E. Dunham

The size, shape, density, and Albedo of Ceres from its occultation of BD+8°471

The occultation of BD+8°471 by Ceres on 13 November 1984 was observed photoelectrically at 13 sites in Mexico, Florida, and the Caribbean. These observations indicate that Ceres is an oblate spheroid having an equatorial radius of 479.6±2.4 km and a polar radius of 453.4±4.5 km. The mean density of this minor planet is 2.7 g/cm3±5%, and its visual geometric albedo is 0.073. While the surface appears globally to be in hydrostatic equilibrium, firm evidence of real limb irregularities is seen in the data.

The diameter of Pallas from its occultation of SAO 85009

The May 29, 1978, occultation of SAO 85009 by Pallas was observed photoelectrically at seven widely spaced sites. The observations are well represented by an elliptical apparent limb profile having semimajor and semiminor axes of 279.5 + or - 2.9 and 262.7 + or - 4.5 km, respectively. Combining these results with published information on the light curve and rotational pole position, Pallass mean diameter is found to be 538 + or - 12 km, which yields a mean density for Pallas of 2.8 + or - 0.5 g/cu cm and a visual geometric albedo of 0.103 + or - 0.005. The diameter of Pallas as determined from this occultation is significantly smaller than the values derived by radiometric, polarimetric, and double-image techniques.

The diameter of Juno from its occultation of AG + 0 deg 1022

The occultation on Dec. 11, 1979, of AG + 0 deg 1022 by Juno was observed photoelectrically from 15 sites distributed across the occultation track. The observations are well represented by a mean elliptical limb profile having semimajor and semiminor axes of 145.2 + or 0.8 and 122.8 + or - 1.9 km, respectively. The corresponding effective diameter of Juno is 267 + or - 5 km, where the uncertainty has been conservatively increased to reflect the presence of limb irregularities clearly seen in the observations. Published radiometric and polarimetric diameters for Juno are 6% to 7% smaller than the occultation result. No secondary occultations attributable to possible satellites of Juno were recorded at any of 23 photoelectrically equipped observing sites.

The thermal structure and energy balance of the Uranian upper atmosphere

Abstract Two occultation observations of the upper atmosphere of Uranus are reported: (i) the 15–16 August 1980 occultation of KM 12 from Cerro Tololo Inter-American Observatory, and (ii) the 26 April 1981 occultation of KME 13 from the Anglo-Australian Telescope and the 1-m telescope of the Australian National University. Mean atmospheric temperatures of 154 ± 15°K for the 1980 event and 132 ± 15°K for the 1981 event are derived from the lightcurves. A comparison of all available Uranus occultation data since March 1977 suggests that the mean atmospheric temperature of Uranus has changed significantly, with a typical variation of 15°K year−1. We investigate plausible energy sources that might account for such large temperature variations. We conclude that molecular and eddy diffusion, and atmospheric dynamics, are potentially as important as radiation to the upper atmospheric heat balance. There is evidence that no significant radiative cooling had occurred at two suboccultation points that had been in darkness for more than 0.5 terrestrial year, suggesting upper limits to the mixing ratios of CH4 and C2H2 of 6 × 10−5 and 5 × 10−7, respectively. The consistently close agreement of immersion and emersion mean temperatures for each occultation, in spite of apparently large secular changes in the atmospheric mean temperature, suggests that effective meridional transport occurs on Uranus. A continuing program of occultation observations in future years should reveal whether this pattern is significant.

The 1983 June 15 occultation by Neptune. II - The oblateness of Neptune

The oblateness and radius of Neptune were determined from an analysis of photoelectric observations of the June 15, 1983 occultation by Neptune at six stations, combined with the results of the Apr. 7, 1968 Neptune occultation of BD - 17 deg 4388. The oblateness is 0.0191 + or - 0.0017 and the equatorial radius is 25,268 + or - 12 km at the level probed by the occultation. The results are consistent with recent determinations of Neptunes rotation period and J2 (the second-order gravitational harmonic coefficient) and suggest that Neptune is far less centrally condensed than Uranus. Temperature profiles for Neptunes upper atmosphere were also derived from these data.

The radius and ellipticity of Uranus from its occultation of SAO 158687

From occultation timings obtained from the Kuiper Airborne Observatory and from Cape Town for Mar. 10, 1977 occultation of SAO 158687 by Uranus, the equatorial radius, Re, of the planet has been determined to be 26,228 + or - 30 km and its ellipticity epsilon = 1 - Rp/Re = 0.033 + or - 0.007. These values refer to the 1.0 x 10 to the 14th/cu cm number-density level, under the assumption that the upper atmosphere is composed of H2 and He with a mean molecular weight mu = 2.20. The dominant source of uncertainty is the position of the center of the ring system, which was used to define the center of Uranus in our analysis. A rotation rate of 12.8 + or - 1.7 hours for the planet is implied by our value for the ellipticity, under the assumption that Uranus is in hydrostatic equilibrium below the 1.0 x 10 to the 14th/cu cm number density level.

Photometry of Phoebe

Abstract Observatios of Phoebe (S9) in the V filter at small solar phase angles (0.2° to 1.2°) with the MIT SNAPSHOT CCD are presented. The value of Phoebes sideral rotational period is refined to 9.282 ± 0.015hr. Assuming the Voyager-derived 110 km radius, Phoebes observed mean opposition V magnitude of 16.176 ± 0.033 (extrapolated from small angles) corresponds to a geometric albedo of 0.084 ± 0.003. A strong opposition effect is indicated by the 0.180 ± 0.035 mag/deg solar phase coefficient observed at these small phase angles. The data are shown to be compatible with a phase function for C-type asteroids ( K. Lumme and E. Bowell, 1981 , Astron. J. 86 , 1705–1721; K. Lumme, E. Bowell, and A. W. Harris, 1984 , Bull. Amer. Astron. Soc. 16 , 684), but give a poorer fit to the average asteroid phase relation of T. Gehrels and E.F. Tedesco (1979 , Astron. J. 84 , 1079–1087). Phoebes rotational lightcurve in the V filter is roughly sinusoidal, with a 0.230-mag peak-to-peak amplitude and weaker higher order harmonics indicating primarily bimodal surface feature contrast. In addition to these photometric results, precise positions on 3 nights are given.

The 1983 June 15 occultation by Neptune. I - Limits on a possible ring system

Observations on 15 June 1983 of an occultation of a star by Neptune from Mauna Kea, Mount Stromlo, Siding Spring, and the Kuiper Airborne Observatory show no evidence for equatorial rings between 25,300 and 200,000 km (R/N/ = 25,269 km). Within most of this region, the upper limit on the optical depth along the line of sight, for rings broader than 6 km , is 0.04, which corresponds to a normal optical depth of 0.016. These results rule out a Neptunian ring system similar to that of Saturn or Uranus, but not a system of low optical depth similar to the Jovian rings. The data show no features that appear likely to have been caused by material in the equatorial plane of Neptune near the Roche limit.

Limit on possible narrow rings around Jupiter

An upper limit to the optical depth of the Jovian ring at high spatial resolution, determined from stellar occultation data, is reported. The spatial resolution of the observation is limited to about 13 km in Jupiters equatorial plane by the projection of the Fresnel zone on the equatorial plane in the radial direction. At this resolution, the normal optical depth limit is about 0.008. This limit applies to a strip in the Jovian equatorial plane that crosses the orbits of Amalthea, 1979J1, 1979J3, and the ring. An upper limit on the number density of kilometer-size boulders has been set at one per 11.000 sq km in the equatorial plane.

Signal-to-noise ratios for possible stellar occultations by pluto

Abstract Signal-to-noise ratios and magnitudes in the Johnson BVR system are presented for mine stars that might be occulted by Pluto during the period 1985–1990. From these calculations of the signal-to-noise ratio that could be achieved with a 1-m telescope, we find that each star (if occulted) is sufficiently bright to give useful information about a possible atmosphere of Pluto.