title: Characterisation of Jupiter-family comet nuclei by time-series photometry
title: The characterisation of Jupiter-family comet nuclei by time-series photometry
titleType: AlternativeTitle
Creators: name: Donaldson, Abbie Louise
Publisher: name: The University of Edinburgh
Year: 2025
Subjects: subject: Jupiter-family comets (JFCs)
subject: Rotational lightcurves
subject: Convex lightcurve inversion
subject: Phase functions
subject: Legacy Survey of Space and Time (LSST)

description: The nuclei of short period comets contain evidence of the processes they have experienced throughout their evolutionary lifetimes. When far from the Sun and unobscured by a coma of gas and dust, their nuclei can be detected and studied directly by ground-based observing facilities. This thesis presents new rotational lightcurves for the nuclei of six large, low-activity Jupiter-family comets. All new lightcurves are calibrated to Pan-STARRS 1 r−band magnitudes and combined with existing lightcurve datasets, thus providing wide-ranging object viewing geometries. For three of these comets, there is sufficient geometry coverage in the lightcurves to derive uniquely-defined models of the shapes and orientations of their nuclei using the technique of convex lightcurve inversion. This thesis provides the first dedicated application of this method to calibrated JFC nucleus lightcurves. The new convex shape models indicate an elongated nucleus for 162P/Siding Spring, and more spheroidal shapes for the nuclei of 137P/Shoemaker-Levy 2 and 143P/Kowal-Mrkos. Artificial lightcurves generated using these convex models suggest lower limits of nucleus elongation of 1.82, 1.20, and 1.54 respectively. If the convex shape model properties are representative of the true nucleus elongations, this reduces the observed bilobed fraction for short period comets to 60 per cent, which is still significantly higher than is currently estimated for other minor planet populations. All three comets have pole obliquity &gt; 90° indicating retrograde nucleus rotation. Using the shape models to remove rotational modulation from the lightcurves, phase functions were measured as ß = 0.048 ± 0.01 mag/deg for 137P, ß = 0.048 ± 0.002 mag/deg for 143P, and ß = 0.051 ± 0.002 mag/deg for 162P. As unique shape models were not possible for comets 10P, 169P or 172P, their phase functions were estimated without correction for rotational variation as ! = 0.050 ± 0.001 mag/deg, 0.060 ± 0.001 mag/deg, and 0.037 ± 0.001 mag/deg respectively. The geometric albedos derived for the targets ranged from 0.022 − 0.046, consistent with the dark surfaces typical of JFCs. None of the three JFCs which were observed at phase angles &lt; 1° exhibited evidence for an opposition surge. This suggests that either this phenomenon occurs only at phase angles extremely close to 0! for JFCs, or potentially that their surface regolith is more compacted than other objects with similarly low albedos. Combining the newly-derived surface properties with those of other well studied JFCs appears to contradict an existing hypothesis regarding their evolution, however I stress the significance of determining nucleus surface properties in a consistent manner before interpreting this result further. In this work I also consider the possible applications of photometric data points acquired by upcoming all-sky surveys for the characterisation of JFC nuclei. The Legacy Survey of Space and Time (LSST) at the Vera C. Rubin Observatory will deliver high quality, temporally-sparse observations of millions of Solar System objects on an unprecedented scale. Such datasets will likely enable the precise estimation of small body properties on a population-wide basis. I simulate sparsein- time lightcurve points acquired with the Baseline 3.0 survey cadence between 2024 − 2033 for the orbit of a typical JFC. Convex lightcurve inversion is used to assess if the simulation input parameters can be accurately reproduced for a sample of nucleus rotation periods, pole orientations, activity onset distances, shapes and sizes. I find that the rotation period and pole direction can be reliably constrained across all nucleus variants tested, and that the convex shape models, while limited in their ability to describe complex or bilobed nuclei, are extremely effective for correcting sparse photometry for rotational modulation - in the tests, the average phase function fit to the uncorrected points deviated from the true value by 4 per cent, which was reduced to 0.9 per cent following rotational correction. Based on this analysis, it is anticipated that photometric data points from the LSST alone will significantly enhance our present understanding of the distributions of JFC spin-states and phase functions distributions of JFC nuclei, though follow-up observations will likely be required for the characterisation of sub-kilometre nuclei.