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A. L. Cochran, E. S. Barker, W. D. Cochran (U. Texas)
Comets delivered some of the volatiles that we see today in the atmospheres of the terrestrial planets, but it is not certain how important a source of volatiles the comets represent. In order to ascertain the relative importance of comets as a source of volatiles, it is necessary to understand the volatile abundances of comets. Owen and Bar-Nun (Icarus 116, 215, 1995) have pointed out that nitrogen is an important guide to the volatile abundances of comets because it is trapped and released by amorphous ice in a manner which is similar to argon. Since nitrogen is cosmically abundant and can exist in a variety of molecules with varying amounts of volatility, it can be compared with several reservoirs of ices in astronomical sources. However, measuring the N2 content of cometary comae is extremely difficult because the available transitions are few, and the Earth's atmosphere contains significant quantities of N2.
In the past, the (0,0) transition of N2+ has been reported in the spectra of comets. This transition occurs at approximately 3900Å. However, care must be taken when observing this transition because N2+ is excited in the atmosphere of the Earth, especially near dusk and dawn when comets are often observed. Thus, accurate measurement of N2+ in cometary spectra requires both good spatial and spectral resolution to separate the features from that of the Earth. We obtained high S/N spectra of Hale-Bopp at R=200,000 and at several distances into the tail in order to place hard limits on the N2+ in the coma of this comet. In addition, we obtained excellent observations of comet de Vico, both of the optocenter and the tail. There is no clear detection of N2+ in either of these comets. We report on our observations and on the limits we can place on N2+ in these comets and compare them with previous observations.
This work was supported by NASA Grant NAG5 4208.