Centaurs and beyond

: analysing serendipitous observations from wide-field surveys

Abstract

The small objects of the Outer Solar System are the material leftover from the construction of the planets and serve as a fossil record of the Solar System’s history. These planetesimals can be divided into populations based on their distance from the Sun and their orbital dynamics. Three such populations include the Kuiper belt, the Centaurs, and the Jupiter-family comets. The Kuiper belt consists of small icy planetesimals residing on orbits beyond Neptune between approximately 30-250 au. Centaurs are cometary precursors that have recently diffused out of the Kuiper belt and form a transitory population on relatively short-lived chaotic, planet-crossing orbits in the giant planet region. The Jupiter-family comets are short-period comets which have been scattered from the Centaur population, and whose orbital dynamics are dominated by Jupiter. By studying the ensemble properties (shapes, sizes, compositions, orbits, and other physical characteristics) of these small icy planetesimals, we can probe the early history and subsequent evolution of the Solar System. Furthermore, as these three populations form an evolutionary continuum, their study can also shed light on the evolution of comets in the Solar System. The main aim of this PhD thesis is to analyse the properties of Kuiper belt objects, Centaurs, and Jupiter-family comets using serendipitous broadband observations from automated wide-field telescope surveys. This thesis focuses particularly on the analysis of these objects’ phase curves, which measure the reflectance of an object at different angles relative to the Sun, and which can serve as a first order probe into its surface properties. Limited telescope time, plus the challenges of Earth-based observations due to poor weather and the limited observability of targets throughout the year, make obtaining accurate phase curves of Solar System objects across a wide range of phase angles difficult. However, automated wide- field telescope surveys, with often long observation baselines, such as the Asteroid Terrestrial-impact Last Alert System (ATLAS), regularly observe many small Solar System objects serendipitously, resulting in datasets large enough to allow accurate analysis of these objects’ phase curves. The first part of this thesis involved the analysis of the phase curves of a sample of bright Kuiper belt objects, Centaurs, and Jupiter-family comets, utilising serendipitous observations from the ATLAS survey. ATLAS’s continuous, multi- year baseline of serendipitous observations has resulted in unprecedentedly large datasets for these objects, preventing the need to compile observations from different telescopes with potentially heterogeneous data reduction pipelines. Phase curves with previously reported negative values in literature are found to become positive with increased numbers of measurements, indicating this phenomenon is due to insufficient sampling of the phase curve profile and not a real physical effect. No strong correlations between phase curve parameters and other object properties such as size and colour were found for any parameter pair, consistent with previous studies. I found the phase curve slopes of several Jupiter-family comets in my sample to closely resemble those of carbonaceous main belt asteroids. Additionally, I utilised the dense sampling of the ATLAS phase curves to search for cometary outbursts in my object sample, which would manifest as unusually bright magnitude measurements above the general phase curve profile. I found two such instances of cometary activity: one previously unreported outburst in 2018 by the Centaur Echeclus, and a new epoch of cometary activity by the large Centaur Chiron commencing in 2021. Analysis of the Centaur Chiron’s new epoch of cometary activity in 2021, which was detected from its phase curve only with no visible coma in ATLAS observations, constitutes the second part of this thesis, in which I utilise serendipitous observations from ATLAS and other wide-field surveys e.g. the Zwicky Transient Facility (ZTF) in addition to targeted-follow-up observations. Chiron was to still be brighter than its pre-outburst magnitude as of February 2023, with rotational effects, phase effects, and surface albedo features precluded as being responsible for Chiron’s observed brightness increase. Previous models of Chiron’s system of debris rings were found to be unable to explain Chiron’s observed brightness evolution during my baseline of observations, though the possibility that these rings are contributing at least partly to the Centaur’s observed brightening cannot be ruled out. An epoch of either new or increased cometary activity is thus deemed the most plausible explanation for Chiron’s observed brightening. No evidence of colour evolution was detected across the observation baseline. No visible coma was detected around Chiron, although a coma either bound to the Centaur’s nucleus too faint to see at Chiron’s geocentric distance remain plausible scenarios. ATLAS’s continuous observations allowed the discovery of a Jupiter-family comet 2023 RN3 in 2023, which was found to be exhibiting cometary activity. Analysis of 2023 RN3’s 2023 cometary outburst constitutes the final part of this thesis, utilising serendipitous observations from ATLAS and ZTF in addition to targeted follow-up observations from the Las Cumbres Observatory (LCO) and Liverpool Telescope. 2023 RN3’s brightness evolution across my baseline of observations was found to be consistent with continuous cometary activity commencing in August 2023, with an increase in brightness of >5.4 mag. I find no evidence of significant evolution in 2023 RN3’s colour across my observations. 2023 RN3’s PSF is visibly extended in LCO and LOOK observations, indicating the presence of a spatially-extended coma, in contrast to my findings for Chiron’s 2021 outburst. Estimates for the maximum dust production rate are consistent with previous measurements for the Jupiter-family comet and Centaur populations.

Date of Award	Dec 2024
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Science & Technology Facilities Council
Supervisor	Megan Schwamb (Supervisor) & Alan Fitzsimmons (Supervisor)