Jonathan H. Elias

Orbits of nine Uranian rings

Observations of a stellar occultation by Uranus and its nine rings are presented and used to examine the structures and kinematics of the rings. The observations of the occultation of the K giant star KM 12 were obtained in the K band with the 4-m CTIO telescope at a signal-to-noise ratio higher than any previously obtained. Ring occultation profiles reveal the alpha ring to possibly have a double structure and less abrupt boundaries than the gamma ring, which exhibits diffraction fringes, while the eta ring is a broad ring with an unresolved narrow component at its inner edge. The present timing data, as well as previous occultation timings, are fit to a kinematic model in which all nine rings are treated as coplanar eclipses of zero inclination, precessing due to the zonal harmonics of the Uranian gravitational potential to obtain solutions for the ring orbits. Analysis of the residuals from the fitted orbits reveals that the proposed model is a good representation of ring kinematics. The reference system defined by the orbit solutions has also been used to obtain a value of 0.022 + or - 0.003 for the ellipticity of Uranus and a Uranian rotation period of 15.5 h.

Structure of the Uranian rings. I - Square-well model and particle-size constraints

A least squares fitting is added to the diffraction model for the Uranian rings and the model is then used to determine the relative optical depths of the rings at visible and near-IR wavelengths. Light transmission is assumed constant on a scale of a few radial km in the rings. The amount of starlight passing through the rings is calculated from occultations with account taken of a monolayer ring, diffraction effects, and multiple ring layers. A diffracted occultation profile is generated with consideration given to the angular diameter of the star, the impulse response of the detector, and atmospheric aberrations. Calculation procedures which remove noise are outlined and results are compared with the observational data base. The wavelength dependence of optical depth is formulated. It is shown that occultation data do not support the presence of a large fraction of submicron particles. No optical depth variations are projected for 0.88-2.2 microns wavelengths. Unchopped data are needed to verify the model predictions.

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.

Five stellar occultations by Neptune : further observations of ring arcs

Abstract Stellar occultations on 18 April 1984 and 20 August 1985 have yielded additional observations of incomplete rings, or “arcs,” around Neptune. The latter represents only the second unambiguous observation of a Neptunian arc to be confirmed by observations at more than one telescope, and thus strongly corroborates the discovery of the first of this class of features by Hubbard et al. (1986, Nature 319, 636). Diffraction model fits to the observed light curves indicate radial widths widths for the arcs of 8–26 km, and normal optical depths of 0.07–0.14, on the assumption that the arcs lie in Neptunes equatorial plane. The 20 August 1985 arc shows a lower optical depth “shoulder” at its inner edge, reminiscent of the Uranian δ ring. The observation of 18 April 1984 shows three distinct features, within a radial range of 180 km. Using the Neptune pole direction determined from Voyager navigation analyses, we confirm that the arc observed on 20 August 1985, together with two previously observed arcs (Hubbard et al. , 1986, Nature 319, 636; Covault et al. 1986, Icarus 67, 126), appear to match in radius the outermost ring (1989N1R) observed in Voyager images (Smith et al. 1989, Science 246, 1422). The 18 April 1984 observation does not match any narrow Voyager feature, but does fall within the 6000-km-wide zone denoted 1989N4R by Smith et al. (1989). A combination of the stellar occultation data and data from Voyager imaging observations of the 1989N1R arcs leads to a refined value for the arc mean motion of 820.1185 deg/day, corresponding to a semi-major axis of 62932.3 km.

The structure of Neptune's upper atmosphere: the stellar occultation of 24 May 1981

Abstract Observations of the 24 May 1981 occultation of an uncatalogued star by Neptune made at the Cerro Tololo Inter-American Observatory have been analyzed to yield temperature profiles of Neptunes upper atmosphere for number densities near 5 × 10 13 cm −3 . The mean temperatures at immersion (latitude −56°) and emersion (latitude −16°) obtained by numerical inversion were 140 ± 10°K and 154 ± 10°K, respectively. The immersion and emersion profiles are remarkably similar in overall shape, suggestive of global atmospheric layering. From the astrometry of the event, precise relative positions of Neptune and the occulted were obtained.

IRAS 00521-7054 - An unusually warm galaxy

IRAS 00521 - 7054 is a galaxy that lies behind the Small Magellanic Cloud. Its infrared energy distribution is quite unusual in that it has exceptionally red 0.6-10-micron colors, while its 12-100-micron spectrum is among the warmest known for a galaxy or quasar, with a peak at 25 microns and an integrated luminosity of close to 10 to the 12th solar luminosity. Its optical spectrum is characteristic of a Seyfert 2 galaxy: low-ionization forbidden lines and H-alpha have FWHM 500 km/s, with some evidence for a much broader component of the H-alpha line. A plausible model is that a central nonthermal source is the dominant source of luminosity, but it is heavily obscured by dust and is thus observed indirectly through reradiation of its flux by the surrounding material. This galaxy may then be a good example of an object in the process of developing into a classical quasar.

The rings of Uranus - Occultation profiles from three observatories

Abstract Occultation profiles for the nine confirmed Uranian rings obtained from Las Campanas, the European Southern Observatory, and Cerro Tololo on 15–16 August 1980 are compared. The α ring shows a “double-dip” structure; the η ring shows a broad and narrow component (similar to Saturns F ring); and the e ring shows six features that appear in the data from all three observatories. Diffraction fringes appear at the edges of several of the occultation profiles.