SPECTROPHOTOMETRY OF THE CONTINUUM IN 18 COMETS

We have studied the continuum emission in spectra of 18 comets. We find that the gas-to-dust ratio (GDR) increases with increasing activity (as measured by the production of CN). This may be explained by having dust production depend on the gas production raised to the 0.55 ± 0.17 power (1σ error). We interpret this to indicate that cometary gas production in the range of heliocentric distances (and gas production rates) for which we have data, is primarily through a porous mantle. Approximately equal numbers of comets show changes in continuum brightness with distance from the optocenter equal to, or significantly slower than, the canonical (distance)-1 falloff. All continua were red compared to the solar spectrum, with an average reddening of 22% per 1000 Å (between 4400 and 5700 Å). There is some evidence for a change in the slope of the continuum with distance from the optocenter in some comets. Trends in these observables throughout the apparition of each comet are discussed, and comparisons among comets are made. © 1992.