Further investigation of changes in cometary rotation

Abstract

Samarasinha & Mueller (2013) related changes of cometary rotation to other physical parameters for four Jupiter family comets defining a parameter $X$, which is approximately constant within a factor of two irrespective of the active fraction of a comet. Two additional comets are added to this sample in this paper and the claim of a nearly constant parameter $X$ for these six comets is confirmed, albeit with a larger scatter. Taking the geometric mean of $X$ for all the comets above excluding 2P/Encke (as $X$ for each comet was determined with respect to that of 2P/Encke), the expected changes in the rotation periods for a sample of 24 periodic comets are derived. We identify comets from this sample that are most likely to show observationally detectable changes in their rotation periods. Using this sample and including the six comets used to determine $X$, we find a correlation between the parameter $\zeta$ (i.e. the total water production per unit surface area per orbit approximated by that inside of 4 au) and the perihelion distance $q$; specifically we derive $\zeta$ $\propto$ $q^{-0.8}$ and provide a theoretical basis for this in Appendix A. This relationship between $\zeta$ and $q$ enables ready comparisons of activity due to insolation between comets. Additionally, a relationship between the nuclear radius $R$ and the rotation period $P$ is found. Specifically, we find that on average smaller nuclei have smaller rotation periods compared to the rotation periods of larger nuclei. This is consistent with expectations for rotational evolution and spin-up of comet nuclei, providing strong observational evidence for sublimation-driven rotational changes in comets.