Characterizing exo-ring systems around fast-rotating stars using the Rossiter–McLaughlin effect

Abstract Planetary rings produce a distinct shape distortion in transit light curves. However, to accurately model such light curves the observations need to cover the entire transit, especially ingress and egress, as well as an out-of-transit baseline. Such observations can be challenging for long period planets, where the transits may last for over a day. Planetary rings will also impact the shape of absorption lines in the stellar spectrum, as the planet and rings cover different parts of the rotating star (the Rossiter–McLaughlin effect). These line-profile distortions depend on the size, structure, opacity, obliquity and sky-projected angle of the ring system. For slow-rotating stars, this mainly impacts the amplitude of the induced velocity shift; however, for fast-rotating stars the large velocity gradient across the star allows the line distortion to be resolved, enabling direct determination of the ring parameters. We demonstrate that by modelling these distortions we can recover ring system parameters (sky-projected angle, obliquity and size) using only a small part of the transit. Substructure in the rings, e.g. gaps, can be recovered if the width of the features (δW) relative to the size of the star is similar to the intrinsic velocity resolution (set by the width of the local stellar profile, γ) relative to the stellar rotation velocity (v sini, i.e. δW/R * ≳ vsini/γ). This opens up a new way to study the ring systems around planets with long orbital periods, where observations of the full transit, covering the ingress and egress, are not always feasible.