The distribution of cometary magnitudes

Summary The distribution of magnitudes for long-period comets with P > 200 yr, intermediate period comets with 15 < P < 200 yr and short-period comets with P < 15 yr were analysed using a maximum likelihood method. It was found that the intermediate comets and short-period comets appear to be distinct cometary populations. On breaking the short-period comets down into smaller period ranges it was found that there was no difference in the brightness distribution indices, confirming that they are all likely to be members of the same population. The long-period comets with P > 200 yr were divided by perihelion distance and also by discovery date. It was found that the brightness distribution does not vary significantly with perihelion distance. The comets discovered during the period 568–1830, however, have a statistically different distribution index from those discovered in the period 1830–1978, giving clear evidence for the presence of observational selection in the earlier observations. Only the more recent observations should be employed in statistical studies. Comparison of the indices for the short-period comets and the long-period comets show that they are not significantly different. A common ancestry for these two groups cannot therefore be ruled out.