Publications (8)7.42 Total impact
- [Show abstract] [Hide abstract] ABSTRACT: We present results from a multi-chord Pluto stellar occultation observed on 29 June 2015 from New Zealand and Australia. This occurred only two weeks before the NASA New Horizons flyby of the Pluto system and serves as a useful comparison between ground-based and space results. We find that Pluto's atmosphere is still expanding, with a significant pressure increase of 5 +/- 2% since 2013 and a factor of almost three since 1988. This trend rules out, as of today, an atmospheric collapse associated with Pluto's recession from the Sun. A central flash, a rare occurrence, was observed from several sites in New Zealand. The flash shape and amplitude are compatible with a spherical and transparent atmospheric layer of roughly 3 km in thickness whose base lies at about 4 km above Pluto's surface, and where an average thermal gradient of about 5 K/km prevails. We discuss the possibility that small departures between the observed and modeled flash are caused by local topographic features (mountains) along Pluto's limb that block the stellar light. Finally, using two possible temperature profiles, and extrapolating our pressure profile from our deepest accessible level down to the surface, we estimate a range of 12.4-13.2 microbar for the surface pressure.
- [Show abstract] [Hide abstract] ABSTRACT: For 40 years, the sizes and shapes of many dozens of asteroids have been determined from observations of asteroidal occultations, and over a thousand high-precision positions of the asteroids relative to stars have been measured. Some of the first evidence for satellites of asteroids was obtained from the early efforts; now, the orbits and sizes of some satellites discovered by other means have been refined from occultation observations. Also, several close binary stars have been discovered, and the angular diameters of some stars have been measured from analysis of these observations. The International Occultation Timing Association (IOTA) coordinates this activity worldwide, from predicting and publicizing the events, to accurately timing the occultations from as many stations as possible, and publishing and archiving the observations.
- [Show abstract] [Hide abstract] ABSTRACT: 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.
Article: Asteroid Occultations V9.0[Show abstract] [Hide abstract] ABSTRACT: This data set is intended to include all reported timings of observed asteroid, planet, and planetary satellite occultation events as well as occultation axes derived from those timings by David W. Dunham and David Herald. This version is complete through April 2011.
- [Show abstract] [Hide abstract] ABSTRACT: Asteroid sizes can be directly measured by observing occultations of stars by asteroids. When there are enough observations across the path of the shadow, the asteroid's projected silhouette can be reconstructed. Asteroid shape models derived from photometry by the lightcurve inversion method enable us to predict the orientation of an asteroid for the time of occultation. By scaling the shape model to fit the occultation chords, we can determine the asteroid size with a relative accuracy of typically ~ 10%. We combine shape and spin state models of 44 asteroids (14 of them are new or updated models) with the available occultation data to derive asteroid effective diameters. In many cases, occultations allow us to reject one of two possible pole solutions that were derived from photometry. We show that by combining results obtained from lightcurve inversion with occultation timings, we can obtain unique physical models of asteroids.
- [Show abstract] [Hide abstract] ABSTRACT: Approximately every six years the orbital plane of the Jovian moons turns edge-on the Earth's line of sight, giving us the opportunity to time the eclipses and occultations arising from this geometry known as Jupiter Mutual Events (JME). These timings help to refine the residuals in the orbital elements of Jovian moons. While taking several tens of minutes of wing data surrounding an occultation by Io in 2009 during that JME cycle, an anomaly was detected in the lightcurve prior to and following the actual occultation. Analysis of this anomaly led to the hypothesis that it was the result of atmospheric extinction of the light from the occulted moon by the atmosphere of Io. The same anomaly was then found when Europa was the occulting body. Occultations by Ganymede showed no dimming anomaly. Eleven observers from four countries contributed 53 data sets for 28 individual events in an observing program to study this phenomenon. This paper details the results including camera response, observing method, reduction method, and atmospheric extinction detection. The atmospheric extinction hypothesis is supported by several independent methods that are also be detailed. Derived atmospheric models are presented, including a noted asymmetry.