Title: Characterising exo-ringsystems around fast-rotating stars using the Rossiter-McLaughlin effect,
Abstract: Planetary rings produce a distinct shape distortion in transit lightcurves.
However, to accurately model such lightcurves 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 modeling 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 ($\delta W$) relative to the size of the
star is similar to the intrinsic velocity resolution (set by the width of the
local stellar profile, $\gamma$) relative to the stellar rotation velocity ($v$
sin$i$, i.e. $\delta W / R_* \gtrsim v$sin$i$/$\gamma$). 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.