156 research outputs found
Comet taxonomies: composition-based classifications and a search for comets in the Main Belt
No full textComets 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
Cometary science with CUBES
Full text linkThe proposed CUBES spectrograph for ESO's Very Large Telescope will be an exceptionally powerful instrument for the study of comets. The gas coma of a comet contains a large number of emission features in the near-UV range covered by CUBES (305-400 nm), which are diagnostic of the composition of the ices in its nucleus and the chemistry in the coma. Production rates and relative ratios between different species reveal how much ice is present and inform models of the conditions in the early solar system. In particular, CUBES will lead to advances in detection of water from very faint comets, revealing how much ice may be hidden in the main asteroid belt, and in measuring isotopic and molecular composition ratios in a much wider range of comets than currently possible, provide constraints on their formation temperatures. CUBES will also be sensitive to emissions from gaseous metals (e.g., FeI and NiI), which have recently been identified in comets and offer an entirely new area of investigation to understand these enigmatic objects
Monitoring of the chemical composition of comets in the framework of the TRAPPIST survey
Full text linkCharacterising small exoplanets
No full textIt was only thirty years ago that the first extrasolar planet, or exoplanet, orbiting a Sun-like star was discovered. Since then (as of October 2025), 6,022 have been
confirmed across 4,490 planetary systems, 1,013 of which host multiple planets. Whilst these exoplanets have been discovered through a range of methods, transit
photometry and radial velocity measurements have proven the most effective, accounting for∼96% of confirmed exoplanet discoveries. Through these two techniques, planetary radius and mass can be constrained to high precision. From these two parameters, planet density can be derived, enabling estimates of both atmospheric and internal compositions. Characterising small (<4 R⊕) exoplanets in this way is crucial for inferring the frequency of true Earth-analogues and assessing the uniqueness of our own planet.
However, there are several compositional trends for small exoplanets that remain poorly understood. The first is the ‘radius valley’ that separates super-Earths and sub-Neptunes, which has been consistently observed from ∼1.5–2 R⊕, and is largely without planets. Debate currently surrounds the origin of this gap, with proposed scenarios including core-powered mass-loss, photoevaporation, or that these planets are primordially rocky. Interpretations differ on the physical mechanism of atmospheric mass-loss, but the result is the same – primordially accreted atmospheres are removed in such a way that different planets are affected in different ways over different timescales, resulting in a ‘valley’ that separates a population of stripped-core planets (super-Earths) from those that have retained their H/He envelopes (sub-Neptunes). Secondly, the internal structure of sub- Neptunes is not just limited to that of a rocky core surrounded by a gaseous atmosphere, it has been theorised that these planets might hold significant fractions of ices or liquid water. It has been suggested that the radii of planets hotter than 900 K and with masses below 20 M⊕ can be reproduced assuming ice-dominated compositions without significant gaseous envelopes. However, it has also been argued that the existence of small planets with hydrogen atmospheres is consistent with the data, once thermal evolution and mass-loss are properly accounted for. This means that there is a strong degeneracy between water-world and silicate/iron-hydrogen models, and that the characterisation of larger sub-Neptunes in this region of the mass–radius diagram can be used to determine planetary evolution and formation pathways.
With our understanding still limited regarding the origins of these compositional trends, taking steps towards improving characterisation methods of bodies and systems in this size range is vital. Improving our understanding of the origins of the radius valley and the diverse pathways of planetary development will finally help us to ascertain the uniqueness of our own solar system and planets, which is a question that humanity has attempted to answer since the beginning of time
Data for "Upper Limits on CN from Falling Evaporating Bodies"
No full textSpectra from beta pictoris and the intermediate cross correlation data products for the paper.Kenworthy, M., de Mooij, E., Opitom, C., Brandeker, A., Kiefer, F., & Fitzsimmons, A. (2025). Data for "Upper Limits on CN from Falling Evaporating Bodies" [Data set]. Zenodo. https://doi.org/10.5281/zenodo.1473641
Orbital architectures of multiple-star systems that host transiting planets
No full textMultiple-star systems provide unique opportunities to study the environments in which planets would have formed. This is under the conservative assumption that planets form after stars, and therefore, the stellar orbits that sculpted the protoplanetary environment are the same orbits observed today. One of the characteristics that could give insights into the formation mechanism of planets in multiple systems is the alignment of the planetary and stellar planes. Alignment of these could indicate that close binaries and their disks preferentially begin as coplanar systems. Transiting planets allow this alignment to be investigated as they must be in edge-on orbits in order to transit their host star. The orientation of the stellar planes in comparison to an edge-on orbit can then be used to test the alignment in these systems.
In this thesis, I present the results of a long-term monitoring programme of Kepler Objects of Interest (KOIs) with at least one stellar companion. The survey uses adaptive optics imaging and non-redundant aperture masking from NIRC2 at the Keck Observatory to monitor the position and separation of planet-hosting multiple-star systems over many epochs. In Chapter 2, I detail the reduction and analysis techniques used to go from raw images of the multiple-star systems to measurements of the separation and position angle of the companions relative to the primary star. With this astrometry, I aim to demonstrate the alignment of stellar planes in both binary and triple-star systems compared to the edge-on orbit of transiting planets.
In Chapter 3, I present the results from the orbit survey for close binaries with a separation of less than 1000 au. I demonstrate how the astrometry of partial orbital arcs can be used to calculate an angle γ. This angle can be used as an indication of the alignment, as edge-on stellar orbits have γ ∼ 0° and face-on orbits have γ ∼ 90°. Low values of γ can also be explained by misaligned systems,
such as ones with high eccentricity. For this reason, the alignment is tested with a statistical sample where an overabundance of low values of γ indicates alignment. In this chapter, I present the γ distribution found for the sample of binaries, which indicates an overdensity of γ values close to 0° and hence illustrates alignment between the stellar and planetary orbits. Finally, I describe how this distribution can be compared to simulated γ results for different alignment scenarios, such as if the orbits of the planet and stellar companion are independent, to statistically determine the significance of the results.
In Chapter 4, I highlight the systems in the orbit survey that have two stellar companions, creating the first-ever statistical sample of orbits of planet-hosting triple systems. I describe how the methods differ when a second stellar orbital plane is introduced in these hierarchical systems. I perform a similar analysis to the binaries, but for each triple system, I calculate two γ values, one for the inner stellar binary and one for the outer stellar companion. For the nine most compact triples, I again calculate a γ distribution and compare it to simulated distributions and the distribution found for the binaries. The statistical sample demonstrates that the observed trend of stellar-planetary orbit-orbit alignment in binaries does not appear to extend to higher-order multiples and that the triples are not completely coplanar systems.
In Chapter 5, I describe how the astrometry for partial orbital arcs can be used for full orbital analysis by fitting complete orbits and calculating posterior distributions for all the orbital parameters. The inclination of the stellar plane can be used as an additional separate measure for the planetary-stellar alignment. However, as the position of the ascending node for the planet is unknown, this again needs to be used as a statistical sample looking for an overabundance of low mutual inclinations. I describe and execute this method for both the binaries and the triple-star systems, highlighting the differences in the methods. Finally, I conclude that for both the binaries and the triples, there is more mutual alignment between the stellar and planetary planes than could be explained by random isotropic orbits where the orbital planes are independent of each other
Data for "Upper limits on CN from exocomets transiting Beta Pictoris"
No full textSpectra from beta pictoris and the intermediate cross correlation data products for the paper.Kenworthy, M., de Mooij, E., Brandeker, A., Opitom, C., Kiefer, F., & Fitzsimmons, A. (2025). Data for "Upper limits on CN from exocomets transiting Beta Pictoris" [Data set]. Zenodo. https://doi.org/10.5281/zenodo.1522687
Modeling of N2+ and 14N15N+ fluorescence spectrum in comets
Full text link1. IntroductionC/2016 R2 (PanSTARRS) was a surprising comet. Detected on September 7, 2016 by Pan-STARRS it showed an unusual composition when it became a bright comet at the end of 2017 and the beginning of 2018. It developed a coma at large (~6 au) heliocentric distance and observations showed that it had a highly unusual composition: no water molecules (or OH radical) could be detected, and the abundances of the usual radicals (CN, C2, C3) were unusually low, with a surprising coma composition dominated by CO, CO2 and N2 molecules with bright CO+ and N2+ emission lines in the visible range. A high CO production rate of about 1029 molecules s-1 was measured (Biver et al. 2018; Wierzchos & Womack 2018) as well as a high CO2 production rate (CO2/CO=1.1 from Opitom et al. 2019), and a high ratio N2/CO varying between 0.06 and 0.09 (Biver et al. 2018; Cochran & McKay 2018a,b; Opitom et al. 2019; Venkataramani et al. 2020).The detection of such bright N2+ emission lines in this comet highlighted the necessity of a good modeling of the N2+ fluorescence spectrum in comets. The high-quality spectra published by Opitom et al. (2019) provided a good opportunity to test such a model. This model also permits to compute the fluorescence spectrum of the 14N15N+ species, leading to the possibility of future measurements of the 14N/15N isotopic ratio in the N2 molecules, one of the main constituant of the solar nebula.2. ObservationsThe spectra used for this work have been obtained with the UVES spectrograph mounted on the ESO 8.2 m UT2 telescope of the VLT. Three different observing nights have been used, corresponding to February 11, 13 and 14, 2018. One single exposure of 4800 s of integration time was obtained during each night and we used a 0.44" wide slit, providing a resolving power R~80,000. The slit length was 8" corresponding to about 14,500 km at the distance of the comet (geocentric distance of 2.4 au). The average heliocentric distance was 2.76 au. Opitom et al. (2019) describe in more details the data processing.From the 2D spectra having a spatial extension of 30 rows, each of them corresponding to a different cometocentric distance, we extracted different 1D spectra for each night. These spectra were then averaged for similar cometocentric distances allowing a detailed comparison of these spectra at different cometocentric distances, the furthest one corresponding to 2x4 rows at the two extremities of the slit (i.e. at a cometocentric distance varying between 4800 and 6600 km).3. Modeling the N2+ fluorescence spectrumWe developed a new fluorescence model for modeling our observational spectra. The transition involved in this spectrum is the first negative group, i.e. the B2Σu+ → X2+Σg+ electronic transition with the (0,0) bandhead appearing near 3914 Å. We considered the first three vibrational levels (v = 0; 1; 2) for both X2+Σg+ and B2Σu+ state, each of them with all the rotational levels from N = 0 to 40.N2+ having no permanent dipole moment, the pure rotational and vibrational transitions are forbidden (or have a very low probability, through quadrupolar transitions, not taken into account in our model). For that reason it takes a long time for this species to reach its fluorescence equilibrium because it needs a few tens of absorption / emission cycles between the X2+Σg+ and B2Σu+ states to reach this equilibrium. A comparison of the spectrum obtained on the nucleus with the one obtained at the edges of the slit revealed clear differences due to different rotational relative populations. For that reason we decided to model the N2+ fluorescence spectrum with a Monte-Carlo simulation. Such a computational method allows to compute a spectrum at different times from an initial relative population distribution. Our model starts with a Boltzmann relative population distribution of 80 K (representing an estimate of the kinetic temperature in the inner coma) and uses 10,000 s of evolution time.We managed to explain satisfactorily the observed N2+ emission spectrum. Fig. 1 presents a close up view around the (0,0) bandhead. This work, presented in more details in Rousselot et al. (2022) also allowed to compute accurate fluorescence efficiencies. Figure 1: Comparison of the observed VLT UVES spectrum of comet C/2016 R2 (blue) obtained at the ends of the slit with our N2+ model (red).4. 14N15N+ fluorescence spectrumOur modeling of the N2+ fluorescence spectrum can be used to compute the 14N15N+ fluorescence spectrum, leading to the possibility of measuring the 14N/15N isotopic ratio in N2 molecules. We will present such a spectrum as well as a search for this isotopologue in the C/2016 R2 spectra. Such comets are rare but future observations will reveal other comets similar in composition to C/2016 R2. With future observing facilities now under construction (such as the ESO ELT) 14N/15N measurements for N2 molecules will probably become possible, leading to new constraints on this isotopic ratio
- …
