The molecular ions, CH(+) ,CO(+) , CO_2(+) and H_2O(+) were identified in optical spectra of the plasma tails of comets Hyakutake (C/1996 B2) and Hale-Bopp (C/1995 O1). In addition the unidentified molecular emission bands first observed in optical spectra of the plasma tail of comet Halley (1P/1982 U1), were observed in low resolution (lambda /Delta lambda ~ 1000-4400), pure ion tail spectra of comet Hyakutake and three other recent comets, but not in comet Hale-Bopp. Of the ten cosmically abundant species with known laboratory molecular ion spectra having electronic transitions in the 4000 - 7000 Angstroms region, none could be attributed to the unidentified cometary emission bands. Based on 1) the asymmetric spatial distribution of the unidentified bands with respect to the comet nuclei, 2) the wavelength coincidence of the strongest unidentified emission feature with a strong unidentified emission line observed in high resolution coma spectra of two of the same comets, and 3) detection of either all or none of the unidentified bands in our sample of six comet tail spectra, we argue that the source of the unidentified bands is probably a single molecular species arising from an ion source located near the nucleus. We suggest that an extended ionopause and/or an extended collision zone (Combi, private communication) arising from the relatively large production rate of comet Hale-Bopp, may account for the absence of the unidentified ions in tail spectra of comet Hale-Bopp, and that these extended cometary zones may also have significantly affected the ion tail abundance ratios observed for other species.
AAS/Division for Planetary Sciences Meeting Abstracts #30
- Pub Date:
- September 1998