Post-Perihelion HST Observations of Comet Hale–Bopp (C/1995 O1)

Abstract Post-perihelion observations of Comet Hale–Bopp (C/1995 O1) were made with the Hubble Space Telescope (HST) on 27 August 1997 ( r =2.48 AU; Δ=2.99 AU), 11 November 1997 ( r =3.38 AU; Δ=3.31 AU), and 19 February 1998 ( r =4.44 AU; Δ=4.40 AU) using the newly installed Space Telescope Imaging Spectrograph (STIS). The STIS CCD was used to image the comet, and useful spectroscopy was obtained between 2000 and 3190 A using the G230LB, G230MB, and G230L gratings. The morphology of the images closely resembled that obtained at similar heliocentric distances preperihelion, but the dust production rates may have been slightly lower post-perihelion. We find no evidence for any companions to the nucleus in the STIS images, but fairly bright objects (up to ∼20% of the brightness of the main nucleus) could be easily “hidden” within the strong coma jets. Emissions from OH and CS were observed in the spectra and were used to derive production rates for H 2 O and CS 2 . As with the dust, the gas production rates appear to be somewhat smaller post-perihelion than preperihelion. The two-dimensional STIS data allowed us to map the spatial distribution of the OH emission with a spatial resolution of ∼0.″1, and the ∼6-A spectral resolution of the G230MB grating permitted a detailed examination of the OH excitation. The relative intensities of the rotational lines in the OH(0,0) band are fairly well matched by a standard fluorescence excitation model. The OH spatial brightness profile was slightly asymmetric (∼20% brightness differences when comparing two different directions), and the intensity peak was offset by ∼3200 km from the continuum peak for both the August and November 1997 observations. Perhaps this offset may be explained by asymmetric ejection of H 2 O molecules from the nucleus, but we have not attempted to model this effect. Except for this offset, conventional models for the spatial distribution of OH provide a good match to the data from November 1997. The observed spatial profile for the August 1997 observation is well matched by our model at large cometocentric distances but is considerably flatter near the nucleus (within ∼2″ of the continuum peak) than predicted. The available evidence strongly suggests that an optical depth effect, rather than the production of OH from a population of icy grains in the coma, is responsible for the observed flattening of the OH spatial brightness profile.