Observations of comet P/Halley at minimum phase angle

Abstract

We present time-series spectrophotometric observations of comet P/Halley obtained over 8 nights in 1985 November in near backscattering geometry (phase angle 1° < α < 10°). We have used the spectra to search for enhanced backscattering of the type reported earlier in comets P/Ashbrook-Jackson, Bowell (1982I), and P/Stephan-Oterma and to study the time dependence of the molecular band and continuum emissions. No opposition surge in brightness greater than ≍20% was observed in P/Halley; instead, the brightening is consistent with a small linear phase coefficient β = 0.02 ± 0.01 mag/deg in the observed phase angle range. The phase functions for P/Halley and the other comets are steeper than the phase function of the interplanetary dust. The comet phase functions are also uniike those of the low albedo asteroids, which typically show an opposition surge. Two continuum brightness outbursts observed on UT 1985 November 15 and 21 are discussed. Temporal variations in the molecular emission bands of the type seen by Millis and Schleicher (1986) have a timescale of variation which is consistent with the period of 7.37 days found in their data; however, the shape of the lightcurve is different.