Mémoire

Abstract

Comet nuclei are the most primitive objects in the Solar System and their study provides clues about its formation. These small, rocky celestial bodies, made up of ices and dust grains, are located in cold, isolated regions beyond the planetary region. However, gravitational disturbances can sometimes alter their trajectory, bringing them closer to the Sun. Under the effect of heat, the ices sublime, releasing dust grains and forming a large luminous coma around the core, observable from Earth.

This study focuses on the activity and composition of two Jupiter-family comets: 67P/Churyumov-Gerasimenko and 103P/Hartley 2, observed during two passages close to the Sun. These two comets were the target of space missions Rosetta and EPOXI, respectively. The data used come from intensive observing campaigns carried out in the visible range with the 60-cm TRAPPIST telescopes located at the La Silla Observatory (Chile) and the Oukaïmeden Observatory (Morocco). These telescopes are equipped with broadband Johnson-Cousins (BVRI) filters and a set of narrow-band Hale-Bopp filters, specially designed for cometary observations.

Observations of 67P cover its perihelion passages in 2015 and 2021, while 103P was observed in 2010 and 2023. From the images obtained, we measured the apparent magnitudes of the comets and followed the evolution of their light curves as a function of heliocentric distance. We also compared the results obtained during the two passages, taking geocentric distance into account. The color indices of the two comets were then calculated, revealing stability for 67P between its two passages, in contrast to 103P, for which a decrease was observed. These indices are consistent with the average values measured for Jupiter-family comets, and redder than the solar colors.

We computed the production rates of OH, NH, CN, C2, and C3 molecular species in the coma of both comets, and analyzed their evolution with heliocentric distance. 67P was found to be more active during its second passage, while 103P was found to be more active during its first passage. Dust activity was estimated using the Afrho parameter, showing a similar trend to that of gas, with 67P being more active during its second passage, and 103P more active during its first passage. Our measurements also showed that 103P is a dust-poor comet as compared to 67P. Finally, an enhancement of some images enabled us to analyze the morphology of the comae, revealing, for 67P, a large gas jet emitted by an active region of the nucleus surface and extending radially, and for 103P, one or two jets winding in a spiral.

This study helped to better characterize the composition of the coma and the activity evolution with respect to the heliocentric distance of these two important Jupiter-family comets.