CCD imaging of Comet Wilson (1987VII) - A quantitative coma analysis

Abstract We have obtained and analyzed narrow-band CCD images of the coma of Comet Wilson (1987VII) in the emission bands of C 2 , C 3 , and CN as well as in the blue and red continua. A detailed structural analysis revealed no presence of distinct features in the coma of Comet Wilson. The images were flux calibrated and converted into either column densities in case of the neutral radicals C 2 , C 3 , and CN or into distance-independent radiation powers in case of the images taken in the blue and red continua. The reduction of the emission band images included a two-dimensional continuum subtraction. We used the continuum images to determine the color of the dust and investigated their temporal and spatial variations. The analysis of radial profiles constructed in the blue and red continua led to the verification of the expected 1/ρ law for the decrease of surface brightness as a function of nuclear distance whereby the deviations from the 1/ρ law in the outer parts of the profiles could be sufficiently explained by the influence of solar radiation pressure. The terminal ejection velocity of grains, assuming β = 1, was determined from the continuum profiles to v gr = (563 ± 68) m/sec. Radial column density profiles were derived from the emission band images and fitted in terms of the vectorial model. The lifetimes and velocities of the radicals as well as of their parent molecules were determined and the corresponding production rates were calculated. The production rate of the C 2 radical is approximately constant during the whole observation period, whereas the CN production rate decreases with increasing heliocentric distance. The comparison of the CN production rate to published values for HCN indicates the presence of at least one further CN parent in Comet Wilson. The CN profiles were best fitted assuming τ p = 27,500 sec for the lifetime of the parent and 1 km/sec v d 2 profiles are flattening much faster in the inner coma than would be expected from the vectorial model. We were able to artificially compensate this flattening of the profiles by introducing an artificial time-dependence to the model.