Abstract
Velocity-resolved ( λ/ Δλ ⩾ 10 5) spectral line profiles of H 2O are used to deduce the spatial distribution and expansion velocity of the neutral gas in Comets P/Halley (pre- and postperihelion) and Wilson (1987 VII). The analysis employs a kinematic model to synthesize line profiles for comparison with observed line shapes. The primary fits are presented as synthetic spectral line profiles and alternate displays include two-dimensional column density maps, empirical expansion velocity curves, and density profiles along hypothetical spacecraft trajectories through the coma. These complementary ways of using the fitted three-dimensional density distributions are used to make detailed comparisons with other observational results and with theoretical models. In general, the results of this study demonstrate that the frequently used assumption of spherically symmetric outflow at constant velocity is not a realistic description of neutral gas outflow in comets. The positions and widths of observed H 2O lines indicate that anisotropy in the neutral gas outflow extends to radial distances greater than 10 4 km and the Gaussian-like line shapes are evidence of velocity dispersion acquired from collisions. The radial dependence of the H 2O expansion velocity is consistent with current trends in theoretical work, but the high H 2O outflow velocity observed in Comet Wilson cannot be reconciled with any kinematic model.
Published Version
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