Doctoral thesis (Dissertations and theses)
Monitoring the activity and composition of comets with TRAPPIST telescopes
Moulane, Youssef
2021
 

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Keywords :
comets; composition; TRAPPIST
Abstract :
[en] Comets are remnants of the early stages of the Solar System and the most primitive Solar System bodies. Understanding their nature and evolution history provides important clues about the formation of the Solar System and the planets. Comets contain complex organic molecules, and may have played a key role in delivering water, organics and noble gases from the interstellar medium to the early Earth, contributing to the origin of life. The strong scientific interest in the Solar System small bodies is well demonstrated by numerous space missions in the recent years, such as Stardust, Deep Impact, Dawn and New Horizon etc. Particularly, ESA’s very successful Rosetta mission recently visited comet 67P/Churyumov–Gerasimenko and made several ground-breaking discoveries that have dramatically increased our knowledge about comets. However, most of the space missions can only perform short-term studies over a narrow time window of single objects. We are currently lacking long-term observations to study variations in the activity and composition in the coma of a comet and how it evolves along its orbit around the Sun. We regularly use the two robotic TRAPPIST telescopes to observe relatively bright comets (V<12 mag) that are visible in both hemispheres. These telescopes are equipped with narrow-band cometary filters that allow us to collect images of a comet at wavelengths where the light is emitted by the main gaseous species accessible in the optical (OH, NH, CN, C2, and C3). In addition, we observe comets in three continuum windows (scattered sunlight) that allow us to characterize the dust component of the comet. Thanks to the large amount of telescope time available on both telescopes for this project, we collected a unique data set of thousands of photometric measurements of 35 comets including 18 Jupiter-family Comets (hereafter JFCs) and 16 Long Period Comets (hereafter LPCs) in addition to the first active interstellar comet 2I/Borisov which was discovered in 2019. Each comet is monitored along an important part of its inner solar system orbit, starting at 3 au from the Sun (where water start to sublimate) and all the way to its perihelion. Post perihelion, we followed the comet again all the way out to 3 au at least. Then, through cometary coma models, the abundances of the main chemical species (accessible in the optical part of the spectrum) and dust production proxy (known as Afrho) and its color were determined. These measurements, thanks to the high quality and homogeneity of the observations and of the verified analysis procedure, allowed us to address important and long debated questions like the existence of comet compositional classes (comet taxonomy), the changes of molecular abundance ratios with the distance to the Sun and the link between chemical composition and dynamical origins, which is a fundamental step in understanding the formation of comets and the Solar System itself. Among 29 comets for which the gas emissions were detected (at least CN and C2), we identified three depleted comets in carbon-chain elements. These comets are 21P/Giacobini–Zinner, 260P/McNaught and 398P/Boattini in addition to the first interstellar comet 2I/Borisov. 21P is known as a depleted comet for a long time , but we confirm its depletion in its 2018 passage. The other three comets were found to be depleted in C2 and C3 for the first time with TRAPPIST. We made an extensive monitoring of the activity of the first interstellar comet 2I/Borisov. We presented an initial characterization of its activity including magnitude, Afrho dust parameter and dust colours during our observation period. 2I/Borisov was found to be depleted in C2 with respect to CN similar to the Solar System comets carbon-depleted group. No LPC was found to be depleted in carbon-chain elements, except for comet C/2019 Y4 which was at the limit of depleted comets before its fragmentation where we see a significant change in its coma composition. We also found four comets (21P, C/2015 V2, C/2017 T2 and C/2018 W2) depleted in NH with respect to OH and CN. In term of dust activity, most comets show a normal dust/gas activity with respect to the heliocentric distance, except for comet C/2017 O1 which shows a higher dust/gas ratio (especially for Afrho/OH) and comet C/2018 Y1 that shows a lower dust/gas ratio (in both Afrho/OH and Afrho/CN) with respect to most comets. We made a follow-up of five JFCs that showed outbursts. These comets include 29P/Schwassmann-Wachmann 1, 123P/West-Hartley, 155P/Shoemaker, 243P/NEAT, and 260P/McNaught. Comet 29P, as usual, shows multiple outbursts with various amplitudes while other comets show an unique outburst. We discussed the evolution of the outbursts for each comet, by measuring the magnitude, Afrho parameter and dust colours. The dense monitoring of comets allows us to investigate new topics like the heterogeneity of the nuclei composition and the determination of their rotation period through the analysis of the flux variations and of the coma features (jets). We found that the rotation period of comet 41P was surprisingly changed by 26 hr in just two months while the rotation period of comet 46P did not change on both sides of perihelion with an average value of 9.10±0.05 hr. The regular measurements of the comets activity and composition from optical measurements are also invaluable to support observing programs on larger ground based or space telescopes that we plan with our collaborators or other groups to perform complementary studies of the most interesting objects through optical and IR spectrometers (e.g, 21P and 66P/du Toit). In this work, we also discussed the reactive collision of electrons with molecular cations in cometary coma. In order to improve our understanding of the kinetics of the cometary coma, theoretical studies of the major reactive collisions in these environments are needed. Deep in the collisional coma, inelastic collisions between thermal electrons and molecular ions result in dissociation and vibrational excitation of the ions, the rates of these processes are especially elevated due to the high ion and electron densities in the inner cometary coma. We presented the study of reactive collisions of electrons with CO+ and H2+ molecular cations using the multi channel quantum defect theory (MQDT), such as dissociative recombination (DR), vibrational excitation (VE) and vibrational de-excitation (VdE), to understand the importance of these reactive collisions in producing carbon, oxygen and hydrogen atoms in cometary activity. The results shows that among all reactive collisions taking place between low energy electrons and CO+/H2+, the dissociative recombination is the most important process at electronic temperatures characteristic of comets, which can be a major source of atoms in the cometary coma at small cometocentric distances.
Research center :
Space sciences, Technologies & Astrophysics Research (STAR) Institute
Disciplines :
Space science, astronomy & astrophysics
Author, co-author :
Moulane, Youssef  ;  Université de Liège - ULiège > STAR
Language :
English
Title :
Monitoring the activity and composition of comets with TRAPPIST telescopes
Defense date :
22 March 2021
Number of pages :
207
Institution :
ULiège - Université de Liège
Degree :
Docteur en Sciences
Promotor :
Jehin, Emmanuel  ;  Université de Liège - ULiège > Unités de recherche interfacultaires > Space sciences, Technologies and Astrophysics Research (STAR)
Benkhaldoun, Zouhair
President :
Grodent, Denis  ;  Université de Liège - ULiège > Unités de recherche interfacultaires > Space sciences, Technologies and Astrophysics Research (STAR)
Secretary :
Hutsemekers, Damien ;  Université de Liège - ULiège > Unités de recherche interfacultaires > Space sciences, Technologies and Astrophysics Research (STAR)
Jury member :
De Keyser, Johan
Opitom, Cyrielle
Rousselot, Philippe
Yang, Bin
Schneider, Ioan
Available on ORBi :
since 31 March 2021

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