Signature of exocomet transits observed with Ariel

Abstract

We investigate the potential of the Ariel space telescope to detect and characterise exocomet transits through their wavelength-dependent photometric signatures. Building on a radiative-transfer framework, we model exocomet transits for three representative dust morphologies, three grain-size distributions, and four homogeneous dust compositions using synthetic photometric bands covering the Ariel wavelength range. The modelled events typically last 1-2 days and produce photometric variations of a few millimagnitudes. We find that the wavelength dependence of the transit signal provides stronger diagnostics of the dust properties than transit morphology alone. In particular, colour variations relative to the VISPhot band and their wavelength-dependent slopes separate the considered compositions into two broad groups: carbonaceous dust and graphite, and olivine and pyroxene. While the compositions within each group show similar behaviour, the two groups remain distinguishable across the dust morphologies and grain sizes considered. We further assess the detectability of these signals using the $β$~Pictoris spectral energy distribution and ArielRad simulations. Integrating over broad synthetic photometric bands substantially improves the signal-to-noise ratio, allowing millimagnitude-level signals to be detected with short integration times for sufficiently bright targets. Ariel's simultaneous optical-to-infrared coverage therefore offers the possibility not only of detecting exocomet transits, but also of characterising their chromatic signatures and constraining broad classes of dust properties. We also discuss the limitations of the modelling assumptions and the potential of coordinated observations with long-baseline photometric missions.