Hint of an exocomet transit in the CHEOPS lightcurve of HD 172555

Abstract

HD172555 is a young ($\sim$20Myr) A7V star surrounded by a 10au wide debris disk suspected to be replenished partly by collisions between large planetesimals. Small evaporating transiting bodies, exocomets, have also been detected in this system by spectroscopy. After ${\hat{I}}^2$Pictoris, this is another example of a system possibly witnessing a phase of heavy bombardment of planetesimals. In such system, small bodies trace dynamical evolution processes. We aim at constraining their dust content by using transit photometry. We performed a 2-day-long photometric monitoring of HD172555 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 lightcurve indicate that HD172555 is a $\hat{I}´$Scuti pulsating star. Once removing those dominating oscillations, we find a hint for a transient absorption. If fitted with an exocomet transit model, it corresponds to an evaporating body passing near the star at a distance of $6.8\pm 1.4$R${}_{\star }$ (or $0.05\pm 0.01$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 ${\hat{I}}^2$Pic system. The nuclei of solar system's Jupiter family comets, with radii of 2-6km, are also comparable in size. This is the first evidence for an exocomet photometric transit detection in the young system of HD172555.

12 pages, 10 figures, 5 tables, accepted for publication in A&A