Evidence for Primordial Alignment: Insights from Stellar Obliquity Measurements for Compact Sub-Saturn Systems
arxiv(2024)
摘要
Despite decades of effort, the mechanisms by which the spin axis of a star
and the orbital axes of its planets become misaligned remain elusive.
Particularly, it is of great interest whether the large spin-orbit
misalignments observed are driven primarily by high-eccentricity migration –
expected to have occurred for short-period, isolated planets – or reflect a
more universal process that operates across systems with a variety of
present-day architectures. Compact multi-planet systems offer a unique
opportunity to differentiate between these competing hypotheses, as their
tightly-packed configurations preclude violent dynamical histories, including
high-eccentricity migration, allowing them to trace the primordial disk plane.
In this context, we report measurements of the sky-projected stellar obliquity
(λ) via the Rossiter-McLaughlin effect for two sub-Saturns in
multiple-transiting systems: TOI-5126 b (λ=1± 48 ^∘) and
TOI-5398 b (λ=-24^+14_-13 ^∘). Both are spin-orbit aligned,
joining a fast-growing group of just three other compact sub-Saturn systems,
all of which exhibit spin-orbit alignment. Our results strongly suggest that
sub-Saturn systems are primordially aligned and become misaligned largely in
the post-disk phase through violent dynamical interactions inherent to
eccentric migration, as appears to be the case increasingly for other exoplanet
populations.
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