Upper limits to the S 2 abundance in several comets observed with the International Ultraviolet Explorer

Abstract

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 S 2 in the May 11, 1983, IUE spectra of Comet IRAS-Araki-Alcock (1983 VII) at a geocentric distance of 0.032 AU; however, S 2 has not been unambiguously identified in the spectrum of any other comet, although these were observed at larger geocentric distances. We reevaluate the S 2 scale length in Comet IRAS-Araki-Alcock using reprocessed IUE spectra and a recent S 2 g-factor calculation (Kim et al. 1990) to derive a new value of 200 km. We also fit a reddened solar spectrum and modeled S 2 steady-state g-factors to a number of IUE cometary spectra in the wavelength region 2600–3240 Å to determine S 2 abundances and use the ratio-of-variances method to estimate uncertainties or upper limits. The only comet surveyed for which S 2 is detected is Comet IRAS-Araki-Alcock, whose S 2 production rate varied relative to the water production rate over several observations, with abundance ratios Q S 2 / Q H 2O 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 Q S 2 / Q H 2O less than ∼10 −3. We derive an upper limit to the Q S 2 / Q H 2O 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 S 2 abundance relative to water, so spatial resolution limitations alone do not account for the lack of S 2 detections in other comets.