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

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 200 723 on 5 October 1996, while the comet was 2.83 AU from the Sun, Compared t o previous occultations by active comets, this is possibly the closest to t he nucleus one has ever observed, Three chords (lightcurves) through the co met's inner coma were measured, though only one chord has a strong indicati on 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 si mple model; there is a large valid section of parameter space. Our data sho w the optical depth of the coma was greater than or equal to 1 within 20 to 70 km of the center of the (assumed spherical) nucleus, depending on the c oma's structure and the nucleus' size. The dependence of the dust coma's op acity on cometocentric distance, rho, was steeper than expected for force-f ree, radial how being probably as steep as or steeper than 1/rho(1.4) withi n 100 km of the nucleus (though it is marginally possible to fit one coma h emisphere 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 s teeper than rho(-2) the upper limit to the radius of the nucleus is about 3 0 km, though relaxing these assumptions limits the radius to 48 km, The cho rd through the coma does not show the same coma structure within 100 km of the nucleus as that which is apparent in larger-scale (similar to 700 km/pi xel) imaging taken just before the event, suggesting that (a) the star's pa th sampled the acceleration region of the dust, and/or (b) azimuthal variat ion in the inner coma is different than that seen in the outer coma. (C) 19 99 Academic Press.