Comet taxonomies: composition-based classifications and a search for comets in the Main Belt

Abstract

Comets are icy small bodies assumed to have remained mostly unaltered since their formation, making them key tracers of the early stages of the Solar System. While comets show a great diversity of dynamical, physical and chemical properties, efforts have been deployed to establish classifications based on these properties, with the aim of identifying different formation and evolution histories. On the one hand, from a dynamical standpoint, it has been found that comets exist in two main reservoirs before being deflected towards the inner Solar System: the Oort Cloud and the Kuiper Belt. However, a third reservoir has recently been identified as some comets have been found in the Main Asteroid Belt. Blurring the traditional divide between asteroids and comets, too few of these objects are known to understand their origin and properties. On the other hand, by quantifying the composition of the gas produced by comets, it has also been shown that classes could be established based on a high or low C₂-to-CN abundance ratio. However, the lack of a clear correlation between carbon-chain depletion and dynamical origins make this divide puzzling. Moreover, previous authors report a decrease of the measured C₂/CN ratio with the heliocentric distance of comets at the time of observation, suggesting that our understanding of C₂ production in comae might be incomplete and that C₂ based taxonomies could be biased. Since these studies typically cover short heliocentric distances (<2au) and different authors do not use consistent modelling parameters (in particular photodissociation scalelengths) to derive these abundance ratios, it is difficult to compare their findings and assess these effects. This thesis looks to bring new insights into these two challenges to established comet classifications.

First, I present a survey of comet volatiles using optical long-slit spectroscopy, aiming to investigate trends and biases in observed compositions. Spectra were acquired for 35 comets using the Isaac Newton Telescope’s Intermediate Dispersion Spectrograph. Having produced a semi-automated pipeline to reduce and analyse this large volume of data, I calculated production rates and upper limits for the main volatile species visible in the near-UV/optical range: OH, NH, C₂, CN, C₃, CH. I present a more focussed analysis of a few targets of interest such as C/2023 H2 (volatile rich) or 12P (outbursting), as well as ensemble results from the study. From these production rates, derived using a Haser outgassing model and commonly used photodissociation scalelengths, I find C₂/CN ratios consistent with a decreasing trend up to 3.5au, making most comets that were observed beyond 2au fall below the depletion threshold. I show that a correlation with perihelion distance is also possible, although I cannot clearly disentangle these two factors. When possible, I also determine and model the spatial distributions of volatiles as seen along the slit and show that a Haser model using literature scalelengths often does not reproduce the measured C₂ profiles, while CN and C₃ show a better agreement between models and observations. Using adjusted scalelengths yields larger C₂ abundances than using literature values, although it could not be determined whether this eliminated heliocentric trends.

Additionally, this thesis presents an imaging survey searching for activity in targeted Main Belt Asteroids in the hope of finding more Main Belt Comets. Using the Isaac Newton Telescope’s Wide Field Camera, r-band observations of 534 asteroids were conducted. These targets were selected based on their closeness to perihelion at the time, and on a hypothesis from previous authors that Main Belt Comets would more likely be found among objects with a longitude of perihelion close to that of Jupiter. After applying wedge photometry and point-spread function analysis methods to detect activity features via an automated pipeline, I made a candidate tail detection on images of asteroid 2001 NL19 (279870). Follow-up observations were conducted with the Liverpool Telescope at the asteroid’s following perihelion but I did not detect recurring activity, implying that the activity of this objects might not be cometary.