D. T. Thompson

Comet Bowell 1980b

Optical filter photometry, and optical and ultraviolet spectrophotometry data collected between November 1980 and June 1982 of Comet Bowell are presented. It was determined that Comet Bowell began producing significant amounts of OH, though not other species, before perihelion at a heliocentric distance near 4.6 AU. As the comet approached perihelion at 3.4 AU, OH production decreased and CN and C2 species were detected at normal concentrations. An outburst in April 1982 was dominated by OH. The grains in 1982 near perihelion showed increases in albedo between 3150 and 4500 A and 1.2 and 1.6 microns. The presence of two grains populations was suggested, together with the conclusion that Comet Bowell was physically a typical new comet entering the inner solar system from the Oort cloud for the first time.

Europa's phase curve: Implications for surface structure

Ground-based photoelectric measurements taken by two of us (W.L. and D.T.) with narrow band filters at 0.47 and 0.55 μm at small phase angles clearly show the existence of a narrow opposition peak. We have combined these data with Voyager images to obtain a range of phase angle coverage from 0.3° to 119°. The integral phase data were analyzed using Hapkes photometric function. The photometric parameters are remarkably similar for both wavelengths and hemispheres. The following values at λ = 0.55 μm for the leading side are typical: single-scattering albedo w = 0.964, opposition effect amplitude B0 = 0.5, opposition effect width h = 0.0016, double-lobed Henyey-Greenstein factors b = −0.429, c = 0.113, and mean roughness angle θ = 10°. The photometric roughness is much smaller than on other bodies of the Solar System and implies that low surface relief seen at kilometer scales in the Voyager images extends down to centimeter scales. If the very narrow opposition surge is due to shadow hiding, the porosity must be large, on the order of 96%, which is much higher than that of a regolith generated by meteoritic gardening. Both the small roughness and high porosity imply that endogenic processes dominate surface formation even at centimeter scales. The single particle scattering function is more back scattering on the leading hemisphere, which contradicts the scattering properties expected of a planetary surface that is being preferentially bombarded by ions on its trailing hemisphere.

Narrowband photometry of Comet P/Stephan-Oterma and the backscattering properties of cometary grains

The results of narrowband filter photometry of Comet P/Stephan-Oterma are presented. For heliocentric distances between 1.95 and 1.58 AU prior to perihelion, OH, CN, and C2 production rates varied approximately as r to the -5th power. Following perihelion, an even steeper dependence of production rate on heliocentric distance was observed for these species. A pronounced brightening of the comets continuum near opposition is interpreted in terms of preferential backscattering by the cometary dust. While Comet P/Stephan-Oterma is one of the dustier comets observed, it shows no indication of compositional anomalies in either the gas or the dust.

UBV photometry of asteroid 433 Eros

Abstract UBV observations of asteroid 433 Eros were conducted on 17 nights during the winter of 1974/75. The peak-to-peak amplitude of the lightcurve varied from about 0.3 mag to nearly 1.4mmag. The absolute V mag at maximum light, extrapolated to zero phase, is 10.85. Phase coefficients of 0.0233 mag/degree, 0.0009 mag/degree and 0.0004 mag/degree were derived for V, B-V, and U-B, respectively. The zero-phase color of Eros (B−V = 0.88, U−B = 0.50) is representative of an S (silicaceous) compositional type asteroid. The color does not vary with rotation. The photometric behavior of Eros can be modeled by a cylinder with rounded ends having an axial ratio of about 2.3:1. The asteroid is rotating about a short axis with the north pole at λ0 = 15° and β0 = 9°.

The albedo of Titan

Photometric observations of Titan since 1972 show a cyclical variation of about 10 percent. A minimum value of brightness and albedo apparently occurred in 1984. Spectrophotometric observations, made annualy since 1980 at 8 A resolution, 3295-8880 A, were used to derive the value p-asterisk = 0.156 + or - 0.010 for the integrated geometric albedo in 1984. Variations of the equivalent widths of spectral features were not seen.

Periodic variations in the activity of Comet P/Halley during the 1985/1986 apparition

A search for periodic variation in the production of gas and dust by Comet Halley has been performed using narrowband photometric measurements from four sites - Lowell Observatory, Mauna Kea Observatory, Perth Observatory, and Cerro Tololo Inter-American Observatory. The method of phase dispersion minimization was applied to observations made during 164 observing nights between September 1985 and June 1986. A clear-cut variation, with a period near 7.4 days, was present throughout the postperihelion window. Less conclusive evidence of a similar period has been found in the pre-perihelion data. No indication of a shorter period or of strong sporadic activity exists in the data. The observations require that Halleys nucleus returns to essentially the same orientation with respect to the sun approximately every 7.4 days except for longer-timescale seasonal evolution. This fact precludes certain proposed models of nuclear motion. 53 refs.

Long-term brightness variations of neptune and the solar cycle modulation of its albedo.

The visual brightness and albedo of Neptune vary periodically during the 11-year solar cycle with an amplitude of 4%, anticorrelated with the variation of solar ultraviolet output. A seasonal trend in color suggests that Neptune, like Uranus, may have a slightly reddened pole.

The photometric variability of solar-type stars. IV Detection of rotational modulation among Hyades stars

High-precision differential photometric observations of 32 Hyades stars, spectral type F4 V to K1 V, were obtained at Lowell Observatory over a five-month season during late 1982 and early 1983. Contemporaneous Ca II H+K emission flux measurements for 30 of these stars were made at Mount Wilson Observatory. Fifteen (possibly 18) of these stars, all later than type F7 V, were found to be photometrically variable. Highly significant correlations between continuum and Ca II emission variations were found in several cases. The photometric variations for nine of the stars were periodic. The authors have interpreted this as rotational modulation, since the measured periods agreed well with rotation periods predicted from measurements of the mean chromospheric emission fluxes for these stars.

The albedo of Uranus

New absolute spectrophotometry of Uranus between 3295 A and 7676 A obtained at spectral resolutions of 4 A and 8 A during its 1981 apparition is reported and compared with previous observations by Younkin (1970) recorded during the 1961-1963 apparitions. Geometric albedos are calculated, and Bond albedos are estimated for both sets of data. An increase in brightness of about 14% and a color change occurred during the interval between the epochs of observation, consistent with the high north-polar haze reported by Price and Franz (1979) and the change in aspect of the planet. An upper limit to the internal heat source is estimated from energy balance considerations.

The disappearance of OH from Comet P/Encke

Photoelectric observations of Comet P/Encke during its 1980 apparition are combined with other published data to relate molecular production rates to the visual lightcurve. In addition to a substantial asymmetry about perihelion which is already well known, there are shorter-term variations in specific molecules which have not been duplicated by models. The most dramatic of these fluctuations is a rapid decrease by more than a factor of 3 in the production of OH at 0.75 AU preperihelion.