UPPER LIMITS TO THE S2 ABUNDANCE IN SEVERAL COMETS OBSERVED WITH THE INTERNATIONAL ULTRAVIOLET EXPLORER

The presence of sulfur-bearing molecules in comets is well-established, with CS detected in almost all comets observed with the International Ultraviolet Explorer (IUE) satellite, and atomic sulfur detected in many of the brighter ones. A'Hearn et al. (1983) discovered the short-lived species S2 in the May 11, 1983, IUE spectra of Comet IRAS-Araki-Alcock (1983 VII) at a geocentric distance of 0.032 AU; however, S2 has not been unambiguously identified in the spectrum of any other comet, although these were observed at larger geocentric distances. We reevaluate the S2 scale length in Comet IRAS-Araki-Alcock using reprocessed IUE spectra and a recent S2 g-factor calculation (Kim et al. 1990) to derive a new value of 200 km. We also fit a reddened solar spectrum and modeled S2 steady-state g-factors to a number of IUE cometary spectra in the wavelength region 2600-3240 Å to determine S2 abundances and use the ratio-of-variances method to estimate uncertainties or upper limits. The only comet surveyed for which S2 is detected is Comet IRAS-Araki-Alcock, whose S2 production rate varied relative to the water production rate over several observations, with abundance ratios QS/QH between 10-4 and 2.5 × 10-3. Analysis of the spectra of Comets Bradfield (1979 X), Černis (1983 XII), P/Giacobini-Zinner (1985 XIII), P/Halley (1986 III), and others results in upper limits in most cases of QS/QH less than ∼10-3. We derive an upper limit to the QS/QH ratio for Comet Austin (1990 V) of 7 × 10-6, an order of magnitude below the lowest detected value in Comet IRAS-Araki-Alcock. This indicates comet-to-comet differences in the S2 abundance relative to water, so spatial resolution limitations alone do not account for the lack of S2 detections in other comets. © 1992.