Mapping Stellar Obliquity Across Space and Time

Abstract

The angle between a star's rotational angular momentum vector and the orbital angular momentum vector of its planets---stellar obliquity---encodes the initial conditions and dynamical history of planetary systems. Variations in obliquity with orbital separation may arise from either primordial disk structure (e.g., broken or warped protoplanetary disks with misaligned inner and outer regions) or subsequent dynamical interactions with planets or external perturbers. Comparing obliquities across orbital distance therefore probes the spatial imprint of these processes, while comparisons across age constrain their temporal evolution. To test for signatures of obliquity evolution in both space and time, we compare stellar inclinations with orbital inclinations for 114 single Sun-like stars spanning four samples that probe distinct regimes of orbital separation and age: 49 protoplanetary disk systems, 24 debris disk systems, 21 warm Jupiter systems, and 20 hot Jupiter systems. We find minimum misalignment rates of $33^{+7}_{-6}$% for protoplanetary disk systems, $21^{+9}_{-7}$% for debris disk systems, $23^{+10}_{-8}$% for warm Jupiter systems, and $20^{+9}_{-8}$% for hot Jupiter systems. These rates are consistent within 1$σ$ across all samples. The absence of significant variation in minimum-obliquity distributions with either orbital separation or system age suggests that stellar obliquities for Sun-like stars may be largely established at formation with limited subsequent evolution in both space and time. These results are consistent with post-formation inclination evolution being either infrequent, confined to small misalignments, or restricted to regions of parameter space not examined here---such as systems with close stellar companions, hot host stars, small planets, and long-period giant planets.