Exploring Disk-Torquing as a Mechanism for Exciting Spin-Orbit Misalignments in Exoplanetary Systems

Abstract

Over the last 15 years, precise detections of exoplanetary systems have given us the ability to measure the relative alignment of host stars and their planets. Observations have shown that unlike in the Solar System, a substantial fraction of exoplanets have orbital angular momentum vectors that are significantly misaligned from the spin axis of their host stars. This gives rise to an interesting question --- assuming a star and its protoplanetary disk begin with a shared angular momentum vector, how do these obliquities arise? One possible mechanism includes a binary companion to the host star, at some distance beyond the disk, which (assuming it has some initial misalignment from the plane of the protoplanetary system) would provide a torque on the disk, causing its line of nodes to recess. In most cases, this leads to a substantial misalignment between the disk and the star by the time the disk has dissipated, presumably leaving behind planets with high spin-orbit misalignments. Here, I apply a Hamiltonian framework to study this system of a host star with a protoplanetary disk and a binary companion. I thoroughly characterize the distribution of possible misalignments that can arise from this setup, creating probability density functions of final star-disk misalignments from this disk-torquing. I find that, while the full range of misalignments is indeed possible from this mechanism, the expected distribution does not match well with the observed distribution of star-planet misalignments. As such, I argue that the disk-torquing method alone cannot be a dominant player in exciting these obliquities.