The Shoemaker-Levy 9 H impact: Some results from the William Herschel Telescope

We present a CCD lightcurve for the H impact observed at 948 nm by the 4.2-m William Herschel Telescope at La Palma (Spain). We compare the results with other lightcurves at visible and near infrared. There appears to be a common pattern in all the CCD lightcurves: An initial relative maximum is followed by a relative minimum and another maximum. Considering the plume as a superposition of material ejected at various speeds, we suggest the first maximum is likely due to reflected solar light by the plume at its widest phase. The minimum may be due to the disappearance of part of the plume, as the material ejected with vertical velocities lower than 9 km/s descends in its ballistic trajectory and penetrates below the level of solar illumination. The following maximum is most likely due to thermal emission from the reimpacting ejecta (either from the particulates or the gas). The timing of the second maximum and the moment of detection of the plume suggest vertical velocities in the range 9-12 km/s for most of the ejecta, with peak particle concentration at 11 km/s for the L impact, according to our interpretation of Schleicher et al. (1994) lightcurve. The atmospheric level where the visible emission takes place is estimated to be P=25+65 -25 mbar and P=35+30 -35 mbar for the L and H impacts, respectively.