2060 Chiron: CCD and electronographic photometry

Abstract

R-band CCD photometry of 2060 Chiron was carried out on nine nights in November and December 1986 and on 23 March 1988. The synodic rotation period is 5.9180 ± 0.0001 hr and the peak-to-peak lightcurve amplitude is 0.088 ± 0.003 mag. From the 1986 data, the absolute magnitude is H R = 6.24 ± 0.02 mag and the slope parameter is G R = 0.70 ± 0.15, though formal errors may not be realistic. The lightcurve has two pairs of extrema, but its asymmetry, as evidenced by the presence of significant odd Fourier harmonics, suggests macroscopic surface irregularities and/or the presence of some large-scale albedo variegation. The 1986 observations show no evidence of periodic or nonperiodic brightness changes that can be ascribed to comet-like activity. In contrast, the 1988 observations indicate a brightening of Chiron by 0.6 ± 0.1 mag, which confirms the finding by D. J. Tholen, W. K. Hartmann, and D. P. Cruikshank 1988, IAU Circ. 4554) and is consistent with 1978 electronographic photometry, published here for the first time. The lightcurve amplitude appears to be unchanged, however, and image profiles are indistinguishable from those of stars in both 1986 and 1988. Chiron may recently have been varying nonasteroidally on time scales greater than a month or two; but it is also possible that the asteroid's intrinsic brightness has been bistable over the past decade, with an excursion of 0.56 ± 0.12 mag on a time scale of some years.