A Determination of The Nuclear Size of Comet Hale–Bopp (C/1995 O1)

The signal of the nucleus was digitally extracted from six images of the innermost coma of this comet, obtained with the Hubble Space Telescope’s Wide-Field Planetary Camera 2 in the planetary mode between October 23, 1995 and October 17, 1996. Two different anisotropic, power-type laws were used to filter out the contribution from the dust coma: one peaking at the center of the elliptical surface brightness distribution (law A), the other peaking at its focus (law B). The nuclear R magnitudes in the Cousins system, reduced to a zero phase angle and to 1 AU from Earth and the Sun with a phase coefficient of 0.035 mag/deg and an inverse square distance power law, are found to average 9.46 ± 0.07 and 9.48 ± 0.18 when law A and law B are applied, respectively. These results become 9.49 ± 0.07 and 9.51 ± 0.17, when the nucleus signal on the October 1995 image is assumed to consist of a sum of the contributions from two unresolved nuclear components. In either scenario, no systematic variations are apparent in the nuclear brightness with time, which suggests the absence of any significant contamination of the extracted nuclear signal by the coma. Assuming a geometric albedo of 4 percent, the corresponding effective nuclear diameter amounts to 71 ± 4 km (formal error). This result substantially exceeds the size estimates published by Weaver et al., which are based only on the October 1995 observation and which were obtained with the help of a different reduction method. Runs in which a power-type law fitting the contribution from the coma was assumed to hold all the way to a small fraction of a pixel from the nucleus led to distinctly inferior solutions and yielded spurious values ll70 km for the nuclear diameter.