Kiefer, F.; Van Grootel, V.; Lecavelier des Etangs, A.; Szabó, Gy. M.; Brandeker, A.; Broeg, C.; Collier Cameron, A.; Deline, A.; Olofsson, G.; Wilson, T. G.; Sousa, S. G.; Gandolfi, D.; Hébrard, G.; Alibert, Y.; Alonso, R.; Anglada, G.; Bárczy, T.; Barrado, D.; Barros, S. C. C.; Baumjohann, W.; ... (2023). Hint of an exocomet transit in the CHEOPS light curve of HD 172555. Astronomy and astrophysics, 671, A25. EDP Sciences 10.1051/0004-6361/202245104
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HD 172555 is a young (~20 Myr) A7V star surrounded by a 10 au wide debris disk suspected to be replenished partly by collisions between large planetesimals. Small evaporating transiting bodies, that is exocomets, have also been detected in this system by spectroscopy. After β Pictoris, this is another example of a system possibly witnessing a phase of the heavy bombardment of planetesimals. In such a system, small bodies trace dynamical evolution processes. We aim to constrain their dust content by using transit photometry. We performed a 2-day-long photometric monitoring of HD 172555 with the CHEOPS space telescope in order to detect shallow transits of exocomets with a typical expected duration of a few hours. The large oscillations in the light curve indicate that HD 172555 is a δ Scuti pulsating star. After removing those dominating oscillations, we found a hint of a transient absorption. If fitted with an exocomet transit model, it would correspond to an evaporating body passing near the star at a distance of 6.8±1.4R★ (or 0.05±0.01 au) with a radius of 2.5 km. These properties are comparable to those of the exocomets already found in this system using spectroscopy, as well as those found in the β Pic system. The nuclei of the Solar System's Jupiter family comets, with radii of 2-6 km, are also comparable in size. This is the first piece of evidence of an exocomet photometric transit detection in the young system of HD 172555.
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