Abstract :
[en] Comets are among the most pristine remnants of the early Solar System, preserving valuable information about the conditions that prevailed during its formation. Studying their physical and chemical properties, as well as their evolution, provides crucial insights into the processes that shaped the Solar System and its planetary bodies. Rich in volatile compounds and complex organic molecules, comets may have played a significant role in delivering water, prebiotic materials, and noble gases from the interstellar environment to the early Earth, potentially contributing to the emergence of life.
Although dozens of potentially observable comets reach perihelion every year, only a small fraction are studied in any detail. Except for extensive astrometric surveys and monitoring of apparent visual magnitudes, homogeneous datasets on comet composition are scarce. Nowadays, it is more and more difficult to obtain observation time on the ever larger and fewer telescopes available. Only a couple of nights at most will usually be available for a given
team to get the detailed chemical information for only one comet, at one given heliocentric distance, usually close to perihelion when the comet is most active. However, comets vary unpredictably for a variety of reasons. Observing them on a limited number of nights could then provide a biased view of their composition and evolution.
Comparison of the measurements of different comets, and even of the same comet observed at different times by different teams using different techniques, is then a very risky matter because of systematic effects. Gathering homogeneous physical data on as large a sample as possible is then very important. Even for comets not studied in detail, homogeneous data are invaluable for understanding the distribution of physical and chemical properties. Only with this information will we be able to infer general conclusions from the detailed information obtained for the very few well-observed comets.
This is exactly my goal with the TRAPPIST telescopes, which I use 50\% of their observing time for a photometric and astrometric survey in both the southern and northern hemispheres. 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 dozens of Jupiter-family comets and more than 40 long-period comets (including the dynamically new), in addition to the third interstellar comet 3I/ATLAS, which was discovered in 2025. To make these observations possible, telescopes are equipped with large, high-quality cometary narrow-band filters. Those filters were built by NASA for the observation campaign of the great comet Hale-Bopp. They isolate small spectral regions where the given cometary species are mostly emitting (emission bands produced by resonance fluorescence of the molecules in the UV sunlight), as well as nearby continuum regions (the dust-reflected solar spectrum). From the different gaseous species production rates, I derive the relative molecular abundances, such as CN/OH because water is usually the reference species, but also other ratios such as CN/C$_2$, etc.
The comparison of those ratios from comet to comet allows finding/confirming compositional groups of comets (comet taxonomy) and possibly a link with their dynamical properties (namely their reservoir of origin). The variations with heliocentric distance will also give important information about ice sublimation, while possible variations for a given comet during a given night could reveal some heterogeneity of the ice in the nucleus. The dust production is also estimated using the quantity $Af\rho$, a parameter that is relatively independent of the size of the aperture and of the wavelength. Dust is a very important part of coma chemistry, and correlations will be searched for, such as the dust-to-gas ratio to heliocentric distance, abundance ratios, dynamical types, etc.
The first study in this work focuses on comet C/2017 K2 (PanSTARRS), a dynamically new comet that displayed remarkable activity at unusually large heliocentric distances. Combining nearly eight years of TRAPPIST photometric monitoring,
comprising more than 270 nights and over 2200 images, with high-resolution spectroscopy from UVES and CRIRES$^+$ at the Very Large Telescope, we trace the evolution of its brightness, colors, and gas production rates (OH, CN,
C$_2$, C$_3$) across a wide range of heliocentric distances. The comet exhibits a smooth, outburst-free brightening, stable optical colors, and a gradual increase in molecular activity approaching perihelion. Combined optical and infrared data identify HCN as the primary source of CN and C$_2$H$_2$ photodissociation as the dominant source of C$_2$, while a
transient pre-perihelion dip in activity is attributed to partial depletion of near-surface supervolatile ices, followed by a resurgence driven by water-ice sublimation within $\sim$3~au.
The second study presents a comparative photometric analysis of eight long-period comets observed between 2010 and 2024, four returning LPCs and four DNCs, examining light curves, broad-band colors, magnitude slopes, the dust proxy $Af\rho$, and gas production rates. DNCs show activity emerging well beyond 3--5~au before perihelion, systematically lower pre-perihelion activity indices ($n\sim1$--3 versus $n\sim5$--13 for LPCs), and elevated dust and gas production at large distances, consistent with supervolatile-dominated activity. LPCs, in contrast, brighten steeply only within $\sim$3~au, consistent with water-ice-driven activity following volatile depletion from prior perihelion passages. Broad-band colors show no significant separation between the two classes, and comet C/2009~P1 (LPC-like DNC behavior), underscoring that individual thermal history, rather than dynamical classification alone, governs the observed activity.
Together, these results confirm that DNCs are compositionally more pristine than the returning LPCs and establish a quantitative framework linking activity onset, dust and gas production, and thermal processing to dynamical history. This framework directly supports the preparation and scientific exploitation of the ESA Comet Interceptor mission, and motivates continued long-term, multiwavelength monitoring of returning LPCs and DNCs.
