to 38μm using the Spitzer Space Telescope Infrared Spectrograph of 28 asteroids representative of all established types of binary groups. Photometric lightcurves were also obtained for 14 of them during the Spitzer observations to provide the context of the observations and reliable estimates of their absolute magnitudes. The extracted mid-IR spectra were analyzed using a modified standard thermal model (STM) and a thermophysical model (TPM) that takes into account the shape and geometry of the large primary at the time of the Spitzer observation. We derived a reliable estimate of the size, albedo, and beaming factor for each of these asteroids, representing three main taxonomic groups: C, S, and X. For large (volume-equivalent system diameter Deq>130km) binary asteroids, the TPM analysis indicates a low thermal inertia (Γ⩽∼100Js−1/2K−1m−2) and their emissivity spectra display strong mineral features, implying that they are covered with a thick layer of thermally insulating regolith. The smaller (surface-equivalent system diameter Deff<17km) asteroids also show some emission lines of minerals, but they are significantly weaker, consistent with regoliths with coarser grains, than those of the large binary asteroids. The average bulk densities of these multiple asteroids vary from 0.7–1.7g/cm3 (P-, C-type) to ∼2g/cm3 (S-type). The highest density is estimated for the M-type (22) Kalliope (3.2±0.9g/cm3). The spectral energy distributions (SEDs) and emissivity spectra, made available as a supplement document, could help to constrain the surface compositions of these asteroids.
Context. The potentially hazardous asteroid (99942) Apophis, previously designated 2004 MN4, is emblematic of the study of asteroids that could impact the Earth in the near future. Orbit monitoring ...and error propagation analysis are mandatory to predict the probability of an impact and, furthermore, its possible mitigation. Several aspects for this prediction have to be investigated, in particular the orbit adjustment and prediction updates when new astrometric data are available. Aims. We analyze Apophis orbit and provide impact predictions based on new observational data, including several orbit propagations. Methods. New astrometric data of Apophis have been acquired at the Pic du Midi one-meter telescope (T1m) during March 2011. Indeed, this asteroid was again visible from ground-based stations after a period of several years of unfavorable conjunction with the Sun. We present here the original astrometric data and reduction, and the new orbit obtained from the adjustment to all data available at Minor Planet Center (until March 2011). Results. We present a new sketch of keyholes and impacts for the next century. Additionally, we discuss observational errors, astrometric reduction, orbit adjustment, and adequacy of the dynamical model used.
Context. We study trans-Neptunian objects (TNOs) from stellar occultations. Aims. We predict stellar occultations from 2012.5 to the end of 2014 by 5 Centaurs and 34 TNOs. Methods. These predictions ...were achieved in two ways: first, we built catalogues with precise astrometric positions of the stellar content around the paths on the sky of these targets, as seen by a ground-based observer; second, the observed positions of the targets were determined with the help of these same catalogues so that we could improve their ephemerides and the reliability of the predictions. The reference system is the International Celestial Reference System (ICRS) as realised by the Fourth US Naval Observatory CCD Astrograph Catalog (UCAC4). All the sky paths as well as the selected targets were observed from Oct. 2011 to May 2013 with the ESO/MPG 2.2 m telescope equipped with the Wide Field Imager (WFI). All astrometric results were obtained with the platform for reduction of astronomical images automatically (PRAIA) after correcting the images for overscan, bias, and flatfield. Results. The catalogues with the stellar content around the sky path of each selected target are complete down to magnitude R = 19 and have an average positional accuracy of about 50 milliarcseconds. This same average accuracy also holds for the observed positions of the targets. In the catalogues from the sky paths, stellar proper motions for non-UCAC4 objects were derived from the combination of the current epoch WFI observations with either the 2MASS or the USNO-B1 catalogues. The offsets between the observed and (JPL) ephemeris positions of the targets frequently reach absolute values of some hundreds of milliarcseconds. Conclusions. We present here stellar occultation predictions for the selected 5 Centaurs and 34 TNOs from 2012.5 to the end of 2014. This work is also an extension of two previous prediction works by us, the first one for Pluto, Charon, Nix, and Hydra, and the second for ten other large TNOs. The use of catalogues from the observations of the sky paths in the astrometry of the TNOs and Centaurs enhanced the coherence between their positions and those of the respective occulted candidate stars. New observations of these TNOs and Centaurs are continuously used to redetermine their ephemerides.
Gomes-Júnior et al. published 3613 positions for the eight largest irregular satellites of Jupiter and 1787 positions for the largest irregular satellite of Saturn, Phoebe. These observations were ...made between 1995 and 2014 and have an estimated error of about 60–80 mas. Based on this set of positions, we derived new orbits for the eight largest irregular satellites of Jupiter: Himalia, Elara, Pasiphae, Carme, Lysithea, Sinope, Ananke and Leda. For Phoebe we updated the ephemeris from Desmars et al. using 75 per cent more positions than the previous one. Because of their orbital characteristics, it is common belief that the irregular satellites were captured by the giant planets in the early Solar system, but there is no consensus for a single model explaining where they were formed. Size, shape, albedo and composition would help to trace back their true origin, but these physical parameters are yet poorly known for irregular satellites. The observation of stellar occultations would allow for the determination of such parameters. Indeed Jupiter will cross the galactic plane in 2019–2020 and Saturn in 2018, improving a lot the chances of observing such events in the near future. Using the derived ephemerides and the UCAC4 catalogue we managed to identify 5442 candidate stellar occultations between 2016 January and 2020 December for the nine satellites studied here. We discussed how the successful observation of a stellar occultation by these objects is possible and present some potential occultations.
Context. We investigate transneptunian objects, including Pluto and its satellites, by stellar occultations. Aims. Our aim is to derive precise, astrometric predictions for stellar occultations by ...Pluto and its satellites Charon, Hydra and Nix for 2008-2015. We construct an astrometric star catalog in the UCAC2 system covering Pluto$\arcmin$s sky path. Methods. We carried out in 2007 an observational program at the ESO2p2/WFI instrument covering the sky path of Pluto from 2008 to 2015. We made the astrometry of 110 GB of images with the Platform for Reduction of Astronomical Images Automatically (PRAIA). By relatively simple astrometric techniques, we treated the overlapping observations and derived a field distortion pattern for the WFI mosaic of CCDs to within 50 mas precision. Results. Positions were obtained in the UCAC2 frame with errors of 50 mas for stars up to magnitude R = 19, and 25 mas up to R = 17. New stellar proper motions were also determined with 2MASS and the USNO B1.0 catalog positions as first epoch. We generated 2252 predictions of stellar occultations by Pluto, Charon, Hydra and Nix for 2008-2015. An astrometric catalog with proper motions was produced, containing 2.24 million stars covering Pluto$\arcmin$s sky path with $30\arcmin$ width. Its magnitude completeness is about R = 18–19 with a limit about R = 21. Based on the past 2005–2008 occultations successfully predicted, recorded and fitted, a linear drift with time in declination with regard to DE418/plu017 ephemerides was determined for Pluto and used in the current predictions. For offset (mas) = A * (t (yr) - 2005.0) + B, we find A = +30.5 ± 4.3 mas yr-1 and B = -31.5 ± 11.3 mas, with standard deviation of 14.4 mas for the offsets. For these past occultations, predictions and follow-up observations were made with the 0.6 m and 1.6 m telescopes at the Laboratório Nacional de Astrofísica/Brazil. Conclusions. Recurrent issues in stellar occultation predictions were addressed and properly overcome: body ephemeris offsets, catalog zero-point position errors and field-of-view size, long-term predictions and stellar proper motions, faint-visual versus bright-infrared stars and star/body astrometric follow-up. In particular, we highlight the usefulness of the obtained astrometric catalog as a reference frame for star/body astrometric follow-up before and after future events involving the Pluto system. Besides, it also furnishes useful photometric information for field stars in the flux calibration of observed light curves. Updates on the ephemeris offsets and candidate star positions (geometric conditions of predictions and finding charts) are made available by the group at www.lesia.obspm.fr/perso/bruno-sicardy/.
Observation of stellar occultation by objects of the Solar System is a powerful technique that allows measurements of size and shape of the small bodies with accuracies in the order of the kilometre. ...In addition, the occultation star probes the surroundings of the object, allowing the study of putative rings/debris or atmosphere around it. Since 2009, more than 60 events by trans-Neptunian and Centaur objects have been detected, involving more than 34 different bodies. Some remarkable results were achieved, such as the discovery of rings around Chariklo and Haumea, or the high albedo of Eris, the lack of global atmosphere around Makemake and the discovery of the double shape of 2014 MU69, among others. After the release of Gaia catalogues, predictions became more accurate, leading to an increasing number of successful observations of occultation events. To keep track of the results achieved with this technique, we created a database to gather all the detected events worldwide. The database is presented as an electronic table (http://occultations.ct.utfpr.edu.br/), where the main information obtained from any occultation by small outer solar system objects are listed. The structure and term definitions used in the database are presented here, as well as some simple statistics that can be done with the available results.
Context. We study large trans-Neptunian objects (TNOs) using stellar occultations. Aims. We derive precise astrometric predictions for stellar occultations by Eris, Haumea, Ixion, Makemake, Orcus, ...Quaoar, Sedna, Varuna, 2002 TX300, and 2003 AZ84 for 2011–2015. We construct local astrometric catalogs of stars complete to magnitudes as faint as R = 18 − 19 in the UCAC2 (Second US Naval Observatory CCD Astrograph Catalog) frame covering the sky path of these objects. Methods. During 2007–2009, we carried out an observational program at the ESO2p2/WFI (2.2 m Max-Planck ESO telescope with the Wide Field Imager) instrument. The observations covered the sky path of the selected targets from 2008 to 2015. We performed the astrometry of 316 GB images using the Platform for Reduction of Astronomical Images Automatically (PRAIA). With the help of field distortion patterns derived for the WFI mosaic of CCDs, we reduced the overlapping mosaics of CCDs. Results. We derive positions in the UCAC2 frame with 40 mas precision for stars up to the catalog magnitude completeness limit (about R = 19). New stellar proper motions are also determined with 2MASS (Two Micron All Sky Survey) and the USNO B1.0 (United States Naval Observatory B 1.0) catalog positions as a first epoch. Astrometric catalogs with proper motions were produced for each TNO, containing more than 5.35 million stars covering the sky paths with 30′ width in declination. The magnitude completeness is about R = 19 with a limit of about R = 21. We predicted 2717 stellar occultation candidates for all targets. Ephemeris offsets with about from 50 mas to 100 mas precision were applied to each TNO orbit to improve the predictions. They were obtained during 2007–2010 from a parallel observational campaign carried out with telescopes from 0.6 m to 2.2 m in size. Conclusions. This extends our previous work for the Pluto system to large TNOs, using the same observational and astrometric procedures. The obtained astrometric catalogs are useful for follow-up programs at small to large telescopes used to improve the candidate star positions and TNO ephemeris. They also furnish valuable photometric information for the field stars. For each TNO, updates on the ephemeris offsets and candidate star positions (geometric conditions of predictions and finding charts) are made available by the group at http://www.lesia.obspm.fr/perso/bruno-sicardy/.
Context. The irregular satellites of the giant planets are believed to have been captured during the evolution of the solar system. Knowing their physical parameters, such as size, density, and ...albedo is important for constraining where they came from and how they were captured. The best way to obtain these parameters are observations in situ by spacecrafts or from stellar occultations by the objects. Both techniques demand that the orbits are well known. Aims. We aimed to obtain good astrometric positions of irregular satellites to improve their orbits and ephemeris. Methods. We identified and reduced observations of several irregular satellites from three databases containing more than 8000 images obtained between 1992 and 2014 at three sites (Observatório do Pico dos Dias, Observatoire de Haute-Provence, and European Southern Observatory – La Silla). We used the software Platform for Reduction of Astronomical Images Automatically (PRAIA) to make the astrometric reduction of the CCD frames. The UCAC4 catalog represented the International Celestial Reference System in the reductions. Identification of the satellites in the frames was done through their ephemerides as determined from the SPICE/NAIF kernels. Some procedures were followed to overcome missing or incomplete information (coordinates, date), mostly for the older images. Results. We managed to obtain more than 6000 positions for 18 irregular satellites: 12 of Jupiter, 4 of Saturn, 1 of Uranus (Sycorax), and 1 of Neptune (Nereid). For some satellites the number of obtained positions is more than 50% of what was used in earlier orbital numerical integrations. Conclusions. Comparison of our positions with recent JPL ephemeris suggests there are systematic errors in the orbits for some of the irregular satellites. The most evident case was an error in the inclination of Carme.
Context. In 2009, the Sun and the Earth passed through the equatorial plane of Jupiter and therefore the orbital planes of its main satellites. It was the equinox on Jupiter. This occurrence made ...mutual occultations and eclipses between the satellites possible. Experience has shown that the observations of such events provide accurate astrometric data able to bring new information on the dynamics of the Galilean satellites. Observations are made under the form of photometric measurements, but need to be made through the organization of a worldwide observation campaign maximizing the number and the quality of the data obtained. Aims. This work focuses on processing the complete database of photometric observations of the mutual occultations and eclipses of the Galilean satellites of Jupiter made during the international campaign in 2009. The final goal is to derive new accurate astrometric data. Methods. We used an accurate photometric model of mutual events adequate with the accuracy of the observation. Our original method was applied to derive astrometric data from photometric observations of mutual occultations and eclipses of the Galilean satellites of Jupiter. Results. We processed the 457 lightcurves obtained during the international campaign of photometric observations of the Galilean satellites of Jupiter in 2009. Compared with the theory, for successful observations, the r.m.s. of O–C residuals are equal to 45.8 mas and 81.1 mas in right ascension and declination, respectively; the mean O–C residuals are equal to –2 mas and –9 mas in right ascension and declination, respectively, for mutual occultations; and –6 mas and +1 mas in right ascension and declination, respectively, for mutual eclipses.
We present the kinematics of hundreds of open clusters, based on the UCAC2 Catalogue positions and proper motions. Membership probabilities were obtained for the stars in the cluster fields by ...applying a statistical method uses stellar proper motions. All open clusters with known distance were investigated, and for 75 clusters this is the first determination of the mean proper motion. The results, including the DSS images of the cluster's fields with the kinematic members marked, are incorporated in the Open Clusters Catalogue supported on line by our group.
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