DUST OUTFLOW VELOCITY AND THE GAS-TO-DUST RATIO IN COMETS

The observational determination of coma outflow velocity for gaseous s pecies is fairly straightforward using high-resolution spectroscopy. T he determination of the outflow speed of the dust is much more difficu lt. Most sources cite Bobrovnikoff (1954). This brief report is not a strictly refereed publication, however, and mixes data from different comets. We present here a simple analysis of some data from the Intern ational Halley Watch (IHW) archive. Differences between continuum imag es from successive nights show dust jets and shells clearly. Their mot ion is apparent to first order from the edges of the features. The com ponent of the dust outflow velocity perpendicular to the observer's li ne of sight may thus be determined. This is of course a lower limit on the dust outflow velocity. Many measurements, at different heliocentr ic distances (R), allow determination of the heliocentric dependence o f the dust outflow velocity. We find that the dust outflow velocity in comet P/Halley varied as R(-0.41). If data from an outburst at 14 AU (Sekanina et al. 1992) is included in the fit, this dependence becomes R(-0.55). This confirms the canonical (e.g. Delsemme 1982) inverse-sq uare-root law, and supports the conclusion of Storrs et al. (1992) on the variability of cometary gas-to-dust ratios.