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Metallicity matters in the tidal damping of stellar obliquities

Presentation #303.05 in the session “Population-Level Exoplanet Demographics”.

Published onJun 01, 2021
Metallicity matters in the tidal damping of stellar obliquities

In our Solar system, the Sun’s spin axis is nearly aligned with the planetary orbit normals; the Sun’s obliquity is small. A growing number of stars hosting extrasolar planets have had their obliquities measured, allowing the Solar system to be placed into a wider context. Among these obliquity measurements, stars hosting hot Jupiters (planets of Jupiter mass with orbits shorter than 10 days), also tend to have low obliquities, but only if the star is cooler than about 6200K. Hotter stellar hosts exhibit a wide range of obliquities, ranging from well-aligned to retrograde spins. A long-standing hypothesis to explain this temperature trend holds that cooler stars efficiently damp their obliquities owing to tidal dissipation in their thick convective envelopes. Though promising, the theoretical viability of the tidal hypothesis remains uncertain. In this talk, I use a combination of stellar models and analytic theory to arrive at a new testable prediction arising from this hypothesis. Specifically, the mass associated with the break between aligned and misaligned stars should depend on metallicity. Concurrently, we help explain the “sharpness” of the observed demarcation between aligned cool stars, and misaligned hot stars. Physically, the distinction arises due to a competition between stellar magnetic spin-down and tidal obliquity-damping. This work provides important observational constraints for future tests of the tidal erasure theory.

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