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Evidence for Efficient Tidal Realignment of Giant Planets Orbiting Evolved Stars

Presentation #616.11 in the session Orbital Dynamics and Planet-Planet Interactions.

Published onApr 03, 2024
Evidence for Efficient Tidal Realignment of Giant Planets Orbiting Evolved Stars

The alignment between the stellar spin-axis and the orbital plane of planets (the obliquity) is a key diagnostic for planet formation. Hot Jupiters orbiting stars hotter than the Kraft break (> 6250 K) display a wide range of obliquities, while similar planets orbiting cool stars are preferentially aligned. A leading theory to explain this trend is stellar tides, which are thought to damp initially high obliquities, and should be more efficient in stars with convective envelopes. Evolved stars, particularly those which have crossed the Kraft break and gained deep convective envelopes, provide a unique test for damping timescales. We present the first systematic study of obliquities using the Rossiter-McLaughlin effect for planet-hosting subgiants that recently developed convective envelopes, including the confirmation of several new hot Jupiters discovered by the TESS Mission. We find that hot Jupiters orbiting subgiants that crossed the Kraft break are well aligned with the spin-axis of their host stars, indicating efficient tidal realignment after the emergence of a stellar convective envelope. We furthermore demonstrate that the observed obliquity pathway for subgiants can be well described by tidal evolution models that include dissipation of orbital angular momentum by inertial waves in the stellar convective envelope, with an enhanced obliquity damping efficiency that is consistent with tidal theory.

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