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

TL;DR

This study reports the first photometric detection of an exocomet transit in the young star HD 172555, providing insights into small body populations and their dynamical evolution in debris disk systems.

Contribution

It presents the first evidence of an exocomet transit in HD 172555 using CHEOPS photometry, linking photometric and spectroscopic observations of exocomets.

Findings

Detected a transient absorption consistent with an exocomet passing near the star.

Estimated exocomet size of approximately 2.5 km, similar to solar system comets.

Found that the exocomet's properties are comparable to those observed in other systems like β Pictoris.

Abstract

HD$\,$172555 is a young ($\sim$20$\,$Myr) A7V star surrounded by a 10$\,$au wide debris disk suspected to be replenished partly by collisions between large planetesimals. Small evaporating transiting bodies, exocomets, have also been detected in this system by spectroscopy. After $\beta\,$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 HD$\,$172555 with the CHEOPS space telescope in order to detect shallow transits of exocomets with a typical expected duration of a few hours. The large oscillations in the lightcurve indicate that HD$\,$172555 is a $\delta\,$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 $6.8\pm1.4\,$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 $\beta\,$Pic system. The nuclei of solar system's Jupiter family comets, with radii of 2-6$\,$km, are also comparable in size. This is the first evidence for an exocomet photometric transit detection in the young system of HD$\,$172555.