Disentangling the stellar inclination of transiting planetary systems: fully analytic approach to the Rossiter-McLaughlin effect incorporating the stellar differential rotation

Abstract

The Rossiter-McLaughlin (RM) effect has been widely used to estimate the sky-projected spin-orbit angle, $λ$, of transiting planetary systems. Most of the previous analysis assume that the host stars are rigid rotators in which the amplitude of the RM velocity anomaly is proportional to $v_\star \sin i_\star$. When their latitudinal differential rotation is taken into account, one can break the degeneracy, and determine separately the equatorial rotation velocity $v_\star$ and the inclination $i_{\star}$ of the host star. We derive a fully analytic approximate formula for the RM effect adopting a parameterized model for the stellar differential rotation. For those stars that exhibit the differential rotation similar to that of the Sun, the corresponding RM velocity modulation amounts to several m/s. We conclude that the latitudinal differential rotation offers a method to estimate $i_\star$, and thus the full spin-orbit angle $ψ$, from the RM data analysis alone.