The inner coma and nucleus of Comet Hale-Bopp: Results from a stellar occultation.

Y. A. Fernandez, D. D. Wellnitz, M. W. Buie, E. W. Dunham, R. L. Millis, R. A Nye, J. A. Stansberry, L. H. Wasserman, M. F. A'Hearn, C. M. Lisse, M. E. Golden, M. J. Person, R. R. Howell, R. L. Marcialis, and J. N. Spitale. Icarus 140, 205-220 (1999). .


We discuss the properties of the nucleus and inner coma of Comet Hale-Bopp (C/1995 O1) as derived from observations of its occultation of Star PPM 200723 on 5 October 1996, while the comet was 2.83 AU from the Sun. Compared to previous occultations by active comets, this is possibly the closest to the nucleus one has ever observed. Three chords (lightcurves) through the comet's inner coma were measured, though only one chord has a strong indication of measuring the occultation, and that was through thin cirrus. We have constrained the radius of the nucleus and properties of the coma using a simple model; there is a large valid section of parameter space. Our data show the optical depth of the coma was >=1 within 20 to 70 km of the center of the (assumed spherical) nucleus, depending on the coma's structure and the nucleus' size. The dependence of the dust coma's opacity on cometocentric distance, rho, was steeper than expected for force-free, radial flow, being probably as steep as or steeper than 1/rho^1.4 within 100 km of the nucleus (though it is marginally possible to fit one coma hemisphere with a 1/rho law). Assuming the dust coma flowed radially from a spot at the center of the nucleus and that the coma's profile was not any steeper than rho^-2, the upper limit to the radius of the nucleus is about 30 km, though relaxing these assumptions limits the radius to 48 km. The chord through the coma does not show the same coma structure within 100 km of the nucleus as that which is apparent in larger-scale (~700 km/pixel) imaging taken just before the event, suggesting that (a) the star's path sampled the acceleration region of the dust, and/or (b) azimuthal variation in the inner coma is different than that seen in the outer coma.

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