The Effect of Protoplanetary Disk Photoevaporation on Disk-Driven Resonantly Excited Stellar Obliquities

The obliquity of a star, the angle between its spin axis and the orbital plane of its surrounding planets, is traditionally expected to be small, such as the seven degree misalignment of the Sun from the invariable plane of the Solar System. However, in exoplanet systems, observed obliquities are often quite large. In the past decade, many dynamical mechanisms have been proposed to explain these large obliquities. One such proposed mechanism that would take place early in the lifetime of an exoplanet system invokes a host star surrounded by a photoevaporating protoplanetary disk and a distant stellar binary companion. As the disk dissipates, the precession rate of the star about the disk and that of the disk about the binary orbit can become equal. If this occurs, a phase space bifurcation (“secular resonance crossing”) causes the stellar spin to become significantly misaligned from the plane of the disk, in which lie the future planets’ orbits. In this talk, we will present new analytical results about this previously-studied resonant excitation process, including an explanation of a retrograde preference in the theoretical cold planet obliquity distribution. We then consider an alternative disk model where photoevaporation opens a gap in the disk, resulting in two independently-precessing inner and outer disks. We analyze the resulting stellar obliquities from such a process and find that the obliquities of warm and hot planets no longer depend on the planets’ masses in such a model. Instead, warm planets become misaligned and cold planets remain aligned.