The Gas Production Rate and Coma Structure of Comet C/1995 O1 (Hale-Bopp)

Abstract

The University of Wisconsin—Madison and NASA—Goddard conducted a comprehensive multi-wavelength observing campaign of coma emissions from comet Hale-Bopp, including OH 3080 Å, [0 I] 6300 Å, H₂O+ 6158 Å, H Balmer-α 6563 Å, NH2 6330 Å, [C I] 9850 Å CN 3879 Å, C₂ 5141 Å, C₃ 4062 Å, C I 1657 Å, and the UV and optical continua. In this work, we concentrate on the results of the H₂O daughter studies. Our wide-field OH 3080 Å measured flux agrees with other, similar observations and the expected value calculated from published water production rates using standard H₂O and OH photochemistry. However, the total [O I] 6300 Å flux determined spectroscopically over a similar field-of-view was a factor of 3 – 4 higher than expected. Narrow-band [O I] images show this excess came from beyond the H2O scale length, suggesting either a previously unknown source of [O I] or an error in the standard OH + v → O(¹ D) + H branching ratio. The Hale-Bopp OH and [O I] distributions, both of which were imaged to cometocentric distances > 1 x 10⁶ km, were more spatially extended than those of comet Halley (after correcting for brightness differences), suggesting a higher bulk outflow velocity. Evidence of the driving mechanism for this outflow is found in the Hα line profile, which was narrower than in comet Halley (though likely because of opacity effects, not as narrow as predicted by Monte-Carlo models). This is consistent with greater collisional coupling between the suprathermal H photodissociation products and Hale-Bopp’s dense coma. Presumably because of mass loading of the solar wind by ions and ions by the neutrals, the measured acceleration of H₂O⁺ down the ion tail was much smaller than in comet Halley. Tailward extensions in the azimuthal distributions of OH 3080 Å, [O I], and [C I], as well as a Doppler asymmetry in the [O I] line profile, suggest ion-neutral coupling. While the tailward extension in the OH can be explained by increased neutral acceleration, the [O I] 6300 Å and [C I] 9850 Å emissions show 13% and > 200% excesses in this direction (respectively), suggesting a non-negligible contribution from dissociative recombination of CO⁺ and/or electron collisional excitation. Thus, models including the effects of photo-and collisional chemistry are necessary for the full interpretation of these data.

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