IUE observations of the evolution of Comet Wilson (1986/): Comparison with P/Halley

Abstract

The ultraviolet spectrum of comet Wilson (1986 l) was observed with the International Ultraviolet Explorer ( IUE) satellite observatory between September 1986 and November 1987. Observations were made at weekly and biweekly intervals near perihelion, making comet Wilson the first dynamically “new” comet to be observed with the IUE through perihelion. This provided the opportunity to compare a “new” comet, Wilson, with a highly evolved comet, P/Halley (1986 III), which was observed both preperihelion and postperihelion at about the same heliocentric distances. The derived production rates of several gases, as well as the visual brightness of comet Wilson, were found to decrease monotonically near perihelion, even though the comet remained at approximately the same heliocentric distance. No short term variability of the type seen in P/Halley was observed in comet Wilson. The water production rates, derived from the observed OH emission, were roughly the same for both comets near 1.2 AU, although earlier (near 2.5 AU inbound), comet Wilson showed significantly greater water production than comet Halley. The column densities of several gas species, relative to OH, were about half as high in comet Wilson as in P/Halley preperihelion at comparable heliocentric and geocentric distances. Previously unidentified spectral features were detected near 1474 and 1425 Å; these were identified as Si emissions, with significant fluorescence efficiencies only when the heliocentric velocity of the comet is near zero. However, the intensity of the λ1474 feature, relative to Si λ1814, is inconsistent with atomic data in the literature.