ALBERT

Description: 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 A, [O I] 6300 A, H2O(+) 6158 A, H Balmer-alpha 6563 A, NH2 6330 A, [C I] 9850 A CN 3879 A, C2 5141 A, C3 4062 A, C I 1657 A, and the UV and optical continua. In this work, we concentrate on the results of the H2O daughter studies. Our wide-field OH 3080 A 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 A 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 + upsilon to O((sup I)D) + H branching ratio. The Hale-Bopp OH and [O I] distributions, both of which were imaged to cometocentric distances greater than 1 x 10(exp 6) 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 H(alpha) 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 A, [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 A and [C I] 9850 A emissions show 13% and less than 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.

Description: Large-aperture photometric observations of comet Hale-Bopp (C/1995 O1) in the forbidden red line of neutral oxygen ([O I] 6300 angstroms) with the 150 mm dual-etalon Fabry-Perot spectrometer that comprises the Wisconsin H-alpha Mapper and a 50 mm dual-etalon Fabry-Perot spectrometer at the McMath-Pierce main telescope from 1997 late February to mid April yield a total metastable O((sup 1)D) production rate of (2.3-5.9) x 10(exp 30)/s. Applying the standard H2O and OH photodissociation branching ratios, we derive a water production rate, Q(H2O), of (2.6-6.1) x 10(exp 31)/s, which disagrees with Q(H2O = 1x10(exp 31)/s determined by independent H2O, OH, and H measurements. Furthermore, our own [O I] 6300 observations of the inner coma (〈 30,000 km) using the 3.5 m Wisconsin-Indiana-Yale-NOAO telescope Hydra and Densepak multi-object spectrographs yield Q(H2O) = 1 x 10(exp 31)/s. Using our [O I] 6300 data, which cover spatial scales ranging from 2,000 to 1x10(exp 6) km, and a complementary set of wide-field ground-based OH images, we can constrain the sources of the apparent excess O((sup 1)D) emission to the outer coma, where photodissociation of OH is assumed to be the dominant O((sup 1)D) production mechanism. From production rates of other oxygen-bearing volatiles (e.g., CO and CO2), we can account for at most 30% of the observed excess O((sup 1)D) emission. Since even less O((sup 1)D) should be coming from other sources (e.g., electron excitation of neutral O and distributed nonnuclear sources of H2O), we hypothesize that the bulk of the excess O((sup 1)D) is likely coming from photodissociating OH. Using the experimental OH photo-dissociation cross section of Nee and Lee at Ly-alpha as a guide in modifying the theoretical OH cross sections of van Dishoeck and Dalgarno, we can account for approximately 60% of the observed O((sup 1)D) excess without requiring major modifications to the other OH branching ratios or the total OH photodissociation lifetime.