Observations of structures in the inner coma of Chiron with the HST Planetary Camera.

K. J. Meech, M. W. Buie, N. Samarasinha, B. E. A. Mueller, and M. J. S. Belton. Astron. J. 113, 844-862 (1997) .


We have obtained a series of images of 2060 Chiron with the HST Planetary Camera on 3 nights near its minimum geocentric distance in 1993 (Delta = 8.342 AU). Extended structure was visible within a radial distance of 0.3'' from the nucleus. We interpret the structure to be the bound ballistic atmosphere and exopause boundary (at R_e~ 1800 km) hypothesized by Meech & Belton (1990). Near simultaneous ground-based photometry of the outer coma showed a steep surface brightness profile with a logarithmic gradient of -3.1 +/- 0.03 (from 1 - 3.3'') and -2.2 +/- 0.01 (from 3.3 - 9.8''). The inner gradient is consistent with the HST observations. The color of the outer coma from the ground-based data is neutral at short wavelengths, and slightly redder than the sun at the longer wavelengths. The outer coma scatters neutrally at much shorter wavelengths than most comets, which suggests that the outer coma is populated by smaller grains (lambda <= 0.6 microns). Combining the grain size of particles escaping into the outer coma with the HST observations, and constraints on grain densities, we infer that the density of Chiron is relatively low, near rho_ {Nuc} < 10(3) kg m(-3) . Within the exopause boundary the nucleus contributes approximately 80% of the total light.

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