Pluto's tenuous nitrogen atmosphere was first detected by the imprint left on the light curve of a star that was occulted by the planet in 1985 (ref. 1), and studied more extensively during a second occultation event in 1988 (refs 2-6). These events are, however, quite rare and Pluto's atmosphere remains poorly understood, as in particular the planet has not yet been visited by a spacecraft. Here we report data from the first occultations by Pluto since 1988. We find that, during the intervening 14 years, there seems to have been a doubling of the atmospheric pressure, a probable seasonal effect on Pluto.
[1] We report the observation of two stellar occultations by Titan on 14 November 2003, using stations in the Indian Ocean, southern Africa, Spain, and northern and southern Americas. These occultations probed altitudes between $550 and 250 km ($1 to 250 mbar) in Titan's upper stratosphere. The light curves reveal a sharp inversion layer near 515 ± 6 km altitude (1.5 mbar pressure level), where the temperature increases by 15 K in only 6 km. This layer is close to an inversion layer observed fourteen months later by the Huygens HASI instrument during the entry of the probe in Titan's atmosphere on 14 January 2005 [Fulchignoni et al., 2005]. Central flashes observed during the first occultation provide constraints on the zonal wind regime at 250 km, with a strong northern jet ($200 m s À1 ) around the latitude 55°N, wind velocities of $150 m s À1 near the equator, and progressively weaker winds as more southern latitudes are probed. The haze distribution around Titan's limb at 250 km altitude is close to that predicted by the Global Circulation Model of Rannou et al. (2004) in the southern hemisphere, but a clearing north of 40°N is necessary to explain our data. This contrasts with Rannou et al.'s (2004) model, which predicts a very thick polar hood over Titan's northern polar regions. Simultaneous observations of the flashes at various wavelengths provide a dependence of t / l Àq , with q = 1.8 ± 0.5 between 0.51 and 2.2 mm for the tangential optical depth of the hazes at 250 km altitude.
Abstract. As mentioned in a previous paper (Viateau & Rapaport 1997), the orbit of asteroid (17) Thetis is strongly perturbed by two large minor planets, (4) Vesta and (11) Parthenope. These strong gravitational perturbations enabled us to determine both the mass of Vesta and the mass of Parthenope. We also independently derived the mass of Vesta from observations of (197) Arete. The weighted mean of the two results gives the value (1.306 ± 0.016) 10 −10 M (solar mass) for the mass of Vesta, and its derived mean density is (3.3 ± 0.5) g/cm 3 . For (11) Parthenope, the values (2.56 ± 0.07) 10 −12 M and (2.3 ± 0.2) g/cm 3 were obtained, respectively for its mass and its mean density.
Abstract. A first CCD 512 × 512 camera working in scan mode (declination field 14 ) was mounted in 1994 on the Bordeaux CCD meridian circle. After a testing period, this camera was installed on the Valinhos CCD meridian circle (near São Paulo, Brazil), as part of a collaboration between Bordeaux Observatory and the Instituto Astronomico e Geofisico of São Paulo. A second improved CCD 1024 × 1024 camera, with a declination field of 28 , was installed on the Bordeaux instrument in June 1996. The mean internal precision of a single observation is about 0.04 in both coordinates for 9 ≤ V ≤ 14. In the same magnitude range, magnitudes can also be obtained with an internal precision of about 0.05 mag. Both instruments can participate efficiently in extending the HipparcosTycho frame, during the next decade. Among other duties, the Bordeaux CCD meridian circle is being used since January 1997 for completing the Méridien 2000 project. The characteristics of both instruments and some results obtained with them are presented in this paper.
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