Comets are icy planetesimals formed in the outer solar system. The nucleus, typically a few kilometers in diameter, is essentially composed of water ice mixed with carbon oxides, methane, ammonia, and of dust particles. When the comet approaches the Sun, the ices sublimate, forming a gaseous and dusty coma. Solar radiation and wind blow this material to form the spectacular cometary tails. Because comets are thought to be preserved remnants of the early stages of the solar system, they are considered as belonging to its most pristine bodies. Understanding their origin, evolution and composition is therefore a clue to the history of our planetary system. Moreover comets contain complex organic molecules, and may have played a key role in the transfer of water and organics from the interstellar medium to the early Earth, then contributing to the origin of life.
Our research group uses the TRAPPIST telescope for a photometric and astrometric survey of Southern comets. For relatively bright comets we measure once a week the gaseous production rates and the spatial distribution of several species among which OH, NH,CN, C2, as well as ions like CO+. The dust production rates are determined, also for fainter comets. Such regular measurements are rare because of the lack of observing time on larger telescopes. Yet they are very valuable as they show how the gas production rate of each species evolves with respect to the distance to the Sun. Those observations allow to determine the composition of the comets and the chemical class to which they belong (rich or poor in carbon for instance), possibly revealing the origin of those classes. Indeed with dozens of comets observed each year, this program provides a good statistical sample after a few years. Thanks to the way the telescope is operated, follow up of split comets and of special outburst events is possible right after an alert is given and can bring important information on the nature of comets. Light curves from those data are useful to assess the gas and dust activity of a given comet in order, for instance, to prepare more detailed observations with larger telescopes.
To make such observations possible, the telescope is equipped with large (5x5cm) high quality cometary narrow band filters. Those filters are on loan from David Schleicher of the Lowell Observatory (Flagstaff, USA) and were built by the NASA for the observing campaign of the famous comet Hale-Bopp (Farnham et al. 2000). Those filters isolate small spectral regions where given cometary species are mostly emitting (emission bands), as well as nearby continuum regions (dust reflected solar spectrum). In addition to providing the productions rates of the different species through a proper photometric calibration, image analysis can reveal coma features (jets, fans, tails), that can lead to the detection of active regions and measure the rotation period of the nucleus.
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