Narrowband Photometry of Comet P/Halley: Variation with Heliocentric Distance, Season, and Solar Phase Angle

Abstract

Narrowband filter photometry of Comet P/Halley from 109 nights in 1985, 1986, and 1987 is analyzed to discern the comet's variation in activity with changing heliocentric distance and season. The scattering phase function of dust in the coma is also investigated. Asymmetry about perihelion in the production of gas and dust is confirmed. However, we find the asymmetry to be significantly less for water than for the trace species and for dust grains. The relative abundances of CN, C2, and C3remain essentially constant with respect to each other throughout the apparition, but change markedly with respect to water. The behavior of NH also more closely tracks that of the carbon-based species than that of water. These phenomena are most readily understood in terms of seasonal effects on a compositionally variegated nucleus. However, this explanation may not work if Halley's nucleus is in a complex long-axis-mode rotational state as many believe to be the case. Observations of Halley's continuum reveal a substantial change near perihelion in the average radial distribution of dust in the inner coma, with the dust displaying a canonical 1/ρ fall-off after perihelion but a much steeper decrease with distance from the nucleus during 1985. The phase function of the dust shows pronounced curvature over the observed range of values (1.5° to 66°). The slope is 0.045 mag/degree near 0° phase angle but becomes nearly flat at angles greater than about 30°. The peak of the phase function at 0° is 2.9 times that at the minimum near 50°, compared with the 2× enhancement typically measured for the zodiacal light.