Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999dps....31.1719c&link_type=abstract
American Astronomical Society, DPS meeting #31, #17.19
Astronomy and Astrophysics
Astronomy
Scientific paper
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 N_2 content of cometary comae is extremely difficult because the available transitions are few, and the Earth's atmosphere contains significant quantities of N_2. In the past, the (0,0) transition of N_2(+) has been reported in the spectra of comets. This transition occurs at approximately 3900 Angstroms. However, care must be taken when observing this transition because N_2(+) is excited in the atmosphere of the Earth, especially near dusk and dawn when comets are often observed. Thus, accurate measurement of N_2(+) 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 N_2(+) 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 N_2(+) in either of these comets. We report on our observations and on the limits we can place on N_2(+) in these comets and compare them with previous observations. This work was supported by NASA Grant NAG5 4208.
Barker Edwin S.
Cochran Anita L.
Cochran William D.
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